Crystal growth of MgB2 from Mg-Cu-B melt flux and superconducting properties

TL;DR

This paper introduces a novel method for growing MgB2 single crystals from a Mg-Cu-B melt flux, overcoming high vapor pressure challenges, and investigates their superconducting properties, including transition temperatures and homogeneity.

Contribution

A new crystal growth technique for MgB2 using Mg-Cu-B melt flux is developed, extending formation conditions and enabling detailed superconducting property analysis.

Findings

MgB2 crystals up to 0.2 x 0.2 x 0.05 mm3 were grown.

Highest Tc observed was 39.2 K with a narrow transition width.

Superconducting transition temperatures varied from 39 K to 7.2 K across samples.

Abstract

A new method for preparation of single crystals of the superconducting intermetallic MgB2 compound from a Mg-Cu-B melt flux is presented. The high vapour pressure of Mg at elevated temperature is a serious challenge of the preparation process. The approximate thermodynamic calculations of the ternary Mg-Cu-B phase diagram show a beneficial effect of Cu, which extends the range of formation of MgB2 to lower temperatures. Within the as-solidified Mg-Cu-B melt flux the MgB2 compound forms plate-like single crystals up to a size of 0.2 x 0.2 x 0.05 mm3 or alternatively rims peritectically grown around MgB4 particles. AC-susceptibility measurements were conducted with specimen selected from different parts of the as-solidified flux containing MgB2 particles. Peritectically formed MgB2-particles display the highest transition temperature of Tc = 39.2 K and a relatively narrow transition width of DTc = 1.3 K. Other sections of the sample exhibit various superconducting transitions from Tc = 39 K to 7.2 K. This variation of Tc is attributed to a finite homogeneity range of the MgB2 compound whereas significant Cu solid solubility in MgB2 can be excluded.