The current-nonlinear Hall effect or second harmonic Hall voltage is widely used as one of the methods for estimating charge-spin conversion efficiency, which is attributed to the magnetization ...oscillation by spin-orbit torque (SOT). Here, we argue the second harmonic Hall voltage under a large in-plane magnetic field with an in-plane magnetization configuration 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}, where it is clearly shown that the large second harmonic voltage is governed not by SOT but mainly by asymmetric magnon scattering without macroscopic magnetization oscillation. Thus, this method does not allow an accurate estimation of charge-spin conversion efficiency in TI. Instead, the SOT contribution is exemplified by current pulse induced nonvolatile magnetization switching, which is realized with a current density of 2.5×10^{10} A m^{-2}, showing its potential as a spintronic material.
Magnetic skyrmions, topological solitons characterized by a two-dimensional swirling spin texture, have recently attracted attention as stable particle-like objects. In a three-dimensional system, a ...skyrmion can extend in the third dimension forming a robust and flexible string structure, whose unique topology and symmetry are anticipated to host nontrivial functional responses. Here we experimentally demonstrate the coherent propagation of spin excitations along skyrmion strings for the chiral-lattice magnet Cu
OSeO
. We find that this propagation is directionally non-reciprocal and the degree of non-reciprocity, as well as group velocity and decay length, are strongly dependent on the character of the excitation modes. These spin excitations can propagate over a distance exceeding 50 μm, demonstrating the excellent long-range ordered nature of the skyrmion-string structure. Our combined experimental and theoretical analyses offer a comprehensive account of the propagation dynamics of skyrmion-string excitations and suggest the possibility of unidirectional information transfer along such topologically protected strings.
Spin-momentum locking in the Dirac surface state of a topological insulator (TI)1, 2, 3, 4, 5, 6 offers a distinct possibility for highly efficient charge-to-spin current (C-S) conversion compared ...with spin Hall effects in conventional paramagnetic metals7, 8, 9, 10, 11, 12, 13. For the development of TI-based spin current devices, it is essential to evaluate this conversion efficiency quantitatively as a function of the Fermi level position EF. Here we introduce a coefficient qICS to characterize the interface C-S conversion effect by means of the spin torque ferromagnetic resonance (ST-FMR) for (Bi1-xSbx)2Te3 thin films as EF is tuned across the bandgap. In bulk insulating conditions, the interface C-S conversion effect via the Dirac surface state is evaluated as having large, nearly constant values of qICS, reflecting that qICS is inversely proportional to the Fermi velocity vF, which is almost constant. However, when EF traverses through the Dirac point, the qICS is remarkably reduced, possibly due to inhomogeneity of kF and/or instability of the helical spin structure. These results demonstrate that fine tuning of EF in TI-based heterostructures is critical in maximizing the efficiency using the spin-momentum locking mechanism.
The spin transfer torque is essential for electrical magnetization switching. When a magnetic domain wall is driven by an electric current through an adiabatic spin torque, the theory predicts a ...threshold current even for a perfect wire without any extrinsic pinning. The experimental confirmation of this 'intrinsic pinning', however, has long been missing. Here, we give evidence that this intrinsic pinning determines the threshold, and thus that the adiabatic spin torque dominates the domain wall motion in a perpendicularly magnetized Co/Ni nanowire. The intrinsic nature manifests itself both in the field-independent threshold current and in the presence of its minimum on tuning the wire width. The demonstrated domain wall motion purely due to the adiabatic spin torque will serve to achieve robust operation and low energy consumption in spintronic devices.
