Induced Ferromagnetism due to Superconductivity in Superconductor-Ferromagnet structures

TL;DR

This paper demonstrates that in superconductor-ferromagnet structures, magnetic moments can penetrate the superconductor below the critical temperature, leading to an inverse proximity effect that influences magnetization and layer interactions.

Contribution

It provides a theoretical model showing magnetic moment penetration into the superconductor below T_c, explaining experimental magnetization reduction and F/S/F layer interactions.

Findings

Magnetic moment penetrates the superconductor below T_c

Magnetization in S opposes ferromagnet's magnetization

Magnetic penetration length exceeds ferromagnet thickness

Abstract

We consider a superconductor-ferromagnet (S/F) structure and assume that above the superconducting transition temperature $T_{c}$ the magnetic moment exists only in F. {In a simple model of the ferromagnet (the exchange field is of the ferromagnetic type for all energies)}we show by an explicit calculation that below $T_{c}$ the magnetic moment may penetrate the superconductor. {In this model} its direction in S is opposite {to the magnetization of free electrons} in the ferromagnet. The magnetization spreads over a large distance which is of the order of the superconducting coherence length $\xi_{S}$ and can much exceed the ferromagnet film thickness. At the same time the magnetic moment in the ferromagnet is reduced. This inverse proximity effect may explain the reduction in magnetization observed in recent experiments and may lead to a strong interaction between the ferromagnetic layers in F/S/F structures.