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2 Basic Elements of Spintronics
types of electron current in a conductor. Therefore, diversified physical mechanisms
can yield only two kinds of spin-polarized currents. These mechanisms are:
1. Orbital moment of a conduction electron: This makes the magnetic field of
spin orbit interaction to be dependent on the direction of electron movement.
2. Skew scattering on defects: This mechanism also makes the magnetic field of
spin–orbit interaction to be dependent on the direction of electron movement.
3. Side-jump scattering on defects: This mechanism makes the magnetic field of
spin–orbit interaction to be dependent on electron spatial position.
4. Side-jump scattering across an interface: This mechanism makes the
magnetic field of spin–orbit interaction to be dependent on electron spatial
position with respect to the interface.
Among these four mechanisms, the first one describes the creation of intrinsic
spin Hall effect and the other three describe the extrinsic spin Hall effect.
Inverse spin Hall effect
Upon application of an electric field to a system consisting of non-magnetic metal
or doped semiconductor with no external magnetic field, the electrons are directed
towards the electric field. If the said system encounters spin–orbit coupling (SOC),
the electrons can experience spin-dependent motion as shown in Fig. 2.21a. The
up-spins and down-spins are deflected in the opposite direction, perpendicular to
the electric field. As a result, there will be a transverse spin current in response to
the electric field. If the system does not have SOC, there cannot be spin-dependent
motions of electrons, and hence no spin Hall effect (SHE) will be observed. The
reciprocal effect to the SHE is the inverse spin Hall effect (ISHE) and is shown in
Fig. 2.21b. In this case, when the spin current is injected into the system, an electric
field is induced. In the injected spin current, the up-spins and down-spins move in the
opposite directions. The spin–orbit coupling causes deflections of the electrons, and
the deflections have the same direction for up- and down-spins, causing a transverse
charge current. The SHE and ISHE enable us to electrically manipulate or detect spin
currents. Charge current can be converted to spin current through spin Hall effect,
whereas spin current can be converted to charge current through inverse spin Hall
effect.
2.12.2 Heat-Driven Seebeck Effect
Interestingly, temperature gradients can also lead to some encouraging effects, as
observed in spin caloritronics (Seifert 2017; http://magnetism). Such heat-driven
phenomenon, also observed in insulators, permits the transport of spin. Two types of
spin caloritronic effects are important in spintronics. One is spin-dependent Seebeck
effect (SDSE) which is mediated by conduction electron. Another one is spin Seebeck
effect (SSE) which is mediated by magnon. These effects can be increased by
breaking the inversion symmetry of magnetic heterostructures. As far as practical
2 Basic Elements of Spintronics
types of electron current in a conductor. Therefore, diversified physical mechanisms
can yield only two kinds of spin-polarized currents. These mechanisms are:
1. Orbital moment of a conduction electron: This makes the magnetic field of
spin orbit interaction to be dependent on the direction of electron movement.
2. Skew scattering on defects: This mechanism also makes the magnetic field of
spin–orbit interaction to be dependent on the direction of electron movement.
3. Side-jump scattering on defects: This mechanism makes the magnetic field of
spin–orbit interaction to be dependent on electron spatial position.
4. Side-jump scattering across an interface: This mechanism makes the
magnetic field of spin–orbit interaction to be dependent on electron spatial
position with respect to the interface.
Among these four mechanisms, the first one describes the creation of intrinsic
spin Hall effect and the other three describe the extrinsic spin Hall effect.
Inverse spin Hall effect
Upon application of an electric field to a system consisting of non-magnetic metal
or doped semiconductor with no external magnetic field, the electrons are directed
towards the electric field. If the said system encounters spin–orbit coupling (SOC),
the electrons can experience spin-dependent motion as shown in Fig. 2.21a. The
up-spins and down-spins are deflected in the opposite direction, perpendicular to
the electric field. As a result, there will be a transverse spin current in response to
the electric field. If the system does not have SOC, there cannot be spin-dependent
motions of electrons, and hence no spin Hall effect (SHE) will be observed. The
reciprocal effect to the SHE is the inverse spin Hall effect (ISHE) and is shown in
Fig. 2.21b. In this case, when the spin current is injected into the system, an electric
field is induced. In the injected spin current, the up-spins and down-spins move in the
opposite directions. The spin–orbit coupling causes deflections of the electrons, and
the deflections have the same direction for up- and down-spins, causing a transverse
charge current. The SHE and ISHE enable us to electrically manipulate or detect spin
currents. Charge current can be converted to spin current through spin Hall effect,
whereas spin current can be converted to charge current through inverse spin Hall
effect.
2.12.2 Heat-Driven Seebeck Effect
Interestingly, temperature gradients can also lead to some encouraging effects, as
observed in spin caloritronics (Seifert 2017; http://magnetism). Such heat-driven
phenomenon, also observed in insulators, permits the transport of spin. Two types of
spin caloritronic effects are important in spintronics. One is spin-dependent Seebeck
effect (SDSE) which is mediated by conduction electron. Another one is spin Seebeck
effect (SSE) which is mediated by magnon. These effects can be increased by
breaking the inversion symmetry of magnetic heterostructures. As far as practical