Spin-orbit torque (SOT) is promising for GHz low-power switching of MTJs with high endurance towards integrated low-power computing because of its attractive non-volatile solutions to currently existing power-hungry volatile cache memory technology(static RAM). Realisation of field-free SOT is the most important challenge towards its practical application. Our works towards various schemes for field-free SOT switching, which have scalability and higher performance promise, involve materials studies, device design and characterisation, materials device simulations and CMOS integration.
We are also exploring the fundamental physics of SOT phenomena and optimisation studies. For example, it is important to study and enhance the charge-to-spin conversion efficiency. We use spin-torque ferromagnetic resonance (ST-FMR) to quantify the spin Hall angle and the damping parameters. In particular, enhanced SOT efficiency may be achieved by engineering FM-based heterostructures and interfaces. We recently demonstrated that substantially large effective spin Hall angle can be achieved with proper stack materials selection, one such example is the Ta/ CoFeB/ Pt stack1. In another study, we investigate the spin-orbit interaction in HM/ CoFe bilayers using first-principles calculations2. Charge transfer at the HM/ CoFe interface results in an interfacial electric field, as well as spin and orbit moments at the interfacial heavy atom. Spin-orbit coupling (SOC) with various materials was studied by comparing the SOC strength at the interfacial heavy atom (IHA) with its bulk value at corresponding transition metal. Our work suggests that interfacial electric field plays an essential role in tuning the SOC effect at HM/ CoFe bilayers, which might be exploited for magnetic switching of SOT-based spin-orbitronic devices.
(a) Figure from Ref [1,2] Typical ST-FMR setup along with its spectra; (b) First principles calculation of SOC strength. The brown bars display the SOC strength of IHA at the HM/ CoFe bilayer. The blue bars represent the SOC strength of heavy elements in its bulk
In addition, our team recently revealed that the existence of interfacial Dzyaloshinskii-Moriya interaction at the HM/ FM interface can induce field-free spin-orbit torque switching of perpendicular magnetisation3. This can be incorporated using a heterostructure free layer (DMI at the interface of FM and SOT channel) to induce switching without sacrificing the free-layer magnetic anisotropy. We discover a regime for deterministic switching against current and DMI variations. This work is directly relevant for field-free SOT by tuning DMI strength through materials engineering.
(c) Schematic of a 3-Terminal SOT device; (d) The sign of DMI may impose boundary conditions at the edge of the MTJs; (e) Dynamics of perpendicular magnetization component, Mz under various D and Hx (in-plane field) conditions (pulse is applied within the shaded region). The dashed curve indicates magnetisation switching is observed without the presence of magnetic field if DMI is introduced
L. Huang et al.,“Engineering magnetic heterostructures to obtain large spin Hall efficiency for spin-orbit torque devices”, Appl. Phys. Lett. 113, 022402 (2018)
- M. Zeng et al.,“Interfacial electric field and spin-orbitronic properties of heavy-metal/CoFe bilayers”, Appl. Phys. Lett. 114, 012401 (2019)
- B. Chen et al.,“Field-free spin-orbit torque switching of a perpendicular ferromagnet with Dzyaloshinskii-Moriya interaction”, Appl. Phys. Lett. 114, 022401 (2019)