Abstract
Spin transport via electrons is typically plagued by Joule heating and short decay lengths due to spin-flip scattering. It is known that dissipationless spin currents can arise when using conventional superconducting contacts, yet this has only been experimentally demonstrated when using intricate magnetically inhomogeneous multilayers, or in extreme cases such as half-metals with interfacial magnetic disorder. Moreover, it is unknown how such spin supercurrents decay in the presence of spin-flip scattering. Here, we present a method for generating a spin supercurrent by using only a single homogeneous magnetic element. Remarkably, the spin supercurrent generated in this way does not decay spatially, in stark contrast to normal spin currents that remain polarized only up to the spin relaxation length. We also expose the existence of a superconductivity-mediated torque even without magnetic inhomogeneities, showing that the different components of the spin supercurrent polarization respond fundamentally differently to a change in the superconducting phase difference. This establishes a mechanism for tuning dissipationless spin and charge flow separately, and confirms the advantage that superconductors can offer in spintronics.
Highlights
It is clear that the polarization of the spin supercurrent along the magnetization direction has a qualitatively different behavior with the length of the system compared with the polarization perpendicular to the exchange field, which oscillates within its typical exponential decay since it is limited by the penetration depth of the short-ranged superconducting correlations
Unlike conventional spin-polarized currents, we find that a spin supercurrent does not decay due to either spin-orbit impurity scattering or spin-flip scattering caused by magnetic impurities
Even though the magnitude of the spin supercurrent is reduced with increasing spin-flip scattering[45], it is remarkable that a spin supercurrent, controllable via the superconducting phase difference, has no decay even if both spin-orbit and magnetic impurities are present in the sample
Summary
Consider the thin-film heterostructure depicted in Fig. 1, which shows a Josephson junction of conventional s-wave superconductive sources with normal and ferromagnetic elements typically utilized in proximity effect experiments. G M R = 0.2 for the normalized interfacial magnetoresistance term and G θ = 1 for the interfacial scattering phase shift on both sides[36] (see Methods for details) In this case, and with a typical superconducting coherence length of ξS = 25 nm, critical current the LR component dominates for ferromagnets of length LF IQC to decay slowly despite the presence of an exchange field h greater than ∼ 10 nm, causing the ∆, remaining orders of magnitude larger than the SR component for increasingly long ferromagnets. It is clear that the polarization of the spin supercurrent along the magnetization direction has a qualitatively different behavior with the length of the system compared with the polarization perpendicular to the exchange field, which oscillates within its typical exponential decay since it is limited by the penetration depth of the short-ranged superconducting correlations Both components decay exponentially, the penetration depth of the parallel component is enhanced greatly by the addition of spin-orbit coupling, and it persists for significantly longer interstitial ferromagnets. Since the long-range correlations are carried by the so-called odd-frequency pairs[38,39], the system in this way reproduces features of unconventional superconductivity[40] using conventional s-wave superconductors
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