Slide 1

Hello

My name is
I am from Spintronics Research Center, AIST,  Tsukuba Japan
In this presentation I will talk about new theory of electron gas in a solid, which includes the fact that spin direction of an electron may be  rotated even during a spin-independent scattering.

I would like to emphasize that  this theory describes very well all-known effects and experimental facts related to the electron gas in a solid  without  any contradiction.

This presentation is only an introduction part for this theory. if you are interested in more details about this theory or have a question or  a suggestion, please visit my web site. The link is shown at bottom of the slide  

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Slide 2
Proposed model vs Classical model of electron gas in a solid

To begin, I would like to compare the presented model with the classical model of electron gas in a solid.

By the classical model I mean the Pauli model for non-magnetic metals, the Zener model,  the  Stoner model and  the s-d model for ferromagnetic metals.

In the classical model it is assumed that spins of all electrons are aliened and all electrons occupy two bands. Electron spins of one band are aligned in opposite direction to the spin direction of electrons of another band. This classical model is called the spin-up/spin-down model. The schematic band diagram assumed in the classical model is shown at upper-right part of the slide.

 It is important to emphasize that there no any proof or fact or argument which would support the assumption of the classical model that all spins in electron gas are aligned into spin-up and spin-down bands.

Another problem of the classical model is that it does not specify the direction along which all spins in spin-up/spin-down bands should be aligned. The direction of spin is free parameter of the classical model. Sometimes it is chosen according to sample geometrical shape or crystal orientation. In the case of a ferromagnetic metal spin alignment is chosen along the magnetization direction. In this case the spin-up and spin-down bands are called the majority and minority bands. For me It is unclear what reasons and conditions should be used to specify a direction along which all spins are supposed to be aligned in the classical model. 

In presented paper I have proved that the basic assumption of the classical model of the electron gas in a solid is incorrect and under any circumstances all spins of electrons can not be aligned into two opposite directions.

The schematic diagram in lower-right side of the slide shows only possible distribution of spin directions in electron gas in a solid. The electrons in the electron gas can be in two groups. In one group there is no spin alignment. In this group an electron spins can have any possible direction with equal probability. In electron gas without spin accumulation all electrons are in this group. In another group all electron spins are aligned in one direction. Electrons can be in this group only in the case when there is a spin accumulation in electron gas.

Since spin-dependent scatterings are very rare in electron gas, in the classical model the spin-up and spin-down bands are almost independent. They may have different Fermi energies and different densities of states. The exchange of electrons between the spin-up and spin-down bands is very rare. The presented model describes the electron gas differently. It proves that there is a frequent exchange of electrons between groups and the Fermi energy is the same for all electrons. 

Once more I would like to emphasize that the classical model of the electron gas  is incorrect.  However, in some cases the classical model may give a correct prediction. If we calculate a projection of all spin on any axis the spin distributions shown in upper and lower parts of the slide may give the same result. Therefore, the classical model may calculate correctly effects in electron gas, only if the effect depends only on spin projections. In the case when an effects depends on spin direction, the classical model can not describe the effect correctly.   

Slide 3

Spin & Electron states in a solid

Next two slides are most important in this presentation. These slides explain reasons why the spin direction may be rotated even after a spin-independent scattering. By a spin-independent scattering I mean a scattering, which is described by a spin-independent Hamiltonian. This laterally means that during scattering there is no any force, which could rotate the spin. Still the spin is rotated after the scattering. It is pure quantum-mechanical effect and a macro -sized object can not experience such rotation. It is a feature of only elemental particles like an electron. 

Since there is no analogous effect in the macro world it might be difficult to understand this effect quickly.  In the case when it is not easy to follow my following arguments and explanations, please visit my web site. There are many animations and mathematical proofs there. They may be helpful to understand better this very interesting effect  

This slide explains a electron state and its spin properties.

According to the Pauli exclusion principle, an electron state can be occupied maximum by two electrons of opposite spins. Therefore, there are three possibilities of occupation of a electron state as it is shown at left-upper side of the slide. When the state is not occupied by any electrons, I define such state as "empty" Obviously, an empty state does not have any spin. When the state is occupied by one electron, the spin of state is one half and the spin have a direction. Therefore I define this state as a spin state. In the case when the state is occupied by two electrons of opposite spins, the spin of the state is zero and there is no spin direction.  I define such state as  a full state.

The schematic diagram in the upper right corner shows the energy distribution of electron state for delocalized electrons in metal. Each electron state is distinguished by its energy, direction of wave vector and symmetry. 

