Effect of Temperature and Spin Torque on the Stoner-Wohlfarth Astroid of a Nanomagnet

We report measurements of the Stoner-Wohlfarth switching astroid curves of the free layer nanomagnet in CoFeB/MgO/CoFeB/Ru/CoFe/PtMn elliptical nanoscale magnetic tunnel junctions made as a function of applied voltage and temperature. Measurements of the astroid area as a function of temperature allow us to determine the magnetic anisotropy energy barrier of the free layer and thereby quantify its thermal stability - an important performance parameter of spin torque nonvolatile magnetic memory. Measurements of the astroid as a function of voltage (V) applied to the junction at the bath temperature of 4 K reveal significant voltage-induced deformations of the astroid curve. We observe a decrease of the hard-axis length of the astroid, which arises from ohmic heating of the junction. Comparison of the hard-axis astroid length measured at T = 4 K and |V| > 0 to the hard-axis astroid length measured at T > 4 K and V = 0 allows us to quantify ohmic heating of nanoscale tunnel junctions by the applied voltage. The applied voltage reduces the easy-axis length of the astroid as well, but the reduction is asymmetric for positive and negative easy-axis directions. This easy-axis asymmetry reverses upon the applied voltage sign reversal and thus it can be attributed to spin transfer torque.