My Research and Inventions

click here to see all content or button bellow for specific topic

 

Spin-Orbit Interaction

Spin and Charge Transport

Relativistic origin of the spin-orbit interaction

An electron moving in an electrical field experiences an effective magnetic field, which acts on the electron magnetic moment (spin). The interaction of the electron magnetic moment with the effective magnetic field is called the spin-orbit interaction.

There are two distinguished classes of effects, which are originated from the spin-orbit interaction:

(class 1): Enhancement of external magnetic field. Effects: (1) perpendicular magnetic anisotropy; (2) magnetostriction; (3) g-factor; (4) fine structure. Localized electrons and atoms of atomic gas experience this class of effects. Time-inverse symmetry is broken by the external magnetic field.

(class 2) Creation of spin polarization by an electrical current. Effects: (1) Spin Hall effect; (2) Inverse Spin Hall effect; (3) Spin relaxation. Conduction electrons experience this class of effects. Time-inverse symmetry is broken by the electrical current.

Breaking of orbital symmetry and time-inverse symmetry are two critical factors, which determine the magnitude of the spin-orbit interaction



Content

click on the chapter for the shortcut

(1). Explanation in short

(2). Relativistic origin of the Spin-Orbit interaction

(3). Lorentz  transformation

(4). Spin-orbit interaction in macro world

(5). Weak and Strong Spin-orbit interaction

(6).Magnitude of the Spin-orbit interaction.

(7). Spin-orbit interaction in the centrosymmetric electric field of atomic nuclear

(8).Spin-Orbit interaction due to the orbital deformation

(9). Time- inverse symmetry and the spin-orbit interaction

(10).Enhancement of magnetic field due to the spin-orbit interaction

(11). g-factor

(12).Perpendicular-to-plane magnetic anisotropy (PMA)

(13). Magneto-elastic effect

(14).Voltage-induced spin-orbit interaction & VCMA effect

(15). Spin-dependent scatterings

(16). Famous misunderstands and misinterpretations of the Spin-Orbit (SO) interaction

(17).Spin-orbit interaction obtained from the Dirac equations

(18). Pauli equation & Spin-Orbit interaction

(19).Merits and demerits of the use of the Hamiltonian approach for calculations of the Spin-Orbit interaction.

(20). Fine structure. Heavy and light holes.

.........


 


Relativistic origin of the Spin-Orbit interaction

Relativistic origin of the Spin-Orbit Interaction
Fig.3 An object (red) is moving in a static electric field. In the coordinate system moving together with the object, the static electric field is relativistically transformed into the effective electric field Eeff and the effective magnetic field Heff In case if the particle has a magnetic moment (spin), there will be a spin precession around the effective magnetic field.

The Theory of Relativity states that a particle moving in an electrical field experiences an effective magnetic field, which is directed perpendicularly to the electrical field and perpendicularly to the particle movement direction. The interaction of this effective magnetic field with the electron spin is called Spin-Orbit interaction. It is important to emphasize that the direction and magnitude of the effective magnetic field does not depend either on the particle charge or on the particle spin.

 

According to the Theory of The Relativity the electric and magnetic field mutually transformed into each other depending on the speed of an observer. For example, if in a coordinate system of static observer there is only a magnetic field, a movable observer will experience this field as both an electrical field and a magnetic field.

A particle moving in a static magnetic field experiences an effective electric field. The effective electrical field acts on the particle charge (the Lorentz force, Hall effect) and forces the particle to move along this field.

A particle moving in a static electrical field experiences an effective magnetic field. The effective magnetic field acts on the particle magnetic moment (spin-orbit interaction) and causes the precession of the magnetic moment around the direction of the effective magnetic field.

 

 

Lorentz  transformation

The electromagnet field is a relativistic object and it is the Lorentz transformation rules as

where Estatic, Hstatic are the electric and magnetic field in the static coordinate system (reference frame) and Emove, Hmove are the electric and magnetic field in the coordinate system, which moves with a constant speed v.

As a result, an electron, which moves in a static magnetic field Hstatic, experience in own reference frame an effective electrical field EHall , which is called the Hall field (Hall voltage). Similarly, when an electron moves in a static electrical field Estatic, it experience in own reference frame an effective magnetic field HSO , which is called the effective spin-orbit magnetic field

For example, when an electron moves in the x-direction

in

The Lorentz force is a tween effect with the effect of the spin-orbit interaction

 

Fig.4. The Lorentz force (Hall effect) and the spin-orbit interaction, both are relativistic effects. The both effects are originated from fact that an electrical and magnetic field are relative to the velocities of observers. A particle, which moves in a static magnetic field, experiences an effective electrical field (The Lorentz force). A particle, which moves in a static electric field, experiences an effective magnetic field.

The Lorentz force (Hall effect) has a similar relativistic origin. An object (red), which is moving in a static magnetic field, experiences an effective electrical field.

Nonrelativistic case (v<<c),

in this case and

The Hall field can be calculated as

The spin-orbit magnetic field can be calculated as

Note: An electron should have velocity component perpendicular to a static electrical field Estatic or a static magnetic field Hstatic in order to experience the magnetic field HSO of spin-orbit interaction or the Hall field EHall

Note: The Hall Effect and the Spin-Orbit interaction are close cousins

the Hall effect ==== results in ====> an effective electrical field

the Spin-Orbit interaction ===results in=====> an effective magnetic field

An electron always moves along the electrical field (but not perpendicularly). Then, how it can experience the spin-orbit interaction?

The trajectory of an electron may be very different. There are many cases when an electron moves perpendicularly to an electrical field. For example, when the electron is orbiting around a nuclear. Another example, an electron current along an interface. Usually, there is an electrical field perpendicularly to the interface, but still the electron may move along the interface (See below)

Which parameter characterizes the spin-orbit interaction? Energy? Spin? Orbital moment? Or any other quantum mechanical parameter?

(It is important): There is only one cause of spin-orbit interaction. It is the magnetic field c of spin-orbit interaction. All other causes are consequences of HSO. For example, the electron spin align itself along HSO. It change electron energy, which can be defined as an energy of the spin-orbit interaction ESO. However, an additional external magnetic field Hext is applied, the electron spin is aligned along total magnetic field HSO+ Hext and ESO has no physical meaning. Therefore, there is only one parameter, which is fully characterizes and describes the spin-orbit interaction. It is HSO.

effective spin-orbit magnetic field HSO

Since the spin-orbit magnetic field HSO is proportional to 1/c2 , the should be very small or almost negligibly small. Should we care about such a tiny effect?

it is correct. Usually, the HSO is very small except cases when the electrical field Estatic is huge. It is the case in close proximity to the nuclear. There the electrical field increase as 1/r, where r is the distance to the nuclear. The nuclear is almost "point-like" object, therefore the electrical field Estatic is huge in close proximity of the nuclear. It makes a large HSO

Any substantial spin-orbit interaction is induced only by an electrical field of a nuclear!!!

Other realistic sources of the electrical field in a solid induce only a very weak spin-orbit interaction

In close vicinity of a nuclear an electron experiences a very strong electrical field of the nuclear. However, this field is very symmetric and the electron experience the opposite signs of the spin-orbit interaction on its path around nuclear. Therefore, the spin-orbit interaction cancels itself and the electron experience no spin-orbit interaction. An externally -applied electrical field or magnetic field or stress field may break the symmetry and the the electron starts to experience very strong effective magnetic field of the spin-orbit interaction. For example, when only only 100 Oe of external magnetic field is applied, an electron may experience an effective magnetic field of 10 000 Oe due to the spin-orbit interaction.


Q. Is the Spin-Orbit interaction a quantum-mechanical effect???

Spin-orbit interaction in macro world
Fig. 5. An imaginary case what would happen if the Sun were charged. In this case the magnetic moment of the Earth would be aligned in respect to the polarity of the Sun charge. The red arrow shows direction of the magnetic moment of the Earth. The blue arrow shows the direction of the effective magnetic field HSO due to the effect of the spin-orbit interaction, which is induced by the electrical field due to the Sun charge. When the the polarity of the charge is reversed, the direction of the effective magnetic field is reversed as well. It follows by the precession of the magnetic moment of the Earth around the effective field until it aligns itself to be antiparallel in respect to the SO magnetic field HSO.

A. No. The Spin-Orbit interaction affects both small objects and large objects. The spin-orbit interaction exists in the macro world as well.

 

 

For example, Figure 5 shows an imaginary case what would happen if the Sun were charged. In this case the magnetic moment of the Earth would interact with the effective magnetic field HSO of the spin-orbital interaction induced by this charge. The magnetic moment of the Earth would be aligned accordingly as it is shown in Fig.5.

 

Is the spin-orbit interaction is a quantum-mechanical effect, because it is only can be derived from the Dirac equations?

A. It is not correct statement. The spin-orbit interaction is fully relativistic effect. It is absolutely not a quantum mechanical effect, even though many quantum- mechanical effects are include the features of the spin- orbit interaction.

See detailed answer in major misunderstandings

In short:

(1) The spin-orbit interaction is a relativistic effect and can be fully described by relativistic equations.

(2) The Dirac is a relativistic quantum-mechanical equation. The Dirac equation describes both the relativistic transformation of the electro-magnetic field and the relativistic transformation of the quantum field of an electron. Calculations of the spin-orbit interaction from the Dirac equation are most precise. (See SO and Dirac equation here)

(3) The Schrödinger equation and Pauli equation, both describe the spin-orbit interaction. Both equations describe the relativistic transformation of the electro-magnetic field, but do not describe the relativistic transformation of the quantum field of an electron. However, using some corrected constants and parameters it is possible to describe the SO interaction by Schrödinger equation and Pauli equations fully precise and fully identical to description by the Dirac equation.

 

 

 

 

 

 

 

 

 

 


 

Energy of spin- orbit interaction

Fig.6 There is a precession of electron spin S around magnetic field HSO of spin-orbit interaction until it aligns parallel to HSO. After the electron spin is aligned, the energy of SO interaction becomes

click on image to enlarge it

Energy of SO interaction

The spin-orbit interaction manifests itself only by the SO magnetic field HSO. The HSO is a very normal magnetic field (almost). There is a precession of electron spin S around magnetic field HSO of spin-orbit interaction until it aligns parallel to HSO. After the electron spin is aligned, the energy of SO interaction becomes

μB is the Bohr magneton

Note: The HSO does not interact with the orbital magnetic moment of electron. It is interact only the spin magnetic moment

It is because of relativistic origin of both the SO interaction and the Lorentz force.

The HSO interacts only with the spin magnetic moment, but not with the orbital magnetic moment or the total magnetic moment

 

Why HSO does not induces the Lorentz force and cannot interact with the orbital magnetic moment?

The HSO is the magnetic field of the relativistic origin. It appears only the coordinate system, which moves together with the electron (See here). The Lorentz force is of the relativistic origin as well and it is originated from the electrical field, which the electron experiences when moves in a magnetic field. In the moving coordinate system, where the electron experiences HSO , the electron does not move and therefore has no experience any Lorentz force

The interaction of the orbital moment with a magnetic field is due to the Lorentz force

 

Note: The total magnetic moment is a quantum- mechanical sum of the orbital magnetic moment and the spin-magnetic moment. The role holds for both cases when spin is align along HSO (along the orbital moment) and when the there is a spin precession around HSO.

Electron spin is aligned along due the spin precession damping (See Fig.6). The spin precession damping is a complex mechanism (See here), which involves an external particle with a non-zero spin (e.g. a photon, a magnon). It could take a relatively a long until full alignment of electron spin along HSO.

 

Is there any cases when the electron spin is not aligned along HSO?

There are many such cases. E.g. the conduction electrons in a metal. The size of a conduction electrons are relatively large. There are many conduction electrons, which simultaneously overlap each other. For this reason, the scattering between quantum states of conduction electrons are very frequent. The time between two consequent scatterings a conduction electron is very short (~ 1 ps). It is far not enough to finish even oven precession period and definitely it is not enough for the electron spin to align along HSO.

 

 


Magnitude of the Spin-orbit interaction.

