Spin-orbit-torque-driven multilevel switching in Ta/CoFeB/MgO structures without initialization

Spin-orbit torque (SOT) has been proposed as an alternative writing mechanism for the next-generation magnetic random access memory (MRAM), due to its energy efficiency and high endurance in perpendicular magnetic anisotropic materials. However, the three-terminal structure of SOT-MRAM increases the cell size and consequently limits the feasibility of implementing high density memory. Multilevel storage is a key factor in the competitiveness of SOT-MRAM technology in the nonvolatile memory market. This paper presents an experimental characterization of a multilevel SOT-MRAM cell based on a perpendicularly magnetized Ta/CoFeB/MgO heterostructure and addresses the initialization-free issue of multilevel storage schemes. Magneto-optical Kerr effect microscopy and micromagnetic simulation studies confirm that the multilevel magnetization states are created by changing a longitudinal domain wall pinning site in the magnet. The realization of robust intermediate switching levels in the commonly used perpendicularly magnetized Ta/CoFeB/MgO heterostructure provides an efficient way to switch magnets for low-power, high-endurance, and high-density memory applications. Published under license by AIP Publishing.