On the Coexistence in RuSr2GdCu2O8 of Superconductivity and Ferromagnetism

TL;DR

This paper reviews the coexistence of superconductivity and ferromagnetism in RuSr2GdCu2O8, analyzing whether these phenomena can coexist and under what conditions, supported by theoretical calculations and considering different pairing mechanisms.

Contribution

It provides a detailed analysis of the conditions under which superconductivity and ferromagnetism coexist in RuSr2GdCu2O8, including theoretical calculations and considerations of pairing types.

Findings

Superconductivity in RuSr2GdCu2O8 occurs below 35 K, below its Curie temperature of 132 K.

The compound has a ferromagnetic RuO2 layer with itinerant magnetism.

Possible superconducting states include singlet and triplet pairing, with implications for vortex phases.

Abstract

We review the reasons that make superconductivity unlikely to arise in a ferromagnet. Then, in light of the report by Tallon and collaborators that RuSr2GdCu2O8 becomes superconducting at approximately 35 K which is well below the Curie temperature of 132 K, we consider whether the objections really apply to this compound. Our considerations are supported by local spin density calculations for this compound, which indeed indicate a ferromagnetic RuO2 layer. The Ru moment resides in t_2g orbitals but is characteristic of itinerant magnetism (and is sensitive to choice of exchange-correlation potential and to the atomic positions). Based on the small exchange splitting that is induced in the Cu-O layers, the system seems capable of supporting singlet superconductivity an FFLO-type order parameter and possibly a pi-phase alternation between layers. If instead the pairing is triplet in the RuO2 layers, it can be distinguished by a spin-polarized supercurrent. Either type of superconductivity seems to imply a spontaneous vortex phase if the magnetization is rotated out of the plane.