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K. Lee et al.
vanishingly small [26], because electrons flow mostly in the Pt than Co layer and the
conduction electrons spins largely depolarize from interfacial scattering in ultrathin
ferromagnets. Moreover, DWs in Pt/Co/oxide moving against the electron flow is
contrary to the action of the STT.
Instead, a series of experiments by Liu et al. [27], have proposed an alternative
mechanism based on the spin Hall effect (SHE) [28]. The SHE is a phenomenon
which appears in systems with spin–orbit coupling. When an electrical current is
applied, a spin current is generated in the transverse direction of the charge current.
This spin current results in an accumulation of spin on the edges of the material. The
SHE has been known to be realized in materials with strong spin–orbit coupling.
It was first proposed by Dyakonov and Perel in 1979 [29]. The actual term SHE
was first introduced by Hirsch [30]. By the semi-classical theory, the SHE could be
explained mainly by three different mechanisms: intrinsic [31, 32], skew scattering
[33, 34] and side-jump mechanisms [35]. The intrinsic SHE is dependent only on the
band structure of the material. The skew scattering mechanism is proportional to the
transport lifetime τ. Since the skew scattering term is proportional to the transport
life time τ, the skew scattering spin Hall term is proportional to the longitudinal
electrical conductivity. The side jump mechanism appears as the contribution to
SHE other than the intrinsic and the skew scattering. The theoretical concept is when
a Gaussian wave packet scatters from spherical impurities, the incident wave vector
will undergo a transverse displacement. This is the cause of a SHE due to the side
jump. With all these mechanisms combined in a system with spin–orbit coupling, the
SHE can result in the non-equilibrium spin accumulation on edges of the nonmagnetic
metal, consequently leading to the generation of a transverse spin current. As shown
in Fig. 4a, the SHE effect occurs in the heavy metal (HM) underlayer accumulating
spins at the interface of the heavy metal and ferromagnet (FM). The accumulated
spins flow into the ferromagnetic layer exerting a torque on the magnetization of the
ferromagnet. The SHE is known to be a bulk effect and is expected to be responsible
for the Slozenski-like torque (or so-called damping-like torque).
Figure 5 displays how the SOTs and STTs differently act on the DW motion. The
DW motion by the STTs in a perpendicularly magnetized thin film can be understood
as the same mechanism described in the previous section. The only difference here is
that the demagnetization field in the perpendicularly magnetized thin films is mainly
attributed to the magnetic volume charge instead of the surface charge. By considering
all the torques acting on the local magnetization, one can notice that the DW can
move in the electron flow direction mostly attributed to the non-adiabatic torque—for
the adiabatic torque, the torques from the adiabatic torque and the demagnetization
field from the canting of the local magnetization (m) are all canceled out as like the
in-plane magnetized counterpart, but, for the case of the non-adiabatic torque, the
net torque is pointing downward, leading to the DW motion in a direction expanding
the magnetic domains saturated downward (see Fig. 5a). In contrast, for the case
of SOTs, the Bloch DW cannot move by the SOTs, as the local magnetization is
always parallel to the spin polarization direction—the spin polarization direction is
transverse to the electron flow direction. Therefore, a Néel DW is needed in order to
explain the DW motion by SOT (the physical origin to stabilize the Néel DW will
K. Lee et al.
vanishingly small [26], because electrons flow mostly in the Pt than Co layer and the
conduction electrons spins largely depolarize from interfacial scattering in ultrathin
ferromagnets. Moreover, DWs in Pt/Co/oxide moving against the electron flow is
contrary to the action of the STT.
Instead, a series of experiments by Liu et al. [27], have proposed an alternative
mechanism based on the spin Hall effect (SHE) [28]. The SHE is a phenomenon
which appears in systems with spin–orbit coupling. When an electrical current is
applied, a spin current is generated in the transverse direction of the charge current.
This spin current results in an accumulation of spin on the edges of the material. The
SHE has been known to be realized in materials with strong spin–orbit coupling.
It was first proposed by Dyakonov and Perel in 1979 [29]. The actual term SHE
was first introduced by Hirsch [30]. By the semi-classical theory, the SHE could be
explained mainly by three different mechanisms: intrinsic [31, 32], skew scattering
[33, 34] and side-jump mechanisms [35]. The intrinsic SHE is dependent only on the
band structure of the material. The skew scattering mechanism is proportional to the
transport lifetime τ. Since the skew scattering term is proportional to the transport
life time τ, the skew scattering spin Hall term is proportional to the longitudinal
electrical conductivity. The side jump mechanism appears as the contribution to
SHE other than the intrinsic and the skew scattering. The theoretical concept is when
a Gaussian wave packet scatters from spherical impurities, the incident wave vector
will undergo a transverse displacement. This is the cause of a SHE due to the side
jump. With all these mechanisms combined in a system with spin–orbit coupling, the
SHE can result in the non-equilibrium spin accumulation on edges of the nonmagnetic
metal, consequently leading to the generation of a transverse spin current. As shown
in Fig. 4a, the SHE effect occurs in the heavy metal (HM) underlayer accumulating
spins at the interface of the heavy metal and ferromagnet (FM). The accumulated
spins flow into the ferromagnetic layer exerting a torque on the magnetization of the
ferromagnet. The SHE is known to be a bulk effect and is expected to be responsible
for the Slozenski-like torque (or so-called damping-like torque).
Figure 5 displays how the SOTs and STTs differently act on the DW motion. The
DW motion by the STTs in a perpendicularly magnetized thin film can be understood
as the same mechanism described in the previous section. The only difference here is
that the demagnetization field in the perpendicularly magnetized thin films is mainly
attributed to the magnetic volume charge instead of the surface charge. By considering
all the torques acting on the local magnetization, one can notice that the DW can
move in the electron flow direction mostly attributed to the non-adiabatic torque—for
the adiabatic torque, the torques from the adiabatic torque and the demagnetization
field from the canting of the local magnetization (m) are all canceled out as like the
in-plane magnetized counterpart, but, for the case of the non-adiabatic torque, the
net torque is pointing downward, leading to the DW motion in a direction expanding
the magnetic domains saturated downward (see Fig. 5a). In contrast, for the case
of SOTs, the Bloch DW cannot move by the SOTs, as the local magnetization is
always parallel to the spin polarization direction—the spin polarization direction is
transverse to the electron flow direction. Therefore, a Néel DW is needed in order to
explain the DW motion by SOT (the physical origin to stabilize the Néel DW will