Effects of the Heating Current Polarity on the Writing of Thermally Assisted Switching-MRAM

In thermally assisted switching-magnetic random access memory (TAS-MRAM), a storage layer is pinned by exchange bias with an antiferromagnet, and is unpinned during writing because of a heating pulse of current injected through the junction. The current densities used for heating are of the order of magnitude of the current densities at which spin-transfer torque (STT) acts as an effective field from the point of view of the thermal activation model. To seek for such combined influence of heating and STT in TAS-MRAM, statistical writing tests were performed on 1-kb test devices. The phase diagrams with the pinned and unpinned regions were obtained. A reduction of the writing field in the unpinned region was evidenced by changing the polarity, which is asymmetrical with respect to the direction of writing. This is consistent with STT influence. The order of magnitude of the effect is in good agreement with previous work on writing field dependence with the current and compares well with numerical estimates using the Neel-Brown-modified model to consider the STT effect. The writing field can be reduced by 15% by properly choosing the current direction for a current density of 2 MA/cm².