Measuring magnetisation reversal in micron-sized Nd₂Fe₁₄B single crystals by microbeam x-ray magnetic circular dichroism

Magnetisation reversal of micron-sized Nd₂Fe₁₄B single crystals with magnetisation as weak as 10⁻⁹ emu (1 µ m size) was studied. Single-crystal specimens (cylinders with diameter and height of 1 to 6 µ m) were prepared by focused-ion beam so that both the magnetic easy and hard axes were included in the basal plane. Their magnetic hysteresis loops were measured when they were rotated with respect to the cylindrical axis by using microbeam hard-x-ray magnetic circular dichroism (XMCD) under transmission geometry. It was found that coercivity is inversely proportional to the cosine of the angle between the magnetocrystalline easy axis and magnetic-field direction and that the magnetisation reversal is dominated by domain-wall pinning in two different modes. One is related to penetration of the reversed domain nucleated in a subsurface soft layer into the bulk hard phase, of which the hysteresis loops exhibit a single-stage abrupt jump in magnetization. The other mode is pinning of the walls within the bulk grain, of which the hysteresis loops exhibit a plateau. The multi-domain structure associated with the pinning was confirmed by XMCD mapping. The proposed method fills the gap between conventional bulk magnetic measurement and submicron-scale electrical-transport measurement for nanofabricated thin films and/or fine particles. It is expected to provide new insights into elemental magnetisation processes in micron-scale regions. (paper)