Spontaneous magnetization changes and nonlocal effects in mesoscopic ferromagnet-superconductor structures

We report magnetization studies of individual ferromagnet-superconductor (FS) structures of submicron size. Upon cooling through the superconducting transition in zero field, such structures are found to change their magnetization spontaneously. We attribute this effect to reshuffling of domains in the ferromagnet, which is caused by temperature-dependent screening of domain's stray fields by the superconductor. The spontaneous magnetization is not localized to the actual contact area between the two materials but can propagate along the ferromagnet as far as several microns from the FS interface.