In magnetic materials with chiral crystal structure, it has been predicted that quasiparticle flows propagating parallel and antiparallel to the external magnetic field can show different propagating ...character, with its sign of nonreciprocity dependent on the chirality of the underlying bulk crystal lattice. This unique phenomenon, termed magnetochiral nonreciprocity, has previously been demonstrated for the propagating light and conduction electrons but seldom for other quasiparticles. In this study, we report the experimental observation of magnetochiral nonreciprocity of propagating magnons for a chiral-lattice ferromagnet Cu sub(2) OSeO sub(3) by employing the spin wave spectroscopy. We found that the sign of nonreciprocity is reversed for the opposite chirality of crystal, and also directly identified the wave-number-linear term in the spin wave dispersion associated with the Dzyaloshinskii-Moriya (DM) interaction as the origin of observed nonreciprocity. Our present results pave a route for the design of efficient spin wave diode based on the bulk crystallographic symmetry breaking and also offer a unique method to evaluate the magnitude of DM interaction in chiral-lattice bulk compounds.
The antiferromagnetic Weyl semimetal Mn3Sn has been shown to generate strong intrinsic anomalous Hall effect (AHE) at room temperature, due to large momentum-space Berry curvature from the ...time-reversal symmetry-breaking electronic bands of the Kagome planes. This prompts us to investigate intrinsic spin Hall effect, a transverse phenomenon with identical origin as the intrinsic AHE. We report inverse spin Hall effect experiments in nanocrystalline Mn3Sn nanowires at room temperature using spin absorption method, which enables us to quantitatively derive both the spin diffusion length and the spin Hall angle in the same device. We observed clear absorption of the spin current in the Mn3Sn nanowires when kept in contact with the spin transport channel of a lateral spin-valve device. We estimate spin diffusion length λs(Mn3Sn)~0.75 ± 0.67 nm from the comparison of spin signal of an identical reference lateral spin valve without Mn3Sn nanowire. From inverse spin Hall measurements, we evaluate spin Hall angle θSH~5.3 ± 2.4 % and spin Hall conductivity σSH~46.99 ± 20.63 ℏe(Ωcm)−1. The estimated spin Hall conductivity agrees with both in sign and magnitude to the theoretically predicted intrinsic σSHint~36-96 (ℏ/e) (Ωcm)−1. We also observed anomalous Hall effect at room temperature in nano-Hall bars prepared at the same time as the spin Hall devices. Large anomalous Hall conductivity along with adequate spin Hall conductivity makes Mn3Sn a promising material for ultrafast and ultrahigh-density spintronics devices.
Thermal properties of submicrometer wires of Pd-doped FeRh alloys are evaluated from measurements of transport properties around the first-order antiferromagnetic to ferromagnetic phase transition. ...Recently, in a submicrometer wire of FeRh alloy, an asymmetric structure between heating and cooling processes was reported in the vicinity of the phase transition temperature. A system where finite-size effects appear is expected to show different magnetic and thermal properties from those in bulk and thin film. However, thermal or magnetization measurements are difficult to perform on samples with such a tiny volume. Here, we demonstrate a quantitative evaluation of thermal properties in thin films and submicrometer wires of B2-ordered Pd-doped FeRh alloy grown on MgO (001). Resistivity measurements on a series of wires with different widths reveal that the anomaly in the resistance change is enhanced in narrow wires. As the transport properties in submicrometer wires sensitively reflect those magnetic states, the entropy change and the irreversible energy loss during the first-order phase transition have been determined via resistance measurements. We find a larger energy loss in smaller samples where wider hysteresis loops appear in temperature-driven measurements. This method can be a probe to evaluate the finite-size effect induced by a restricted magnetic domain nucleation process during the phase transition.
We have investigated spin relaxation and spin-to-charge conversion at the Cu surface decorated with lead (II) phthalocyanine (PbPc) molecules. The study on the frequency dependence of ferromagnetic ...resonance linewidth revealed that the Cu/PbPc interface exhibits more pronounced spin relaxation than that of the Cu/H 2 Pc (metal-free phthalocyanine) interface. A clear peak owing to the spin-to-charge conversion was observed in the spin-pumping-induced voltage spectrum for a NiFe/Cu/PbPc trilayer film, whereas no trace of peak was observed in the film with NiFe/Cu/H 2 Pc. These results might be attributed to the Rashba effect at Cu surface by the decoration with PbPc, which indicates the new possibility of molecular spintronics.
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