Spin torque switching and scaling in synthetic antiferromagnet free layers with in-plane magnetization

Abstract

We study the field-induced and the current-induced switching of synthetic antiferromagnet free layers with in-plane magnetization. In practical cases, the energy barrier relevant for the thermal stability depends mainly on the magneto-crystalline anisotropies of the layers. We derive the spin-flop and the direct overwrite fields and their dependence on the asymmetry of the thickness of the two layers of the SyF. This is used to explain why the SyF magnetizations are much more robust to external fields than the anisotropy would tell. We then calculate the spin-torque instability threshold current densities for the acoustical and the optical excitations of the SyF, taking into account that there are spin torques acting on the two layers of the SyF. The acoustical excitation has the lowest instability current. Based on these findings, we finally discuss the scaling of SyF free layers by combining thermal stability requirements and dielectric breakdown avoidance constraints. Scaling can be conducted with either the aim of minimizing the write current or with the aim of minimizing the free layer surface. An absolute minimum of write current can be reached for relatively large cells (166×166 nm2) provided a zero effective magnetization is induced by a proper choice of the free layer thicknesses. Depending on how close the technology can reliably approach to the thickness yielding the easy axis reorientation transition while keeping bulk damping, this current minimum could in principle be as low as 14 μA in the macrospin approximation. If in contrast one aims at small free layer surfaces, then the present CoFeB alloy-based technology cannot reach dimensions below 90×90 nm2.