Spin Torque Switching in Asymmetric Fe$_{60}$Co$_{20}$B$_{20}$/MgO/ Ni$_{60}$Fe$_{20}$B$_{20}$ Magnetic Tunnel Junctions

Spin-torque (ST) studies involving MgO-based magnetic tunnel junctions (MTJs) generally have utilized junctions with fixed and free electrodes of similar or identical CoFe-based composition. Here we report the bias-dependent conductance, TMR, and ST behavior of nanopillar MTJs with asymmetric Fe$_{60}$Co$_{20}$B$_{20}$/MgO/ Ni$_{60}$Fe$_{20}$B$_{20}$ electrodes (AMTJ) having good performance (TMR$\sim $100{\%}, RA=10$\Omega \mu $m$^{2})$.We compare to symmetric (SMTJ) Fe$_{60}$Co$_{20}$B$_{20}$/MgO/Fe$_{60}$Co$_{20}$B$_{20}$ junctions (TMR $\sim $ \textbf{120{\%}}, RA = \textbf{10}$\Omega \mu $m$^{2})$. The magnitude of the bias dependence of TMR for the AMTJ is markedly less than for the SMTJ. In the AMTJs, using bias and field dependent measurements of thermally-activated ST-assisted reversal, we have observed a very significant asymmetry in the polarity-dependent ST critical currents and thermal activation barriers for reversal. Accordingly, the ST phase diagram, measured under high bias, shows a strong asymmetry with bias polarity, which we attribute to a polarity-dependent asymmetry in the field-like ST term at high bias. We will discuss the possible causes of these results based on the different band structures and work functions of FeCo and NiFe.