Except of a few weak effects, all spin-orbital effects are induced by an electrical field of an atomic nuclear and the election movement (rotation) in the close proximity of the nuclear !!

Magnitude of the spin-interaction in is small when an conduction electron moves in any realistic extrinsic or intrinsic electrical field in a solid!!!.

a moderate electrical field + a moderate electron speed => result: a very small spin-orbit interaction

Except for an electron, which moves in a close vicinity of an atomic nuclear

a very strong electrical field + a moderate electron speed => result: a strong spin-orbit interaction


 

Example 1.

 

Even in the of the highest-possible electron speed in solid and largest-possible applied electrical field, the effective magnetic field of the spin-orbit interaction is small!!

Estimation: Maximum electron speed + Maximum applied electrical field

Electron Speed: Saturation Velocity :1E7 m/s (GaAs Si )

It is maximum drift speed of electrons in a solid.

Experimentally I have measured the saturation velocity (See here). An electron can not go faster, because above the saturation velocity the electron intensively illuminate phonons. It is similar to the case when a supersonic plane flies faster than the speed of sound.

The applied voltage: Breakdown voltage: 5E5 V/cm(GaAs, Si)

It is maximum voltage, which could be applied to a semiconductor (a oxide). For higher voltage the avalanche breakdown occurs.

Result:

The effective magnetic field of the spin-orbit interaction is only 0.5 Gauss

It is too small!!! For example, Earth's magnetic field at at the Earth's surface ranges from 0.25-0.65 Gauss.


When the orbital of an electron is center-symmetrical, the orbital moment and spin-orbit interaction can be zero despite of electron rotation

s- orbit of an atom

 
electron orbital can be represented as 2D electron rotation around a rotated axis electron orbital can be represented as 3D electron rotation around a point (nuclear position)  

Both representation of the electron orbital are equivalent.

Fig.8. s-orbital. An electron rotates around a nuclear in spherical orbital. Since at the same time the electron rotates in clockwise and anti clockwise directions, the spin-orbit interaction for opposite rotations cancels each other and the electron does not experience the spin-orbit interaction.
It is not unusual that the electron rotates in opposite directions. Just look at the animation carefully.
click on image to enlarge it

Example 2.

An electron rotating around a nuclear.

Electron Speed: linear speed of electron rotating around atom : ~2.1E6 m/s

The applied voltage:Coulomb Electrical Field in H atom at 1st orbital (r=0.053 nm) 5.1E9 V/cm

The high voltage is because the orbit is very close to the nuclear !!!

Result:

The effective magnetic field of the spin-orbit interaction is 125 kGauss=12.5 T

It is rather large!!!. Such large magnetic field can only be obtained by a superconducting magnet.

It is important: An electron may experience such large magnetic field only when it is very near to the nuclear and only when the electron is rotating around the nuclear.

Spin-orbit interaction and orbital moment

Is the spin-orbital interaction proportional to the orbital moment?

A. Actually, not. Even though there are common tendencies between the spin-orbital interaction and the orbital moment. E.g. When orbital moment is zero, the SO interaction is zero. When orbital moment changes its sign, the SO interaction changes its sign as well.

Even though the "orbital" is a part of name of the SO interaction, the relation between orbital moment and the HSO is complex and not straightforward.

The spin-orbit interaction:

for centrosymmetric electrical field of a nuclear:

orbital momentum vs rotation symmetry

Case 1.

Case 2.

Case 3.

orbital momentum is zero. there is no SO interaction orbital momentum is largest & SO interaction is strongest. Direction of orbital momentum is up orbital momentum is largest & SO interaction is strongest. Direction of orbital momentum is down
Full rotation only 1st-half rotation only 2d-half rotation

Look carefully on a difference of the rotation symmetry

Orbitals

Case 1.

Case 2.

Case 3.

All 3 rotation electron symmetry gives the same spherical orbital

Orbital moment

Case 1.

Case 2.

Case 3.

no orbital moment

The total electron orbital (left) is sum of equal but opposite half - rotations (center & right). Therefore, it is zero

Different symmetry of rotation produces a different spin-orbit magnetic field

Fig.9. click on image to enlarge it

where qnuclear is the nuclear charge.

HSO is proportional to ~1/r. As a result, the main contribution to HSO is from region in proximity of the nuclear. The symmetry of electron distribution function and electron rotation symmetry in close vicinity of the nuclear mainly contribute to HSO

The orbital moment:

or in quantum-mechanical representation

L is proportional to ~r. As a result, both regions, which are close and far from the nuclear, give a substantial contributions to L.

Even though a formal relation between HSO and L is very simple:

The integration over electron distribution gives very different value of HSO and L depending on the symmetry and details of electron wavefunction.

Orbital momentum vs rotation symmetry vs spin-orbit interaction

Orbital rotation of conduction electrons

When a conduction electron moves along the crystal lattice. Additionally to the movement, simultaneously the conduction electron rotates around stationary atomic nuclears. Electrical field of each nuclear induces the magnetic field interaction HSO

 

Yellow wavy ellipse shows the wave function of the conduction electron. Blue circle show the direction of rotation of the conduction electron around each atomic nuclear (dark spheres).Electrical field of each nuclear induces the magnetic field interaction HSO. The electron experiences the accumulative strong HSO. Even though the electrons moves along stationary nuclears,accumulative HSO remains constant.

The size of a conduction electron (size of its wave function) is relatively large. A conduction electron can cover simultaneously hundreds or thousands of nuclears.
camera moves with the electron
click on image to enlarge it

Q. How is it possible that an electron, while rotating around a nuclear, does not experience the Spin-Orbit Interaction ????

A. It is because, for the spherical orbit an electrons makes an equal number of rotations in two opposite directions. Since for opposite rotation directions the directions of the effective magnetic field of the spin-orbit interaction are opposite, an electron does not experience any spin-orbit interaction.

Is HSO always equal to zero for a spherical orbital?

No, See center and right pictures

The s-orbital can be divided to the sum of two spherical orbital, for which HSO is a no zero and opposite between two orbitals.

 

 

 

 

Q. In case of s-orbital a half of rotations an electron experience the field of the spin-orbit interaction in one direction and on another half in the opposite direction. This case should be different from the case when the does not experience the spin-orbit interaction at all. Therefore, the spin-orbit interaction still does affect the electron of s-orbital. Is it correct?

A. No, it is not correct. The spin-orbit interaction does not affect an electron of s-orbital for the following reason: An electron is an elementary particle, which could not be divided into the parts (See here). Therefore, it is impossible that the spin of one part of the electron rotates in one direction and the spin of another part rotates in a different direction. A whole electron has only one direction of the spin. In the case when along the length of electron (the mean-free path) or along the electron orbit the magnetic field changes, the spin interact with an average magnetic field. It is important there is always one defined spin direction for one electron.

 

 

quenched and unquenched orbital moments (See details here))

unquenched orbital: orbital moment can be freely rotated in any direction. E.g. orbital moment can align alone an external magnetic field.

Localized electrons of a paramagnetic material have a non-zero unquenched orbital moment. A non-zero orbital moment is mainly feature of a paramagnetic gas. In most of ferromagnetic and paramagnetic metals,the orbital moment of localized electrons is zero and the paramagnetic properties is determined by the electron spin without influence of the orbital moment.

quenched orbital: orbital moment cannot be freely rotated. Its orbital direction either is fixed or its orbital momentum is zero.

An unique spacial electron distribution each orbital moment. When the orbital moment is changed, the orbital spatial distribution is changed as well. However, the bonding of atom in a solid fixes the spacial electron distribution. The orbital moment of bonding electrons cannot be rotated. Otherwise, the bonding would be destroyed. As a result, the total orbital momentum of bonding electrons is zero.

 

orbital moment of electrons in a metal (common case)

localized electrons: the orbital moment is zero and orbital is quenched

conduction electrons: the orbital moment is a non-zero and orbital can be either quenched or unquenched

 

 


Three type of Spin-orbit interaction

It is convenient to divide effects related to the SO into 3 classes depending on a source of electrical field and a source of breaking of the time-inverse symmetry

Three types of the Spin-Orbit (SO) interaction

SO interaction. Type 1. Example: the Spin Hall effect

Fig.2 (left) 2D scan of Kerr rotation angle θKerr in GaAs 30-um-wide wire. θKerr is proportional to number of spin-polarized electrons nS , when an electrical current flows through the wire. T=30 K; (right) top- view optical optical image of same GaAs wire
The red and blue regions at wire side are regions of spin accumulation. Red and blue colors corresponds to spin-up and spin-down directions of spin accumulation. White color corresponds to regions without any spin accumulation.
Effect Origin 2:: There is a Schottky barrier at each edge of the GaAs wire and therefore an electrical field perpendicularly to the edge. Since an electrical current flows in the wire perpendicularly to that electrical field, the electrons experience the SO magnetic field HSO , which creates the spin accumulation. The direction of the electrical field is opposite on opposite sides. As a result, the polarity of and spin accumulation is opposite on opposite sides.
Y. K. Kato, R. C. Myers, A. C. Gossard, D. D. Awschalom, "Observation of the Spin Hall Effect in Semiconductors". Science 306, 1910-1913 (2004)
click on image to enlarge it. See details here
Key requirements for the SO to exist:
(requirement 1): electrons should move perpendicularly to the electrical field
(requirement 2): The time-inverse symmetry should be broken. (The SO cannot break time-inverse symmetry by itself, but breaking ids required in order to have HSO )

(type 1: weak SO) SO interaction induced by an external electrical field , which is perpendicular to electron current

(source of electrical field): electrical field at interface; electrical field of a Schottky barrier:

(source of breaking of time-inverse symmetry): an electrical current flowing along interface and perpendicularly to the interface electrical field:

(induced effects): Spin Hall effect & Inverse Spin Hall effect (weak contributions)

(type 2: strong SO) Enhancement of an external magnetic field

(source of electrical field): centrosymmetric electrical field of atomic nuclear

(source of breaking of time-inverse symmetry): an external magnetic field

(induced effects): Perpendicular magnetic anisotropy (PMA), voltage-controlled magnetic anisotropy (VCMA)

(type 3: moderate SO) creation of spin polarization by an electrical current

(source of electrical field): centrosymmetric electrical field of atomic nuclear

(source of breaking of time-inverse symmetry): an electrical current

(induced effects):Spin Hall effect & Inverse Spin Hall effect (main contributions), Spin-Orbit Torque

 

 



(type 1: weak SO) SO interaction induced by an external electrical field , which is perpendicular to current

only conduction electrons experiences this type of SO effect

(source of electrical field): electrical field at interface; electrical field of a Schottky barrier:

(source of breaking of time-inverse symmetry): an electrical current flowing along interface and perpendicularly to the interface electrical field:

(induced effects): Spin Hall effect & Inverse Spin Hall effect (weak contribution)

It is weakest, but easiest to understand SO type.

Explanation of effect: An electrical field, which exists at interface, or an external electrical field is applied (exists) perpendicularly to the electron current. The conduction electrons are confined in a 2D structure (e.g. a quantum well). Therefore, they do not flow in the perpendicular direction along the perpendicular magnetic field. The geometry of this type of SO effect is nearly the same as the geometry explaining the relativistic origin of the SO interaction, when electrons move perpendicularly

 

The reasons why the type of SO interaction is small, see here

 

 

 


(type 2: strong SO) Effect of Enhancement of external magnetic field

Enhancement of external magnetic field

Fig. 14. The external magnetic field Hext induces the effective magnetic field of the spin-orbit interaction HSO, which is is in the same direction as the external magnetic field.. Therefore, the total magnetic field, which the electron experiences, becomes larger.

note: Magnitude of HSO depends significantly on the direction, in which Hext is applied with orbit to orbital symmetry.
click on image to enlarge it
only localized electrons (e.g. d- or f- electrons) experience this type of SO effect

(source of electrical field): centrosymmetric electrical field of atomic nuclear

(source of breaking of time-inverse symmetry): an external magnetic field

(induced effects): Perpendicular magnetic anisotropy (PMA), voltage-controlled magnetic anisotropy (VCMA)

localized electrons experience this type of SO

Explanation of effect:

The type 2 of spin-orbit interaction is induced be an external magnetic field. E.g. in absence of a external magnetic field a localized electron does not experience any HSO. However, when external magnetic field Hext is applied, it induces strong HSO parallel to Hext and the electron experiences a stronger total magnetic field Htotal =Hext+HSO. E.g. when Hext=1 kG is applied, it induces HSO.=5 kG. Therefore, in total electron experience Htotal=6 kG.