The states of higher energy (at least 5kT above the Fermi energy), are not occupied by electrons and almost all of them are “empty” states with no spin . Almost all states of lower energy (at least 5kT below the Fermi energy) are occupied by two electrons and almost all of them are the “full” states with no spin. A state, energy of which is near the Fermi energy, may be filled by one electron and have spin one half. Also, there is a probability that  the state is not filled or it is filled by two electrons of opposite spins

The following is very important.

Only a spin state has non-zero spin and spin state has a spin direction. Spin of Full and empty states is zero. Since these states do not have a spin, they do not have a spin direction as well. 

If spin states is described by a spinor, in contrast the wavefunction of a full state is as a scalar. That means that the wavefunction of full state is an invariant under any spatial rotations. This literally means that 3 full states depicted in left-lower part of the slide are identical and undistinguishable. It is an very important feature of an electron, which lead to effect of spin rotation during spin-independent scatterings.

You may say “ A Hey Why they are not distinguishable. Just look at them. We can easily distinguish all three of them”. You may say that only because I depicted the electrons as microsize objects with surface color, surface structure, surface roughness. Remember an electron is an elementary particle, which does not have these all attributes. The electron has only one feature, which could make  distinguishable its orientation in the space. It is its spin direction. When the electron get into the full state, it loses this feature. All we know about the electrons in the full state is that their spins are in opposite directions. But there is nothing inside a full state, which could memorize or record the information, in which particular direction spins are pointed.

Let us take as an example electrons of the inner shells of an atom or electrons of an atom of an inert gas. We know that in this case the total spin of all electrons is zero. That means that numbers of electron with spin parallel and anti parallel to any axis are equal. This statements is valid not for some specific axis, but for all axes. In case of two electrons this feature is just depicted at lower-left part of the slide.

In next slide I will show how this important quantum-mechanical feature causes the spin rotation. 

Slide 4

Spin rotation after a spin-independent scattering.

This slide explains how spin may rotate after spin-independent scatterings in case when there is no any force, which could possibly turn the spin.

Let us consider elastic spin-independent scatterings between 3 delocalized states, which all have the same energy and this energy is close to the Fermi energy.  The states are labeled as “A”, “B” and “C”  and they are only distinguished only by slight difference in direction of their wavevector  in the Brillion zone. Let us assume that at time t1 the state “A” is filled by one electron, which spin is up, the state “B” is filled by one electron, which spin is down, and the state “C” is not filled. At time t2 after 10 ps the electron of the state “A” experiences an elastic scattering. Since the spin-up place of the state “B” is not occupied, the electron may be scattered into state “B”. In this case, at time t1 state “B” becomes occupied by two electrons, its spin is zero and it does not have a spin direction. At time t3 after 20 ps an electron of the state “B” experiences again an elastic scattering into an unoccupied state “C”. Since there was no electron in state “C” at time, the electron scattered into state “C” may have any spin direction. For example, it may have spin-right direction. Than, spin of the electron, which remains on the state “B”, is spin-left. With equal probability the electrons of states “B” and “C” may have spin-front and spin-back directions. Therefore, the electrons have experienced a random spin rotation after two consecutive spin-independent scatterings. At time t1=0 ps there were two spin states with spin up and down. Result of two scatterings is again two spin states but with spin directions left and right.

   The random spin rotation occurs because of a quantum nature of an electron. At time t2, the spin of state “B” is zero and inside of the electrons of this state there is no record, which spin directions they had before at time t1. Therefore, when the electron scattered out of full state at time t3 the spin direction of the electron is random without any correlation to the spin direction the electron had at time t1. 

Slide 5

This slide explains different scatterings and how they are influenced by electron spin.

There are scatterings, which change and which do not change the number of states of each type.

At upper-right part it is shown a scattering of an electron from spin state into "empty" state.  The result of this scattering again one spin state and one empty stat as it was before the scattering. Therefore, this scattering does not change the  number of either spin states or empty states.

 The similar scattering is the scattering of an electron from a full state to a spin state. The result of this scattering is one spin state and one full state. Again the scattering does not change the  number of either spin states or full states.

Since these two kind of scatterings do not change the number of states, they do not influence the spin properties of the electron gas.

At left -lower part it is shown a scattering of an electron from full state into "empty" state. The result of the scattering is two spin states of opposite spin directions. As was described in the previous slide, the spin direction of the resulting spin states can be in any direction with equal probability. It is important that this scattering reduces the number of the full and empty states and increases number of the spin states. 

