Orientation of a MgB2 Powder in a Magnetic Field

Abstract

We have attempted magnetic orientation of MgB2 powder grains in an epoxy resin under a 7-T horizontal magnetic field. The horizontal field was crucial to get rid of gravitational sedimentation by rotating the mixture of MgB2 powder grains and the epoxy resin along the horizontal axis. Since we did not know the principal axes and the magnitude of the magnetic anisotropy tensor for MgB2, we devised different alignment schemes assuming two cases: case I for the easy axis along the c-axis and case II for the two easy axes perpendicular to the c-axis. For each case, we built different rotation devices: namely, for case I, the rotation axis was parallel to the magnetic field whereas, for case II, it was perpendicular to the magnetic field. For both cases, the powder grains should be aligned along the rotation axis. For assessment of the c-axis alignment, X-ray diffraction (XRD) and B-11 nuclear magnetic resonance (NMR) measurements were utilized. However, the XRD and B-11 NMR data for both cases were the same as those for the powder sample and showed no evidence of orientation. For possible scenarios, we suggest two possibilities: negligible anisotropy of the magnetic susceptibility tensor and the large size of the ground MgB2 powder particles still in multi-grains.