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We demonstrate the interplay of pure spin current, spin-polarized current, and spin fluctuation in 3d Ni_{x}Cu_{1-x}. By tuning the compositions of the Ni_{x}Cu_{1-x} alloys, we separate the effects due to the pure spin current and spin-polarized current. By exploiting the interaction of spin current with spin fluctuation in suitable Ni-Cu alloys, we obtain an unprecedentedly high spin Hall angle of 46%, about 5 times larger than that in Pt, at room temperature. Furthermore, we show that spin-dependent thermal transport via anomalous Nernst effect can serve as a sensitive magnetometer to electrically probe the magnetic phase transitions in thin films with in-plane anisotropy. The enhancement of spin Hall angle by exploiting spin current fluctuation via composition control makes 3d magnets functional materials in charge-to-spin conversion for spintronic application.

The demagnetizing field associated with shape anisotropy originates from the magnetic field generated through magnetization of magnetic materials. The shape and demagnetizing effects must be considered as characteristics of a magnetic object in the presence of a magnetic field. In this study, we demonstrated the strong influence of the demagnetizing field on the spin current transports in Pt/YIG, including the spin Seebeck effect, spin Hall effect, spin Hall magnetoresistance, and planar Hall resistance. The calculated effective demagnetizing factors were closely related to the thickness- and width-dependent anomalous plateau behaviors of the spin-dependent electrical and thermal transports as well as magneto-optical measurements. Moreover, we demonstrated that the width of the plateau manifested from the shape of the YIG sample. Furthermore, through magnetic force microscopy, we directly visualized the evolution of the magnetic domains with an abrupt 90\ifmmode^\circ\else\textdegree\fi{} magnetic rotation, which we further corroborated by the 90\ifmmode^\circ\else\textdegree\fi{} resistance phase shift in the angular-dependent planar Hall effect.

The authors demonstrate the magnetization dependent spin Hall effect where the spin polarization of pure spin current can be additionally manipulated by the magnetization.

In this study, we used the thermally driven spin current to investigate the spin frustrations and spin fluctuations in spin-glass (SG) $\mathrm{C}{\mathrm{u}}_{1\ensuremath{-}x}\mathrm{M}{\mathrm{n}}_{x}$ alloys. Tuning the $\mathrm{C}{\mathrm{u}}_{1\ensuremath{-}x}\mathrm{M}{\mathrm{n}}_{x}$ composition results in a transition of the alloys from the SG state to the antiferromagnetic state; these states have different spin-freezing temperatures $({T}_{f})$. Most spins randomly freeze at temperatures lower than the ${T}_{f}$ of the alloy. For each alloy composition, we obtained a temperature-dependent inverse spin Hall voltage with a peak at ${T}_{p}$. Crucially, ${T}_{p}$ had nearly identical composition dependence as that of ${T}_{f},$, with ${T}_{p}$ being nine times larger than ${T}_{f}$. Similar behavior was captured using the SG insulator, amorphous ${\mathrm{Y}}_{3}\mathrm{F}{\mathrm{e}}_{5}{\mathrm{O}}_{12}$. These results indicated that the maximum spin fluctuation in both conducting and insulating SGs occurred at temperatures considerably higher than the ${T}_{f}$ of each. In addition, we demonstrated the importance of the effective number of valence electrons in tailoring the spin Hall angle in binary alloys.

Much theoretical and experimental attention has been focused on the electrical switching of the antiferromagnetic (AFM) Néel vector via spin-orbit torque. Measurements employing multiterminal patterned structures of Pt/AFM show recurring signals of the supposedly planar Hall effect and magnetoresistance, implying AFM switching. We show in this Letter that similar signals have been observed in structures with and without the AFM layer, and of an even larger magnitude using different metals and substrates. These may not be the conclusive evidence of spin-orbit torque switching of AFM, but the thermal artifacts of patterned metal structure on substrate. Large current densities in the metallic devices, beyond the Ohmic regime, can generate unintended anisotropic thermal gradients and voltages. AFM switching requires unequivocal detection of the AFM Néel vector before and after SOT switching.

Perpendicular magnetization switching induced by spin-orbit torque (SOT) continues to attract great attention as a promising writing method for ultrafast, high-density, energy-efficient spintronic devices. Unfortunately, this otherwise very attractive switching scheme often comes with an unfavorable external magnetic field. Here the authors achieve polarity-controlled, $f\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}d\ensuremath{-}f\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}e$ SOT switching via oriented columnar microstructures in sputtered films. It is demonstrated that Cr---a $3d$, not $4d$, metal---can induce strong perpendicular magnetic anisotropy and generate large spin current to deliver SOT. These findings point to highly efficient, nonvolatile SOT spintronic devices.

