FIG. 1.

Schematic of the electrical circuit used to couple two superparamagnetic tunnel junctions and the four metastable configurations they take. Each panel shows the constant voltage source $V_0$, the series resistor $R_0$, the two SMTJs in parallel with each other, and a voltmeter representing the oscilloscope measurement of the shared voltage across the SMTJs. The yellow arrows connecting the four panels represent thermally driven transitions between the different configurations of the SMTJs. Panel (a) shows both SMTJs in the parallel (P) state, with the free layer (bottom) magnetization parallel to the fixed layer (top) magnetization. When the magnetization in the free layer of either SMTJ flips, panels (b) and (c), the resistance of the now antiparallel SMTJ (indicated by a purple free layer) increases so the voltage across both SMTJs increases. The voltages in these two cases differ due to differences in the magnetoresistance of the two devices. If both free layers flip, the system goes to the configuration in panel (d), with both SMTJs in the antiparallel (AP) state and the largest magnitude voltage across both SMTJs.