Reason why an external magnetic field Hext induces the spin-orbit interaction HSO

(without external magnetic field): The orbital moment of the localized electrons is zero (or unquenched (See details here)). Any 3D orbital can be divided as a sum of two 2D orbitals of CCW and ACW electron rotation. Since the total moment of the localized electrons is zero, the CCW and ACW orbitals are identical. As a result, the electron experience the same but opposite HSO for the CCW and ACW orbitals , therefore in total it experiences no HSO

(with external magnetic field):Since electron rotation in the CCW and ACW orbitals is opposite, the Lorentz force is opposite for CCW and ACW orbitals. As a result, the CCW and ACW orbitals are deformed differently in an external magnetic field, HSO becomes different for CCW and ACW orbitals and in total the electron experiences a non-zero HSO.

Origin of SO interaction of type 2

Without external magnetic field, a localized electron does not have any orbital moment. It means the electron rotates in CCW and ACW directions on exactly the same orbital and experiences the same, but opposite HSO. The HSO is induced by the electrical field of nuclear. Therefore, in total the electron experiences no SO, HSO=0.
Under external magnetic field, CCW orbital becomes slightly closer to nuclear and CCW orbital becomes more distant due to the Lorentz force FLor. As result, HSO becomes different for CCW and ACW orbitals and in total HSO becomes a non-zero
click on image to enlarge it

How an external magnetic field affect a localized electron and electron orbital?

(influence 1) Electron energy is changed. (less important for SO)

The electron energy changes in a magnetic field according to its orbital moment. The orbital moment is aligned along magnetic field minimizing magnetic energy. The energy of s- electrons (orbital moment L=0) does not change. The energy of p-, d-, f- electrons (orbital moment L=1,2,3) changes.

(influence 2) Time- inverse symmetry is broken. (very important for SO)

The magnetic field changes the spacial distribution of an electron orbital, which breaks the time-inverse symmetry for the orbital. The part of electron distribution, which corresponds to the electron rotation in ACW direction with respect to magnetic field, is becomes closer to the nuclear. The part of electron distribution, which corresponds to the electron rotation in CCW direction with respect to magnetic field, is shifted away from the nuclear.

 

Note: The breaking of the time - inverse symmetry of the orbital does not depend whether the electron energy or electron orbital moment is changed or not. For example, a magnetic field breaks the time- inverse symmetry even for the s-orbital, even though the magnetic field does not change either energy or orbital moment of the s- orbital.

 

Effects, which are originated from Spin-orbit interaction of type 2:

(effect 1): Perpendicular magnetic anisotropy (PMA) (also see here)

(effect 2): g-factor

(effect 3):. Magneto-elastic effect

(effect 4): VCMA effect ? (one possible origin)

(effect 5): Interface sensing

effect 6): Magnetic anisotropy

 

 

 



(type 3: moderate SO) creation of spin polarization by an electrical current

Spin Hall effect. Band- current origin

No electrical current

electrical current J (yellow arrow) flows

rotational moment of a conduction electron

In total, average HSO, which experience by all conduction electrons, is zero. It is because there are equal numbers of conduction electrons moving in each direction. As a result, their rotational moments are compensate each other. In total, average HSO, which experience by all conduction electrons (shown in left-top corner), is a non-zero. It is because, there are more along y-direction than in opposite direction. As a result, the rotational moment of conduction electrons is not compensated and the whole electron gas experience a non-zero HSO in z- direction  

Fig. 3. Distribution of electrons with orbital moment and distribution of HSO.

The length of a vector from axis origin to sphere is proportional to the number of electrons moving in the vector direction.
Red arrow shows the direction of electron movement. Blue circle shows the direction of orbital moment. Violet arrow shows direction of HSO induced by the orbital moment.
Each election moving in a different direction experience HSO in different direction
 
click on image to enlarge it

(source of electrical field): centrosymmetric electrical field of atomic nuclear

(source of breaking of time-inverse symmetry): an electrical current

(induced effects):Spin Hall effect & Inverse Spin Hall effect (main contribution), Spin-Orbit Torque

Only conduction electrons experience this type of SO

Explanation of the effect:

(effect 1): Spin Hall effect

When electrical current flows in a metallic wire, it generates a spin current flowing perpendicularly to the electrical current

(effect 2): Spin pumping

When electrical current flows in a metallic wire, it creates spin-polarized conduction electrons. As a result, initially spin-unpolarized gas of conduction electrons becomes spin-polarized.

(effect 3): Inverse Spin Hall effect (ISHE)

When a spin-polarized current flows in a metallic wire, it generates a charge current (conventional current)flowing perpendicularly to the spin current.

(effect 4): Spin damping

When the electron gas is spin-polarized, there are spin-polarized conduction electrons, which spins is directed in one direction. When the conduction electron has a non-zero rotational (orbital) moment, it experiences a non-zero HSO and there is a spin precession around HSO. Since the direction of HSO is different for electrons moving in different directions, the spin precession is along different directions for electrons moving in different directions. As a result, the spins of spin-polarized electrons is misaligned from one direction and degree of the spin polarization is reduced.

there are two contributions to current-induced spin-orbit effects:

(contribution 1) band current

It occurs only when conduction electrons have non-zero rotational moment.
It is a feature of a metal of a high conductivity (See here)

(explanation of effect):

(step 1) The conduction electron have a non-zero rotational (orbital) moment,which created magnetic field HSO. There is a spin precession around HSO and the spin is aligning along HSO due to the damping of the spin precession.

(step 2) When there is no electrical current, there are equal numbers of electrons moving in any two opposite directions. Since the rotational (orbital) moment and HSO are equal and opposite for electrons moving in opposite direction, both the total rotational (orbital) moment and total are zero for the electron gas and scattering probabilities are independent on electron movent direction and electron spin

(step 3) When there is an electrical current, the number of conduction electrons moving along current is larger than number of electrons moving in the opposite direction. As a result, the rotational (orbital) moment of electrons moving along current is not fully balanced by the opposite moment of electrons moving in the opposite direction and the total the electron gas experience a non-zero HSO and the electron gas becomes spin-polarized.

(step 4) When there is an electrical current, the scattering probability of spin-up electrons to the left becomes different from the scattering probability to the right. As a result, e.g. the spin-up polarized current flows to the left and the spin-down polarized current flows to the right.

(contribution 2) scattering current

It is originated from electrical field of defects and from direction dependence of scattering probability in the vicinity of an interface.

It is a feature of a metal of a low conductivity (See here)

(explanation of effect):

(step 1) There is an electrical field in close vicinity of a defect in a metal and an interface between two metal or at edge of a metal wire. The conduction electrons are screening any electrical field in a metal. However, in close proximity of a defect or interface the electrical field is not fully screened. Especially it is the case of a metal of a low conductivity

(step 2) When a conduction electron moves along the electrical field of the defect or interface, it experience HSO and its spin is aligned along HSO

(step 3) Since direction of the electrical field is opposite from left and right sides

 

The conduction electrons move simultaneously in the forward direction along lattice and around each atom (nuclear) of the lattice.

 

 

 

 

 

 


 

Spin-orbit (SO) interaction. Facts in short.

(fact 1) SO describes the fact that an electron moving perpendicularly to electrical field experiences a magnetic field HSO. The magnetic field HSO affects the electron spin. As a result, the electron properties become spin-dependent.

(fact 2)There are three types of SO interaction: the "weak- type" SO , "non- zero orbital" type, "strong- type" and "moderate - type" SO. Their properties and features are completely different

Properties distinguish each type of SO interaction

-What is the origin of electrical field?

- Direction of electron movent

- What (an electrical current or an external magnetic field) breaks the time- inverse symmetry.

 

Type 1: "Weak- type"SO interaction. (HSO=0.01-1 Gauss). Facts in short.

note: HSO of the Weak SO interaction is comparable to Earth magnetic field 0.25-9.65 Gauss (See here)

(Origin): It is originated by an electrical current flowing in quantum well (QW) or in the vicinity of the interface.

An external magnetic field is not required to create the weak SO effect.

note: In the case of weak SO effect the time-inverse symmetry is broken by the electrical current

(Origin of breaking of time-inverse symmetry): electrical current

(Source of electrical field (field is weak)): (1) electrical field of charge accumulation at interface (Schottky type or due to a difference of work functions of materials at sides of the interface). (2) electrical field of defects

(Reason why an electron moves perpendicularly to electrical field) Electrical current in the perpendicular direction is blocked by the interface or 2D confinement.

(Source of electron movement (movement is slow)): electron current along a 2D QW or along an interface

(Symmetry): (1)Polarity of HSO is reversed when is the electrical current is reversed. (2) Asymmetrical structure perpendicularly to current direction is often required. Otherwise, SO interaction compensate itself at two opposite interfaces and HSO becomes zero.

Effects, originated by "Weak- type" SO:

(1) Spin Hall effect (weak contribution)

It describes the fact that a spin-polarized current is generated perpendicularly to a flow of electrical current in a nonmagnetic material.

(2) Anomalous Hall effect (weak contribution)

It describes the fact that a Hall voltage is generated at side of a ferromagnetic wire when the wire magnetization is perpendicular to the electrical current

(3) Effect of Anomalous Magneto - Resistance (AMR) (weak contribution)

It describes the fact that

(4) Rashba effect (weak contribution)

It describes the fact that there is a spin precession for a conduction electron when there is an electrical current in a quantum well (QW)

 

Type 2: "non- zero orbital" SO interaction. (HSO=0.01-20 000 Gauss). Facts in short.

(Origin): It is originated from orbital movement of electron, when the orbital moment of electron is a non-zero.

(Origin of breaking of time-inverse symmetry): orbital alignment (spontaneous, local, by a magnetic field etc.)

(Source of electrical field (field is strong)): electrical field of the nuclear

(Reason why an electron moves perpendicularly to electrical field): Orbital movements,

(Source of electron movement (movement is fast)): Orbital movements

(Symmetry): (1) HSO is linearly proportional to the orbital moment of electron. (2)

Effects, originated by "non- zero orbital" SO:

(1) Fine structure

Type 3: "Strong- type" SO interaction. (HSO=1 000 -20 000 Gauss). Facts in short.

(Origin): It is originated by an electrical field of nuclear due to the orbital rotation of an electron around nuclear and an external magnetic field, which breaks the times-inverse symmetry of the orbital

An external magnetic field is required to create the strong SO effect. Without an external magnetic the effect does not exists!

note: The strong-type SO effect cannot break the time-inverse symmetry by itself. It requires an external magnetic field to break it.

It is only type of SO interaction, which exists without an electrical current

(Origin of breaking of time-inverse symmetry): external magnetic field

(Source of electrical field (field is strong)): electrical field of the nuclear

.(Reason why an electron moves perpendicularly to electrical field): Orbital movements,

(Source of electron movement (movement is fast)): Orbital movements

(Symmetry): (1) HSO is linearly proportional to the external magnetic field. (2)

Effects, originated by "Strong- type" SO:

(1) Perpendicular magnetic anisotropy (PMA)

It describes the fact that a spin-polarized current is

Type 4: "Moderate- type" SO interaction. (HSO=1 0 - 500 Gauss). Facts in short.

(Origin): It is originated by an electrical field of nuclear due to the orbital rotation of an electron around nuclear and an electrical current, which breaks the times-inverse symmetry of the orbital

note: The moderate-type SO effect cannot break the time-inverse symmetry by itself. It requires an electrical current flow to break it.

