Heavy metal/ferromagnetic metal bi-layered structures that exhibit both strong perpendicular magnetic anisotropy (PMA) and large spin-orbit torque (SOT) efficiency have high potential in high-density, low-power memory, and logic device applications. Here, we report the enhancement of PMA and SOT in Ta/Pt/Co/Ta multi-layered heterostructures through interfacial diffusion. The structures can exhibit PMA fields of 9100 Oe at 300 K and 14100 Oe at 10 K and an effective spin Hall angle (SHA) of 0.61 ± 0.03 at 300 K. These values are larger than the corresponding values reported previously for similar heterostructures. The current-induced magnetization switching was demonstrated. The critical switching current density is on the order of 106 A/cm2, and the corresponding switching efficiency is higher than that reported for similar structures. X-ray absorption spectroscopy and high-angle annular dark-field scanning transmission electron microscopy analyses suggest a strong correlation between the observed PMA and SOT enhancement and the interfacial diffusion during the sputtering growth of the samples. It is very likely that the interfacial diffusion gives rise to enhanced spin-orbit coupling at the interface, while the latter results in enhancement in the PMA, SHA, and switching efficiency in the structure.
Thin films with perpendicular magnetic anisotropy (PMA) play an essential role in the development of technologies due to their excellent thermal stability and potential application in devices with high density, high stability, and low energy consumption.
We report a current-induced four-state magnetization reversal under zero magnetic field in a wedged Ta/MgO/CoFeB/MgO heterostructure with a perpendicular magnetic anisotropy. Anomalous Hall effect and magneto-optical Kerr effect microscopy measurements were performed to demonstrate that the field-free multi-level reversal is jointly determined by the spin–orbit torque effective field that originates from the lack of the lateral inversion symmetry in the wedged stacking structure and the current-induced Oersted field. Moreover, the creation of robust intermediate Hall resistance states in the multi-state switching strongly depends on the current-induced Joule heating. Our results provide a route for the field-free multi-level state reversal, which is significant for fabricating the non-volatile and energy-efficient multi-level memories or artificial neuron devices.
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