Switching of Exchange-Coupled Perpendicular Magnetized Layers Driven by Spin Orbital Torque With Low Power Consumption

Extensive experiments have been devoted to study the deterministic switching of perpendicularly magnetized layers in heavy metal/ferromagnet devices driven by spin orbital torque by the spin Hall effect [1–4]. A perpendicular magnetized layer has been proved to be successfully and deterministically switched under certain circumstances experimentally and theoretically [5–8]. To obtain high perpendicular anisotropy, the thickness of the film needs to be sufficiently small (<1 nm). To resist the thermal fluctuations during operation, we proposed a multilayer structure including exchange-coupled perpendicularly magnetized layers to switch at relatively low currents and maintain thermal stability, inspired by the ECC media in HDD systems [9]. Without loss of generality, we simply used an in-plane field along the charge current direction (y) to describe the effective field to break the symmetry of rotation in response to the spin orbital torque in our simulation. Fig.1(a) illustrates our design: the bottom magnetic layer is softer $(K_{1} < K_{2})$ and is relatively vulnerable to the reversal torque. We used typical magnetic parameters for each layer: the saturation magnetization $M_{s1}=1200$ emu/cm3 and $M_{s2}=800$ emu/cm3, and the effective anisotropy constants $K_{1}= 0.5 \times 10 ^{6}$ erg/cm3 and $K_{2}= 2 \times 10 ^{6}$ erg/cm3. We assume only the bottom magnetic layer is subject to the spin orbital torque as the torque originates from spin orbit interaction. Without any applied currents the multilayer relaxes to its equilibrium state and the average magnetization is slightly tilted towards y axis (about 12°). In the switching process, the softer magnetic layer tends to reverse first and the harder layer follows driven by the exchange interaction. The critical spin current density is 5MA/cm2. Our new structure provides a way to design and optimize the spintronic device.