Comparative material issues for fast reliable switching in STT-RAMs

With its fast write and read, small cell size, non-volatility and excellent endurance, Spin Transfer Torque-RAM (STT-RAM) has a high potential of dominating the embedded and standalone memory world in the near future. In this paper, the suitability of different classes of magnetic materials constituting the STT-RAM free layer is reviewed for faster switching and thermal stability. We identify the following material classes for faster switching in the thermally stable free-layer of a STT-RAM: (a) In-plane materials with high H_K and low M_S. While the high H_K deters the magnetization during the easy to hard axis switching, it helps with switching past the equator, making the switching speeds for high and low H_K materials comparable. However, high H_K materials benefit from higher thermal stability. (b) Perpendicular materials with low damping have the same switching speed as in-plane materials but greater switching probability because of a lower critical current. The demagnetization field helps the free layer to start switching to the hard axis, but hinders it from switching further to the easy axis beyond the equator. (c) Anti-ferromagnetically capped partially-perpendicular materials. Capping with a Va layer decreases the demagnetization field, which promotes faster switching.