Enhancement of critical current density in superconducting/magnetic multi-layers with slow magnetic relaxation dynamics and large magnetic susceptibility

TL;DR

This paper proposes a superconductor-magnet multi-layer structure to significantly enhance critical current density by leveraging slow magnetic relaxation and large magnetic susceptibility, leading to improved vortex pinning and a depinning transition.

Contribution

It introduces a novel multi-layer design utilizing polaronic pinning mechanisms with slow magnetic relaxation to boost critical current density in superconductors.

Findings

Estimated critical current density of ~10^9 A/m^2 at 1 T magnetic field.

Identification of a depinning transition characterized by a jump in the I-V curve.

Enhanced vortex viscosity due to polaron formation in the proposed structure.

Abstract

We propose to use superconductor-magnet multi-layer structure to achieve high critical current density by invoking polaronic mechanism of pinning. The magnetic layers should have large magnetic susceptibility to enhance the coupling between vortices and magnetization in magnetic layers. The relaxation of the magnetization should be slow. When the velocity of vortices is low, they are dressed by nonuniform magnetization and move as polarons. In this case, the viscosity of vortices proportional to the magnetic relaxation time is enhanced significantly. As velocity increases, the polarons dissociate and the viscosity drops to the usual Bardeen-Stephen one, resulting in a jump in the I-V curve. Experimentally the jump shows up as a depinning transition and the corresponding current at the jump is the depinning current. For Nb and proper magnet multi-layer structure, we estimate the critical current density $J_c\sim 10^{9}\ \rm{A/m^2}$ at magnetic field $B\approx 1$ T.