Spin Alignment in the Superconducting State

Abstract

It is argued that spin alignment can and will occur for ion-core spins in superconductors, but that the alignment is in the form of extremely small domains. Central to the argument is the concept of the nonlocal susceptibility $\ensuremath{\chi}(r\ensuremath{-}{r}^{\ensuremath{'}})$, which leads to a positive short-range Kittel-Ruderman-Yosida interaction of ion-core spins, but a negative long-range interaction of range ${\ensuremath{\xi}}_{0}$.Very general arguments suggest that purely ferromagnetic alignment should not be observed in preference to this domain-like "cryptoferromagnetic" alignment.