Large Superconducting Spin Valve Effect and Ultrasmall Exchange Splitting in Epitaxial Rare-Earth-Niobium Trilayers

Epitaxial $\mathrm{Ho}/\mathrm{Nb}/\mathrm{Ho}$ and $\mathrm{Dy}/\mathrm{Nb}/\mathrm{Dy}$ superconducting spin valves show a reversible change in the zero-field critical temperature ($\mathrm{\Delta }{T}_{c0}$) of $\sim 400\mathrm{mK}$ and an infinite magnetoresistance on changing the relative magnetization of the Ho or Dy layers. Unlike transition-metal superconducting spin valves, which show much smaller $\mathrm{\Delta }{T}_{c0}$ values, our results can be quantitatively modeled. However, the fits require an extraordinarily low induced exchange splitting which is dramatically lower than known values for rare-earth Fermi-level electrons, implying that new models for the magnetic proximity effect may be required.

Figure 1

In-plane (IP) $M(H)$ loop of a $\mathrm{Ho}(10\mathrm{nm})/\mathrm{Nb}(20\mathrm{nm})/\mathrm{Ho}(40\mathrm{nm})$ SSV at 10 K. Arrows indicate the magnetization orientation of two epitaxial Ho films. Inset: The structure of an epitaxial multilayer with crystal orientations and thicknesses.

Figure 2

(a) Difference between the zero-field critical temperature following a set field and the as-cooled critical temperature for which the Ho is in the spin spiral state versus set field [$T_{c0}(H_{\text{set}})\text{−}{T}_{c0}$(S) versus ${\mu }_0{H}_{\text{set}}$] of a $\mathrm{Ho}(10\mathrm{nm})/\mathrm{Nb}(20\mathrm{nm})/\mathrm{Ho}(40\mathrm{nm})$ SSV. (b) $R_0(T)$ curves of the SSV at different ${\mu }_0{H}_{\text{set}}$ (0, $\pm 1$, and $\pm 0.1\mathrm{T}$), which correspond to different states (S, P, and AP). Inset: $d{R}_0(T)/dT$. (c) $R(H)$ measurements of the SSV at three different temperatures.

Figure 3

(a) Summary of $T_{c0}$ for Ho-based SSVs (S, spiral; P, parallel; AP, antiparallel). (b) Summary of $T_{c0}$ for Dy-based SSVs (V, virgin). (c) Comparison of experimental and theoretical results for Ho-based SSVs. (d) Comparison of experimental and theoretical results for Dy-based SSVs. Insets: (a) Out-of-plane (OOP) $M(H)$ loop of a $\mathrm{Ho}(10\mathrm{nm})/\mathrm{Nb}(20\mathrm{nm})/\mathrm{Ho}(70\mathrm{nm})$ SSV. (b) $T_{c0}$ versus $d_{\text{Dy}}$ at P state. (c) $\mathrm{\Delta }{T}_{c0}$ in different states for Ho-based SSVs. (d) $\mathrm{\Delta }{T}_{c0}$ in different states for Dy-based SSVs.