Shape Anisotropy Enabled Field Free Switching of Perpendicular Nanomagnets

TL;DR

The paper tackles the challenge of achieving deterministic, field-free spin-orbit torque switching in perpendicular magnetic anisotropy devices for SOT-MRAM. It introduces a geometry-driven approach by using triangular Co PMA nanomagnets atop a Pt Hall bar to generate an internal demagnetizing field through shape anisotropy, thereby breaking switching symmetry. Experimentally, 10 ns voltage pulses enable high switching probabilities (>90%) with appropriate amplitudes, and micromagnetic simulations with MuMax3 corroborate that the triangular geometry induces the desired bias, while a square geometry does not. The work presents a practical, CMOS-friendly route toward scalable field-free SOT-MRAM and motivates further optimization of geometry for robust device integration.

Abstract

Spin Orbit Torque-Magnetic Random Access Memory (SOT-MRAM) is being developed as a successor to the Spin transfer torque MRAM (STT-MRAM) owing to its superior performance on the metrics of reliability and read-write speed. SOT switching of perpendicularly magnetized ferromagnet in the heavy metal/ferromagnet bilayer of SOT-MRAM unit cell requires an additional external magnetic field to support the spin-orbit torque generated by heavy metal to cause deterministic switching. This complexity can be overcome if an internal field can be generated to break the switching symmetry. We experimentally demonstrate that by engineering the shape of ferromagnet, an internal magnetic field capable of breaking the switching symmetry can be generated, which allows for deterministic switching by spin-orbit torques. We fabricated nanomagnets of Cobalt with triangular shape on top of Platinum and showed external magnetic field free switching between the two stable states of magnetization by application of nano-second voltage pulses. The experimental findings are consistent with the micro-magnetic simulation results of the proposed geometry.

Figures (4)

• Figure 1: Schematic of the proposed triangular shaped Co PMA nanomagnets over Pt Hall bar. The DLT arising from charge current in Pt aligns the magnetization in-plane, shown by the dotted black arrows. The magnetic charges induced on the periphery of triangle are denoted by plus and minus signs. The resultant demag field is shown by red arrow. The value of $m_z=\pm 1$ indicates the final magnetization state after current is switched off. (a), (b) the component of internal demag field along $J_C$ is positive and magnetization relaxes to -$\hat{z}$ direction when the charge current is switched off. (c), (d) the component of internal demag field along $J_C$ is negative and magnetization relaxes to +$\hat{z}$ direction when the charge current is switched off.
• Figure 2: (a) Schematic of the proposed device – triangular shaped Co PMA nanomagnet over Ta/Pt Hall bar (b) SEM image of the device (c) AHE hysteresis loop of the device
• Figure 3: Field Free Switching of magnetization of triangular shaped devices, established through AHE measurements. (a), (b) The measurement sequences. AHE hysteresis loops are measured by passing charge current either along $\hat{x}$ or $\hat{y}$ direction, with magnetic field swept along $\hat{z}$ direction as indicated in (a) or (b). The star symbol denotes application of 10 $\mu s$ pulse with amplitude of 2 V or -2 V, when the external magnetic field is zero. (c) to (j): The black dot indicates starting point of the magnetic field sweep. The direction of current $\pm x/y$ is indicated on the top. The polarity of voltage pulse is also indicated in each panel. Fig (c)-(f) show that $m_z=-1$ state is reached when charge current is passed along $\hat{x}$ or -$\hat{x}$ direction. Fig (g)-(j) show that $m_z=+1$ state is reached when charge current is passed along $\hat{y}$ or -$\hat{y}$ direction.
• Figure 4: (a) Switching Probability as a function of pulse amplitude for 10 ns pulses (b) Micromagnetic simulation of field free switching of triangular nanomaget due to SOT. Current along $\hat{y}$ leads to a state with $m_z>0$ when current is on. Magnetization switches to +$\hat{z}$ when the current is switched off. Current along $\hat{x}$ results in a state with $m_z<0$ when current is on and magnetization switches to -$\hat{z}$ when the current is switched off. The inset shows micromagnetic simulation of square shaped FM. Application of current leads to a state with $m_z=0$, indicating that magnetization can switch to $\pm \hat{z}$ with equal probability when the current is turned off.