Perpendicular STT-MRAM Switching at Fixed Voltage and at Fixed Current

The magnetization dynamics of a free layer in spin-transfer torque MRAM is usually determined by the torque created by a position-independent current density. In circuits, the voltage, not current density, is fixed during switching. Therefore, the approximate evaluation of the torque based on the fixed current density becomes questionable in modern magnetic tunnel junctions with a tunneling magnetoresistance ratio of about 200%, where the current densities across the structure can vary by a factor of three. We compare the switching times obtained within a fixed voltage assumption with those from the approximate fixed current density approach. We demonstrate that the fixed current approach can reproduce the correct switching also in the case of high TMR, if the current is appropriately adjusted. It is shown that the correction to the current is not universal and depends on the temperature and the switching speed.