Modeling Spin-Orbit Torques
in Advanced Magnetoresistive Devices
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Chapter 8 SOT Driven Magnetization Dynamics
Although much insight can be gained from the 1D treatment of spin transport and SOTs in bilayer and trilayers, ultimately, a full 3D treatment is required to accurately capture the resulting magnetization dynamics in realistic
device geometries. This is particularly important for modeling SOT-MRAM devices, where the magnetization dynamics are strongly influenced by the interplay of magnetic anisotropy, stray fields, exchange interaction, and DMI,
which together give rise to complex domain wall dynamics. The micromagnetic model presented in chapter 3 provides a powerful tool to study the magnetization
dynamics in 3D, as it can capture the spatial variation of the magnetization and the various torques acting on it. Coupling the micromagnetic model with the full 3D spin transport model presented in chapter 4 and the BCs from chapter 5 enables comprehensive modeling of the switching behavior and
performance of SOT-MRAM devices.
In this chapter, the SOT-driven magnetization dynamics are explored through switching simulations of various FM systems that have been proposed for SOT-MRAM applications. All the presented simulations were performed using
the extended ViennaSpinMag software [86], and the meshes on which the simulations were performed were generated using NetGen [88]. In all cases, the SOT currents are applied along the \(x\)-axis, and the material layers are
stacked along the \(z\)-axis. To reduce the computational load, the MCT combined with the interfacial Rashba perturbation theory approach BCs are used for the drift-diffusion module.
8.1 Simulation Parameters
.
Parameter
CoFeB
Units
\(\alpha \)
\(0.02\,\)[141]
1
\(Ms\)
\(0.81\,\)[121]
MA/m
\(A\)
\(20\,\)[142]
pJ/m
\(D\)
\(0.3-1.2\)
mJ/m\(^2\)
\(K_1\)
\(0.65\,\)[121]
MJ/m\(^3\)
Table 8.1: Typical micromagnetic material parameters for CoFeB. The interfacial DMI constant depends strongly on the choice of the HM layer. For Pt/CoFeB/MgO a DMI constant of \(\approx 1.2\,\si {mJ/m^2}\) has
been reported [143], while for W/CoFeB/MgO a DMI constant of \(\approx 0.3\,\si {mJ/m^2}\) has been reported [144].
To achieve realistic simulations of SOT-MRAM devices, a suitable choice of material parameters is crucial. The material parameters used in the micromagnetic simulations are summarized in Table 8.1 . The FM layers used in the simulations are given material parameters consistent with CoFeB, as it is the most widely adopted material in MRAM devices due to high spin
polarization, good thermal stability, and high TMR in MgO-based MTJs [9]. CoFeB does not exhibit any bulk DMI, however, in contact with HMs such as W and Pt, the magnetization in thin CoFeB layers can experience a
significant interfacial DMI through the SOC which must be included in the simulations [143, 144].
.
Parameter
Value
Units
TMR
\(200\%\)
1
\(R_P\)
\(14\)
k\(\Omega \)
\(R_{AP} \)
\(42\)
k\(\Omega \)
\(\mathcal {G}_0\)
\(0.476\)
\(\mu \)S
\(P_{RL} = P_{FL} \)
\(0.707\)
1
\(a_{RL} = a_{FL} \)
\(1.0\)
1
\(P_{RL}^\eta = P_{FL}^\eta \)
\(0.2\)
1
When the whole MTJ is included in the simulations, it is modeled in the drift-diffusion module using the tunneling current model presented in Section 4.3.2 ,
with the parameters summarized in Table 8.2 . The parameters are chosen to represent a typical CoFeB/MgO/CoFeB MTJ [74].
The NM SOT layer is treated as either Pt or W, as both have a high spin Hall angle and are commonly used in SOT devices. The drift-diffusion parameters used for Pt and W are the same as the ones used to obtain the fits in
Section 7.2 , which are summarized in Tables 7.2 and 7.3 . To avoid repetition, these parameters are used for all subsequent simulations, unless otherwise specified.