Investigations of spin currents in elemental, alloyed and multilayer systems using spin-torque ferromagnetic resonance technique

Current-induced spin-orbit torques (SOT) are a vital research field within the realm of spintronics. They have attracted significant attention due to the vast opportunities for applications, e.g., in neuromorphic computing or memory devices, and their potential to reduce energy consumption in said applications. Energy efficient generation of SOT is an enormous part of today’s spintronics research, and a simple yet powerful way to generate them is the spin Hall effect (SHE), which converts charge currents into spin currents due to the spin-orbit coupling induced spin-dependent scattering in a nonmagnetic material (NM). When the spin currents accumulate at the interface to an adjacent ferromagnet (FM), they can exert a fieldlike SOT, and when they are absorbed into the FM, they exert a damping-like SOT onto the magnetization. It is thus essential to examine the spin-charge conversion ratio, which can be achieved by determining the ferromagnetic resonance (FMR) efficiency with spin-torque ferromagnetic resonance (STFMR) measurements, which are based on measuring the rectification of an alternating current and oscillating resistance due to two magneto-resistive effects, namely, the anisotropic magnetoresistance (AMR) and the spin Hall magnetoresistance (SMR). Further, it allows simultaneous characterization of the damping parameter and, by applying an additional bias current, the individual SOT efficiencies. Achieving a high spin-charge conversion ratio is highly desired as it can improve SOT efficiencies and reduce energy consumption in applications. One method to enhance the spincharge conversion ratio is to increase the scattering cross-section by introducing impurities into the NM. However, this results in an increase in resistivity and, thus, a drop in spin Hall conductivity (SHC), which is not advantageous regarding energy efficiency. Therefore, I studied the FMR efficiency and the SHC in a CuW alloy and its effect on the damping in an adjacent Fe layer. I found that at a W concentration of 60%, the spin-charge conversion ratio and the SHC are enhanced. Additionally, the ratio of the FMR efficiency to damping, which is proportional to the charge current density needed to switch an in-plane magnetization, is decreased by a factor of 4 compared to a pure W layer. I further investigate the spin-charge conversion enhancement using a multilayer structure of alternating W and Cu layers on a Fe layer. By increasing the number of interfaces in the NM, a significant enhancement up to a factor of 84 was observed, compared to a single layer of W given the same thickness. Further, it was shown that while the Cu layers did not much affect the spin current in the NM, a spin accumulation arose in the W layers, explaining a rather large field-like SOT and a reduction in damping-like SOT. Finally, out-of-plane STFMR measurements were performed on a conventional Co/Pt heterostructure to entangle the contributions of AMR and SMR. These measurements revealed the potential to control the spin current that crosses the FM/NM interface and the SOT by frequency and bias current dependent STFMR measurements in the out-of-plane direction.Doctor of Philosoph

Investigation of spin-orbit torque performance with W/Cu-multilayers as spin current source

We study the W/Cu multilayers as a spin current source and the coherent spin-orbit torques in a Fe layer using the spin-torque ferromagnetic resonance (STFMR) technique. With increasing numbers of layers, the line shape of the STFMR signals changes from predominantly antisymmetric to predominantly symmetric. When using [W(0.5)/Cu(0.5)]5 as a spin current source, the symmetric amplitude increases by a factor of 5 compared to a single W layer. Simultaneously, the effective damping parameter also increases slightly due to enhanced spin pumping. Along with an increasing trend in the damping-like torque efficiency, this suggests that the extrinsic spin Hall effect is enhanced. Concurrently, the antisymmetric amplitude decreases significantly by a factor of 27, which indicates an increase in the field-like torque when multilayers are used as a spin current source.Agency for Science, Technology and Research (A*STAR)Published versionThis work was supported by the RIE2020 ASTAR AME IAF-ICP grant via Grant No. I1801E0030

