Zero-field spin transfer oscillators combining in-plane and out-of-plane magnetized free layers

Abstract

Spin-transfer-torque driven magnetization dynamics in a spin-valve device consisting of an in-plane magnetized polarizer and an out-of-plane magnetized free layer were studied numerically. Such devices hold promise for nanoscale wireless transmitters operating at gigahertz frequencies, compatible with current mobile telephone and wireless local area network technologies [1]. In traditional spin-transfer-torque devices, with applications as memory elements (spin-transfer-torque MRAM), the magnetic easy axes of both the free and reference layers are co-linear (either in-plane magnetized or perpendicularly magnetized) in order to give the maximum difference in magnetoresistance between the two available storage states i.e. fully parallel or fully anti-parallel alignment. For spin-transfer-oscillators the situation is somewhat different. The criterion for having two stable static states with well separated resistance values is no longer an important factor. What is desired is a precessional orbit that passes through both the fully parallel and fully anti-parallel state as well as the maximisation of the torque in the initial state. For this, the most efficient geometry is one in which the free layer is magnetized out-of-plane and the polarizing layer is magnetized in-plane. For the ground state, the spin-transfer-torque efficiency is close to maximum as the angle between the two layers is 90°. The amplitude of oscillation is maximised as precession around the film normal allows passage through the parallel and anti-parallel states in one precession cycle [2,3].