(Origin of breaking of time-inverse symmetry): electrical current

(Source of electrical field (field is strong)): electrical field of the nuclear

(Reason why an electron moves perpendicularly to electrical field): Orbital movements

(Source of electron movement (movement is fast)): Orbital movements

(Symmetry): (1) HSO is linearly proportional to the current (2)

Effects, originated by the "Moderate- type" SO:

(1)

It describes the fact that a spin-polarized current is

 


 

Key properties of the spin-orbit interaction

The spin-orbit interaction is relativistic effect (not quantum-mechanical).

The spin-orbit interaction is described by the effective magnetic field HSO of the spin-orbit interaction. The HSO is a real magnetic field, which is indistinguishable from a common magnetic field (An exception, see here)

Only the electrical field of a nuclear can induced a substantial spin-orbit interaction

In the case of a spherical orbital, the electron experiences opposite-sign of HSO in equal amounts. As a result, the electron does not experience any spin-orbit interaction

In order to experience the spin-orbit interaction in the electrical field of an atomic nuclear, two condition should be satisfied.

(condition 1): The electron orbital should be deformed. It should not be spherical. It is better it should be not centrosymmetric.

(condition 2): The time-inverse symmetry for the orbital should be broken. It means that an external magnetic field should be applied.

.....


 

What difference the spin-orbit interaction does make? What does the spin-orbit interaction affect and influence?

Effect 1:

-- Enhancement (magnification) of the applied magnetic field.
Due to the spin-orbit interaction, an electron experiences the effective magnetic field, which is larger than the actual applied magnetic field.
where it is the case: (1) changing of g-factor; (2) perpendicular magnetic anisotropy; (3) magnetostriction

Effect 2:

-- Spin-dependent scatterings.
Due to the spin-orbit interaction, the scattering probability for electrons with opposite spins becomes different.
where it is the case: (1) Anomalous Hall effect; (2) Spin Hall effect

Effect 3:

-- Spin precession. Spin relaxation.

When electron moves across a strong electrical field, the effective magnetic field of the spin-orbit interaction causes a spin precession.

where it is the case: a electrical current flowing along an interface or a junction.

When an electron may move at different angles, it may cause different directions of the precession, therefore the spin relaxation.


Which specific changes the spin-orbit interaction does?

In a non-magnetic material (paramagnetic or diamagnetic)

(Effect 1) -g-factor becomes larger than g-factor of an electron in the free space ;

(Effect 2)-The bulk-type Spin Hall effect due to scatterings on non-magnetic and magnetic impurities

(Effect 3)- The interface-type Spin Hall effect due to interface scatterings

(Effect 4)- spin relaxation becomes larger. Especially for delocalized electrons (conduction electrons) of p- symmetry (-d or -f as well)

In a ferromagnetic material

(Effect 5)- saturation magnetization becomes larger (exchange interaction is enhanced due to the spin-orbit interaction)

(Effect 6)-interface-induced perpendicular anisotropy (for example, Co/Pt). It is due to a large difference in the spin-orbit enhancement for magnetic field directed along and across the interface

(Effect 7)- changing the magnetization and magnetization direction due to the stress. Magnetostriction (magneto- elastic) effect. The stress in a metallic single-crystal multilayer structure.

(Effect 8)- Anomalous Hall effect (AHE)



Q1. The Spin-Orbit interaction. What it is ??

The Spin-Orbit interaction describes the fact that an electron experiences an effective magnetic field when it moves in an electrical field.

Q2. The Spin-Orbit interaction. How does it affect an electron??

The effective magnetic field HSO of the Spin-Orbit interaction affects only the electron spin. Interaction of HSO with electron spin leads to

1) There can be a spin precession

2) There can be a damping of the spin precession, which aligns the electron spin along the effective magnetic field of the spin-orbit interaction

3) Electron transport can become spin-dependent

4) The electron energy becomes spin-dependent.

Important Note 1: The effective magnetic field HSO of the Spin-Orbit interaction cannot induce the Lorentz force or the Hall effect.

Important Note 2: The effective magnetic field HSO of the Spin-Orbit interaction does not interact with the magnetic moment induced by the orbital moment of the electron.

 



Example to understand how the spin-orbit interaction works in a solid

Increase of spin-orbit interaction due to crystal deformation.

click on image to enlarge it

Fig. 20. Due to the crystal deformation, the orbital are deformed and the nuclears are shifted out of the center of the orbital. It makes HSO (green arrow) larger and the magnetic energy larger.
Green arrows show the effective magnetic field HSO of the spin-orbit interaction. It is large only when the orbitals are deformed.
Blue arrows show the intrinsic magnetic field Hinside. It the total magnetic field, which electron experience: external magnetic field, magnetic field from neighbor orbitals. The total magnetic field except HSO
White spheres show the orbitals of the localized d-electrons

The magnetic energy and HSO increase only when the magnetization and Hinside (blue arrow) are perpendicular to the deformation and film surface. There is no enhancement of HSO when the magnetization is in-plane. This effect is called the perpendicular magnetic anisotropy (PMA)

 

 

 

 



Q. Both localized (-d,-f) and delocalized (-s,-p) electrons are rotating around nuclears (atoms), is it sufficient for them to experience a strong spin-orbit interaction?

A. No. It is far not sufficient. There are several other conditions the electron should satisfy in order to experience the spin-orbit interaction:
1) The orbital moment (symmetry) should be non-zero. Only electrons, which wave function has the -p,-d,-f- like spacial symmetry may experience the spin-orbit interaction.
2) The time-inverse symmetry should be broken!!

The following describes the reasons why an electron does not experience the spin-orbit interaction when the electron orbit is spherical and why it does experience the spin-orbit interaction for other shapes of the orbital.

 

Spin-orbit interaction. Type 2. Magneto-elastic effect

This effect described facts that the elastic stress may enhance the spin-orbit interaction and increase the energy of the perpendicular magnetic anisotropy (PMA).

see details here

When a pressure applied to the film, the atomic orbitals are deformed. There are two types of deformations. (type 1): The orbital becomes more elliptical. (type 2): nuclears are shifted out of the center of the orbital. Both deformation makes the effective magnetic field HSO of the spin-orbit interaction larger.

In a ferromagnetic material the localized electrons have a non-compensated spin, which creates a magnetic field Hmag

At an interface between a magnetic and non-magnetic material, the demagnetization field Hdemag is created due to uncompensated magnetic moment at the interface. The direction of Hdemag is perpendicular to the interface and opposite to Hmag.

The magnetic field Hinside inside of the ferromagnetic field equals Hmag- Hdemag. The Hinside is the total magnetic field except HSO. It includes the external magnetic field

Important fact: Additionally, the electron experience the effective magnetic field HSO of the spin-orbit interaction, which is always directed along Hinside. The magnitude of HSO is proportional to Hinside and the degree of the orbital deformation.

Without a deformation the orbitals of the localized electrons is nearly spherical and the effective magnetic field HSO of the spin-orbit interaction is small.

When the pressure applied, the orbitals are deforms in the direction of the applied pressure and the effective magnetic field HSO of the spin-orbit interaction increases.

 

The magnetic energy of an electron equals to a product of the electron spin and Hinside+HSO.

When magnetization is perpendicular to the film, the orbital deformation is larger, HSO is larger and the magnetic energy is larger.

When magnetization is in-plane, the orbital deformation is smaller, HSO is smaller and the magnetic energy is smaller.

The dependence of the magnetic energy on the magnetization direction is called the magnetic anisotropy. In the case when the difference of the magnetic energy are with respect to the interface, the effect is called the perpendicular magnetic anisotropy (PMA)

 

Spin-orbit interaction in the centrosymmetric electric field of atomic nuclear.

Animated figure 6. Spin-Orbit Interaction in the center-symmetrical electrical field of atom nuclear. It is an imaginary case when electron moving in a circular orbit around nuclear (for s- symmetry electron, the orbit is spherical). Red arrow indicates the spin direction of electron. Blue arrow indicates the effective magnetic field of the spin-orbit interaction. The effective field appears only when the electron moving. The directions of the effective field is opposite for the opposite direction of the electron movement. There is a spin of precession around the effective magnetic field of the spin-orbit interaction. The electrons spin slowly aligns itself along the effective magnetic field because of the precession damping.

click here to enlarge

 

 

 

 

 

 

 

 

 

 

 

 

 

 


Distribution of conduction electrons with orbital moment

Fig.

click on image to enlarge it

Spin-orbit interaction (type 3: moderate SO) creation of spin polarization by an electrical current

(source of electrical field): centrosymmetric electrical field of atomic nuclear

(source of breaking of time-inverse symmetry): an electrical current

(induced effects):Spin Hall effect & Inverse Spin Hall effect (main contribution), Spin-Orbit Torque

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Movement of a delocalized (electron in a metal

Simultaneously with movement along the crystal the delocalized electron rotates a couple of times around each nuclear. The rotation and the linear movent cause two different types of the spin-orbit interaction.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


Q. How to make an electron to rotate in one direction more than in the opposite direction??? How to make the spin-orbit interaction stronger??

Simple Answer: It is necessary to deform the electron orbital.

 

The deformation or distortion of the electron orbital can be done be an external electrical field or stress.

See VCMA effect

The orbital can be distorted by an electrical field. In this case, the electron experiences the effective magnetic field due to the spin-orbit interaction.

When the orbital is distorted by an external electrical field, the existence of the effective magnetic field due to the spin-orbit interaction is called the Rashba effect.

When the orbital is distorted by an axial crystal field, the existence of the effective magnetic field due to the spin-orbit interaction is called the Dresselhaus effect.

Note: The external magnetic field may deform the orbit. However, the deformation is very small. The magnetic field has another important function for the SO. The magnetic field breaks the time-inverse symmetry, which is a key condition for SO to occur (See below).

 

orbital momentum in
orbital momentum is zero. there is no SO interaction orbital momentum is largest & SO interaction is strongest and directed up  
     

 

 
click on image to enlarge it

 

 

 

 

 


Direct (weak) and indirect (strong) spin-orbit interaction (SO)

There are two kinds of the spin-orbit interaction in a crystal lattice. In both cases an electron experiences an effective magnetic field of the spin-orbit interaction.

Direct (weak)

An electron moves perpendicularly to an electrical field. The electrical field directly induces the magnetic field. For example, such electron movement across an electrical field is possible in in a quantum well. The electrical field could be an externally-applied electrical field, an axial crystal field or/and an electrical field across interface or junction due to a charge accumulation. Only delocalized (conduction) electrons may experience the direct SO.

Indirect (strong)

In this case the magnetic field of the spin-orbit interaction is induced not by an external electrical field, but by the electrical field of a nuclear. The external electrical field just deforms the electron orbital making the spin-orbit interaction stronger.

In contrary to the direct SO, in the case of the indirect SO it is not necessary for an electron to move along the crystal lattice. Therefore, the indirect spin-orbit interaction may experience localized electrons, delocalized (conduction) electrons and standing-wave electrons.

In contrast to direct SO, the indirect SO can occurs only when the time-inverse symmetry is broken. It can be broken by an external magnetic field or a local magnetic field. (See below)

 


The spin-orbit interaction in compound metals and semiconductors.

When a crystal consists of different atoms, the electrons are distributed asymmetrically. Some electron orbit is shifted from a cation to be closer to anion. , the orbital becomes deformed. That causes a stronger spin-orbit interaction. This is reason, for example, why the spin-orbit interaction is significantly stronger in GaAs than in Si.

In an ionic crystal the covalent electrons are nearly-fully transformed from a cation to a anion and the electron orbital becomes again more center-symmetrical with a weak spin-orbit interaction. This is reason, for example, why the spin-orbit interaction is significantly weaker in ZnO than in GaAs.


Q. Why the spin orbit interaction is larger in a heavy element with a larger atomic number??

Simple answer: The strength of the spin-orbit interaction is directly proportional to the electric field of the nuclear. The nuclear charge is larger for an element of a larger atomic number. Therefore, the electrical field of the nuclear and the spin-orbit interaction, which is induced by this field, becomes larger as well.

Another reason: the screening by inner electrons becomes weaker and asymmetrical (See below)


Screening of spin-orbit interactions by inner electrons.

