Spin-current coherence peak in superconductor/magnet junctions

Abstract

Coherence peak effects in a superconductor induced by a thermal spin current are reported. We measured inverse spin Hall effects induced by spin injection from a ferrimagnetic insulator Y3Fe5O12 into a superconductor NbN using longitudinal spin Seebeck effects. In the vicinity of the superconducting transition temperature of the NbN, a large enhancement of the spin Seebeck voltage is observed, whose sign is opposite to that for the vortex Nernst effect, but is consistent with a calculation for a coherence peak effect in the superconductor NbN.Superconducting spintronics is an emerging research field which explores new spintronic functions by combining superconducting and magnetic orders [1][2][3][4][5]. One of the key ingredients in the superconducting spintronics is generation of spin-polarized carriers in superconductors [6]. In conventional s-wave superconductors, a spinsinglet condensate does not carry spin angular momentum. However, thermally excited quasiparticles (QPs) can carry spin angular momenta even in s-wave superconductors [7]. It was theoretically proposed that spin currents due to the spin-polarized QPs are deflected by spin-orbit scattering to yield a charge imbalance along the Hall direction [8], which can be detected using the technique of the inverse spin Hall effect (ISHE) [9][10][11]. A giant ISHE due to the QP spin current was indeed observed recently by electrical spin injection into an s-wave superconductor NbN in a lateral spin valve structure [12].A spin Seebeck effect (SSE) is one of the useful ways to generate spin current in magnetic heterostructures by applying a temperature gradient [13]. In bilayer systems made up of a ferro-or ferri-magnet (F) and a nonmagnetic metal (N), the SSE enables spin injection from the F layer into the attached N layer through a thermal spin pumping originating in a non-equilibrium spin dynamics [14]. Several theories have been proposed for the mechanism of the SSE [14-17]. Adachi et al. formulated the SSE using the linear-response theory [15,16], where the spin current flowing across the N/F interface reflects spin susceptibilities of both layers and the interface s-d exchange coupling. The injected spin current into the N layer is converted into charge current by ISHE; the ISHE voltage arises in the direction of j s × σ, where j s is the spin current and σ is the spin-polarization vector of electrons in the N layer. Hence ISHE voltage may reflect the spin dynamics of spin-detection layers in SSE measurements according to the theory [15].In this study, we have investigated ISHE induced by SSE in a superconductor/ferrimagnet (S/F) bilayer comprising a superconductor NbN and a ferrimagnetic insulator Y 3 Fe 5 O 12 (YIG). We show that the generated ISHE voltage exhibits an anomalous enhancement just below the superconducting transition. The voltage enhancement can be attributed to a coherence peak effect [18]; according to a theoretical calculation, singularity in the QP density of states at superconducting transition temperature (T c ) leads to an enhance...

Spin diffusion equation in superconductors in the vicinity of Tc

“…Given the present result of an increased spin relaxation time below T c , it is rather easy to discuss the physics behind the appearance of the coherence peak found in the spin pumping into superconductors [15,16,37]. In short, this is a kind of the inverse of motional narrowing [35,53] in that a reduction of the spin-flip scattering rate in superconductors below T c results in a broadening of the magnon damping in the adjacent magnet.…”

“…Note that in the present approach the impurity spin-orbit scattering preserves the spin symmetry of the single-particle Green's function [Eq. (16)], but it modifies the spin symmetry of the two-particle Green's function [Eqs. (18) and (21)] through the vertex corrections.…”

Microscopic theory of spin transport at the interface between a superconductor and a ferromagnetic insulator

“…We formulate the spin current at the interface, and study its temperature dependence above and below the superconducting transition temperature. We also discuss the noise power of the pure spin current following the theory developed by three of the present authors and one collaborator 43 , and estimate it using the experimental parameters for the yttrium-iron-garnet(YIG)-NbN interface 29,34 .…”

Spin transport parameters of NbN thin films characterized by spin pumping experiments

“…Quasiparticle spin transport has also been investigated by spin pumping and by monitoring the spin Seebeck effect [28,29]. To the best of our knowledge, spin transport parameters in NbN such as and θSH have only been extracted by Wakamura et al [27] by the spin absorption method in lateral spin-valves, and it is vitally important to extract these parameters also by other [27][28][29]. By accumulation of a body of results, we will then be able to understand the fundamental nature of SHE and spin transport in NbN which can be useful and transferable to future spintronics research using SC NbN [21,30].…”

“…To study the spin transport properties in NbN across Tc, by spin-pumping technique, would require an improvement of YIG thin-film quality to overcome the observed ΔH enhancement. We note that a very recent work by Umeda et al exploited the spin-Seebeck effect as a spin-injection method, demonstrating an interesting coherent peak in spin-Seebeck coefficient related to the quasi-particle spin transport[29]. * rogdakis7@gmail.com † h.kurebayashi@ucl.ac.ukFigure captionsFIG.1: Structural and magnetic properties of a bare (111)-oriented YIG film (nominally 60-nmthick) used in this work and deposited onto GGG.…”