Effect of subvolume excitation and spin-torque efficiency on magnetic switching

Recently developed magnetic tunnel junctions with full perpendicular magnetization that are spin-torque switchable allow for quantitative comparison of spin-torque switching statistics with a macrospin model. For typical devices above 50 nm in lateral size, the comparison suggests the presence of subvolume magnetic excitations which often dominate the switching process and which degrade the spin-torque switching efficiency. A simple model of subvolume spin-torque-driven magnetic switching is presented to account for the experimental observations. The origin of the subvolume thermal excitation is traced to a competition between the macrospin fluctuation within a simple uniaxial anisotropy potential and that of thermal magnon excitation. The subvolume excitation problem highlights the importance of improving the magnetic exchange stiffness of the junction free layer, and the reduction of junction lateral sizes below 50 nm where an improved spin-torque efficiency is seen as the switching dynamics cross over to a more macrospin-like process.