Modeling Spin-Orbit Torques
in Advanced Magnetoresistive Devices
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3.3 Current-Induced Torques
In addition to the effective field contributions described in the previous section, the magnetization can also be manipulated by fields generated by electrical currents flowing through the magnetic domains in a circuit containing FM
components. A part of these contributions comes from Oersted fields and the magnetic fields generated by eddy currents, which become increasingly ineffective as the dimensions of the system are reduced [56, 15]. Since this work
focuses on nanoscale thin film systems and devices, these contributions are neglected. Another contribution arises when the current induces a net spin polarization. In this case, the conduction electrons can transfer their angular
momentum to the localized spins in the FM regions, resulting in a torque on the magnetization. This is accounted for by adding a spin torque term to the LLG equation [57, 58, 54]:
\(\seteqnumber{0}{3.}{28}\)
\begin{equation}
\frac {\partial \bm {m}}{\partial t} = -\gamma _0 \bm {m} \times \bm {H} + \alpha \bm {m} \times \frac {\partial \bm {m}}{\partial t} + \frac {1}{M_s}\bm {T_s}.
\end{equation}
The form of the torque term depends on the system and the origin of the spin-polarized current. A general form of the spin torque is described by Eq. (2.6), where the FL and DL components get their name from their magnetization dependence having the same form as the effective field and Gilbert damping
term in Eq. (3.9), respectively. Typically, \(T_\mathrm {DL}\), \(T_\mathrm {FL}\) and the spin polarization direction \(\bm {p}\) are treated
as constant across the magnetic domain; however, in reality, they depend on the local current density distribution and, therefore, on the materials and geometry of the total current-carrying part of the system. Furthermore,
spin-dependent interactions and scattering processes can change the polarization direction and the degree of polarization of the current, leading to a spatially varying \(\bm {p}\). Consequently, to accurately capture the effect of
current-induced torques on the magnetization dynamics, a detailed model of the current flow and spin transport in the system is required.