Based on the nudged elastic band model, we investigate the thermal energy barrier and its field dependence of a circular-shaped perpendicular nanomagnet for various exchange constants and cell sizes. We find that the energy barrier depends not only on the cell size but also on the exchange stiffness constant when the switching is governed by the domain-wall nucleation and propagation. We show that analyzing the switching probability curve based on the field-dependent energy barrier equation for the single-domain switching can underestimate the energy barrier so that one has to use the correct field dependence of energy barrier, which can be obtained numerically.Index Terms-Energy barrier, perpendicular nanomagnet, spin-transfer torque magnetic random access memory (STT-MRAM).

We numerically investigate the effect of magnetic and electrical damages at the edge of a perpendicular magnetic random access memory (MRAM) cell on the spin-transfer-torque (STT) efficiency that is defined by the ratio of thermal stability factor to switching current. We find that the switching mode of an edgedamaged cell is different from that of an undamaged cell, which results in a sizable reduction in the switching current. Together with a marginal reduction of the thermal stability factor of an edge-damaged cell, this feature makes the STT efficiency large. Our results suggest that a precise edge control is viable for the optimization of STT-MRAM.

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