 

Because of the screening of an electrical field of a nuclear by inner electrons , the strength of spin-orbit interaction reduces.

The effects of screening:

(effect 1) The spin-orbit interaction (SO), which is induced by a anion, is smaller than the SO, which is induced by a cation.

Since there are more electrons in the vicinity of an anion than in the vicinity of cation, the screening of nuclear field of anion is larger. Therefore, the spin-orbit interaction induced by the nuclear of anion is smaller.

(effect 2) In atoms of unfilled inner shells the spin-orbit interaction is stronger.

In the case when the inner shell of atom is not fully filled, the screening of the nuclear by the electrons of the inner shell is not centrosymmetric. It makes the spin-orbit interaction stronger.


It is good to know.

(fact) The effective spin-orbit magnetic field HSO acts only on spin and it does not effect the magnetic moment due to the orbital moment.

The magnetic moment of an electron is a quantum- mechanical sum of magnetic moments induced by the spin and induced by the orbital moment.

 



Spin-Orbit interaction due to the orbital deformation

The increase of the spin-orbit interaction due to deformation of the electron orbital

click here or on image to enlarge it

Fig. 9. Electron orbital. When the orbital is spherical the effective magnetic field of the spin-orbit interaction HSO is zero. Only when the orbital is deformed, there is the magnetic field of the spin-orbit interaction. The effective magnetic field is the largest in the case of a circle or elliptical orbital.

 

The effective magnetic field of the spin-orbit interaction for localized electrons due to a deformation of electron orbit may be very large. It may reach 1-30 kOe and larger. The effective magnetic field for the delocalized electrons is smaller, but still it may be large.

The type of orbit deformation, which may enlarge the spin-orbit interaction

(1) The electron orbit should be deformed along one direction

(2) The electron orbit should be deformed asymmetrically in respect to its nuclear

 

The orbital is significantly deformed in compound materials with covalent bonding (like GaAs). Therefore, they have a larger spin-orbit interaction.

In materials with ionic bonding, the orbital is less deformed and they have a smaller spin-orbit interaction (like ZnO).

The p- , d- and f- orbitals are inherently asymmetrical. For each individual p- , d- and f- orbital, the spin-orbit interaction may be strong.

For each individual p- or d- or f- orbital, the time-inverse symmetry is broken. However, in a non-magnetic metal or a semiconductor, where the time-inverse symmetry is not broken, the electron wavefunction is a combination of the wave functions of different moments and it is more symmetric. Therefore, in a crystal the spin-orbit interaction of electrons of p- or d- or f- symmetry may be not as strong as in the case of a separated atom.


 

 

 

 

 

 

 

 

 

 

 

 

 



Enhancement of magnetic field due to the spin-orbit interaction

 

Along an applied external magnetic field, an additional magnetic field is induced due to the spin-orbit interaction. Therefore, an electron experience a larger magnetic field than externally applied due to the spin-orbit interaction

This is the most important property of the spin-orbit interaction !!!.

This property determines how the spin-orbit interaction affects electrons in a solid

In fact, it is the joint work of two relativistic effects: 1) the Lorentz force 2) the spin-orbit interaction

- The Lorentz force, which is induced by an external magnetic field, deforms the electron orbital and breaks the time-inverse symmetry;

- Because of the broken time-inverse symmetry, the strong effective magnetic field is induced by the spin-orbit interaction.

 


Time- inverse symmetry and the spin-orbit interaction

Time-inverse symmetry is not broken

click here to enlarge

Fig. 12. Electron orbital. The green arrows show the direction of the effective magnetic field of the spin-orbit interaction HSO

 

Time-inverse symmetry: not broken

Average effective magnetic field of the spin-orbit interaction: zero

 

When the time-inverse symmetry in the material is not broken, there is an equal probability that electron circulating around the nuclear in the clockwise and anti clockwise directions. Since the electron experiences equal and opposite effective magnetic field of the spin-orbit interaction, in the average the electron does not experiences any effective magnetic field of the spin-orbit interaction. (See Fig. above)

Even in the case when the orbital moment of the electron is not zero, when the time-inverse symmetry in the material is not broken, there is an equal probability for an electron to occupy the orbit with opposite orbital moment and again the average effective magnetic field of the spin-orbit interaction: zero

note: in this case the spin-orbit interaction affects the spin relaxation

---------------------------------------------

( important fact:) The spin-orbit interaction cannot break time- inverse symmetry by itself. E.g. the object cannot be magnetized due to spin-orbit interaction only. The spin-orbit interaction enhances already-existed magnetic field, but it cannot create its own magnetic field. The effective spin-orbit magnetic field HSO is always proportional to the external magnetic field. It does not exist when there is no external magnetic field. This condition is the consequence of the conservation law of the time-inverse symmetry.

This fact contradicts with the existence of a fine structure in the hydrogen gas and the existence of the heavy and light holes in a semiconductor? In both case, the spin-orbit interaction causes a spliting of the energy levels without any external magnetic field.

There is no contradiction. Globally, there is no magnetic field in both cases. However, locally each atom experience a magnetic field, which is magnified by the spin-orbit interaction. The magnification is different for the orbital of a different symmetry. As a result, the orbitals experience a different HSO and the orbital energies become different. (See more details Here)

(fact) HSO does not exist when there is no external magnetic field

(fact) HSO is always proportional to an external external magnetic field

(fact) The spin-orbit interaction cannot break the time inverse symmetry

How the time-inverse symmetry is broken:

Time-inverse symmetry is broken

click here to enlarge

Fig. 13. Electron orbital in the presence of external magnetic field (blue arrow). The green arrows show the direction of the effective magnetic field of the spin-orbit interaction HSO The red arrows show the direction the Lorentz force

It results: Average effective magnetic field of the spin-orbit interaction becomes non-zero

 

An external magnetic field breaks the time-inverse symmetry and it causes a non-zero average effective magnetic field of the spin-orbit interaction in the direction of the external magnetic field.

Since the electron moves around the nuclear, it experiences the Lorentz force in the magnetic field. The Lorentz force is in opposite directions for electron moving in the clockwise and anticlockwise directions around the magnetic field. The Lorentz force modifies the orbital of electrons. When an electron moves in the anticlockwise direction, it moves closer to the nuclear and it experiences the larger electrical field from the nuclear and the larger corresponded effective magnetic field of the the spin-orbit interaction. When an electron moves in the clockwise direction, it moves more distant from the nuclear and it experiences the smaller electrical field from the nuclear and the smaller corresponded effective magnetic field of the the spin-orbit interaction. In the average, the average the electron experiences a non-zero effective magnetic field of the the spin-orbit interaction in the direction of the external magnetic field.

note: The effective electrical field of the Lorentz force can not induce the spin-orbit interaction, because of its relativistic nature (See here)

 

Fig. 13 shows the diamagnetic response of the atom to the external magnetic field. Therefore, a material with the largest diamagnetic constant should have the largest spin-orbit interaction.

Notice: all electrons have the diamagnetic response shown in Fig. 13, including electrons of the inner orbitals and electrons of the inert gases. However, the electrons of the the external orbitals have uncompensated spin and only they experiences the spin-orbit interaction.

 

 

 

 

 

 

 


 

Enhancement of magnetic field due to the spin-orbit interaction

The increase of the effective magnetic field due to the spin orbit interaction

Fig. 14. The external magnetic field Hext induces the effective magnetic field of the spin-orbit interaction HSO, which is is in the same direction as the external magnetic field.. Therefore, the total magnetic field, which the electron experiences, becomes larger.

click on image to enlarge it

 

 

When a magnetic applied to the material, it breaks the time inverse symmetry. As result, the electron starts to experience non-zero effective field of the spin-orbit interaction.

The effective magnetic field HSO of the spin-orbit interaction is in the same direction as the applied external magnetic field.

The total magnetic field, which the electron experiences, becomes larger. In some cases, the total effective magnetic field may be a significantly larger than the external magnetic field.

 

The induced effective magnetic field of the spin-orbit interaction may be significantly different for different directions of the applied external magnetic field. It is the largest in the direction, in which the electron orbit is deformed (See Fig. 14).

 


Spin relaxation in gas of conduction electrons

Spin relaxation due to non-zero orbital moment of a conduction electron

 

Alignment of HSO with respect to the spins of spin-polarized conduction electrons

Spin precision around HSO

precession of spin of conduction electron around HSO

E.g. spins of spin- polarized conduction electrons are aligned along the same direction (along x-axis). The alignment of HSO and electron spin are different for different movement direction of the conduction electrons. There is a precession of spin of conduction electrons around HSO. The precession direction different for a different movement direction of a conduction electron. It results of misalignment of spin- polarized electrons and therefore the spin relaxation. Spin precession of a conduction electron. The pink circle shows the orbital (rotational) moment of a conduction electron. Blue arrows shows HSO, which is due to this orbital moment. Red arrow shows movement direction of conduction electron.
  Note: the scatterings in electron are rather frequent. As a result, the electron is scattered out much earlier before even one period spin precession is finished. In fact, the precision angle is rather small (less than 1 degree). The scatterings are constantly re-aligning spins into spin- direction distributions of two groups of spin-polarized and spin- unpolarized electrons (See details here) when rotational moment of conduction electron is non-zero, it experiences a magnetic field HSO of spin- orbit interaction. There are a precession of electron spin around HSO. The direction of and consequently direction of the spin precession is fixed to movement direction of the conduction electron

Fig.

click on image to enlarge it

The spin relaxation reduces the amount of spin-polarized conduction electrons in electron gas (See here)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


 

g-factor

wiki page about g-factor is here

The g-factor describes the ratio between the spin or the orbital moment and the magnetic moment of an electron

For an electron in free space the value of g equals to 2.002319

------------------

There are two cases where the g-factor is used and it can be measured:

1) Ferromagnetic resonance and electron paramagnetic resonance. The g-factor describes the precession frequency (Larmor frequency) of the spin in an external magnetic field. The external magnetic field is applied at an angle with respect to the spin direction.

2) Zeeman effect. The g-factor describes the energy difference for electrons, which spins are along and opposite to the direction of the magnetic field.

-------------------

Important notice: The g-factor, which is measured from the ferromagnetic or paramagnetic resonance, is not always same as the g-factor, which is measured from the Zeeman spliting.

The reason of the difference: In a solid there is no precession of the orbital moment in a magnetic field (See here) , but the orbital moment contributes to the Zeeman spliting.

-----------------------------

Orbital moment and the g-factor

In atoms when the spin is compensated and the magnetic moment is only due to the orbital moment, the g-factor equals to 1. The g-factor of atoms of gas is between 2 and 1.

In crystal:

1) The orbital moment of localized and delocalized electrons in a crystal does not contribute to the ferromagnetic or paramagnetic resonance, because the external electron orbits are fixed by the crystal structure and the interactions with neighbor atoms.

2) orbital moment is contributes to the Zeeman splitting.

------------------------

The spin-orbit interaction and the g-factor

non-magnetic materials (paramagnetic and diamagnetic materials)

When an external magnetic field is applied, an electron in crystal experiences a larger magnetic field, because the effective magnetic field is enlarged due to the spin-orbit interaction.

