Easy plane magnetic tunnel junction as an entropy source for computing

Abstract

A nano-structured magnetic tunnel junction (MTJ) operating in superparamagnetic limit with easy-plane dominant anisotropy can convert thermal fluctuation of the magnetic moments into electric tunnel conductance fluctuation with nearly white spectrum beyond a GHz in bandwidth and could be useful as a compact electronic entropy source for computing. We explore the base-line behavior of such easy-plane dominant MTJs using materials and fabrication routes similar to spin-transfer-torque magnetic random-access memory (STT-MRAM). The measurement observations, both on analog time-series and the corresponding binary digitized bit-streams are analyzed for their randomness and spectrum content, and compared to behaviors generated from a 4-moment coupled macrospin Langevin model in comparable materials and operating parameters space. We show experimentally that stochastic bit-streams can be generated using these MTJs at a bit-rate around 1Gb/sec, and we establish the likely important factors in need for further materials and device optimization – chief of them the reduction of any unintended in-plane magnetic anisotropy energy in such junctions in comparison to thermal energy.