I. Introduction
Spin-transfer torque magnetoresistance random access memory (STT-MRAM) is a nonvolatile resistive memory. A magnetic tunnel junction (MTJ), schematically shown in Fig. 1, is used to store data. It has two stable states with a high and a low resistance. When the pinned layer and the free layer magnetization are aligned in parallel (P), the MTJ has a low resistance . When they are aligned antiparallel (AP), the MTJ has a high resistance . The MTJ is switched from one state to the other (AP2P or P2AP) by sending a current larger than the critical writing current ( or ) through the MTJ. For the AP2P switching, the current needs to flow from the free layer to the pinned layer and for the P2AP switching, a generally larger current needs to flow in the other direction. The tunnel magnetoresistance ratio (TMR) is a metric for the difference in resistance of these two states as compared with the lowest resistance.
Schematic of an MTJ with the pinned layer on top and the free layer at the bottom. (a) Low resistive or P-state. (b) High resistive or AP-state.