Even though in the reality the effective electron g-factor does not change and only the effective magnetic field changes due to the spin-orbit interaction, it is convenient to assume the g-factor of the material is changed, but the magnetic field remains unchanged. Therefore, the Larmor frequency can be calculated as

where kSO is coefficient, which described the enhancement of the magnetic field due to the spin-orbit interaction. From Eq. (g4), the Larmor frequency is calculated as

where the g-factor is

Often the g-factor is defined and measured for the external magnetic field strength H instead of the magnetic induction B. In this case the effective g-factor can be used

g-factor and Specific magnetic susceptibility for non-magnetic materials

Content

g-factor

conduction band (bulk):

GaAs : -0.3 (300 K) -0.45 (50 K)

InAs: -15

InP: 1.5

GaSb=-8

InSb=-51.3

n-Si: =1.9985

p-Si=2

 

Cu=

 

 

Specific magnetic susceptibility (CGS-emu=Si-unit/4pi)

Ge Si InAs GaAs InSb GaSb Al Ag Cu            
-1.15 -1.08 -1.2 -1.25 -1.25 -1.35 1.75×10−6 −1.84×10−6 −0.083×10−6            

Paramagnetic (Si unit)

FeO Pt Al W Cr Ti    
720×10−5 26×10−5 2.2×10−5 6.8×10−5 3.13×10−4 1.81×10−4    

 

Diamagnetic (Si unit)

Ag Cu Au Si Al2O3      
-2.6×10−5 -1×10−5 -3.44×10−5 -0.41×10−5 -1.81×10−5      

 

EPR for Ge=2

9.3882 GHz-> 3.35 kG

 

 

ferromagnetic metals

In ferromagnetic metals

g-factor, saturated magnetization and width of FMR peak in ferromagnetic metals. Click to extend

 

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
g factor
free electron
g=2.0023
Fe g=2.088
Co g=2.18
Ni g=2.2
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
4piM=
Ni= 6.2 kG
Py=17.1 kG
Co=17.8 kG
Fe=21.4 kG
;;;;;;;;;;;;;;;;;;

;;;;;FMR

;;;;;;;;;;;;;;;;;;;;;;;;;
FMR magnetic field for 9.8 GHz
Ni= 1600 G
Py=700 G
Co=690 G
Fe=600 G
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; ;;;;;;;;;;;;;;;;;;

relaxation parameters
Fe 57 MHz ( alfa= 0.002)
Ni 220 MHZ
Co 170 MHz
Py 114 MHz
;;;;;;;;;;;;;;;;;;;;;;;;;

for YIG width of FMR resonance
0.15 Oe -0.2 Oe

 

 

 

 

Q. Why the spin-orbital enhancement of the magnetic field cannot be included into Magnetic susceptibility

In the of paramagnetic metals, the spin-orbit interaction does not produced any additional magnetic field or magnetization inside material. It only makes larger the effective magnetic field, which the electron experiences.

Note: Both the magnetic susceptibility and the spin-orbit interaction enhance the effective magnetic field, which an electron experiences. Except ferromagnetic metals, the enhancement due to the magnetic susceptibility is much weaker than the enhancement due to the spin-orbit interaction. For example (See above), in diamagnetic materials the enhancement is only about -0.001 %, in paramagnetic it is 0.01 %. As can be evaluated from the g-factor, the enhancement due to the spin-orbit interaction is 1-10 % for the most of materials. In the case of III-V semiconductors (GaAs,InAs), the enhancement may be more than 100 %.


 


Perpendicular-to-plane magnetic anisotropy (PMA)

Detailed description of the PMA is here

 

Perpendicular-to-plane magnetic anisotropy at a Fe/Pt interface

Magnetization is perpendicular to the interface

click here or on image to enlarge it

Magnetization is along the interface

click here or on image to enlarge it
Fig.15. Schematic diagram of Pt/Fe interface. The blue and red spheres show the electron orbitals in Fe and Co, respectively. Blue arrows shows the magnetization (spin of localized d-electrons). The green arrows show the effective magnetic field HSO of the spin-orbit interaction. The orbital of the localized electrons are shown.

The magnetization of a single-material ferromagnetic film is in - plane. In the case when the film consists of a thin layers of different metals, the magnetization could be out of plane. The example of such multi-layered films are Co/Pt, Fe(fcc)/Pt, Co/Tb, Fe(fcc)/Tb.

Since the strength of the spin-orbit interaction depends of the shape of the electron orbit in a crystal, the perpendicular-to-plane magnetic anisotropy only a feature of a specific crystal orientation and only a specific crystal orientations of the interfaces. For example, in all above-mentioned cases the perpendicular-to-plane magnetic anisotropy is feature of only fcc(111) interfaces or very similar hcp interfaces

Perpendicular-to-plane magnetic anisotropy occurs due to a strong effective magnetic field of the spin-orbit interaction at the interface . The enhancement of the effective field of the spin-orbit interaction occurs because of a deformation of the orbital of the ferromagnetic and non-magnetic metals in the close vicinity of the interface.

 

In the bulk of the metals, the shape of the orbitals are close to a sphere (shown as the red and blue-colored spheres). In the vicinity of the contact, the orbitals are deformed.

 

 

 

 

Thickness-dependence of PMA

Thicker metal.

Magnetization is in-plane

Thinner metal

Magnetization is perpendicular to plane

Fig.16. Magnetization of CoFeB film grown on MgO. The magnetization of a thick film (thickness >1.5 nm) is in-plane , but the magnetization of a thinner film is perpendicular-to-plane

Detailed description of the dependence of PMA on film thickness is here

 

 

 

It should be noticed that the magnetizations of a thin Fe(bcc)(001) on Cu(bcc)(001), on Ta (001), on W(100) is also is perpendicular to plane.

 

Since the strength of the spin-orbit interaction depends of the shape of the electron orbit in a crystal, the perpendicular-to-plane magnetic anisotropy only a feature of a specific crystal orientation and only a specific crystal orientations of the interfaces.

For example, the magnetization of a thin Co(hcp) or Co (fcc) film on MgO or on Cu is in-plane.

 

See features of the PMA for different interfaces here
M. T.Johnson et. al. Reports on Progress in Physics(1996) ; P.Bruno PRB (1989);

 

 

 


 

Magneto-elastic effect (Villari effect) and the Spin-orbit interaction

This effect described facts that the elastic stress may enhance the spin-orbit interaction and increase the energy of the perpendicular magnetic anisotropy (PMA)

Increase of spin-orbit interaction due to crystal deformation.

click on image to enlarge it

Fig. 20. Due to the crystal deformation, the orbital are deformed and the nuclears are shifted out of the center of the orbital. It makes HSO (green arrow) larger and the magnetic energy larger.
Green arrows show the effective magnetic field HSO of the spin-orbit interaction
Blue arrows show the intrinsic magnetic field Hinside
White spheres show the orbitals of the localized d-electrons

The magnetic energy and HSO increase only when the magnetization and Hinside (blue arrow) are perpendicular to the deformation and film surface. There is no enhancement of HSO when the magnetization is in-plane. This effect is called the perpendicular magnetic anisotropy (PMA)

Wikipedia page is here

When a pressure applied to the film, the atomic orbitals are deformed. There are two types of deformations. (type 1): The orbital becomes more elliptical. (type 2): nuclears are shifted out of the center of the orbital. Both deformation makes the effective magnetic field HSO of the spin-orbit interaction larger.

Without a deformation the orbitals of the localized electrons is nearly spherical and the effective magnetic field HSO of the spin-orbit interaction is small.

When the pressure applied, the orbitals are deforms in the direction of the applied pressure and the effective magnetic field HSO of the spin-orbit interaction increases.

In a ferromagnetic material the localized electrons have a non-compensated spin, which creates a magnetic field Hmag

At an interface between a magnetic and non-magnetic material, the demagnetization field Hdemag is created due to uncompensated magnetic moment at the interface. The direction of Hdemag is perpendicular to the interface and opposite to Hmag.

The magnetic field Hinside inside of the ferromagnetic field equals Hmag- Hdemag

Additionally, the electron experience the effective magnetic field HSO of the spin-orbit interaction, which is always directed along Hinside. The magnitude of HSO is proportional to Hinside and the degree of the orbital deformation.

The magnetic energy of an electron equals to a product of the electron spin and Hinside+HSO.

When magnetization is perpendicular to the film, the orbital deformation is larger, HSO is larger and the magnetic energy is larger.

When magnetization is in-plane, the orbital deformation is smaller, HSO is smaller and the magnetic energy is smaller.

The dependence of the magnetic energy on the magnetization direction is called the magnetic anisotropy. In the case when the difference of the magnetic energy are with respect to the interface, the effect is called the perpendicular magnetic anisotropy (PMA)

 

 

Strains

The perpendicular-to-plain magnetization may also increase (decrease) due to strain.

 

When a thin film is grown on a substrate of different lattice constant, the film is strained.

When the lattice parameter of the film is larger than that of the substrate, the strains are tensile. The effective magnetic field of the spin-orbit interaction, which induced by the strains, is directed perpendicularly to the film plane.

When the lattice parameter of the film is larger than that of the substrate, the strains are compressive. The effective magnetic field of the spin-orbit interaction, which induced by the strains, is directed in plane.

Lattice constant of some metals. Click to expand

Fe (BCC) = 2.870Å (along [110] 2.03 Å )

Ta (BCC)= 3.310 Å (along [110] 2.34 Å )

Cr (BCC) =2.880 A (along [110] 2.036 Å )

V (BCC)=3.020 A (along [110] 2.135 Å )

W(BCC) =3.160 A

Cu (BCC metastable) =2.88 Å

 

Co (hcp) =2.59 Å

Ru (hcp) =2.700 Å

Ti(hcp)= 2.950 A

 

Pt (FCC) =3.920 Å (a/2=1.96)

Au (FCC)=4.080 Å (a/2=2.040Å)

Cu(FCC) =3.610

Al (FCC)=4.050 (a/2=2.025)

Pd (FCC) =3.890

 

notice: hcp and fcc structures are very similar (See here),

BCC cell consists of 2 net atoms, The bcc unit cell has a packing factor of 0.68.

FCC cell consists of 4 net atoms. The bcc unit cell has a packing factor of 0.74.

hcp cell consists of 6 net atoms. The bcc unit cell has a packing factor of 0.74.

 

MgO lattice constant a = 4.212Å (a/2=2.106Å)

Si=5.431 Å (a/2=2.7155) (along [110] 1.92 Å )

Ge=5.66 Å (a/2=2.83) (along [110] 2.001 Å )

GaAs= 5.65325 Å (a/2=2.826625 ) (along [110] 1.9987 Å )

 

Fe:GaAs (GaAs(110) easy axis)

TiN (cubic)


Young's modulus (tensile strain) & Bulk module (compressive strain)

 

SiO2= 68 GPa -- &--35 GPa

Al= 69 GPa -- &--76 GPa

Au= 79 GPa--&--220 GPa

Ti=110 GPa ---&--110 GPa

Cu=118 GPa-- &--140 GPa

Pt=168 GPa-- &--230 GPa

Ta= 186 GPa-- &-- 200 GPa

Fe=210 GPa-- &--170 GPa

Co= 209 GPa-- &--180 GPa

W=400 GPa-- &--310 GPa

Ru= 447 GPa -- &--220 GPa

Ir=528 GPa -- &--320 GPa

MgO= 270-330 GPa-- &--250 GPa


MgO bulk elastic properties

Compressive Strength 800-1600 MPa

Elastic Limit 80-166 MPa

Hardness 5-7 GPa

Breakdown Potential= 6-10 MV/m=0.006-0.01 V/nm


Conductivities (S/m)

Silver 6.30E+07
Copper 5.96E+07
Gold 4.10E+07
Aluminium 3.50E+07
Tungsten 1.79E+07
Co    1.66E+07
Nickel 1.43E+07
Ru 1.40E+07
Iron 1.00E+07
Platinum 9.43E+06
Tin 9.17E+06
Cr  7.87E+06
Ta    7.40E+06
Carbon steel (1010) 6.99E+06
Lead 4.55E+06
Titanium 2.38E+06
Stainless steel 1.45E+06
titanium Nitride   1.42-3.33E6

 

 

 

 

Strain relaxation and the critical thickness.

The strain field, which acts on the film-substrate interface, is linearly proportional to the film thickness. The thin film has the in-plane lattice parameter the same as that of the substrate. As the film thickness increases the strain field, which acts on the interface, increases. At some thickness the strain field becomes sufficient to create a dislocation at interface. This thickness is called the critical thickness. The dislocations reduce the strain in the film. The process of the creation of the dislocation is called the stain relaxation mechanism.

The critical thickness depends on the crystal quality of the film and the strain relaxation mechanism. Approximately, the critical thickness hcritical can be calculated from relation:

notice: Eq. (3) is valid only for high-crystal quality low-defect-density materials. Otherwise, the ratio (3) becomes smaller than 0.7.