At right -lower part it is shown a scattering of an electron from one spin state into another spin state of opposite spin direction. The result of the scattering is full and empty states. Contrary to the previously-described scattering, this scattering reduces the number of the spin states and increases number of the full and empty states. As will be shown in the following slides, the balance between these two scattering determines the number of the spin, full and empty states in electron gas or spin statistics.

Slide 6

This slide explains properties of  two groups to which all electrons in electron gas can be divided, These groups  I call electron assemblies, because they contains different types of states with different properties..
   
First group, shown in yellow color, represents only group which exists in case when there is no spin accumulation in a metal. I named this group the time-inverse symmetrical assembly or in short TIS assembly. Assembly because it contains full, empty and spin states. The time inverse symmetry is a key property of this group of electrons. In the TIS assembly spin states can have any possible spin direction with equal probability.

You may recall from the last slide that there are electron scatterings which converts two spin states into empty and full states and there are scatterings, which do reverse conversion. Therefore, in the TIS assembly there is a continuous conversion between spin, full and empty states.

There is another group of electrons, which only exists in the case when there is a spin accumulation in a metal. It is shown in blue color.  I named this group the time inverse asymmetrical assembly or in short TIA assembly. It contains only the spin states with spins directed in the same direction. 
The number of electrons and its spin direction are two parameters describing the TIA assembly.

The figure at right side compares the distributions of spin directions for spin states of the TIS and TIA assemblies. The length of a vector from the origin to the surface corresponds to the number of electrons, the spin direction of which is the same as the direction of the vector. All spins in the TIA assembly are directed in one direction. Spins in the TIS assembly are directed in all directions with equal probability.

Slide7

In next two slides I will describe the spin statistics. 

It is well know that the energy distribution of electrons in the electron gas is described by the Fermi-Dirac statistics, which is shown at upper part of slide. 

The spin statistics describes the energy distribution of the full, empty and spin states. It is additional information to the Fermi-Dirac statistics. 

As was described in the previous slides, there are scatterings, after which the numbers of the full, empty and spin states are changed.

After the scattering shown at the right side, an electron moves from a full state empty state The results of the scattering are more spin states and less full and empty states. Contrary after the scattering shown at the left side the number of spin states decreases and the number of full and empty states increases. In an equilibrium, the numbers of spin, full and empty states do not change. Therefore, these two scatterings should balance each other. From this condition the formula describing the spin statistics is obtained as it is shown at the lower part of the slide.

Slide 8

The left graph shows the energy distribution for full, empty and spin  states in the case of a electron gas without any spin accumulation. At energy lower than the Fermi energy, most of the states are full.   At energy higher  than the Fermi energy, most of the states are empty. The spin states are mainly near the Fermi energy. 
The graph at right side shows the spin statistic in case of 75 % of spin accumulations. The distribution of spin states of TIA assembly is similar to the distribution of the spin states of the TIS assembly. Please notice the decrease of the number of the full and empty states near the Fermi energy in the latter case

At lower part,  I compare the spin statistic in the classical an presented model for the case of spin accumulation in a non-magnetic metal.

In the classical model the spin/up and spin/down bands are independent, When there is a spin accumulations, the band have different Fermi energies, which means the two bands are described by two different statistics. 

In the presented model, electrons frequently interacts. Therefore the both assemblies have the same Fermi energy and they are described by the same spin statistic.

Slide 9

In the following two slides I will talk about the interaction of a magnetic field with an electron gas, about the Pauli paramagnetism and the properties of the electron gas in a ferromagnetic metal.

In short, the effect of a magnetic field on an electron gas is simple. Under a magnetic field the electrons of the TIS assembly are converted into the TIA assembly. Therefore, even if an electron gas is not spin-polarized without a magnetic field, it becomes spin-polarized under applied magnetic field..

Before explaining the details, I would like to compare descriptions of an electron gas in magnetic field as given by the classical and presented models.

At the upper part of the slide the schematic diagrams of spin-up and spin-down bands of the classical model are shown for the cases of a non-magnetic metal without a magnetic field, for a non-magnetic metal under a magnetic field and for a ferromagnetic metal. All these three spin distributions are incorrect, because they ignore the fact that the spin direction may rotates after a spin-independent scattering.

