Magnetic coupling in superconducting spin valves with strong ferromagnets

Abstract

We investigate the magnetotransport behavior of ferromagnet (F)/superconductor/ferromagnet trilayers made of ferromagnetic ${\text{Ni}}_{80}{\text{Fe}}_{20}$ (Permalloy, Py) and superconducting Nb for temperatures both above and below the superconducting transition temperature ${T}_{c}$. In such devices, and for weak ferromagnets, ${T}_{c}$ depends on the relative magnetization directions of the two F layers in such a way that ${T}_{c}^{\text{P}}$ of the parallel (P) alignment is lower than ${T}_{c}^{\text{AP}}$ of the antiparallel (AP) alignment (the so-called superconducting spin-valve effect). For strong magnets, the suppression of Andreev reflection may alter this picture, but also stray field effects become important, as is known from earlier work. We compare large-area samples with microstructured ones, and find blocklike switching in the latter. We show this not to be due to a switch between the P and AP states, but rather to dipolar coupling between domains which are forming in the two Py layers, making a stray-field scenario likely. We also present measurements of the depairing (critical) current ${I}_{dp}$ and show that a similar depression of superconductivity exists far below ${T}_{c}$ as is found around ${T}_{c}$.