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.

中文翻译：

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楔形Ta/MgO/CoFeB/MgO异质结构中无场自旋轨道扭矩驱动的多态反转

我们报告了在具有垂直磁各向异性的楔形 Ta/MgO/CoFeB/MgO 异质结构中零磁场下电流诱导的四态磁化反转。进行了异常霍尔效应和磁光克尔效应显微镜测量，以证明无场多级反转是由自旋轨道扭矩有效场共同决定的，该场源于楔形堆叠结构中缺乏横向反转对称性和电流感应的奥斯特场。此外，在多态切换中产生稳健的中间霍尔电阻状态在很大程度上取决于电流引起的焦耳热。我们的结果为无场多级状态反转提供了一条途径，