Spin–orbit torque (SOT) is an emerging candidate for electrically controlled magnetization switching in low-power and nonvolatile spintronic devices. However, SOT switching of perpendicular magnetization requires an auxiliary field or additional lateral symmetry breaking, which is difficult to achieve in practical applications. In particular, the mechanism of field-free switching through vertical symmetry breaking still lacks a quantitative description. In this work, a vertically asymmetric Co/Pt bilayer has been constructed through quantitative engineering of anisotropy gradient, while keeping the total magnetic thickness of the bilayer constant. Interestingly, the enhanced asymmetry with greater anisotropy gradient would induce higher SOT efficiencies and larger field-free switching ratios. Field-free switching can be attributed to the slight lateral asymmetry caused by the perpendicular anisotropy gradient. The SOT effective-field enhancement and field-free switching through quantitative engineering of the anisotropy gradient not only offer a deeper understanding of current-induced magnetization switching in perpendicularly asymmetric systems but also provide a potential avenue for practical applications of SOT devices at the wafer level.
Step-induced antiferromagnetic (AFM) uniaxial anisotropy and its effects on the exchange coupling have been systematically investigated in the epitaxial $\mathrm{Fe}/\mathrm{CoO}$ bilayers on a $\mathrm{MgO}(001)$ vicinal surface. X-ray magnetic linear dichroism measurements proved that the atomic steps induced a strong in-plane AFM uniaxial anisotropy in the CoO film. We found that the thermal activation induced in-plane 90\ifmmode^\circ\else\textdegree\fi{} switching of CoO AFM in-plane spins. The competition among the step-induced AFM anisotropy, the interface exchange coupling, and thermal activation generate a novel multiple in-plane spin reorientation transition of the Fe magnetization, which can further provide new insights on the exchange coupling in FM/AFM systems.
Spin-flip probabilities from bulk defects and phonons are determined for the mesoscopic Cu channels of nonlocal spin valves (NLSVs). The NLSV spin signals are analyzed by a new approach to consistently extract the in situ values of the Cu spin diffusion length, effective spin injection (detection) polarization, and Cu resistivity. The size variations of the spin injectors (detectors) between NLSVs are treated by normalizing the spin signals to be commensurate with a standard injector (detector) size. The microstructure variations are statistically accounted for by measuring many NLSVs on the same substrate. From the Cu resistivity and spin diffusion lengths at 10 K and 295 K, the spin-flip probabilities from bulk defects and phonons are determined to be (3.9 ± 0.8) × 10−4 and (2.8 ± 1.2) × 10−4, respectively.
The energy efficiency of the spin Hall effects (SHE) can be enhanced if the electrical conductivity is decreased without sacrificing the spin Hall conductivity. The resistivity of Pt films can be increased to 150-300 μΩ*cm by mesoscopic lateral confinement, thereby decreasing the conductivity. The SHE and inverse spin Hall effects (ISHE) in these mesoscopic Pt films are explored at 10 K by using the nonlocal spin injection/detection method. All relevant physical quantities are determined in-situ on the same substrate, and a quantitative approach is developed to characterize all processes effectively. Extensive measurements with various Pt thickness values reveal an upper limit for the Pt spin diffusion length: λ_pt<0.8 nm. The average product of λ_pt and the Pt spin Hall angle α_H is substantial: α_H*λ_pt=(0.142 +/- 0.040)nm for 4 nm thick Pt, though a gradual decrease is observed at larger Pt thickness. The results suggest enhanced spin Hall effects in resistive mesoscopic Pt films.
Abstract Magnetic proximity effect between two magnetic layers is an important focus of research for discovering new physical properties of magnetic systems. Antiferromagnets (AFMs) are fundamental systems with magnetic ordering and promising candidate materials in the emerging field of antiferromagnetic spintronics. However, the magnetic proximity effect between antiferromagnetic bilayers is rarely studied because detecting the spin orientation of AFMs is challenging. Using X-ray linear dichroism and magneto-optical Kerr effect measurements, we investigated antiferromagnetic proximity effects in epitaxial CoO/NiO/MgO(001) systems. We found the antiferromagnetic spin of the NiO underwent a spin reorientation transition from in-plane to out-of-plane with increasing NiO thickness, with the existence of vertical exchange spring spin alignment in thick NiO. More interestingly, the Néel temperature of the CoO layer was greatly enhanced by the adjacent NiO layer, with the extent of the enhancement closely dependent on the spin orientation of NiO layer. This phenomenon was attributed to different exchange coupling strengths at the AFM/AFM interface depending on the relative spin directions. Our results indicate a new route for modifying the spin configuration and ordering temperature of AFMs through the magnetic proximity effect near room temperature, which should further benefit the design of AFM spintronic devices.
