Multilevel magnetization switching of magnetic tunnel junctions induced by spin-orbit torque

Abstract

Magnetization switching of magnetic tunnel junctions (MTJs) via spin-orbit torque (SOT) is gaining interest in cache memory applications due to its benefits in lowering power consumption. Although SOT switching of perpendicular MTJs offers better scalability, in practice the requirement of an external magnetic field to assist SOT switching is a fundamental bottleneck. Meanwhile, in-plane (IP) MTJs, fabricated in the form of an ellipse, have been demonstrated to switch by SOT without an external field. Here, the easy axis of the elliptical MTJ is perpendicular to the charge flow in the spin-hall channel, and the magnetization trajectory is argued to undergo several precessions before magnetization flip similar to spin-transfer torque switching. [1] However, in this work we experimentally show that these IP-SOT MTJs, can in fact switch as fast as 400 ps, shorter than the precessional trajectory. The IP MTJs, which implements Pt/CoFeB/MgO as the free layer, are fabricated on a 200 mm wafer using stepper lithography and ion-beam etching. Interestingly, by applying asymmetric SOT pulse segments at substantially small voltage steps, we found that a significant number of MTJs demonstrated multilevel resistance states. While multilevel states have been reported in antiferromagnet/ferromagnet structures, the underlying mechanism arising from variation in exchange bias is not applicable in our case. [2] Using micromagnetic simulations, we formulate that the existence of meta-stable domain states inside the MTJ free-layer is a likely explanation in our case. In addition, we also probe the role of CoFeB thickness, aspect ratio, and switching probabilities of these devices. Multilevel resistance states in SOT devices could find potential in neuromorphic circuits.