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Register a new userSpin-torque effect on spin wave modes in magnetic nanowires
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Authors
Voicu O. Dolocan
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The interaction between a spin polarized dc electrical current and spin wave modes of a cylindrical nanowire is investigated in this report. We found that close to the critical current, the uniform mode is suppressed, while the edge mode starts to propagate into the sample. When the current exceeds the critical value, this phenomenon is even more accentuated. The edge mode becomes the uniform mode of the nanowire. The higher spin wave modes are slowly pushed away by the current until the propagating mode remains.
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Spin-orbit effects [1-4] have the potential of radically changing the field of spintronics by allowing transfer of spin angular momentum to a whole new class of materials. In a seminal letter to Nature [5], Kajiwara et al. showed that by depositing Platinum (Pt, a normal metal) on top of a 1.3 $mu$m thick Yttrium Iron Garnet (YIG, a magnetic insulator), one could effectively transfer spin angular momentum through the interface between these two different materials. The outstanding feature was the detection of auto-oscillation of the YIG when enough dc current was passed in the Pt. This finding has created a great excitement in the community for two reasons: first, one could control electronically the damping of insulators, which can offer improved properties compared to metals, and here YIG has the lowest damping known in nature; second, the damping compensation could be achieved on very large objects, a particularly relevant point for the field of magnonics [6,7] whose aim is to use spin-waves as carriers of information. However, the degree of coherence of the observed auto-oscillations has not been addressed in ref. [5]. In this work, we emphasize the key role of quasi-degenerate spin-wave modes, which increase the threshold current. This requires to reduce both the thickness and lateral size in order to reach full damping compensation [8] , and we show clear evidence of coherent spin-orbit torque induced auto-oscillation in micron-sized YIG discs of thickness 20 nm.
We report a giant spin Hall effect (SHE) in {beta}-Ta that generates spin currents intense enough to induce efficient spin-transfer-torque switching of ferromagnets, thereby providing a new approach for controlling magnetic devices that can be superior to existing technologies. We quantify this SHE by three independent methods and demonstrate spin-torque (ST) switching of both out-of-plane and in-plane magnetized layers. We implement a three-terminal device that utilizes current passing through a low impedance Ta-ferromagnet bilayer to effect switching of a nanomagnet, with a higher-impedance magnetic tunnel junction for read-out. The efficiency and reliability of this device, together with its simplicity of fabrication, suggest that this three-terminal SHE-ST design can eliminate the main obstacles currently impeding the development of magnetic memory and non-volatile spin logic technologies.
The spin-orbit torque induced by a topological insulator (TI) is theoretically examined for spin wave generation in a neighboring antiferromagnetic thin film. The investigation is based on the micromagnetic simulation of N{e}el vector dynamics and the analysis of transport properties in the TI. The results clearly illustrate that propagating spin waves can be achieved in the antiferromagnetic thin-film strip through localized excitation, traveling over a long distance. The oscillation amplitude gradually decays due to the non-zero damping as the N{e}el vector precesses around the magnetic easy axis with a fixed frequency. The frequency is also found to be tunable via the strength of the driving electrical current density. While both the bulk and the surface states of the TI contribute to induce the effective torque, the calculation indicates that the surface current plays a dominant role over the bulk counterpart except in the heavily degenerate cases. Compared to the more commonly applied heavy metals, the use of a TI can substantially reduce the threshold current density to overcome the magnetic anisotropy, making it an efficient choice for spin wave generation. The N{e}el vector dynamics in the nano-oscillator geometry are examined as well.
Current-induced spin-transfer torques (STT) and spin-orbit torques (SOT) enable the electrical switching of magnetic tunnel junctions (MTJs) in nonvolatile magnetic random access memories. In order to develop faster memory devices, an improvement of the timescales underlying the current driven magnetization dynamics is required. Here we report all-electrical time-resolved measurements of magnetization reversal driven by SOT in a three-terminal MTJ device. Single-shot measurements of the MTJ resistance during current injection reveal that SOT switching involves a stochastic two-step process consisting of a domain nucleation time and propagation time, which have different genesis, timescales, and statistical distributions compared to STT switching. We further show that the combination of SOT, STT, and voltage control of magnetic anisotropy (VCMA) leads to reproducible sub-ns switching with a spread of the cumulative switching time smaller than 0.2 ns. Our measurements unravel the combined impact of SOT, STT, and VCMA in determining the switching speed and efficiency of MTJ devices.
Precise estimation of spin Hall angle as well as successful maximization of spin-orbit torque (SOT) form a basis of electronic control of magnetic properties with spintronic functionality. Until now, current-nonlinear Hall effect, or second harmonic Hall voltage has been utilized as one of the methods for estimating spin Hall angle, which is attributed to the magnetization oscillation by SOT. Here, we argue the second harmonic Hall voltage in magnetic/nonmagnetic topological insulator (TI) heterostructures, Cr$_x$(Bi$_{1-y}$Sb$_y$)$_{2-x}$Te$_3$/(Bi$_{1-y}$Sb$_y$)$_2$Te$_3$. From the angular, temperature and magnetic field dependence, it is unambiguously shown that the large second harmonic Hall voltage in TI heterostructures is governed not by SOT but mainly by asymmetric magnon scattering mechanism without magnetization oscillation. Thus, this method does not allow an accurate estimation of spin Hall angle when magnons largely contribute to electron scattering. Instead, the SOT contribution in a TI heterostructure is exemplified by current pulse induced non-volatile magnetization switching, which is realized with a current density of $sim 2.5 times 10^{10} mathrm{A/m}^2$, showing its potential as spintronic materials.
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