Size Dependence of Current Spin Polarization through Superconductor/Ferromagnet Nanocontacts

The spin polarization $P$ of the transport current through the interface between superconducting Al and ferromagnetic Fe is determined by means of Andreev reflection at nanostructured point contacts. We observe a systematic decrease of $P$ with decreasing contact resistance. Our data provide evidence for the reduction of $P$ by spin-orbit scattering and thus establish a link between density of states and transport spin polarizations.

Figure 1

Differential conductance $G=dI/dV$ normalized to the normal-state value $G_N=G(T>T_c)=54.3\mathrm{mS}$ for a nanostructured Al/Fe point contact (sample no. 5) for different $T$. For clarity, the curves are shifted upwards successively by 0.2 units with decreasing $T$. The conductance curves calculated with the ${\tau }_{\uparrow }-{\tau }_{\downarrow }$ model match the experimental curves exactly except for the lowest $T$ in the region of the $G$ maximum (solid line).

Figure 2

Normalized differential conductance $G/G_N$ of nanostructured Al/Fe point contacts at $T\approx 0.1\mathrm{K}$. For clarity, the curves are shifted upwards successively by 0.15 units. The conductance curves (solid lines) are calculated with the ${\tau }_{\uparrow }-{\tau }_{\downarrow }$ model (see text).

Figure 3

(a) $T$ dependence of the zero-bias resistance $G_0^{-1}$ of sample no. 5 together with a theoretical curve (see text). (b) $T$ dependence of the superconducting gap $\Delta$ for all samples investigated in this study, obtained from a least-squares fit of the data and normalized to $\Delta (T<200\mathrm{mK}\ll T_c)$. (c) $T$ dependence of the current spin polarization $P=|{\tau }_{\uparrow }-{\tau }_{\downarrow }|/({\tau }_{\uparrow }+{\tau }_{\downarrow })$. $T$ is normalized to the experimental $T_c^{\exp }$ of each sample in order to account for the small variation of $T_c$ from sample to sample.

Figure 4

Dependence of the current spin polarization on the contact radius. The solid line shows $P=P_0\exp (-a/{\ell }_{\mathrm{so}})$, with $P_0=0.496\pm 0.01$ and ${\ell }_{\mathrm{so}}=255\pm 91\mathrm{nm}$. Over the short distance $a\ll {\ell }_{\mathrm{so}}$, the exponential decay of $P(a)$ looks almost like a straight line.