Spin Injection and Relaxation in a Mesoscopic Superconductor

We study spin transport in a superconducting nanowire using a set of closely spaced magnetic tunnel contacts. We observe a giant enhancement of the spin accumulation of up to 5 orders of magnitude on transition into the superconducting state, consistent with the expected changes in the density of states. The spin relaxation length decreases by an order of magnitude from its value in the normal state. These measurements, combined with our theoretical model, allow us to distinguish the individual spin-flip mechanisms present in the transport channel. Our conclusion is that magnetic impurities rather than spin-orbit coupling dominate spin-flip scattering in the superconducting state.

Figure 1

(a) SEM image of the sample together with the nonlocal measurement configuration. The electrical circuit schematic illustrates the measurement arrangement used for directly measuring ${\lambda }_{\mathrm{sf}}$. (b) ${\lambda }_{\mathrm{sf}}$ is the exponential decay length of the spin accumulation away from the injection point. (c) Schematic density of states, illustrating spin accumulation ($\Delta \mu ={\mu }_{\uparrow }-{\mu }_{\downarrow }$) due to tunneling between a ferromagnet and a superconductor.

Figure 2

Normalized spin signal ($R_S$) for sample 1, as a function of normalized temperature. $T_C\approx 1.6\mathrm{K}$ and $R_S(4\mathrm{K})\approx 50\mathrm{m}\Omega$.

Figure 3

Normalized differential conductance of the injection junction measured at 22 mK, together with a theoretical fit [27]. The best fit was obtained for $T_{\mathrm{eff}}\approx 0.2T_C$.

Figure 4

Normalized spin diffusion length (${\lambda }_{\mathrm{sf}}$) for two samples, 20–25 nm in thickness, as a function of normalized temperature ($T/T_C$). $T_C\approx 1.6\mathrm{K}$ and ${\lambda }_{\mathrm{sf}}(4\mathrm{K})\approx 1\mu \mathrm{m}$. The error bars (not shown) are approximately of the size of the symbols representing the data points.