Without magnetic field the spin-up and spin-down band are absolutely identical. The classical model assumes that in a magnetic field the spin-up/spin-down bands are immediately aligned themselves along the magnetic field.  Electrons, which spin is along the magnetic field, have a smaller energy than electrons, which spin is opposite to magnetic field. Therefore, in a magnetic field the spin-up band is shifted down and spin-down band is shifted up. In the classical model the spin-up and spin-down bands may have different Fermi energies during time about the spin relaxation time. For a time substantially longer than the spin relaxation time some electrons from the spin-down band are scattered into the spin-up band making the Fermi energies the same for the both bands. Since the energies of bottom of the spin-up and spin-down bands are different, the number of electrons in each band is different as well. Therefore, the classical model predicts a spin accumulation in electron gas under a magnetic field. Upper-right schematic diagrams shows spin distribution of an electron gas in a ferromagnetic metal as it is assumed in a classical model. The exchange interaction of localized d- or f-electrons with delocalized electrons of an electron gas can be represented as an effective magnetic field. Since this effective magnetic field is very strong, additionally to a significant differences in bottom energies of the spin-up and spin-down bands and the number of electrons in each bands, there is a difference of the density of the states at the Fermi energy between the bands. Therefore, the conductivity of a ferromagnetic metal is spin-dependent. 

As I mentioned several times already, the classical model ignores the fact that spin may be rotated after a spin-independent scattering.

The correct spin distributions are shown at the lower part of the slide.

In non-magnetic metal without magnetic field, all electrons are in the TIS assembly and they have spins directed in all directions with equal probability. Under a magnetic field the electrons from the TIS assembly are transformed into the TIA assembly, so they become aligned along the magnetic field. All electrons were transformed into the TIA assembly, if there would be no spin relaxation. Due the spin relaxation the electrons are transformed back from the TIA to TIS assembly. Therefore, the balance between transforming rates between assemblies determines the spin polarization of the metal. The spin polarization is percentage of spin states in TIA assembly related to all amount of the spin states.

The spin distribution of electron gas in a ferromagnetic is similar to that in a non-magnetic metal under a magnetic field. Some spins are aligned and some are evenly distributed.

It is important to emphasis the spin polarization of a ferromagnetic metal is determined by a balance between a spin relaxation and spin pumping induced by the exchange interaction. For example, iron with many defects and contaminations has a smaller spin polarization of an electron gas than single-crystal high-crystal-quality iron.

Back to the classical model I would like to note that the spin accumulation induced by a magnetic field and a spin accumulation induced by spin injection are of different kinds. You can compare this and previous slides. In contrast, in presented model these two spin accumulations are identical and undistinguishable. 

Slide 10.

As was mentioned in the previous slide, the spin polarization in non-magnetic or ferromagnetic metal is determined by a balance of rates of spin relaxation and spin pumping. The upper equation describes this fact.

There are many causes of the spin relaxation, but the spin relaxation is always proportional to the number of the spin states in the TIA assembly. In contrary the conversion rate of electrons from TIS to TIA assembly or spin pumping is proportional to the number of the spin states in TIS assembly and of course the magnitude of the magnetic field. Plugging these two conditions into upper equations the spin polarization of the metal can be found.

Next, I would like to explain the reason why under a magnetic field the electrons are converted from the TIS to TIA assembly.

When a magnetic field is applied the spin precession starts around the magnetic field. You may guess that the spin precession may change significantly the spin distribution, but it is not true. Spin precession does not affect the spin distribution. The spin directions of electrons of the TIS assembly already were evenly distributed. It is impossible to mix them up more.

Except spin precession there is one more interesting effect the spins experience in the magnetic field. It is a damping of spin precession or the Gilbert damping. Because of interaction of spin with environment, for example with phonons, during the spin precession the spin slowly turns towards direction of the magnetic field. The Gilbert damping is the effect which converts electrons from the TIS assembly to the TIA assembly or you may say align the electron spins along the magnetic field.. In an imaginary case if there were no scatterings, after switching on a magnetic field, all spins of electrons of the TIS assembly, which were initially evenly distributed, eventually would turn themselves along the magnetic field. Therefore, the TIS would become the TIA assembly. However, the scatterings do not allow that. 

In electron gas at same time at the same place only one TIS and one TIA assembly can coexist together. Any spin distribution is almost instantly transformed by scatterings into the distribution, which is a sum of one TIS and one TIA assembly.

Therefore, after spins of TIS assembly turned only a little towards the magnetic field, the scatterings redistribute the electron in one TIA assembly with spin along the magnetic field and new TIS assembly with smaller number of electrons and evenly distributed spins.

