A fast-track characterization protocol of spin-orbit torque efficiencies

   Spin-orbit coupling is a source of many exotic physical phenomena found in various condensed matter systems; these phenomena, such as the current-induced spin Hall effect (SHE) and the spin-orbit torque (SOT), originate from transition metal-based and emergent material-based magnetic heterostructures. The SHE-induced SOT and its related effects have been widely studied both theoretically and experimentally and demonstrated by harnessing the magnetization dynamics in certain material systems (e.g., Pt-, Ta-, and W-based heterostructures). Therefore, the SOT effect might become a major mechanism for controlling next-generation spintronic devices, such as magnetic random access memory (MRAM)/SOT-MRAM and spin logic devices.

   
In recent years, several measurement techniques have been adopted to characterize the current-induced effective fields originating from the SOTs in material systems with perpendicular magnetic anisotropy (PMA). These techniques include harmonic voltage measurements, the spin Hall torque magnetometry of the chiral domain wall, and calibrated current-induced magnetization switching measurements. The goal of all these different approaches is nevertheless the same: To determine the sign and magnitude of the current-induced effective field per current density, χ, from which the spin Hall efficiency and the spin Hall angle of the system can be further estimated.

   
In our recent work, in collaboration with Professor Geoffrey Beach’s team at Massachusetts Institute of Technology, we have shown that by measuring the current-induced hysteresis loop shift versus the in-plane bias magnetic field, as shown in Figure 1, the SHE contribution of the current-induced effective field per current density χSHE can be estimated for transition-metal-based magnetic heterostructures with PMA (Ref. 1). A rough estimation of the Dzyaloshinskii-Moriya interaction effective field HDMI can also be achieved by this technique. Furthermore, while applying this method to samples with a wedge-deposited ferromagnetic layer, we observed an extra contribution (χWedged) due to the asymmetric nature of the deposited ferromagnetic layer, as shown in Figure 2. The origin of χWedged might be related to the asymmetric depinning process in the ferromagnetic layer during magnetization switching; this process was checked by magneto-optical Kerr microscopy (MOKE). The applicability of this newly proposed characterization method has also been further verified in several recent studies regarding SOTs in nonmagnetic (NM)/ferromagnetic systems. These results indicate the possibility of achieving deterministic field-free spin-orbit torque switching by controlling the symmetry of the domain expansion, which will be beneficial for the development of next-generation SOT-MRAM devices. Our method also provides a fast-track and non-invasive method for SOT efficiency determination.

Schematic illustration of the measurement protocol and representative data

Figure 1. (a) Schematics of the current-induced hysteresis loop shift measurement setup. (b) Representative hysteresis loops under different currents obtained by anomalous Hall voltages. (c) The switching phase diagram of the measured device under an in-plane bias field of 2500 Oe. (d) Without any biasing field, the SHE-induced SOT efficiency is virtually zero. (Reproduced from Ref. 1)

Extracting the structural-induced and SHE-induced SOT efficiencies

Figure 2. (a) Schematics of the wedge-deposited ferromagnetic layer (cobalt (Co) in this case) sample. (b) Extracted SOT efficiencies from the wedged sample. The contributions from the structural effect χWedged and the spin Hall effect χSHE can be easily deciphered. (c) The relative strengths of the two SOT contributions. (d) The MOKE-obtained switching dynamics for the uniformly deposited and wedge-deposited samples, which indicate different switching modes. (Reproduced from Ref. 1)

Reference
Chi-Feng Pai, Maxwell Mann, Aik Jun Tan, and Geoffrey S. D. Beach (2016). Determination of spin torque efficiencies in heterostructures with perpendicular magnetic anisotropy. Physical Review B, 93(14), 144409. DOI:10.1103/physrevb.93.144409.

Assistant Professor Chi-Feng Pai
Department of Materials Science and Engineering
cfpai@ntu.edu.tw

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