Analysis of Switching Under Fixed Voltage and Fixed Current in Perpendicular STT-MRAM

Abstract

In spin-transfer torque magnetoresistive random access memory, the magnetization dynamics of a free layer is usually assumed to be determined by the torque created via a position-independent current density. In circuits, however, it is the voltage, not the current density, which stays fixed during switching. Therefore, the approximate evaluation of the torque based on a fixed current density becomes questionable in modern magnetic tunnel junctions with a tunneling magnetoresistance ratio of about 200%, where the current density across the structure can vary by a factor of three. In this work, we compare the switching times obtained within a fixed voltage assumption with those from the approximate fixed current density assumption. We demonstrate that the assumption of a fixed current can reproduce the correct switching if the current is appropriately adjusted. It is shown that the correction to the current is not universal and depends on various factors such as TMR, temperature, the size of the structure and resistance area.

Keywords

• Spin-transfer torque
• MRAM
• perpendicular magnetization
• tunneling magnetoresistance