Third Harmonic Susceptibility and the Irreversibility Line of Fe/MgB2 Tapes

TL;DR

This paper investigates the vortex pinning and irreversibility line in Fe/MgB2 tapes using ac magnetic susceptibility, revealing how ball-milling affects vortex dynamics and the irreversibility field.

Contribution

It introduces a detailed analysis of third harmonic susceptibility to understand vortex pinning and irreversibility in Fe/MgB2 tapes, including numerical simulations of vortex dynamics.

Findings

Third harmonic onset criterion determines irreversibility line.

Ball-milling enhances vortex pinning and irreversibility field.

Numerical simulations match experimental vortex motion regimes.

Abstract

We report a study of the pinning properties of Fe/MgB2 tapes performed by means of ac magnetic susceptibility measurements. In particular, the third harmonic response has been measured as a function of the dc magnetic field up to 9 T, at different temperatures, for various frequencies and amplitudes of the ac field. The irreversibility line has been determined using the third harmonic onset criterion for samples made with different ball-milled MgB2 powders. The study of the harmonic response allows us to probe the nature of the vortex pinning and to analyse its influence on the enhancement of the irreversibility field in strongly ball-milled samples. We have also investigated the dynamical regimes governing the vortex motion in the tapes by comparing the experimental curves with numerical simulations of the non-linear diffusion equation for the magnetic field.