Influence of the carbon substitution on the critical current density and AC losses in MgB2 single crystals

TL;DR

This study investigates how carbon substitution affects the critical current density and AC losses in MgB2 single crystals, revealing a decrease in Jc at low fields and analyzing AC losses near the transition temperature.

Contribution

It provides new insights into the effects of carbon doping on the electromagnetic properties of MgB2, especially regarding critical current and AC losses.

Findings

Carbon substitution decreases Jc at low magnetic fields.

AC losses depend on amplitude and bias magnetic field.

Results are analyzed using the critical state model.

Abstract

The DC magnetization and AC complex magnetic susceptibilities were measured for MgB2 single crystals, unsubstituted and carbon substituted with the composition of Mg(B_0.94C_0.06)2. The measurements were performed in AC and DC magnetic fields oriented parallel to the c-axis of the crystals. From the DC magnetization loops and the AC susceptibility measurements, critical current densities (Jc) were derived as a function of temperature and the DC and AC magnetic fields. Results show that the substitution with carbon decreases Jc at low magnetic fields, opposite to the well known effect of an increase of Jc at higher fields. AC magnetic losses were derived from the AC susceptibility data as a function of amplitude and the DC bias magnetic field. The AC losses were determined for temperatures of 0.6 and 0.7 of the transition temperature Tc, so close to the boiling points of LH2 and LNe, potential cooling media for magnesium diboride based composites. The results are analyzed and discussed in the context of the critical state model.