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Spintronic phenomena to date have been established in magnets with collinear moments, where the spin injection through the spin Seebeck effect (SSE) is always along the out-of-plane direction. Here, we report the observation of a vector SSE in a noncollinear antiferromagnet (AF) LuFeO_{3}, where temperature gradient along the out-of-plane and also the in-plane directions can both inject a pure spin current and generate a voltage in the heavy metal via the inverse spin Hall effect (ISHE). We show that the thermovoltages are due to the magnetization from canted spins in LuFeO_{3}. Furthermore, in contrast to the challenges of generating, manipulating, and detecting spin current in collinear AFs, the vector SSE in LuFeO_{3} is readily viable in zero magnetic field and can be controlled by a small magnetic field of about 150 Oe at room temperature. The noncollinear AFs expand new realms for exploring spin phenomena and provide a new route to low-field antiferromagnetic spin caloritronics and magnonics.

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.

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.

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.

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.

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.

The inverse spin Hall effect (ISHE) has been observed only in nonmagnetic metals, such as Pt and Au, with a strong spin-orbit coupling. We report the observation of ISHE in a ferromagnetic permalloy (Py) on ferromagnetic insulator yttrium iron garnet (YIG). Through controlling the spin current injection by altering the Py-YIG interface, we have isolated the spin current contribution and demonstrated the ISHE in a ferromagnetic metal, the reciprocal phenomenon of the anomalous Hall effect. A large spin Hall angle in Py, determined from Py thin films of different thicknesses, indicates many other ferromagnetic metals may be exploited as superior pure spin current detectors and for applications in spin current.

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

The Pt/YIG structure has been widely used to study spin Seebeck effect (SSE), inverse spin Hall effect, and other pure spin current phenomena. However, the magnetic proximity effect in Pt when in contact with YIG, and the potential anomalous Nernst effect (ANE) may compromise the spin current phenomena in Pt/YIG. By inserting a Cu layer of various thicknesses between Pt and YIG, we have separated the signals from the SSE and that of the ANE. It is demonstrated that the thermal voltage in Pt/YIG mainly comes from spin current due to the longitudinal SSE with negligible contribution from the ANE.

The thickness and magnetic field dependence of resistance in Co/Nb multilayers with current perpendicular to plane geometry were measured at 4.2 K. When the number of bilayers increased, the resistance increased linearly. This linear behavior enables us to use a series resistance model to analyze our data for the thicknesses we used, 20 and 80 nm for Nb. When sandwiched between Co, Nb is a normal metal when thinner than 30 nm, otherwise it is superconducting. Assuming bulk resistivities for normal Nb and Co, we got unit area resistance values of 6.3 and 3.5 fΩ m2 for superconducting and normal Nb/Co interfaces. There was no magnetoresistance observed, at a resolution of about 0.1%. The applicability of a diffusive regime model is discussed.

Platinum (Pt) metal, being nonmagnetic and with a strong spin-orbit coupling interaction, has been central in detecting the pure spin current and establishing most of the recent spin-based phenomena. Magnetotransport measurements, both electrical and thermal, conclusively show strong ferromagnetic characteristics in thin Pt films on the ferromagnetic insulator due to the magnetic proximity effects. The pure spin current phenomena measured by Pt, including the inverse spin Hall and the spin Seebeck effects, are thus contaminated and not exclusively established.

Most studies of spin caloritronic effects to date, including spin-Seebeck effect, utilize thin films on substrates. We use patterned ferromagnetic thin film to demonstrate the profound effect of a substrate on the spin-dependent thermal transport. With different sample patterns and on varying the direction of temperature gradient, both longitudinal and transverse thermal voltages exhibit asymmetric instead of symmetric spin dependence. This unexpected behavior is due to an out-of-plane temperature gradient imposed by the thermal conduction through the substrate and the mixture of anomalous Nernst effects. Only with substrate-free samples have we determined the intrinsic spin-dependent thermal transport with characteristics and field sensitivity similar to those of the anisotropic magnetoresistance effect.

A series of NbTi single thin films, Co/NbTi/Co trilayers, and multilayers are made by dc sputtering to investigate the superconductive dimensional crossover between two-dimensions (2D) and three dimensions. From the temperature dependence of the upper critical fields measured resistively, we see a nonlinear changing to linear dependence of Hc2(T) from 60 to 80 nm of NbTi thickness for trilayers. Insertion of 0.6 nm thick Co between 30 nm NbTi films also shows 2D behavior, which suggests confinement of the superconducting wave function within individual layers. Magnetic field response is measured to study the interplay between the superconductor and ferromagnet. We inferred a magnetic penetration depth of ∼35 nm into NbTi.

In this study, we showed that a magnetic coercivity (Hc) as high as 37 kOe was obtained in a CoPt thin film that contains separated nanometer-size CoPt crystallites. We prepared CoPt thin films with thicknesses of 5 and 175 nm by magnetron sputtering. After annealing in an Ar/H2 atmosphere at temperatures from 650 to 750 °C for 3–12 h, we measured the magnetic properties and found that magnetic Hc relates to annealing temperature, annealing time and film thickness. From atomic force microscopy and magnetic force microscopy studies, the magnetic single domain size of CoPt is in the range of 100–200 nm. The high Hc is likely due to the well-separated nanometer-size crystallites and the well-ordered fct phase of CoPt alloy.

