Nucleation of superconductivity and vortex matter in superconductor - ferromagnet hybrids

TL;DR

This paper reviews the theoretical and experimental understanding of how superconductivity nucleates and vortex matter behaves in superconductor-ferromagnet hybrids, emphasizing magnetic interactions and phase behavior.

Contribution

It provides a comprehensive analysis of the magnetic and superconducting interactions in S/F hybrids, including effects near the transition and in the superconducting state, and discusses the coupling when ferromagnet magnetization is dynamic.

Findings

Magnetic stray fields influence superconductivity nucleation.

Critical temperature T_c depends on external magnetic field and ferromagnetic configuration.

Interaction between vortices and magnetic inhomogeneities affects superconducting properties.

Abstract

The theoretical and experimental results concerning the thermodynamical and low-frequency transport properties of hybrid structures, consisting of spatially-separated conventional low-temperature superconductor (S) and ferromagnet (F), is reviewed. Since the superconducting and ferromagnetic parts are assumed to be electrically insulated, no proximity effect is present and thus the interaction between both subsystems is through their respective magnetic stray fields. Depending on the temperature range and the value of the external field H_{ext}, different behavior of such S/F hybrids is anticipated. Rather close to the superconducting phase transition line, when the superconducting state is only weakly developed, the magnetization of the ferromagnet is solely determined by the magnetic history of the system and it is not influenced by the field generated by the supercurrents. In contrast to that, the nonuniform magnetic field pattern, induced by the ferromagnet, strongly affect the nucleation of superconductivity leading to an exotic dependence of the critical temperature T_{c} on H_{ext}. Deeper in the superconducting state the effect of the screening currents cannot be neglected anymore. In this region of the phase diagram various aspects of the interaction between vortices and magnetic inhomogeneities are discussed. In the last section we briefly summarize the physics of S/F hybrids when the magnetization of the ferromagnet is no longer fixed but can change under the influence of the superconducting currents. As a consequence, the superconductor and ferromagnet become truly coupled and the equilibrium configuration of this "soft" S/F hybrids requires rearrangements of both, superconducting and ferromagnetic characteristics, as compared with "hard" S/F structures.