Controlled pulse shape cooling in planar TAS-STT-MRAM for improved writeability

Abstract

In field written thermally assisted (TAS) MRAM, the storage layer is pinned with an antiferromag-netic layer. The writing of TAS-MRAM consists of heating the storage layer above the blocking temperature of the antiferromagnet using an injected current pulse through the tunnel barrier. This pulse can be used to assist the writing either combined to an external magnetic field or to a spin transfer torque (STT) [1] effect coming from the spin polarized current flow. After setting the storage layer direction during the write step, the current pulse is removed, pinning the storage layer in the set direction. The actual temperature decay occurs in a timescale of few tens of nanoseconds [2]. STT is efficient while the current is flowing through the junction but disappears during cooling, once the heating pulse is removed. This assumption is valid if the temperature gradients across the barrier, giving rise to spin accumulation of thermal origin are negligible. In these conditions, when the temperature is above or close to the blocking temperature of the antiferromagnetic layer pinning the storage layer, the written state might be thermally unstable. In this paper, we have investigated the possibility of controlling the temperature decay, so that the spin polarized current and temperature decay at the same rate. We show that there is an improvement in writing reproducibility using a linear transition at the end of the current pulse during the cooling phase. This is especially evident in cases where STT influence on the field writing is more significant. The write error rate dependence with the voltage transition duration was measured, and we find an optimum value for a 70ns transition, corresponding to a linear pulse amplitude decay of 18mV/ns.