Ferromagnetic resonance (FMR) in yttrium iron garnet (YIG)/Pt with a suitable YIG thickness in the submillimeter range show pure spin current contributions from the uniform FMR (UFMR) mode and various spin-wave resonance (SWR) modes, each with a different absorption intensity, inverse spin Hall voltage, and temperature dependence. The higher order mode of SWR with nonzero moment transferring along the thickness direction induces a maximum dc voltage among all modes. The spin current driven by UFMR and SWR has a linear $T$ and power-law $T$ dependence, respectively. In addition, the mixing conductance is derived for both modes, indicative of power-law $T$ dependences. There is an absence of evidence for coherent spin pumping in YIG/Pt.

Thermal generation of magnonic spin current by the spin Seebeck effect (SSE) and spin-polarized current by the anomalous Nernst effect (ANE) are fascinating phenomena in the field of spin caloritronics. In this paper, we investigate the thermal spin current driven by light in the framework of the SSE and the ANE. Unlike heating by electrical means, the spin current can be enhanced or reduced, and its sign can even be reversed, thanks to the intrinsic characteristic of the penetrating light. Most importantly, by reversing the direction of the incident light, we are able to separate the contributions from the interfacial and bulk temperature gradients to the transverse spin accumulation. Notably, although the overall spin signals are strongly influenced by the frequency of light, the interfacial and bulk spin Seebeck coefficients are intrinsic and frequency independent. Furthermore, with light excitation, we observe a nearly instantaneous steplike response for both the SSE and the ANE, highlighting the importance of spin-charge conversion by spin-orbit coupling for ultrafast spin caloritronics.

Gold (Au) and tantalum (Ta) metals are two important spin Hall materials with diametrically different crystalline structure, resistivity, and spin Hall angle. By vapor quenching, $\mathrm{A}{\mathrm{u}}_{x}\mathrm{T}{\mathrm{a}}_{1\ensuremath{-}x}$ alloys of different crystalline and amorphous structures, stable and metastable, have been formed over the entire composition range. We have studied the inverse spin Hall voltage induced by the longitudinal spin Seebeck effect in $\mathrm{A}{\mathrm{u}}_{x}\mathrm{T}{\mathrm{a}}_{1\ensuremath{-}x}$ alloy films. We have found that the inverse spin Hall voltage $\mathrm{\ensuremath{\Delta}}{V}_{\mathrm{ISHE}}$, resistivity $\ensuremath{\rho}$, and the spin Hall angle ${\ensuremath{\theta}}_{SH}$ vary smoothly with composition. Both the spin Hall angle and the spin Hall conductivity exhibit a quasilinear dependence on composition in the concentrated alloy regime, despite drastic differences in other properties. As corroborated by first-principles calculation, we show that the contribution to spin Hall effect in $\mathrm{A}{\mathrm{u}}_{x}\mathrm{T}{\mathrm{a}}_{1\ensuremath{-}x}$ alloy is dominated by the number of valence electrons. We also show that the magnetoresistance correlates with the spin Hall angle.

The anomalous Nernst effect (ANE) is one of the most important mechanisms to explore the anomalous Hall heat current in ferromagnets. In this work, we studied the ANE in various ferromagnetic materials with in-plane anisotropy. Surprisingly, we show that the thickness dependence of the ANE on the magnitude and sign is nontrivial, even in conventional ferromagnetic metals (FMs), including Fe, Co, Ni, and Py ($\mathrm{N}{\mathrm{i}}_{80}\mathrm{F}{\mathrm{e}}_{20}$). While the sign of the ANE of Fe is opposite to that of Co, Ni, and Py in thicker films, it can even be reversed via decreasing thickness. Most importantly, the anomalous Nernst angles ${\ensuremath{\theta}}_{\mathrm{ANE}}$ for these FMs show a unified behavior. They can be significantly enhanced by up to one order of magnitude in ultrathin films. By systematically studying the thickness dependence of the electrical and thermal transport properties, we show that the enhanced ANE of FMs is dominated by spin-orbit coupling through the intrinsic and side-jump mechanisms.

High-entropy alloys (HEAs) with three to five nearly equimolar components have garnered interest in the past few years, due to their great material strength and tremendously tunable composition. Most work so far has focused on mechanical and structural properties rather than electrical transport, but recent studies indicate that HEAs possess unexpected superconductivity and magnetism. The authors' experiments demonstrate that amorphous Ta-Nb-Hf-Zr-Ti can provide a sizable spin Hall effect, and supports magnetization switching via the induced spin-orbit torque, making it an interesting candidate for spintronic memory devices.

