Observation of Odd-Parity Superconductivity with the Geshkenbein-Larkin-Barone Composite Rings.

Phase-sensitive measurements on a composite ring made of a superconductor of interest connected by a known singlet s-wave superconductor can unambiguously determine its pairing symmetry. In composite rings with epitaxial ß-Bi_{2}Pd and s-wave Nb, we have observed half-integer-quantum flux when Nb is connected to the opposite crystalline ends of ß-Bi_{2}Pd and integer-quantum flux when Nb is connected to the same crystalline ends of ß-Bi_{2}Pd. With ascending temperature, the half-integer-flux quantization transits to integer-flux quantization, before the eventual loss of phase coherence. These findings point to odd-parity pairing symmetry in superconducting ß-Bi_{2}Pd.

Unequivocal Identification of Spin-Triplet and Spin-Singlet Superconductors with Upper Critical Field and Flux Quantization.

Spin-triplet superconductors play central roles in Majorana physics and quantum computing but are difficult to identify. We show the methods of kink-point upper critical field and flux quantization in superconducting rings can unequivocally identify spin-singlet, spin-triplet in centrosymmetric superconductors, and singlet-triplet admixture in noncentrosymmetric superconductors, as realized in γ-BiPd, ß-Bi_{2}Pd, and α-BiPd, respectively. Our findings are essential for identifying triplet superconductors and exploring their quantum properties.

Exploiting Spin Fluctuations for Enhanced Pure Spin Current.

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.

Observation of Vector Spin Seebeck Effect in a Noncollinear Antiferromagnet.

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.

Spin-Triplet Pairing State Evidenced by Half-Quantum Flux in a Noncentrosymmetric Superconductor.

A prime category of superconducting materials in which to look for spin-triplet pairing and topological superconductivity are superconductors without inversion symmetry. It is predicted that the broken parity symmetry gives rise to an admixture of spin-triplet and spin-singlet pairing states. However, experimental confirmation of pairing mixing in any material remains elusive. In this work, we perform a phase-sensitive experiment to examine the pairing state of noncentrosymmetric superconductor α-BiPd. The Little-Parks effect observed in mesoscopic polycrystalline α-BiPd rings reveals the presence of half-integer magnetic flux quantization, which provides a decisive evidence for the spin-triplet pairing state. We find both half-quantum fluxes and integer-quantum fluxes of different proportions, consistent with the scenario of an admixture of singlet-triplet pairing.

Voltage-Controlled Antiferromagnetism in Magnetic Tunnel Junctions.

We demonstrate a voltage-controlled exchange bias effect in CoFeB/MgO/CoFeB magnetic tunnel junctions that is related to the interfacial Fe(Co)O_{x} formed between the CoFeB electrodes and the MgO barrier. The unique combination of interfacial antiferromagnetism, giant tunneling magnetoresistance, and sharp switching of the perpendicularly magnetized CoFeB allows sensitive detection of the exchange bias. We find that the exchange bias field can be isothermally controlled by magnetic fields at low temperatures. More importantly, the exchange bias can also be effectively manipulated by the electric field applied to the MgO barrier due to the voltage-controlled antiferromagnetic anisotropy in this system.

Absence of Evidence of Electrical Switching of the Antiferromagnetic Néel Vector.

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.

Spin-to-Charge Conversion in Bi Films and Bi/Ag Bilayers.

Thermally injected pure spin current phenomena have been investigated in Bi/Y_{3}Fe_{5}O_{12} and Bi/Ag/Y_{3}Fe_{5}O_{12} structures at room temperature. We show that although pure spin current has been injected into the Bi layer and the Bi/Ag bilayer, there is little detectable signal of spin-to-charge conversion, except the distinctive Nernst signal from the Bi layer, in sharp contrast to the inverse Rashba-Edelstein effect claimed in these systems.

Switching a Perpendicular Ferromagnetic Layer by Competing Spin Currents.

An ultimate goal of spintronics is to control magnetism via electrical means. One promising way is to utilize a current-induced spin-orbit torque (SOT) originating from the strong spin-orbit coupling in heavy metals and their interfaces to switch a single perpendicularly magnetized ferromagnetic layer at room temperature. However, experimental realization of SOT switching to date requires an additional in-plane magnetic field, or other more complex measures, thus severely limiting its prospects. Here we present a novel structure consisting of two heavy metals that delivers competing spin currents of opposite spin indices. Instead of just canceling the pure spin current and the associated SOTs as one expects and corroborated by the widely accepted SOTs, such devices manifest the ability to switch the perpendicular CoFeB magnetization solely with an in-plane current without any magnetic field. Magnetic domain imaging reveals selective asymmetrical domain wall motion under a current. Our discovery not only paves the way for the application of SOT in nonvolatile technologies, but also poses questions on the underlying mechanism of the commonly believed SOT-induced switching phenomenon.

Nontrivial Nature and Penetration Depth of Topological Surface States in SmB_{6} Thin Films.

The nontrivial feature and penetration depth of the topological surface states (TSS) in SmB_{6} were studied via spin pumping. The experiments used SmB_{6} thin films grown on the bulk magnetic insulator Y_{3}Fe_{5}O_{12} (YIG). Upon the excitation of magnetization precession in the YIG, a spin current is generated in the SmB_{6} that produces, via spin-orbit coupling, a lateral electrical voltage in the film. This spin-pumping voltage signal becomes considerably stronger as the temperature decreases from 150 to 10 K, and such an enhancement most likely originates from the spin-momentum locking of the TSS and may thereby serve as evidence for the nontrivial nature of the TSS. The voltage data also show a unique film thickness dependence that suggests a TSS depth of ∼32 nm. The spin-pumping results are supported by transport measurements and analyses using a tight binding model.