The electrical control of vortex polarity and chirality is in high demand for spintronic oscillators, memory, and computing. Previous studies have shown that vortex polarity can be efficiently switched by electrical current, while chirality is not. Here the authors report that the dynamic transformation of edge solitons is able to switch both vortex polarity $a\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}d$ $c\phantom{\rule{0}{0ex}}h\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}y$, at low current and high speed. This work not only provides insight into the fundamental aspects of magnetic edge solitons, but also offers an efficient means to control vortex properties for low-power memory and computing applications.
The recent discovery of bulk spin–orbit torques (SOTs) in magnetic single layers has attracted much recent attention. However, currently, it remains elusive as to how to understand and how to tune such bulk SOTs. In this study, we study the tunability of the bulk SOTs in the CoPt films, by the annealing temperature. Our results show that the field-free switching can be realized after annealing and optimized at 450 °C. The switching performance is consistent with the out-of-plane SOT efficiency, which also appears after annealing and maximized at 450 °C. The crystal-axis dependence of the switching performance reveals that besides the threefold modulation by the crystal-axis, the switching ratio also contains a contribution that is independent of the crystal-axis, which is different from that in single-crystal films. Our results can help the understanding of the mechanism of SOT and point to the developing of SOT devices.
This spin-stand measurement study focuses on recording characteristics at submicron scale track width. The pole tips of a set of identical thin film heads were trimmed from the air-bearing surface by focused ion beam etching. A set of thin film heads with track widths ranging from W=2 μm to W=0.5 μm were produced. Recording experiments were performed on a high precision spin-stand tester using these heads. Both on-track and off-track performances were studied and analyzed. As the track width is scaled down, degrading of recording performance is observed. When the width of a recording head is decreased, the onset of partial erasure occurs at a lower density, and the noise power per unit head track width increases. Further investigation on the track profiles reveals that the extent of partial erasure is higher at the track edge as density increases, and this phenomenon is more pronounced in narrower track width heads.
Summary form only given. The interface magnetic phenomenon, which forms the backbone of modern information technology, is of great interest for several decades. When two or more dissimilar materials with different long-range magnetic orderings and/or functionalities are combined together, it may give rise to new interfacial properties, such as the proximity effect. The exchange coupling between two FM layers, as well as that between a FM layer and an AFM layer, has been widely studied before, but the exchange coupling between the AFM spins in two AFM layers has not been paid enough attention due to the technique difficulty to directly measure the AFM properties in AFM layers. In this report, antiferromagnetic proximity effect was systematically studied in epitaxial CoO/NiO/ MgO(001) system with x-ray linear dichroism (XMLD) and magneto-optic Kerr effect (MOKE) measurement. NiO AFM spin undergoes a spin reorientation transition from in-plane to out-of-plane or spin canting with increasing NiO thickness owning to the competition between the strain-induced out-of-plane anisotropy of the NiO AFM spin and the AFM interfacial exchange coupling driven by the in-plane CoO AFM spins. The Néel temperatures of CoO layer and NiO layer can be determined directly by temperature dependent XMLD effect. We found the Néel temperature of CoO layer could be greatly enhanced by the adjacent NiO layer, but the enhancement closely depends on the spin orientation of adjacent NiO AFM. The T N of the 2nm single CoO layer is ~220K, and increases to ~400K while grown on 2nm NiO layer with the in-plane anisotropy, then CoO's TN decreases to ~300K while grown on 12nm NiO layer with the out-of-plane anisotropy. Moreover, we found the Néel temperatures of CoO layer strongly depends on the CoO thickness, and the T N of top CoO interface decreases when it is away from CoO/NiO interface. Through the CoO-thickness-dependent T N measurement, we can estimated the antiferromagnetic correlation length, which is ~2 .2nm. Our results give clear evidence that magnetic proximity effect between antiferromagnetic bilayers could influence the spin direction and the ordering temperature.
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