Vortex dynamics and pinning properties analysis of MgB2 bulk samples by ac susceptibility measurements

TL;DR

This study investigates vortex dynamics and pinning in MgB2 bulk superconductors using ac susceptibility measurements, revealing vortex glass behavior and flux creep characteristics through experimental and numerical analysis.

Contribution

It provides new insights into vortex motion regimes and flux creep activation energy in MgB2, supported by combined experimental and numerical approaches.

Findings

Vortex glass behavior observed in MgB2

Frequency response of third harmonic linked to flux creep

Numerical models match experimental susceptibility curves

Abstract

The flux lines dynamics have been investigated on MgB2 bulk superconductors obtained by reactive liquid infiltration by measuring the ac magnetic susceptibility. The fundamental and third harmonics have been studied as a function of temperature, dc magnetic field, ac field amplitude and frequency. In order to determine the dynamical regimes governing the vortex motion, the experimental results have been compared with susceptibility curves obtained by numerical calculations of the non-linear diffusion equation for the magnetic field. The frequency behaviour of the third harmonic response, that cannot be explained by frequency dependent critical state models, has been related to the current dependence of the flux creep activation energy U(J) in the diffusion coefficient. In this way we have shown that the measured curves are correctly interpreted within the framework of a vortex glass description.