The conversion rate of electrons of TIS assembly into TIA assembly can be calculated by the formula shown at lower part of the slide.

Slide 11

This slide explains the effect of the spin torque. Please, note that the classical model is unable to explain this effect.

The spin torque is defined as a torque, which is rotates the spin direction of the TIA assembly toward the spin direction of electrons of another TIA assembly, which may be injected into or generated in a metal.
 
As I mentioned in previous slide, in electron gas at same time at the same place only one TIS and one TIA assembly can coexist together. 

But what happens, if electrons are injected into an electron gas and the spin polarization of the injected electrons is different from the spin direction of existing spin accumulation. Two TIA assemblies with different spin directions can not coexist together. The scatterings instantly convert them into one TIA assembly. It is important that the spin direction of the new TIA assembly comparing to old TIA assembly is turned towards spin direction of the injected electrons.

In the case when the electrons are injected at a constant rate, the spin direction of the spin accumulation is constantly forced to rotate toward the spin direction of the injected electrons. It can be interpreted as a torque acting on spin polarization, which I have defined as the spin torque.

As another example let us consider a metal film, in which an electron gas is spin-polarized and the spin direction is in-plane. Circularly-polarized light shines on the film from the top. The circularly-polarized light photo-excites spin-polarized electrons in the metal. The spin of the photo-exited electrons is directed normally to the film surface. In the case when light shines a sufficient time, due to spin torque the spin direction of electron gas may turn from in-plane to perpendicular-to-plane direction. 

It should be noted that the spin torque is always accompanied by an additional spin relaxation.

The formulas which can be used to calculate the spin torque and the additional spin relaxation, are shown.

Slide 12

This slide explains the effect of the spin-torque current. Please, note that the classical model is unable to explain this effect.

The spin-torque current is defined as a current flowing between regions, which have different spin directions of a spin accumulation. The spin-torque current induces a spin torque on the electrons of the TIA assembly. The spin-torque current is trying to align the spin of all electrons of the TIA assembly in one direction over the whole sample.

As I have mentioned several times already, in electron gas at same time at the same place only one TIS and one TIA assembly can coexist together. 

But what happens, if in different points in an electron gas the spin direction of spin accumulation is different. In the electron gas the electrons diffuse from point to point. If at a neighbor point the spin direction is different, the electrons diffused from neighbor point cause the spin torque. The direction of the spin torque is such that it tries to align spin accumulation at neighbor points to be the same.

This slide compares different kinds of the spin currents, which may flow in electron gas. The conventional spin current is the current of spins from regions of larger spin accumulation shown as a big spin towards regions of smaller spin accumulation shown as a small spin. The conventional spin current is trying to make the amount of the spin accumulation to be the same over whole sample.

The spin-torque current is the current of spins between regions of different spin directions of the spin accumulation. The spin-torque current is trying to align spin direction of the spin accumulation to be the same over whole sample.

It should be noted that the spin-torque current is always accompanied by an additional spin relaxation.

The formulas which can be used to calculate the spin-torque current and the additional spin relaxation, are shown.

Slide 13

Following two slides explain the origin of the spin-transfer torque. 

The spin-transfer torque is the torque acting on the magnetization of a ferromagnetic electrode of the magnetic tunnel junction or MTJ in short, when the electrical current flows between the electrodes of the MTJ. The spin-transfer torque may cause either a magnetization precession in the ferromagnetic electrode or a reversal of the magnetization of the electrode. The spin-transfer torque is utilized as a recording mechanism in the spin-transfer-torque magnetic random access memory or  STT-MRAM in short. It might be possible that in the future the STT-MRAM will be a universal memory, which may replace high-density non-volatile memory (like hard disks, Flash memory) and high-density fast-speed memory (like DRAM, SRAM).

The MTJ is a basic cell of the STT-MRAM. As it is shown at upper part of the slide the MTJ consists of two ferromagnetic metals and a thin isolator or a tunnel barrier between them. The magnetization of one ferromagnetic metal is pinned by an exchange field with an antiferromagnetic layer, which is not shown in these pictures, and the magnetization of pinned layer can not be reversed. The magnetization of the second ferromagnetic layer may have two stable directions along its easy axis. This ferromagnetic region is called the “free” layer. A bit of data in the MTJ cell is stored by means of two opposite magnetization directions of the “free” layer. Due to the spin transfer torque the magnetization of the “free” layer can be reversed by an electrical current. Therefore, a bit of data can be recorded.

