Superconducting spin valves based on epitaxial Fe/V superlattices

Abstract

In superconducting spin valves of the type S/F1/N/F2 or F1/S/F2 with a superconducting layer S, two ferromagnetic layers F1 and F2, and a normal metallic layer N, the superconducting transition temperature ${T}_{\text{S}}$ depends on the relative magnetization direction of the ferromagnetic layers F1 and F2. The difference of the transition temperature $\ensuremath{\Delta}{T}_{\text{S}}={T}_{s}^{\text{AP}}\ensuremath{-}{T}_{s}^{\text{P}}$ with the magnetization direction of F1 and F2 either antiparallel or parallel is called the superconducting spin valve effect. We have prepared both types of spin valves by growing Fe/V thin-film heterostructures with epitaxial quality on MgO(001) substrates. In the S/F1/N/F2-type spin valves the ferromagnetic layers were the first two Fe layers of a [Fe/V] superlattice coupled antiferromagnetically via the interlayer exchange interaction. Here we observed a superconducting spin valve shift of up to $\ensuremath{\Delta}{T}_{\text{S}}\ensuremath{\approx}200\text{ }\text{mK}$ when aligning the sublattice magnetization in an external magnetic field. In the F1/S/F2-type spin valves the ferromagnetic layer F1 was either a [Fe/V] or a $[{\text{Fe}}_{x}{\text{V}}_{1\ensuremath{-}x}/\text{V}]$ superlattice, the F2 layer was a Fe-, a Co-, or a ${\text{Fe}}_{x}{\text{V}}_{1\ensuremath{-}x}$ film. Using weakly ferromagnetic ${\text{Fe}}_{x}{\text{V}}_{1\ensuremath{-}x}$ alloy layers as F1 and F2 we find a spin valve effect of up to $\ensuremath{\Delta}{T}_{\text{S}}\ensuremath{\approx}20\text{ }\text{mK}$, which is more than a factor of 2 larger than reported in the literature before for spin valves with comparable transition temperatures. Our results indicate that a high interface transparency and a large superconducting correlation length are prerequisites for the observation of a sizable superconducting spin valve effect.