Numerical Analysis of Deterministic Switching of a Perpendicularly Magnetized Spin-Orbit Torque Memory Cell

R L De Orio,V Sverdlov,W Goes,S Fiorentini,S Selberherr,J Ender

doi:10.1109/jeds.2020.3039544

Abstract

We propose a magnetic field-free spin-orbit torque switching scheme based on two orthogonal current pulses, for which deterministic switching is demonstrated via numerical simulations. The first current pulse selects the cell, while the second current pulse ensures deterministic switching of the selected cell. 100% switching probability has been obtained for a wide range of amplitudes and durations of the pulses, thus precise timings are not required. This has also been verified considering a variability of ±5% of the saturation magnetization and anisotropy constant. An important feature of the scheme is that the magnitude of the second current is lower than the critical current for spin-orbit torque switching. The lower second current pulse improves the efficiency of the switching, reducing the corresponding pulse power by 75% and the total writing power by 40%, while maintaining the same switching time. Due to the sub-critical current, the corresponding spin-orbit torque is weak and does not disturb the bits of non-selected cells. Therefore, a single additional wire can be routed through several cells in a row, reducing the number of transistors per cell, and simplifying the cell integration in a memory array.

Highlights

Spin-orbit torque (SOT) magnetoresistive random access memory (MRAM) is a promising future nonvolatile memory solution for ultra-fast operation beyond the spin-transfer torque MRAM
The second current pulse is applied through the write source lines 2 (WSL2) and the second heavy metal wire (NM2 routed through several cells in an array), when the access transistors are enabled by the word write lines 2 (WWL2) signal, and the switching of the cell is completed
These results indicate that the operation window for the second current pulse yielding 0% switching probability, when the cell is not selected by the first pulse, is very large and its amplitude can be even higher than the critical current

Summary

INTRODUCTION

Spin-orbit torque (SOT) magnetoresistive random access memory (MRAM) is a promising future nonvolatile memory solution for ultra-fast operation beyond the spin-transfer torque MRAM. An alternative field-free scheme is based on purely electrical switching using two orthogonal current pulses [10] In this scheme a second heavy metal layer is grown on top of the FL, but with the advantage that no modification of the layers or of the fabrication processes is required. In order to perform switching, the current densities of both pulses were larger than the critical value for SOT switching This yields a fairly high power consumption, and. In this work we show that decreasing the current density of the second pulse below the critical value considerably supports the cell integration and reduces its power consumption. After a selection of the cell by the first current pulse, the current of the second pulse can be reduced to 50% of the critical value, while still guaranteeing deterministic and very fast switching of a perpendicularly magnetized FL. The cell integration in a cross-bar architecture becomes simpler, as the second current can be applied to a single heavy metal line connected to several cells in an array without disturbing the information of non-selected cells

SOT CELL SWITCHING

RESULTS AND DISCUSSION

CONCLUSION