The structural and magnetic properties of high κ dilute magnetic oxides (DMOs) have been studied in two types of samples: thin films and nanocrystals, including Co doped HfO2 films, Co doped Y2O3 films, Co doped Y2O3 nanocrystals, and Mn doped Y2O3 nanocrystals. The characterizations were conducted by extended x-ray absorption fine structure, x-ray absorption near edge structure, and superconducting quantum interference device vibrating sample magnetometer. Oxygen vacancies are shown to play a crucial role in ferromagnetic ordering, as defect centers in the bound magnetic polaron model to account for DMOs with medium band gap and low carrier concentration. The observation of room temperature ferromagnetism in tri-valence oxide of Y2O3 as well as in tetra-valence oxide of HfO2 suggests a generic feature of transition metal doped high κ oxides as good candidates for DMO. The ability to modulate the magnetic behavior of DMO via oxygen vacancy concentration by means of post anneals can be exploited for potential applications in spintronics.

We report magneto-optical Kerr microscopy study of the magnetic reversal behavior of Fe3Si/GaAs(100) epitaxial thin films grown by molecular beam epitaxy. Results are described in the context of superconducting quantum interference device and Kerr effect vector magnetometry to study the magnetic switching behavior. By tuning the Fe concentration of the samples from 75% to 83%, we found that the ratio of uniaxial to cubic anisotropy constants γ (= |Ku/K1|) would tremendously vary from 0.06 to 0.3. When the field was applied along the easy axis of Fe3Si samples, a step feature was observed in the M-H loops. This feature has been resolved in the domain image by Kerr microscopy, in which we observed a two-switching behavior at the field where the step feature occurred. These can be qualitatively understood by considering the ratio γ.

We study the spin dependent transport at the junctions between two superconducting Nb leads connected by two ferromagnetic NiFe bars in parallel, which have various separations. The separation distance l between NiFe bars was varied from 150 to 700 nm to study the spin relaxation effect in Nb. In small l (150 and 250 nm) samples, a spin-valve-like behavior related to the inverse proximity effect was observed, which manifest itself as charge accumulation due to spin imbalance near the interface. However, for samples with large l, the spin-valve behavior vanishes and the anisotropic magnetoresistance effect dominates.

Point-contact Andreev reflection measurements on Ga0.94Mn0.06As were investigated using Pb tips. The observed conductance versus voltage spectra exhibited behaviors which were described by the modified Blonder–Tinkham–Klapwijk model but with significant broadening. We propose a model by introducing a spreading resistance and compare with an existing model which included effective temperature as a parameter. In as-grown/annealed Ga0.94Mn0.06As samples, we extracted conduction electron spin polarization of 76%/74% from our model, instead of 90%/82% from the one with the effective temperature.

We quantitatively study the interface resistance in Ni/Nb multilayers fabricated by sputtering system. For a fixed Ni layer thickness in Ni/Nb/Ni trilayers, the superconducting temperature Tc decreases with decreasing Nb thickness. By analyzing the data with the proximity effect, the critical thickness below which superconductivity vanished was deduced. From current perpendicular to plane (CPP) measurement interpreted with a one-band series-resistor model, we obtained the CPP resistivities of Nb and Ni and the unit area resistances of 4.2±0.2 and 1.5±0.4 fΩ m2 for superconducting and normal Ni/Nb interfaces. The transparency parameter is directly calculated in terms of interface resistance.

The influence of the local structure distortion on the magnetic transition in La doped HoMn2O5 has been investigated systematically. The orthorhombic crystal structure of LaxHo1−xMn2O5 is maintained up to x⩽0.2 but decomposed into multiphases for x⩾0.25. By doping La ions to a concentration of 0.1⩽x⩽0.2, the formation of the RMnO3 (113) phase can be suppressed and single-phase LaxHo1−xMn2O5 (0.1⩽x⩽0.2) compounds can be formed under 1atm flowing oxygen. For x=0.2, a ferromagnetic (FM) transition at 150K is superimposed on the paramagnetic background, which implies that the compound undergoes a ferromagnetic to antiferromagnetic (AFM) transition. This unique FM to AFM transition is observed for the first time. The FM transition is attributed to the formation of magnetic clusters in a host paramagnetic matrix.

The behaviors of superconducting transition temperature Tc and upper critical field Hc2 as a function of different superconductor thicknesses have been investigated in Cu50Ni50∕Nb trilayers and multilayers. We deduced superconductor critical thickness, below which superconductivity vanishes, by analyzing the data in terms of proximity theory. The temperature dependence of Hc2 measurement reveals the spatial dimensional crossover and the flux pinning mechanism in the superconductor. A strong pair-breaking effect was observed in the weak ferromagnetic Cu50Ni50 layers from both measurements. We attribute our observation to the high interface transparency between Cu50Ni50 and Nb.

In this study, we investigated the electron transport properties of interface between a ferromagnet and a superconductor with flowing current perpendicular to plan (CPP) at 4.2K in Fe∕Nb multilayers. The CPP resistance increased linearly with layer thickness and with bilayer number. We extracted the unit area resistance value of one pair of interface for superconducting and normal Fe∕Nb by using a series resistor model. Hence, we can quantitatively analyze the interface resistance between Fe and Nb in the diffusive regime.

We report here the resistance of Co∕NbxTi1−x multilayers, for x=0.4 and 0.6, with current flowing perpendicular to the layer plane at 4.2K. When NbTi films are sandwiched between Co, we found that there are critical thicknesses of 20nm for x=0.4 and 27nm for x=0.6, below which no superconducting transition could be found. Using a series resistor model, we extracted the unit area resistance of one pair of Co∕NbTi interfaces and compared with the pure Nb case we have reported. The influence of mean free paths and superconducting coherence lengths is analyzed.