Example 1. AlGaAs (001)on GaAs(001)

The lattice constant of AlGaAs (x=0.5) is 0.069 % larger than the lattice constant of GaAs.

The strains are compressive. The critical thickness approximately equals to 1 um.

Example 2. InGaAs(001) on GaAs(001)

The lattice constant of InGaAs (x=0.5) is 3.582 % larger than the lattice constant of GaAs.

The strains are compressive. The critical thickness approximately equals to 19.5 nm.

Example 3. Fe(001) on MgO(001)

The lattice spacing of MgO (001) in [110] direction is 3.74 % smaller than lattice spacing of Fe (001) in [100] direction.

For Fe film on MgO, the strains are tensile. For MgO film on Fe, the strains are compressive.

The critical thickness in both cases approximately equals to 18.7 nm.

3.74% of tensile strains in Fe correspond to mechanical tensile stress of 7.44 GPa

3.74% of strains in MgO correspond to mechanical compressive stress of 9.3 GPa. It is significantly larger than the elastic limit of MgO of 160 MPa, and compressive Strength of 1.6 GPa

for more details about Fe:MgO:Fe MBE grown click to expand

bulk

MgO lattice constant a = 4.212Å (a/2=2.106Å)

Fe (BCC) lattice constant a = 2.870Å (along [110] 2.03 Å )


 

Case of MgO (1.8nm) on Fe (See Yuasa et al. Nature Material (2004))

MgO

Even the is much less than the critical thickness, the 2/3 of strains is relaxed (from 3.74% to 1.2 % (2.54% of strains are relaxed))

experiment:

the lattice spacing is elongated along the [001] axis by 5% and is compressed along the [100] axis by 1.2% compared with the lattice of bulk MgO (compressive stress 3 GPa . It is larger than compressive Strength of 1.6 GPa).

Fe

tensile strained (max 2.54%)

experiment

the lattice of the top Fe electrode is expanded by 1.9% along the [110] axis, which means that 0.64% is relaxed. (2.54%-1.9%)

tensile stress is 4 GPa

 

Example 4 Ta on Fe

The lattice constant of Ta is 13 % larger than lattice constant of Fe.

Ta is compressively strained. A thin Ta can be used with tensile-strained Fe in order to reduce the strain field and to increase the critical thickness of the tensile-strained Fe.

 

Magnetostriction

The mechanical stress σ can be calculated

where ε is the total strain, E is the Young’s modulus at magnetic saturation and λ is the magneto elastic strain

 

Magnetostriction. Click to expend

Magnetostriction

wiki page is here

The effect describes the change of shape of a ferromagnetic material when its magnetization changes.

The origin of the effect

Magnetostatic interaction between domains in the ferromagnetic materials. When shape, size, magnetization inside domains changes, the strength of the magnetostatic interaction between domains changes and the lattice contracts or expands.

Note: in a single-domain nanomagnet the magnetostriction of this type does not exists.

Materials

Terfenol-D (TbxDy1-xFe2)

The magnetostriction of the Terfenol-D generates strains 100 times greater than traditional magnetostrictive, and 2-5 times greater than traditional piezoceramics.

For typical transducer and actuator applications, Terfenol-D is the most commonly used engineering magnetostrictive material.

Elastic properties (Tb0.3Dy0.7Fe1.92)

Young's Modulus=25-35 GPa

 

 


Voltage-induced spin-orbit interaction & VCMA effect

details about VCMA effect are here

Enhancement of the spin-orbit interaction due to electrically induced orbital polarization

Without electrical field

When electrical field is applied

 

Fig.17. The yellow mesh shows the electron orbital; the blue arrow shows direction and magnitude of external magnetic field; the green arrow shows direction and magnitude of the effective field of the spin-orbit interaction; the red arrows shows direction and magnitude of the applied electrical field; the oval at left- bottom corner shows the induced dipole polarization of the orbital.

 

The external magnetic field induces the magnetic field along its direction. In the case of near-spherical orbit (Fig. 17), the enhancement is small and the magnetic field of the spin-orbit interaction is small.

 

In the external electrical field the positively-charged nuclear moves a little toward the direction of the electrical field. The negatively-charged electrons move in the opposite direction.

Without the electrical field the charge was symmetrically distributed (Fig. 17 left). When the electrical field is applied there is more positive charge at right side and there is more negative charge at the left side. Therefore, the electrical field induces a dipole polarization in the material. The dipole polarization is described by permittivity of the material.

Also, the magnetic field of the spin-orbit interaction becomes larger. Under the electrical field the electron orbit is deformed so that at the left side the electron distribution becomes denser in the close vicinity of the nuclear. Therefore, at the left side of the nuclear the electron experiences a larger electrical field and a larger corresponded magnetic field of the spin-orbit interaction. Even at the right side of the nuclear the spin-orbit interaction is reduced, in total the spin-orbit interaction becomes larger in the electrical field. It is because the electrical field of nuclear decays as 1/r^2 and at left side it increases sharply, but at the right the decrease is small.

It is important: In absence of an external magnetic field, there is no spin-orbit interaction and HSO=0.

The spin-orbit interaction by itself cannot break the time inverse-symmetry!

Enhancement of the spin-orbit interaction due to electrically induced orbital polarization

click here or on image to enlarge it

Fig.

 

 

 

 

 

 

 

 

 

 

 

 


Spin-dependent scatterings

Due to the spin-orbit interaction the electron scattering may become the spin-dependent.

The total spin of the electron gas is not conserved after each spin-dependent scattering.

An electron scattering becomes spin-dependent when the spin-orbital interaction connected the spin of the electron gas to a spin particle outside of the equilibrium of the electron gas

 

The Spin Hall effect is the effect describing accumulation of the spins at a surface of a metallic wire, when an electrical current flows through the wire due to the Spin-Orbit interaction.

The effects which are originated by spin-dependent scatterings

The effective electrical field, which is induced by the defect, is shown by the green arrows. The electrical field induces the spin-orbit interaction. The direction of the effective magnetic field of the spin-orbit interaction is different for electrons scattered into the left and into the right. This makes different the probabilities of scattering into the left and into the right.

Anomalous Hall effect

The charge is accumulated, when a spin-polarized drift current flows

click here or on picture to enlarge it

Spin Hall effect

The spin is accumulated, when a spin-unpolarized drift current flows

click here or on picture to enlarge it or another version

Inverse Spin Hall effect

The charge (and spin) is accumulated, when a spin diffusion current flows.

click here or on picture to enlarge it

Under an applied voltage the drift current flows in the metal wire. When the metal is ferromagnetic, the drift current is spin-polarized. Therefore, there are more electrons with spin directed up. It causes more electrons be scattered into the left than into into the right. This is the reason for the charge accumulation at the right side of the wire. Under an applied voltage the drift current flows in the non-magnetic metal wire. The drift current is spin-unpolarized and the electrons have spin in any direction with an equal probability. Since the probability to be scattered to the left is higher for electrons with spin directed up and the probability to be scattered to the right is higher for There is a region of spin accumulation at backside of the wire. The diffusive spin current flows from the region of a higher spin accumulation to the region of a lower spin accumulation. This means that spin polarized electrons (spin directed up (TIA assembly)) flow from back to front of the wire. In the opposite direction the spin-unpolarized electrons (spin directed in all directions (TIS assembly)) flow. The scattering probability of spin-up electrons into the right is higher. This is the reason for the charge accumulation at the left side of the wire.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Detailed explanation of this Figure is here

 



Famous misunderstands and misinterpretations of the Spin-Orbit (SO) interaction

misinterpretations (1): The Spin-Orbit interaction is the Quantum-Mechanical effect, because it can be derived from the Dirac equations. The SO is only the feature of very small quantum-mechanical objects. A larger object does not experience any SO interaction.

The SO interaction is important for a large objects as well. For example, the influence of SO interaction is very strong in case of a Giant object (like a neutron star or a black hole). In the vicinity of these giant objects the particles move at near the speed of light and there is a substantial electrical field. As a result, the SO magnetic field is very strong in the vicinity of a neutron star or a black hole.

The Spin-Orbit interaction is a relativistic effect, but not a Quantum-Mechanical effect. The Dirac equations are relativistic quantum-mechanical equations describing the quantum field of electrons. As any relativistic equations, they contain the information about the SO interaction. Any relativistic equations, which describe the photon-electron interaction, should contain a description of the SO interaction. For example, the Maxwell's equations contain the description of the SO interaction as well.

e.g. See Landau, Lifshitz. The Classical Theory of Fields

misinterpretations (2): There is a "special" "quantum-mechanical" field of the Spin-Orbit interaction

Only a field of the SO interaction is the magnetic field. The SO magnetic field HSO is a very "normal" magnetic field, which is undistinguished from any other magnetic fields (e.g. the magnetic field created by an electrical current). There is only one difference between HSO and "normal" magnetic field, HSO cannot induce the Lorentz force

misinterpretations (3): Since the SO interaction can "interact" only with the electron spin, which is a quantum mechanical object, but not with the orbital moment, the SO interaction has quantum-mechanical origin. Additionally, the SO magnetic field does not induce the Lorentz force. It is an additional proof of the quantum-mechanical origin of the SO interaction.

An electron experiences the SO magnetic field only in the coordinate system, which moves together with the electron. In this coordinate system, the electron does not move. It stay still. Therefore, in this coordinate system the electron does have any orbital moment or any movement-relative property and the SO magnetic field can only interact with the electron spin. The spin is only one magnetic property remaining for a motionless still object. For the same reason, the SO magnetic field does not create the Lorentz force. The Lorentz force is created when an electron moves in a magnetic field. In the coordinate system where the SO magnetic field HSO exists, the electron does not move. This property is related to the Quantum Mechanics.

misinterpretations (4): Since the strength of the SO interaction is proportional to 1/c2, the influence of SO interaction always can be calculated as a tiny perturbation.

The SO interaction is not small at all. In a ferromagnetic metal with perpendicular magnetic anisotropy (PMA), HSO may reach tens of kGauss. For example, in a thin Fe film it can override the demagnetization field of 20-30 kGauss and align the magnetization perpendicularly to the film surface.

misinterpretations (5): The spin-orbit is proportional to the orbital moment of an electron.

The SO interaction is not directly related to the orbital moment (see here). Even though in same specific cases, such relation can be established.

As was shown above, the strength of the spin orbit interaction substantially depends on the breaking on the orbital spatial symmetry and the time-inverse symmetry. The orbital symmetry is different for an electron of a different orbital moment. As was shown above, the dependence of the SO interaction on the orbital spatial symmetry is more rich and complex than just its dependence on its orbital moment.

It is the case (the most common case) of substantial strength of the SO interaction, when the SO interaction is induced by an electrical field of atomic nuclear.

Note: the formulas for orbital momentum and the spin-orbit interaction are very similar. The difference between them is only coefficient 1/r2. In close vicinity of the nuclear, the 1/r2 is huge and it makes a huge difference.

Can the Schrödinger equation be used to describe the spin-orbit interaction?

A. Yes. Even though the Schrödinger equation is not a relativistic equation, its combination with the Maxwell's equations, which are relativistic equations, gives a correct description of the SO interaction. The spin properties should be included into the solution. Such description includes the relativistic features of the electromagnetic field, but does not include the relativistic features of the quantum field of an electrons. However, for a description of effects in a solid state these features can be included by adjusting some parameters and constants.

 

Difference between description of the spin-orbit interaction from the Dirac equations or Schrödinger equation

Dirac equations calculates both contributions to the spin-orbit interaction (contribution 1) due to relativistic nature of the electromagnetic field. (contribution 2) due to relativistic nature of the quantum field of an electron.

Schrödinger equation calculates only contribution 1 due to relativistic nature of the electromagnetic field. It does not calculate contribution 2 due to relativistic nature of the quantum field of an electron.


Spin-orbit interaction obtained from the Dirac equations

The Dirac equations include the full and correct description of the SO interaction.

