Enhanced trapped field in MgB2 bulk magnets by tuning grain boundary pinning through milling

Abstract

The effects of the grain size, lattice strain, and microstructure on the trapped field properties of ball-milled MgB2 bulks were studied. A decrease in the in-plane lattice parameters and transition temperature and an increase in the in-plane x-ray diffraction peak widths occurred with milling as a result of grain refinement and carbon substitution. Microstructural analysis revealed that sub-micron-size fine MgB2 grains were well connected, resulting in increased grain boundary density. The trapped field of the magnetized bulk samples exhibited a large improvement with milling and reached 3.72 T at 5 K, which is the highest among MgB2 bulks prepared using pressureless synthesis. Excess milling resulted in degradation of the trapped field because of Tc reduction. The increased grain boundary density and electron scattering are considered to contribute to the high trapped field through quantitatively and qualitatively enhancing the grain boundary flux pinning.