Electrical Detection of Spin Backflow from an Antiferromagnetic Insulator/Y_{3}Fe_{5}O_{12} Interface.

Spin Hall magnetoresistance (SMR) has been observed in Pt/NiO/Y_{3}Fe_{5}O_{12} (YIG) heterostructures with characteristics very different from those in Pt/YIG. This phenomenon indicates that a spin current generated by the spin Hall effect in Pt transmits through the insulating NiO and is reflected from the NiO/YIG interface. The SMR in Pt/NiO/YIG shows a strong temperature dependence dominated by effective spin conductance, due to antiferromagnetic magnons and spin fluctuation. Inverted SMR has been observed below a temperature which increases with the NiO thickness, suggesting a spin-flip reflection from the antiferromagnetic NiO exchange coupled with the YIG.

Enhancement of Thermally Injected Spin Current through an Antiferromagnetic Insulator.

We report a large enhancement of thermally injected spin current in normal metal (NM)/antiferromagnet (AF)/yttrium iron garnet (YIG), where a thin AF insulating layer of NiO or CoO can enhance the spin current from YIG to a NM by up to a factor of 10. The spin current enhancement in NM/AF/YIG, with a pronounced maximum near the Néel temperature of the thin AF layer, has been found to scale linearly with the spin-mixing conductance at the NM/YIG interface for NM=3d, 4d, and 5d metals. Calculations of spin current enhancement and spin mixing conductance are qualitatively consistent with the experimental results.

Electronic properties of nanoentities revealed by electrically driven rotation.

Direct electric measurement via small contacting pads on individual quasi-one-dimensional nanoentities, such as nanowires and carbon nanotubes, are usually required to access its electronic properties. We show in this work that 1D nanoentities in suspension can be driven to rotation by AC electric fields. The chirality of the resultantrotation unambiguously reveals whether the nanoentities are metal, semiconductor, or insulator due to the dependence of the Clausius-Mossotti factor on the material conductivity and frequency. This contactless method provides rapid and parallel identification of the electrical characteristics of 1D nanoentities.

Physical origins of the new magnetoresistance in Pt/YIG.

A new type of magnetoresistance (MR) observed in Pt/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 Pt/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.

A BCS-like gap in the superconductor SmFeAsO0.85F0.15.

Since the discovery of superconductivity in the high-transition-temperature (high-T(c)) copper oxides two decades ago, it has been firmly established that the CuO(2) plane is essential for superconductivity and gives rise to a host of other very unusual properties. A new family of superconductors with the general composition of LaFeAsO(1-x)F(x) has recently been discovered and the conspicuous lack of the CuO(2) planes raises the tantalizing question of a different pairing mechanism in these oxypnictides. The superconducting gap (its magnitude, structure, and temperature dependence) is intimately related to pairing. Here we report the observation of a single gap in the superconductor SmFeAsO(0.85)F(0.15) with T(c) = 42 K as measured by Andreev spectroscopy. The gap value of 2Delta = 13.34 +/- 0.3 meV gives 2Delta/k(B)T(c) = 3.68 (where k(B) is the Boltzmann constant), close to the Bardeen-Cooper-Schrieffer (BCS) prediction of 3.53. The gap decreases with temperature and vanishes at T(c) in a manner consistent with the BCS prediction, but dramatically different from that of the pseudogap behaviour in the copper oxide superconductors. Our results clearly indicate a nodeless gap order parameter, which is nearly isotropic in size across different sections of the Fermi surface, and are not compatible with models involving antiferromagnetic fluctuations, strong correlations, the t-J model, and the like, originally designed for the high-T(c) copper oxides.

Intrinsic spin Seebeck effect in Au/YIG.

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.

Inverse spin Hall effect in a ferromagnetic metal.

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.

Pt magnetic polarization on Y3Fe5O12 and magnetotransport characteristics.

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.

Electric-field-assisted switching in magnetic tunnel junctions.

The advent of spin transfer torque effect accommodates site-specific switching of magnetic nanostructures by current alone without magnetic field. However, the critical current density required for usual spin torque switching remains stubbornly high around 10(6)-10(7) A cm(-2). It would be fundamentally transformative if an electric field through a voltage could assist or accomplish the switching of ferromagnets. Here we report electric-field-assisted reversible switching in CoFeB/MgO/CoFeB magnetic tunnel junctions with interfacial perpendicular magnetic anisotropy, where the coercivity, the magnetic configuration and the tunnelling magnetoresistance can be manipulated by voltage pulses associated with much smaller current densities. These results represent a crucial step towards ultralow energy switching in magnetic tunnel junctions, and open a new avenue for exploring other voltage-controlled spintronic devices.

Unified formalism of Andreev reflection at a ferromagnet/superconductor interface.

We present a unified formalism of Andreev reflection of a partial polarized current at a ferromagnet/superconductor interface instead of assuming a linear combination of unpolarized and polarized currents. The Andreev reflection is limited by the states of minority spins and the extra majority spins become evanescent wave. We further study the effects of spin polarization, inelastic scattering, and interfacial scattering on the Andreev reflection, normal reflection, and transmitted probabilities in equilibrium as well as under a bias. Our model, which reduces to those of Blonder, Tinkham, and Klapwijk, Mazin, and Dynes in three limiting cases, provides a significantly better description of the experimental results.