Microresonator-integrated magnetic tunnel junctions allowing for thermal gradient sign reversal designed using COMSOL simulations

In magnetic tunnel junctions (MTJs), it was theoretically predicted that magnetization dynamics can be induced by thermal gradients via thermal-spin transfer torques (T-STTs), similar to STTs induced by applied voltages[1]. We recently proposed an approach based on microresonators (µR) in order to detect T-STT terms acting on the free layer of an MTJ device by means of investigating the ferromagnetic response of an MTJ exposed to a cw laser in open-circuit conditions. The linewidth of the ferromagnetic signal is modified by the damping-like torque induced by thermal gradient, while the frequency is subject to changes induced by the field-like torque[2]. Here, we used COMSOL simulations to determine the temperature profile of Co2FeAl/MgO/CoFeB MTJs, when the MR structure is considered, see Fig 1. In this design, the structure is defined with laterally larger Co2FeAl (CFA) layer below CoFeB (CFB) layer. This allows for the laser beam to be focused on the CFA layer surface, in order to heat the bottom electrode of the MTJ rather than the top, as commonly investigated[3-6]. The temperature of both layers and temperature differences (ΔT) as a function of lateral width are shown in Fig 2. The temperature on the CFA layer decreases laterally about 25K towards to center of the structure and yields a ΔT over the barrier of about 680mK, if a gold pad is placed on the CFA layer, in the position where the laser is shined on (Fig 2). Without the gold pad and leaving the MgO on the CFA unpatterned, the ΔT over the barrier is reduced almost by factor of 2, and is estimated to be around 320mK and 270mK respectively. When both layers have gold contacts, ΔT drops to 20mK (Fig 2). Consequently, the magnitude and the sign of ΔT over the barrier can be engineered by appropriately designing the design of the device, which would allow for T-STTs induced by thermal gradients of different sign to be investigated experimentally in a single MTJ.

*Figure 1 2D sketch of an MTJ structure considered for COMSOL modelling, where the loop of µR was taken into account and only the bottom layer is exposed to the laser.

*Figure 2 Temperature profile of the MTJ versus lateral size, ΔT dependence on designed structure.
References
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