Enhanced spin Hall conductivity in tungsten-copper alloys

We report on the enhancement of the spin Hall conductivity in tungsten by alloying with copper, measured by using the spin-torque ferromagnetic resonance technique. The alloying leads to an increase in spin-dependent scattering events and results in an enhancement of the contributing extrinsic spin Hall effects. The measured damping property shows a slight increase with higher tungsten concentration, due to spin current losses from the ferromagnetic layer into the tungsten-copper alloy. At a tungsten concentration of 60%, the spin Hall conductivity reaches a maximum of 3.68±0.68×105Ω-1m-1, corresponding to an enhancement of 120% compared to the pure tungsten sample. At the same concentration, the ratio of the spin Hall angle to the damping of the ferromagnetic layer, which offers a quick estimation for the critical switching current density, is found to be four times smaller as compared to pure tungsten.Agency for Science, Technology and Research (A*STAR)National Research Foundation (NRF)The work was supported by the Singapore National Research Foundation, under a Competitive Research Programme (Non-volatile Magnetic Logic and Memory Integrated Circuit Devices, NRF-CRP9-2011- 01), and an Industry-IHL Partnership Program (NRF2015- IIP001- 001). The support from an RIE2020 ASTAR AME IAF-ICP Grant (No. I1801E0030) is also acknowledged

The influence of Ti ultrathin insertion layer on the effective magnetic damping and effective spin Hall angle

We report the influence of ultrathin Ti insertion layer on the effective magnetic damping and effective spin Hall angle in Co/[Pt/Ti]n/Pt structures via spin-torque ferromagnetic resonance measurements. The effective magnetic damping shows a non-monotonic variation as a function of insertion layers number n, reaching a minimum at n = 5. Our analysis shows that when n is less than 5, the damping is mainly related to the thickness of the bottom Pt layer, and when it is greater than 5, the attenuation of the spin currents leads to increased damping. The effective magnetic damping first decreases as the number of layers n increases, reaching a minimum at n=5, and then increases with further increases in n. The observation can be ascribed to a competition between the increased longitudinal resistivity, which is due to the strong interfacial scattering, and the reduced effective spin Hall conductivity that originates from the shortening of the carrier lifetime. Additionally, the extracted interfacial spin transparency is found to be improved with the effect of the insertion layer.Agency for Science, Technology and Research (A*STAR)Economic Development Board (EDB)Published versionThis work was supported by a ASTAR AME IAF-ICP Grant (No. I1801E0030). This work was also supported by an EDB-IPP Grant (No. RCA-17/284). Z. Xu gratefully acknowledges financial support from the China Scholarship Council

Magnetically directed co-nanoinitiators for cross-linking adhesives and enhancing mechanical properties

Magnetically directed localized polymerization is of immense interest for its extensive impacts and applications in numerous fields. The use of means untethered from an external magnetic field to localize initiation of polymerization to develop a curing system is a novel concept, with a sustainable, efficient, and eco-friendly approach and a wide range of potential in both science and engineering. However, the conventional means for the initiation of polymerization cannot define the desirable location of polymerization, which is often exacerbated by the poor temporal control in the curing system. Herein, the copper-immobilized dendrimer-based magnetic iron oxide silica (MNPs-G2@Cu2+) co-nanoinitiators are rationally designed as initiators for redox radical polymerization. The nanoinitiators are magnetically responsive and therefore enable localized polymerization using an external magnetic field. In this work, anaerobic polymerization of an adhesive composed of triethylene glycol dimethacrylate, tert-butyl peroxybenzoate, and MNPs-G2@Cu2+ as the magnetic co-nanoinitiators has been investigated. The use of a magnet locates and promotes redox free radical polymerization through the synergistic functions between peroxide and MNPs-G2@Cu2+ co-nanoinitiators. The mechanical properties of the resulting polymer are considerably reinforced because the MNPs-G2@Cu2+ co-nanoinitiators concurrently play another crucial role as nanofillers. This strategy provides a novel approach for magnetically tunable localized polymerization, which allows new opportunities to govern the formulation of advanced adhesives through polymerization under hazard-free conditions for various promising applications.Ministry of Education (MOE)Nanyang Technological UniversityH.K. is supported by a SINGA scholarship awarded by Nanyang Technological University. The project is funded by Ministry of Education AcRF Tier 1, Project RG63/20, Award 2020-T1-001- 165