Spin pumping (SP) and the spin Seebeck effect (SSE) are widely used to generate a spin-wave spin current from ferromagnetic insulators. In this work, the authors show that while SP is significantly reduced in a polycrystalline yttrium iron garnet (YIG), the SSE is insensitive to the crystal structure. This discovery not only offers new insights into the mechanisms between the coherently driven SP and the noncoherently excited SSE but also demonstrates that the robust spin current generated by the SSE is a uniquely powerful tool to study the physics of the pure spin current in spintronics.

The longitudinal spin Seebeck effect (LSSE) with a vertical temperature gradient is one of the most important mechanisms to generate pure spin current. Previous studies of the LSSE excited spin current focus mainly on the magnetic insulators, a little on ferromagnetic metals, and rarely on ferromagnetic half metals. In this work, we demonstrate a significant spin current injected from the highly spin polarized ferromagnetic half metal $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ by the LSSE. The sign of the thermal voltage can be reversed by using the spin current detector Cr with a large negative spin Hall angle. The ratio of the inverse spin Hall voltage to the total thermal signal in $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ is much larger than that in ferromagnetic metals, such as permalloy and CoFeB. The nontrivial temperature-dependent voltage suggests that the thermal transport in $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ is carried by magnons. This study provides insight into the mechanism of thermally excited spin current in ferromagnetic half metals and recommends the highly spin polarized $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ as a promising candidate for metal-based spin caloritronics devices.

The anomalous Nernst effect (ANE) and the spin Seebeck effect (SSE) in spin caloritronics are two of the most important mechanisms to manipulate the spin-polarized current and pure spin current by thermal excitation. While the ANE in ferromagnetic metals and the SSE in magnetic insulators have been extensively studied, a recent theoretical work suggests that the signals from the thermal Hall effect (THE) have field dependences indistinguishable from, and may even overwhelm, those of the ANE and SSE. Therefore, it is vital to investigate the contribution of the THE in the ANE and SSE. In this work, we systematically study the THE in a ferromagnetic metal, Permalloy (Py), and magnetic insulator, an yttrium iron garnet (YIG), by using different Seebeck coefficients between electrodes and contact wires. Our results demonstrate that the contribution of the THE by the thermal couple effect in the Py and YIG is negligibly small if one includes the thickness dependence of the Seebeck coefficient. Thus, the spin-polarized current in the ANE and the pure spin current in the SSE remain indispensable for exploring spin caloritronics phenomena.

Upon the injection of a pure spin current, a ferromagnet, similar to a nonmagnetic metal, also exhibits inverse spin Hall effect (ISHE). We show in Co/Cu/YIG, where the thin Cu layer allows transmission of spin current from YIG into Co but decouples the two ferromagnets, that the interaction between ISHE and ferromagnetic ordering in Co can be unambiguously investigated. By switching on and off the pure spin current contribution, we demonstrate that the ISHE in Co is independent of the direction of the Co magnetization, which clearly suggests that the ISHE in Co is dominated not by the extrinsic impurity scatterings, but from the intrinsic origin.

Yttrium iron garnet (YIG, ${\mathrm{Y}}_{3}\mathrm{F}{\mathrm{e}}_{5}{\mathrm{O}}_{12}$) is a magnetic insulator that has been widely used to generate spin-wave spin current via the longitudinal spin Seebeck effect. Spin current can be converted to charge current by the inverse spin Hall effect (ISHE) in an attached metal layer with a spin direction given by the magnetization of the YIG. However, both the ISHE voltage from the thermal transport measurement and the magnetoresistance (MR) from the electrical transport measurement of the metal/YIG structure show a clear plateau behavior in the low-field range, which is inconsistent with the magnetization reversal behavior of the YIG slab. In this work, we provide direct evidences by using the highly sensitive micro-magneto-optic Kerr effect (micro-MOKE) measurement to demonstrate that the plateau behavior in the thermal and electrical transport measurement of metal/YIG is due to the noncollinear magnetization configuration between the bulk and surface of YIG. In addition, keeping the measured surface of YIG unaltered, we show that its surface magnetization can be systematically controlled by varying the thickness. We further demonstrate that the magnetic coupling between the surface magnetization of YIG and an attached ferromagnetic layer exhibits long-range interaction due to the magnetic dipole-dipole interaction.

