Spin–Orbit Torque-Controlled Magnetic Tunnel Junction With Low Thermal Stability for Tunable Random Number Generation

Abstract

Spin-orbit torque has emerged as an alternative to spin-transfer torque switching of thermally stable magnetic tunnel junctions (MTJs). For MTJs with low thermal stability, i.e., low energy barrier MTJs, spin-orbit torque devices have been proposed, which allow for tunable random number generation and decoupled WRITE and READ paths. Such stochastic devices can then be interconnected to realize computational systems, such as Ising networks, neural networks, invertible logic, etc. In this letter, we experimentally demonstrate such a stochastic spin device called a “p-bit” that is characterized by a spin-orbit torque-controlled MTJ with low thermal stability as the WRITE unit, and where the tunneling magnetoresistance is employed as the READ unit. We first demonstrate deterministic switching of stable in-plane MTJs using the spin-orbit torques generated by tantalum. Next, we employ spin-orbit torques to tune the stochasticity of the MTJ with low thermal stability to generate tunable random numbers. National Institute of Standards and Technology randomness tests are performed to evaluate the quality of our random number generator. The results are then quantitatively analyzed using the standard model for thermally activated switching and the theory of thermal fluctuation in superparamagnetic particles. The devices presented in this letter consisting of a WRITE path (spin-orbit torque) and a READ path (MTJ) are the key building blocks for probabilistic spin logic applications.