Switching Characteristics of Electrically Connected Stochastically Actuated Magnetic Tunnel Junction Nanopillars

Stochastic magnetic tunnel junctions (MTJs) are promising candidates for constructing Ising machines to solve optimization problems efficiently. Our group has demonstrated that stable magnetic tunnel junctions actuated with nanosecond pulses (SMART-MTJs) can generate large quantities of high-quality random bits. A feasible realization of the exchange coupling is required to use SMART-MTJs to solve problems based on the Ising Hamiltonian. In this study, we investigate electrically coupling two SMART-MTJs. When the junctions are connected in parallel, due to the slight variations in MTJ resistance, we can distinguish four distinct states (00, 01, 10, 11), where 0 is parallel, and 1 is antiparallel MTJ states. By varying the applied electrical pulse across the junctions, we study the distribution of the system occupying each state and calculate the correlation between the two junctions. We observe antiferromagnetic-like coupling (01 and 10) with a Pearson correlation of -0.78, and ferromagnetic-like (00 and 11) coupling with a Pearson correlation of 0.97. We discuss methods for tuning the coupling strength and other approaches for creating interactions between MTJs.