Influence of disorder on Hc2-anisotropy and flux pinning in MgB2

TL;DR

This study investigates how disorder affects the upper critical field, anisotropy, and flux pinning in MgB2, revealing that irradiation-induced defects can enhance performance but have limited impact on flux pinning compared to natural grain boundaries.

Contribution

It demonstrates that neutron irradiation modifies the upper critical field and anisotropy in MgB2, providing insights into optimizing flux pinning and transport properties for applications.

Findings

Disorder from irradiation enhances Hc2 and reduces anisotropy.

Flux pinning improvement from irradiation is minor compared to grain boundary pinning.

Application range of MgB2 is limited by the smallest Hc2 in certain directions.

Abstract

The upper critical field and flux pinning in MgB2 single crystals were investigated. The implications of these properties for technical applications are discussed and compared with transport properties of polycrystalline bulk samples and wires. In these untextured materials current percolation is important, especially at high magnetic fields. It is shown that the anisotropy of the upper critical field influences the "irreversibility line" and that the application range of MgB2 is limited by the smallest upper critical field (i.e., for the field direction perpendicular to the boron planes). Disorder, introduced by irradiation with neutrons, enhances the upper critical field, reduces the anisotropy and drastically changes flux pinning. While the enhanced Hc2 and the reduced anisotropy generally improve the transport properties of the polycrystalline samples, the contribution of the radiation-induced defects to flux pinning is small compared to the as-grown defect structure (grain boundary pinning).