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Numerical Analysis and Innovative Simulation
Techniques for Designing Advanced MRAM

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7.2 Computational Setup

The simulations in this chapter employ the coupled micromagnetic and spin-charge drift-diffusion framework developed in Chapter 6, integrated with the FE discretization described in Section 4.2. The spin-transfer torque density at each spatial point is computed using the drift-diffusion formalism (Section 6.1)  [32], with tunneling boundary conditions at each MgO barrier derived from the NEGFs analysis in Section 6.3  [118].

Table 7.1: Standard simulation parameters for the switching studies presented in this chapter. Values are representative of CoFeB\(|\)MgO-based magnetic tunnel junctions and are sourced from the Boris Computational Spintronics material database and published experimental literature.
.
Parameter Symbol Value
Gilbert damping constant \(\alpha \) 0.015–0.02
Gyromagnetic ratio \(\gamma \) 1.76 · 1011rad/(s T)
Saturation magnetization (CoFeB) \(M_S\) 0.81 · 106A/m to 1.2 · 106A/m
Exchange stiffness constant (CoFeB) \(A\) 1 · 10−11J/m to 2 · 10−11J/m
Interface anisotropy \(K_i\) 1.53 · 10−3J/m2
Current spin polarization \(\beta _\sigma \) 0.52–0.7
Diffusion spin polarization \(\beta _D\) 0.7–1.0
Electron diffusion coefficient (CoFeB) \(D_{e,\text {FM}}\) 1 · 10−2m2/s to 2 · 10−2m2/s
Exchange length \(\lambda _{J}\) 0.8nm to 1nm
Spin dephasing length \(\lambda _{\varphi }\) 0.4nm
Spin-flip length (CoFeB) \(\lambda _{sf,\text {CoFeB}}\) 10nm
Spin-flip length (Ta) \(\lambda _{sf,\text {Ta}}\) 1.9nm
Spin-flip length (Ru) \(\lambda _{sf,\text {Ru}}\) 4nm
Resistance (P state) \(R_P\) 410kΩ (ultra-scaled)
Resistance (AP state) \(R_{AP}\) 750kΩ (ultra-scaled)

Interlayer exchange coupling between non-adjacent magnetic layers is incorporated via the weak-form interface mapping described in Section 6.4. Magnetization dynamics are solved using the tangent-plane scheme (Section 5.3). Demagnetization fields are computed with the hybrid FE-BEM approach from Chapter 4.

The standard simulation parameters employed throughout this chapter are summarized in Table 7.1. Material set differences are detailed in Table 7.2.

The simulated resistance values yield tunnel magnetoresistance ratios consistent with experimental CoFeB\(|\)MgO devices: \(\text {TMR} = (R_{AP} - R_P)/R_P = 83\%\) for the ultra-scaled composite FL (\(R_P = \SI {410}{\kilo \ohm }\), \(R_{AP} = \SI {750}{\kilo \ohm }\)), and for the 70nm SAF-enhanced stacks the input \(\text {RA} = \SI {6.5}{\ohm \micro \meter \squared }\) with \(\text {TMR} = 150\%\) lies within the experimentally reported range (\(\text {RA} \approx \SIrange {3.5}{7}{\ohm \micro \meter \squared }\), \(\text {TMR} \approx 100\)–\(250\%\)) for optimized MgO barriers at room-temperature  [185, 186, 187]. The Gilbert damping constant \(\alpha = 0.015\)–\(0.02\) is representative of sputtered CoFeB thin films  [53]. In the macrospin limit, the critical switching current scales linearly with damping (\(I_{c0} \propto \alpha \))  [31], so the absolute switching times scale accordingly. However, the qualitative features identified in this chapter, namely the sequential switching order, the back-hopping threshold, and the IEC role progression, are governed by the torque hierarchy and coupling geometry rather than the damping magnitude.

A critical insight emerges from investigating multi-layer composite FL structures: the spin-transfer torques in a multi-segment stack are inherently non-additive. The coupled partial differential equation system governing spin transport exhibits essential non-linearity: the spin-accumulation profile in each magnetic layer is modified by the magnetization state of neighboring layers through spin-transport coupling across the shared interfaces, creating strong feedback that cannot be decomposed into independent contributions. This non-additivity has direct practical implications: superposing single-layer simulation results will produce systematically incorrect switching times and potentially miss qualitative features such as the sequential switching mechanism  [154, 188].

All simulations presented in this chapter are performed at zero temperature (\(T = 0\) K). This deliberate choice isolates the deterministic switching dynamics from thermally activated processes, enabling distinct identification of torque-driven phenomena. In particular, the back-hopping mechanism characterized in Section 7.4 is identified as a deterministic consequence of the composite FL torque hierarchy precisely because stochastic thermal fluctuations are excluded from the simulations. The approximation is justified quantitatively: the characteristic switching times of 1ns to 3ns and back-hopping cycling periods of approximately 0.5ns are many orders of magnitude shorter than the thermally activated reversal time \(\tau _{\text {th}} = \tau _0 \exp (\Delta )\), which for \(\Delta > 60\) exceeds \(10^{17}\) s. Thermal fluctuations would add stochastic jitter to the trajectories without altering the qualitative switching sequence or the back-hopping mechanism.