Consequences of an interface-concentrated perpendicular magnetic anisotropy in ultrathin CoFeB films used in magnetic tunnel junctions

We examine the consequences of a strongly interface-concentrated perpendicular magnetic anisotropy (PMA) energy in CoFeB thin films currently in wide use in magnetic tunnel junctions (MTJs) for spin-torque-related memory applications. The direct consequence of such an anisotropy energy distribution, in combination with a moderate exchange coupling of the interface moment to the rest of the film, is a phenomenological appearance of a fourth-order anisotropy term as the film is viewed by ferromagnetic resonance. The presence of a fourth-order anisotropy also affects the apparent thermal activation energy of a patterned nanomagnet with such thin films, and it could lead to an apparent increase in the spin-torque switching efficiency as represented by the ratio of the thermal activation energy and the threshold switching current. However, for interface-sensitive quantities such as tunnel magnetoresistance's hard-axis behavior, as well as for spin-torque excitation processes, the specifics of such separation of interface versus film-interior moment rotation could become important.