A new type of magnetoresistance (MR) observed in $\mathrm{Pt}/\mathrm{YIG}$ when nominally nonmagnetic Pt comes in contact with a ferrimagnetic insulator yttrium iron garnet (YIG) has drawn intense experimental and theoretical interest. In this Letter, we experimentally demonstrate two physical origins of the new MR: a spin current across the $\mathrm{Pt}/\mathrm{YIG}$ interface and the magnetic proximity effect. The new MR can also be reproduced when Pt is in contact with a nonmagnetic insulator doped with a few percent of Fe impurities. By tuning the YIG surface and inserting an Au layer between the Pt and YIG, we are able to separate the two contributions.

We report room temperature electrical detection of spin injection from a ferromagnetic insulator (YIG) into a ferromagnetic metal (Permalloy, Py). Non-equilibrium spins with both static and precessional spin polarizations are dynamically generated by the ferromagnetic resonance of YIG magnetization, and electrically detected by Py as dc and ac spin currents, respectively. The dc spin current is electrically detected via the inverse spin Hall effect of Py, while the ac spin current is converted to a dc voltage via the spin rectification effect of Py which is resonantly enhanced by dynamic exchange interaction between the ac spin current and the Py magnetization. Our results reveal a new path for developing insulator spintronics, which is distinct from the prevalent but controversial approach of using Pt as the spin current detector., 5 pages, 4 figures

We demonstrate a self-consistent and versatile method to determine the relative spin Hall angle ${\ensuremath{\theta}}_{\text{SH}}$ and spin diffusion length ${\ensuremath{\lambda}}_{\text{SF}}$ of selected $5d$ metals (Pt, Au, Ta, and W) by thermal spin injection from a common ferromagnetic insulator, yttrium iron garnet, into thin metal films of various thicknesses. The values of ${\ensuremath{\theta}}_{\text{SH}}$ for Pt and Au are opposite in sign to those of Ta and W. The inverse spin Hall effect voltage depends on spin Hall resistivity, which is the product of ${\ensuremath{\theta}}_{\text{SH}}$ and electrical resistivity. We also show evidence of magnetic proximity effects including magnetoresistance and the anomalous Hall effect in these $5d$ metals, except for Au.

We report a new magnetoresistance (MR) effect observed in a nominally nonmagnetic metal (Pt) thin film in contact with either a ferromagnetic insulator or a ferromagnetic metal. The resistivities with in-plane magnetic fields parallel (rho(parallel to)) and transverse (rho(T)) to a current and a perpendicular field (rho(perpendicular to)) show the behavior of rho(perpendicular to) approximate to rho(parallel to) > rho(T), distinctly different from all other known MR effects, including anisotropic MR with rho(parallel to) > rho(T) approximate to rho(perpendicular to). We termed the new MR as hybrid MR, which appears in a metal in close proximity with a ferromagnet either insulating or metallic, and is associated with the induced magnetic moments at the interface.

Thin Pt films on an yttrium iron garnet (YIG = Y(3)Fe(5)O(12)) show ferromagneticlike transport properties, which may impact the functionality of Pt in spin current detection, but do not provide direct quantitative information on the Pt magnetization. We report magnetic x-ray magnetic circular dichroism measurements of YIG/Pt(1.5 nm) showing an average Pt moment of 0.054 μ(B) at 300 K and 0.076 μ(B) at 20 K. This observation indicates strong proximity effects and induced magnetic ordering in Pt on magnetic insulators and their contribution to the spin-related measurements should not be neglected. The transport characteristics also suggest considerable modifications in the Pt electronic structure due to magnetic ordering.

The acute magnetic proximity effects in Pt/YIG compromise the suitability of Pt as a spin current detector. We show that Au/YIG, with no anomalous Hall effect and a negligible magnetoresistance, allows the measurements of the intrinsic spin Seebeck effect with a magnitude much smaller than that in Pt/YIG. The experiment results are consistent with the spin-polarized density-functional calculations for Pt with a sizable and Au with a negligible magnetic moment near the interface with YIG., 15 pages, 4 figures

A theoretical model for the electron-hole exchange interaction in three-dimensionally (3D) confining semiconductor nanostructures is presented to explain the observed decreasing tendency of the fine-structure splittings (FSSs) of small InGaAs/GaAs self-assembled quantum dots (QDs) with increasing the emission energies. The experimentally revealed FSS reduction is shown to be highly associated with the significant 3D spreading of electronic orbitals and reduced overlap of electron and hole wave functions in small and/or Ga-diffused QDs. The combination of quantum size and Ga-diffusion effects substantially reduces the averaged $e\text{\ensuremath{-}}h$ exchange interaction and leads to the reduced FSSs in the regime of high emission energy.