Quantitative characterization of spin-orbit torques in Pt/Co/Pt/Co/Ta/BTO heterostructures due to the magnetization azimuthal angle dependence

Substantial understanding of spin-orbit interactions in heavy-metal (HM)/ferromagnet (FM)heterostructures is crucial in developing spin-orbit torque (SOT) spintronics devices utilizing spin Hall and Rashba effects. Though the study of SOT effective field dependence on the out-of-plane magnetization angle has been relatively extensive, the understanding of in-plane magnetization angle dependence remains unknown. Here, we analytically propose a method to compute the SOT effective fields as a function of the in-plane magnetization angle using the harmonic Hall technique in perpendicular magnetic anisotropy (PMA) structures. Two different samples with PMA, a Pt/Co/Pt/Co/Ta/BaTi O3 (BTO) test sample and a Pt/Co/Pt/Co/Ta reference sample, are studied using the derived formula. Our measurements reveal that only the dampinglike field of the test sample with a BTO capping layer exhibits an in-plane magnetization angle dependence, while no angular dependence is found in the reference sample. The presence of the BTO layer in the test sample, which gives rise to a Rashba effect at the interface, is ascribed as the source of the angular dependence of the dampinglike field

Deterministic Spin-Orbit Torque Induced Magnetization Reversal In Pt/[Co/Ni]_n/Co/Ta Multilayer Hall Bars

Spin-orbit torque (SOT) induced by electric current has attracted extensive attention as an efficient method of controlling the magnetization in nanomagnetic structures. SOT-induced magnetization reversal is usually achieved with the aid of an in-plane bias magnetic field. In this paper, we show that by selecting a film stack with weak out-of-plane magnetic anisotropy, field-free SOT-induced switching can be achieved in micron sized multilayers. Using direct current, deterministic bipolar magnetization reversal is obtained in Pt/[Co/Ni]2/Co/Ta structures. Kerr imaging reveals that the SOT-induced magnetization switching process is completed via the nucleation of reverse domain and propagation of domain wall in the system

Simultaneous determination of effective spin-orbit torque fields in magnetic structures with in-plane anisotropy

The strength of spin-orbit torque in ferromagnetic structures is characterized by fieldlike and dampinglike effective fields. Conventionally, two distinct measurement approaches are employed to quantify the magnitude of the respective effective fields in structures with in-plane magnetic anisotropy. Here, we propose and demonstrate a self-validating method, which enables simultaneous quantification of both the fieldlike and dampinglike terms in structures with in-plane magnetic anisotropy. An analytical expression is derived and validated by harmonic Hall resistance measurement. Both the fieldlike and dampinglike effective fields are extracted from a single measurement using the derived fitting functions for the second harmonic Hall resistance. The first harmonic Hall resistance acts as a reference to confirm that the experimental parameters are consistent with the derived equations

Spin-orbit torque effective fields and spin accumulation in TA/CO/PT structure with in-plane magnetic anisotropy

The magnetic recording technique of magnetoresistive random access and domain-wall memories requires a clear understanding of spin-orbit torque (SOT) which is applied for writing. In the writing unit where the structure of heavy metal (HM)/ferromagnetic metal (FM)/HM or oxide is commonly used, the SOT strength is represented by an effective field comprised of fieldlike and dampinglike terms. Generally, two distinct measurement approaches are used to characterize the effective field magnitude. In this thesis, a self-validating approach is proposed and demonstrated, which enables concurrent quantification of both the fieldlike and dampinglike terms in structures with in-plane magnetic anisotropy. Based on this method, in Ta/Co/Pt structure the dependences of the effective field on the thickness of Ta layer and the magnetization magnitude of Co layer are investigated. The investigation results reveal that the thickness of Ta varies the magnitude of the effective field. The fieldlike term decreases while the dampinglike term increases with the increase of the Co magnetization magnitude, indicating that the SOT effective field is tunable by the magnetization amplitude of FM layer. Angular dependence of the effective field is investigated. The results show that the fieldlike term consists of a component with fixed value and another component with azimuthal angle dependence, which experimentally supports that both Rashba and spin Hall effects contribute to the SOT. The dampinglike term is independent of the angle, indicating that the dampinglike term cannot be tuned by the azimuthal orientation of the magnetization. The SOT effective field, which has been characterized, is caused by spin accumulation in HM/FM interfaces. In this thesis, the spin accumulation is quantified by means of harmonic Hall resistance measurement. Spin accumulation up to 10% of the local magnetization is recorded when the applied electric current density is 10 to the power of 11 Amperes per square meter.​Doctor of Philosophy (SPMS

Aerodynamic Characteristics When Trains Pass Each Other in High-Speed Railway Shield Tunnel

The characteristics of the aerodynamic effects of high-speed trains passing in a shield tunnel were studied using the three-dimensional, compressible, unsteady Reynolds-averaged Navier-Stokes (RANS) equations for the simulation analysis. Numerical calculations were compared with dynamic model tests to verify the reliability of the numerical simulations. The results showed that the compression wave characteristics of high-speed trains in shield tunnels were consistent with those in molded concrete tunnels. When high-speed trains met in the middle of the shield tunnel, the positive and negative peak attenuation rates of shield tunnels were higher than the positive and negative peak attenuation rates of molded lining tunnels, and the maximum pressure attenuation rate could reach 57.8%. At the same time, the micro-pressure wave of the former was reduced by 10.78%, compared with those of the latter. When meeting cars at different locations, the maximum pressure at the intersection in the center of the tunnel was significantly higher than those at other intersection points in the tunnel. Different intersection positions and different tunnel lining structures had relatively little influence on the aerodynamic drag and lateral force, while train speed had a significant influence

Analysis of aerodynamic effects and load spectrum characteristics in high-speed railway tunnels

To study the aerodynamic effects and load spectra caused by high-speed trains passing through double-track tunnels, this paper uses unsteady, viscous, compressible Navier-Stokes equations and the Re-Normalization Group k-e turbulence model with sliding grid technology for simulation. A dynamic model test is carried out to verify the calculation method and grid. This study considers the impact of the three main factors of the tunnel aerodynamic effect when the train passes through the tunnel. The peak of the aerodynamic load spectrum when the train is coupled to the tunnel is mainly concentrated in the range of 0-5 Hz. The results show that as the train speed increases, the peak pressure and pressure gradient increase significantly, and the maximum pressure gradient appears in the time between the peak and trough of the head wave. When two trains meet in the middle of the tunnel, the peak pressure and its position change significantly, and the maximum pressure gradient peaks reach 80.58 kPa/s. When two trains exit the tunnel, the pressure presents a fixed period of fluctuations, and the maximum pressure peak is only 0.09 kPa less than the peak when a single train passes through the tunnel at the same monitoring point

Spin-orbit torque induced magnetization anisotropy modulation in Pt/(Co/Ni)4/Co/IrMn heterostructure

In this work, we show that domain wall (DW) dynamics within a system provide an alternative platform to characterizing spin-orbit torque (SOT) effective fields. In perpendicularly magnetized wires with a Pt/(Co/Ni)4/Co/IrMn stack structure, differential Kerr imaging shows that the magnetization switching process is via the nucleation of the embryo state followed by domain wall propagation. By probing the current induced DW motion in the presence of in-plane field, the SOT effective fields are obtained using the harmonic Hall voltage scheme. The effective anisotropy field of the structure decreases by 12% due to the SOT effective fields, as the in-plane current in the wire is increased.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Published versio