Is the nature of itinerant ferromagnetism playing a role in the competition between spinpolarization and singlet pair correlations?

Abstract

We consider the competition between spin singlet pairing and itinerant ferromagnetismwhose magnetization is yielded by a relative shift of the bands with opposite spinpolarization or by asymmetric spin-dependent bandwidths. Within the framework of theexact solution of an extended version of the reduced BCS model, the structure of thecoexisting state is shown to have general features that are not related to the character ofthe ferromagnetism. The role of different types of ferromagnet is then investigated for theproximity effect in a system made of a bilayer junction with a spin singlet superconductorinterfaced with a ferromagnet in the clean limit. We show that the qualitative behaviour ofthe proximity effect does not depend on the nature of the ferromagnetism. Differencesemerge at the borderline with the half-metallic regime. For the spin-dependentbandwidth type of ferromagnetism the pairing amplitude exhibits an oscillatingbehaviour until the density of the minority spin carrier becomes almost zero. Thecrossover from an oscillating to an exponentially damped profile occurs awayfrom the half-metallic limit when a spin exchange type ferromagnet is considered.