Physical Review B

Easy-plane dominant stochastic magnetic tunnel junction with synthetic antiferromagnetic layers

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We experimentally explore a synthetic-antiferromagnetically (SAF) coupled easy-plane stochastic magnetic tunnel junction (EP-SMTJ) for a baseline device behavior assessment. The tunnel conductance fluctuates over time due to magnetoresistance reflecting the superparamagnetic state of the EP-SMTJ's layers. Binary digitizing at its median gives a stochastic bit stream, which is analyzed using the NIST SP800-22r1a test suite. When the tunnel conductance is sampled at different times for producing the binary bit stream, a best bit rate of ∼250 Mb/s is seen. If two independently taken bit streams are xored, the effective bit rate can be >1 Gb/s, provided that the in-plane anisotropy is small compared to ambient temperature (kBTa). The SAF design for both the free and the reference layer allows for a device conductance fluctuation median that is well centered and only weakly voltage-bias dependent. Comparison of experimental observations with a four-moment coupled finite-temperature macrospin model reveals the role of in-plane anisotropy as causing additional telegraphing fluctuations, which is associated with a longer timescale for stochasticity - a behavior that needs to be optimized for high-speed stochastic bit-stream applications.