Demonstration of four-state memory structure with perpendicular magnetic anisotropy by spin-orbit torque

Abstract

Current, instead of magnetic field, induced magnetization switching is very important for future spintronics in information storage or/and information processing. As one of the effective current-induced magnetization methods, spin-orbit torque (SOT) has aroused considerable interest because it has low-power consumption and can improve the device endurance. Normal metal (NM)/ferromagnetic metal (FM) are the common materials used for SOTs, where the NM denotes the materials with strong spin-orbit coupling such as Pt, Ta, W, etc. Owing to the spin Hall effect, the in-plane current in NM layer can be converted into a vertical spin current that exerts torques on the adjacent FM layers. Spin current can also come from the NM/FM interface charge-spin conversion due to interfacial asymmetry, exerting torques on the adjacent FM layers. Materials with in-plane and perpendicular magnetic anisotropy are used to study the SOT-induced magnetization switching. Compared with the memories using the in-plane ferromagnetic films, the magnetic memories using NM/FM multilayers with perpendicular magnetic anisotropy can have much high integration density. Currently the used information storage was based on the two-state memory cell. Owing to more than two states contained in one memory cell, multiple states memory manipulated by electric current could further benefit the higher-density memory. In this paper, a four-state memory unit is demonstrated by the influence of TaOx buffer layer on the magnetic anisotropy of Pt/Co/Pt multilayers. The memory unit consists of two regions. One is directly deposited on thermal oxide Si substrate[Pt(3 nm)/Co(0.47 nm)/Pt(1.5 nm)] and the other has a buffer layer of TaOx[TaOx(0.3 nm)/Pt(3 nm)/Co(0.47 nm)/Pt(1.5 nm)], thus leading to the difference in magnetic property between these two regions. According to the Z axis magnetic hysteresis loops of two regions, measured by polar magneto-optical Kerr effect, the coercivity of the region with TaOx is obtained to be 23 Oe and that without TaOx is 11 Oe. At the junction between two regions, the magnetic hysteresis loop shows the superposition of hysteresis loops of two regions, resulting in switching four times as the magnetic field changes. Under a fixed magnetic field along the current direction, the magnetization orientation of region with TaOx and that of region without TaOx are switched by spin-orbit torques with threshold currents of 5 mA and 1.5 mA respectively. The switching direction can be changed as the in-plane magnetic field changes to the opposite direction, which is one of the typical features of SOTs-induced magnetization switching. At the junction between two regions, through applying different-form current pulses to one conductive channel of the device, the magnetic state of the memory cell can be switched between four clear states. This kind of structure provides a new idea to design SOT multi-state memory devices.