Coexistence of Long-Range Magnetic Order and Superconductivity from Campbell Penetration Depth Measurements

TL;DR

This study uses tunnel-diode resonator measurements to explore how different types of magnetic order coexist with superconductivity, revealing that weak magnetic orders can enhance vortex pinning and critical currents.

Contribution

It demonstrates the application of TDR to probe vortex response in magnetic superconductors and shows how magnetic fluctuations influence critical current enhancements.

Findings

Full local-moment ferromagnetism destroys superconductivity.

Weak ferromagnetic and antiferromagnetic orders coexist with superconductivity.

Critical magnetic fluctuations enhance vortex pinning and critical currents.

Abstract

Application of a tunnel-diode resonator (TDR) technique for studies of the vortex response in magnetic superconductors is described. Operating at very small excitation fields and sufficiently high frequency, TDR was used to probe small-amplitude linear AC response in several types of single crystals where long-range magnetic order coexists with bulk superconductivity. Full local - moment ferromagnetism destroys superconductivity and can coexist with it only in a narrow temperature range ($\sim 0.3$ K). In contrast, weak ferromagnetic as well as antiferromagnetic orders can coexist with bulk superconductivity and may even lead to enhancements of vortex pinning. By analyzing the Campbell penetration depth we find sharp increase of the true critical current in the vicinity of the magnetic phase transitions. We conclude that critical magnetic fluctuations are responsible for this enhancement.