Magnetic Nonvolatile SRAM Based on Voltage-Gated Spin-Orbit-Torque Magnetic Tunnel Junctions

Abstract

This article proposes two different magnetic nonvolatile-static random access memory (MNV-SRAM) cell circuits for low-power, high-speed, and high-reliable backup operation with a compact cell area. They employ perpendicular magnetic tunnel junctions (p-MTJs) as nonvolatile backup storage elements and explore the spin-orbit torque (SOT) with the assistance of the voltage-controlled magnetic anisotropy effect (VCMA), referred to voltage-gated SOT (VGSOT), to perform the backup operation. By using an antiferromagnetic (AFM) layer that provides both an exchange bias and the SOT, no external magnetic field is required, making it suitable for practical applications. In addition, owing to the aid of the VCMA effect, the critical SOT write current (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SOT</sub> ) for 1-ns backup operation can be reduced significantly, thus resulting in high speed, low power consumption, and high reliability. Moreover, such resulted I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SOT</sub> allows to be driven by the cross-coupled inverters in the SRAM cell, instead of a dedicated write driver, thereby leading to low cell area overhead. By using a commercial CMOS 40-nm design kit and a physics-based VGSOT-MTJ model, we have demonstrated their functionalities and evaluated their performance. Compared with previous SOT-based MNV-SRAM cell circuits, the proposed MNV-SRAM cell circuits can achieve lower backup energy dissipation, smaller backup delay, lower backup error rate and less cell area overhead without the assistance of the external magnetic field.