One oversimplified description of the spin-transfer torque is that the electrical current transfers a magnetic moment from pin layer to free layer. It may explain why magnetization of the free layer turns from to antiparallel to parallel direction, but it can not explain the reason why the magnetization turns back to be antiparallel when the polarity of the current is reversed.

The physics of the spin-transfer torque is richer, more complicated and more interesting than the simple transfer of the magnetic moment. It is amazing how such simple structure may have such a rich physics. The structure is extremely simple. It is  just a contact between two metal, through which an electrical current is flowing. 

To make understanding of the origin of the spin-transfer torque easier, in the following I will explain only main effects, which induce the spin-transfer torque. Please, note that I will ignore some effects, which contribute to the spin-transfer torque. If you would like to understand the full picture, please, visit my web site.

For better understanding, the effects leading to the spin-transfer torque may be divided into several events or steps. The spin of the localized d-electrons is shown in blue, the spin of delocalized electrons of the TIA assembly are shown in green. I consider an example when before the voltage is applied, the angle between magnetizations directions of ferromagnetic layers is 90 degrees. The electron gas in each layer is spin polarized. This means the conduction electrons are in both TIS and TIA assemblies. I assume the sign of the exchange interaction between d-electrons and conduction electrons is such that spins of electrons of TIA assembly aligned parallel to spins of d-electrons 
Step 1, under applied voltage the drift current flows from one ferromagnetic electrode to other electrode. Since electrons of both TIA and TIS assemblies participate in the transport, the drift current is spin-polarized. 
At boundary between electrodes the drift current changes its spin direction. It leads to the spin accumulation at the contact. Perhaps, you may imagine this fact as following.  As electrical current passes through the contact, the electron of  TIA assembly are decoupled from the drift current and  the electron of  another TIA assembly of another spin direction is coupled to the drift current. Since electrons can not just disappear or appear from nowhere, the electrons of TIA assemblies accumulated and depleted at sides of the contact.

Slide 14

After been accumulated at sides of contact, the electrons of TIA assemblies starts to diffuse from contact deep into electrodes. This diffusion occurs simply because there more electrons of the TIA assembly near contact than inside electrodes. This diffusion makes the gradient of the spin accumulation from the contact across each electrodes.

The spin direction of the TIA assembly is different in each ferromagnetic electrode. Also, there is a gradient of the spin accumulation.  These two conditions are sufficient for a spin-torque current to flow between the electrodes. The spin-torque current is linearly proportional to the gradient of the spin accumulation, it is largest near the tunnel barrier and decays exponentially as it flows away from the tunnel barrier. 

What does the spin-torque current do? It tries to turn spin of electrons of both TIA assemblies towards each other. However, the exchange interaction between the conduction electrons of the TIA assembly and the localized d-electrons is trying to align the spin of the conduction electron back to be parallel to the spin of the d-electrons. Therefore, there is a trade-off between opposite forces originated from the spin-torque current and the exchange interaction. As result the spin of the electrons of TIA assembly turn out from spin direction of the d-electrons.

The exchange interaction can be treated as an effective magnetic field. When the spin has non-zero angle in respect to this effective magnetic field, the spin precession is started.  When there is a non-zero angle between the d-electrons and the conduction electrons of the TIA assembly, the effective magnetic field due exchange interaction is non-zero for both the d-electrons and the conduction electrons. Therefore, there is a precession of both types of electrons around this effective magnetic field as it is shown at lower part of the slide. In the case of a sufficiently large spin-torque current, the spin direction of the d-electrons may be reversed.

In simplify view, the magnetization of a ferromagnetic metal simply may be defined as the total spin of the d-electrons. Therefore, from above explanation I hope you can understand physical reasons why the magnetization precession occurs in  "free" layer or even the magnetization may be reversed when the electrical current flows through the MTJ. 

The origin of spin transfer torque, as I have just explained,  is still a very simplified story. The physics of the spin transfer torque is more complicated.  The spin transfer torque has contributions from several physical mechanisms and effects. In order to calculate the spin transfer torque, the dynamics of the precision of the local d-electrons and the delocalized s-electrons of the TIA assembly, the electron conversion from the TIS assembly to the TIA assembly, spin relaxation, spin diffusion and spin-torque diffusion should be calculated self-consistently.

Slide 15

This slide concludes this talk.

 I would like to emphasize once more that  this proposed model describes very well all-known effects and experimental facts related to the electron gas in a solid  without  any contradiction.

This model gives simple and understandable explanations of the most of spntronics effects. 

If you would like to follow new development and see details 

Please visit my web site.