Nonlocal effects in angular dependence of in-plane magnetization of tetragonal superconductors

Abstract

The reversible magnetization M of $\mathrm{Lu}({\mathrm{Ni}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{)}_{2}{\mathrm{B}}_{2}\mathrm{C}$ with $x=0$ and $0.06$ is measured in a broad temperature domain as a function of field orientation $\ensuremath{\theta}$ in the basal crystal plane. The data are interpreted within London theory extended for nonlocality of the current-field relation in superconductors. The dependence $M(\ensuremath{\theta})$ diminishes on warming, vanishes at ${T}^{*}<{T}_{c},$ and changes sign for $T>{T}^{*}.$ The low-$T$ sign is opposite to what is expected from the known angular dependence of the upper critical field and therefore cannot be attributed to the latter. Upon doping with Co, the effect disappears with decreasing mean-free path in agreement with theory.