Thermal dependence of magnetic properties in nanocrystalline FeSiBCuNb wires and microwires

Abstract

The devitrification process for Fe 73.5Si 13.5B 9Cu 1Nb 3 amorphous samples leads to the possibility of obtaining rather different microstructural structures depending on the annealing temperature before and after the optimum `nanocrystalline’ state is achieved. In the present work, the magnetic properties of as-cast and annealed FeSiBCuNb wires and microwires have been reviewed and compared. The parallel modifications of structure and magnetic behaviour of this kind of samples can be summarised as follows: (a) structural relaxation within the amorphous state (annealing temperature up to around 400°C) leads to a reduction of coercivity, (b) the relative magnetic hardening when annealing at around 460°C is ascribed to the segregation of Cu-enriched clusters as well as of first nanocrystalline nuclei, (c) drastic magnetic softening related to the homogeneous nanocrystalline structure and (d) strong magnetic hardening as consequence of the growing of crystallites and appearance of new phases. The influence of the geometry of the samples and the microstructure present in each case in the bistable magnetic behaviour has been also analysed. Magnetic properties have been studied in the range of measuring temperatures between 30 and 660 K. The samples were heat treated previously at different temperatures up to 820 K. The magnetic behaviour of the samples has been explained considering the contributions mainly of the magnetoelastic and exchange anisotropy.