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

Abstract

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 ($\mathrm{\ensuremath{\Delta}}{T}_{c0}$) of $\ensuremath{\sim}400\text{ }\text{ }\mathrm{mK}$ 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 $\mathrm{\ensuremath{\Delta}}{T}_{c0}$ 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.