The Einstein's relativistic equation for the energy is

which should describe the electron field as well. The quantum-mechanical for the energy and the momentum are

Substituting Eq.(3.2) into Eq.(3.2) gives Klein-Gordon equation as

Considering limitation on the possible symmetries of the wave function, the wave equation should be 1st order differential equation with respect of time and space. Dirac has found that the Klein-Gordon equation can be represented as a product of 1st order differential equation and its conjugate. Therefore, such the wave equation fully describes the electron field.

The Dirac equation (classical form) is

where the gamma matrices (2 × 2 sub-matrices taken from the Pauli matrices)

The Dirac equation, which includes the gauge potential, is

Does Klein-Gordon equation include information about the conservation of the time-inverse symmetry and spin?

Probably not. The Dirac equation and the Pauli equation, both do include the conservation of the time-inverse symmetry and spin. It is difficult to answer about the Klein-Gordon equation.

 

Non-relativistic form of Dirac equation

case v<<c

In this case wave function can be represented as a sum of a large "electron" part and a small "positron" part.

a perturbation method using 1/c2 as a small parameter.

 

 


Pauli equation & Spin-Orbit interaction

The Pauli equation is the wave equation describing interaction of electron spin with the electromagnetic field. Wiki page is here

Does the Pauli equation describe the spin-orbit interaction?

Yes, the effective magnetic field HSO should be used in Eq.(3.2) additionally to the external magnetic field. Then, the Pauli equation correctly describes the SO interaction

In the Pauli equation the gauge invariant A is used, which is the invariant for the Lorentz transformation, therefore the Pauli equation (Eq.(3.1))should automatically include the spin-orbit interaction without usage of HSO?

The gauge invariant A is the invariant for the Lorentz transformation, but the wave function of the Pauli equation is not a invariant. Therefore, in contrast to the Dirac equation the Pauli equation is not an invariant for the Lorentz transformation. The reason why HSO is not included into the Pauli equation, but should be input as additional magnetic field, is following. The Pauli equation is the equation, which is valid in only one static coordinate system. When an electron moves in this static coordinate system, the Pauli equation becomes not valid. The relativistic transformation of the quantum field of an electron are missing in Pauli equation. However, the adding of HSO fixes the problem and the Pauli equation becomes valid again.

The Pauli equation is the extension of Schrödinger equation, where electron spin properties are included

The Pauli equation can be considered as a semi- relativistic equation. They place is between simpler, but approximate Schrödinger equation and full, but more complex Dirac equation.

The Pauli equation can be obtained from the Dirac equation.

The Pauli equation (classical form) is

where A is the magnetic vector potential and is φ the electric scalar potential. σ is the Pauli matrices

The magnetic field can be calculated as

Substituting Eq.(4.2) into Eq.(4.2) gives

The Pauli equation, which include the Spin-orbit interaction, is


Merits and demerits of the use of the Hamiltonian approach for calculations of the Spin-Orbit interaction

This method has limitations and restrictions, which are discussed below

 

merit (1): The Spin-Orbit interaction can be easily included into a more general Hamiltonian as an additional term

merit (2): It allows to use a calculation based on a minimization of Hamiltonian. It does not require any assumption about orbital symmetry.

 

demerits:

demerit (1): It often approximates that the spin-orbit interaction is small. It is a very rough approximation and it is often incorrect.

 

demerit (2): a small calculation error may lead to a substantial error in the final result. It does not use the important features of the orbital symmetry for the calculation of SO.


HSO and external magnetic field

Fine structure

The fine spliting of the energy levels of a hydrogen atom. The fine structure correction predicts that the Lyman-alpha line (emitted in a transition from n=2 to n=1) must split into a doublet.
Image taken from here. click on image to enlarge it

(important fact:) The spin-orbit interaction cannot break time- inverse symmetry by itself. E.g. the object cannot be magnetized due to spin-orbit interaction only. The spin-orbit interaction enhances already-existed magnetic field, but it cannot create its own magnetic field. The effective spin-orbit magnetic field HSO is always proportional to the external magnetic field. It does not exist when there is no external magnetic field. This condition is the consequence of the conservation law of the time-inverse symmetry.

(fact) HSO does not exist when there is no external magnetic field

(fact) HSO is always proportional to an external external magnetic field

Two effects: (1) the fine structure and (2) the difference between energies of heavy and light holes) exists due to the spin orbit interaction. In both cases there is no external magnetic field, but there is a substantial spin-orbit interaction. Why?

Fine structure. Heavy and light holes.

(possible contradiction) The spin-orbit interaction cannot break the time-inversion symmetry. As a result, the spin-orbit interaction does not manifest itself until the time-inverse symmetry is broken by another mean. E.g. the external magnetic field is applied or an electrical current flows. This fact contradicts with the existence of a fine structure in the hydrogen gas and the existence of the heavy and light holes in a semiconductor? In both case, the spin-orbit interaction causes a spliting of the energy levels without any external magnetic field.

There is no contradiction. Globally, there is no magnetic field in both cases. However, when the orbital moment of an atom is a non-zero, locally the time-inverse-symmetry is broken for each atom and each atom experiences a magnetic field HSO of the spin-orbit interaction. Since the directions of the orbital moments are equally distributed in all directions, the gas is non-magnetic and globally there is no breaking of the time-inverse symmetry. Still changes the electron energy and there is a difference of electron energies of electrons of different orbital moments.

Difference between Global and Local breaking of the time-inverse symmetry (TIS)

It can be understood from the following example, which describes the Zeeman splitting of a gas of atoms in a magnetic field and in which the more complex effect of the spin-orbit interaction is not involved. Let us consider a gas of atoms. Each atom has a magnetic moment and a non-zero spin. Both the local and global breaking of TIS lead to the energy splitting. However, only the global breaking creates the directional- dependency of gas or material properties.

(global symmetry breaking): The symmetry is broken globally, when a sufficiently-strong external magnetic field is applied. As a result, the magnetic moment of all atoms is aligned along the magnetic field. The magnetic field field breaks the time-inverse symmetry. It results in two changes. The first change the energy of electrons with spins along and opposite the magnetic field becomes different. As a result, one energy level splits into two levels. The second result of the time-inverse symmetry breaking is that the properties of the atomic gas become direction- dependent. E.g. left- and right- circular polarized light is absorbed differently, when light propagates along the magnetic field and the absorption is the same when the light propagates perpendicularly to the field.

(local symmetry breaking): The symmetry is broken locally, but not globally, when there is a magnetic field in each atom along the atom magnetic moment, but the magnetic moments of all atoms are not aligned. It is the case when there is no external magnetic field and the magnetic moments of atoms of the gas are distributed equally in all directions. In this case there is no any directional asymmetry, but the energy splitting still remains. Each atom has the equal energy splitting independently on direction of its magnetic moment. Even though globally there is no magnetic field, the atoms of the gas experiences the Zeeman splitting.

Spin-orbit interaction makes properties of light and heavy holes different

The the light and heavy holes experience a different magnitude of the spin-orbit interaction, due to the different symmetry of their the spacial distribution. The spin-orbit interaction causes the difference of properties between the light and heavy holes.
click on image to enlarge it

 

The fine structure :

It describes the fact that in the hydrogen gas the p- energy level is splits in two levels: the lower-energy level (J=1/2) and the higher-energy level (J=3/2).(More details see here)

(The reason of the energy splitting:) The orbital moment of a p- electron is non-zero. As a result, the p- electron experiences the additional spin-orbit magnetic field HSO and corresponding additional energy ESO=HSO*S, where S is the electron spin. HSO depends on the spacial symmetry of the orbital. The spacial symmetry of the orbitals of J=1/2 and J=3/2 are different. As a result, the electrons experience a different HSO and have a different energies.

In a molecular or atomic gas, all values of the orbital moment, HSO and electron spin are non-zero. The orbital moment, HSO and electron spin are directed in one direction specific for each individual atom. Each atoms creates a dipole magnetic field around itself and theretofore it can be considered as a tiny magnet. However, the atomic gas in total is not magnetic. It is because the directions of the orbital moments, HSO and spins are equally distributed in all direction. Still HSO changes the energy of each atom, which is independent on the direction of magnetic moment of each atoms and remains as a feature of the whole gas. The dependence of HSO on the orbital moment (orbital symmetry) causes the fine energy splitting.

Light and heavy holes

In a semiconductor, the holes have p- spacial symmetry. The holes are divided into two classes: the light (J=1/2) and heavy (J=3/2) holes.

(reason of difference between light and heavy holes) The the light and heavy holes experience a different magnitude of the spin-orbit interaction, due to their different orbital symmetry (orbital moment). The spin-orbit interaction causes the difference of energies (properties) between the light and heavy holes.

The orbital moment of a hole is non-zero. As a result, the hole experiences the spin-orbit magnetic field HSO and corresponding additional energy ESO=HSO*S, where S is the electron spin. HSO depends on the spacial symmetry of the orbital. The spacial symmetry of a light hole (J=1/2) and a heavy hole (J=3/2 ) are different. As a result, the the light and heavy holes experience a different HSO and have a different energies.


Questions & Comments about Spin-Orbit interaction

Q. Why does the Spin-orbit interaction increase when the electronic orbital is deformed and becomes less symmetrical?

A. There are several reasons why:

(reason 1): In the case of a deformed orbital, the electron distribution become more close to the atomic nuclear (at least some of the distribution). In order for the SO be large, the electron should move in a very large electrical field, which is only exists in a very close proximity of the nuclear. Only this region mainly contributes to SO. If the ionic or atomic radius (see here or here) is about 50-100 pm, the region with radius only 1-2 pm contributes about 99% to SO (number might be slight different from case to case due to the symmetry and frequent cancelation of SO in the proximity of the nuclear).Usually a deformation of orbital moves electron (electron distribution) closer to the nuclear, which makes the SO larger.

(reason 2): In the case when the orbital moment is zero, the SO is zero, because it has two opposite contributions, which of compensate and cancel each other(see here). The deformation makes the orbital moment larger. There is no balance between two opposite contributions and the SO is enlarged. The localized electrons and the conduction electrons of the s-symmetry (e.g. n-type electrons in a semiconductor) have zero orbital moment.

(reason 3): The SO is enhanced by an external magnetic field, because of the symmetry breaking due to the Lorentz force induced by the external magnetic field. This enhancement is more efficient for less symmetrical orbital.

Q. Why an external magnetic field does not break symmetry for a spherical orbital? (Jan asked): So i guess i understand the Point why an external field is making a net- spin -orbit- interaction magnetic field, but i DONT see why this is NOT the case for a spherical Orbit.

A. You are right. An external magnetic field enhances the SO in the case of the spherical orbital as well. However, the enhancement is more efficient for a deformed orbital than for a spherical orbital. This can be understood as follows. A deformed orbital has some part of electron distribution, which is very close to the nuclear.

Q. You always depict an electron as a point-like particle. Should you use a wave function instead and full quantum-mechanical description of the SO interaction?

A. The spin-orbit interaction is relativistic effect (it is not a quantum- mechanical effect). All effects due to the spin- orbit interaction exist for both a small object (which should be described by a wave- function) and a large object ( which can be approximated as a point- like classical object).

Q. Why the spin- orbit interaction cannot break the time- symmetry by itself?

A. The spin- orbit interaction is a relativistic effect, which just describes the transformation of the electro - magnetic field between coordinate systems moving with different speeds (Also, it describes the transformation of the quantum field of electron between different coordinate systems (See Dirac Eq.)). If the time- inverse symmetry is not broken in one coordinate system, it is not broken in any other coordinate system. It doesn't matter whether the coordinate system is moving or not. It is the reason why the spin- orbit interaction cannot break the time- inverse symmetry. In order to manifest itself, the spin- orbit interaction always requires an external breaking of the time- inverse symmetry.

 


 

Content of this page represents my personal view and it is reflected my own finding. It may slightly different from the "classical" view on the spin-orbit interaction, which is described in following references

M. T.Johnson et. al. Reports on Progress in Physics(1996) ; P.Bruno PRB (1989);

 

 

 

 

 

 

 

I truly appreciate your comments, feedbacks and questions

I will try to answer your questions as soon as possible

 

Comment Box is loading comments...