Back hopping in spin-transfer-torque devices, possible origin and counter measures

摘要

The effect of undesirable high-frequency free-layer switching in magneticmultilayer systems, referred to as back hopping, is investigated by means ofthe spin-diffusion model. A possible origin of the back-hopping effect is foundto be the destabilization of the pinned layer which leads to perpetualswitching of both layers. The influence of different material parameters on thecritical switching currents for the free and pinned layer is obtained bymicromagnetic simulations. It is found that the choice of a free-layer materialwith low polarization $\beta$ and saturation magnetization $M_s$, and apinned-layer material with high $\beta$ and $M_s$ leads to a low free-layercritical current and a high pinned-layer critical current and hence reduces thelikelihood of back hopping. While back hopping was observed in various types ofdevices, there are only few experiments that exhibit this effect inperpendicularly magnetized systems. However, our simulations suggest, that thisis likely to change due to loss of pinned-layer anisotropy when decreasingdevice sizes.