Coercivity of glass-coated Fe73.4-xCu1Nb3.1Si13.4+xB9.1 (0≤x≤1.6) microwires

Abstract

The effects of the geometry, thermal treatment and the amplitude of applied magnetic field on the coercivity behaviour of glass covered Fe_73.4-xCu₁Nb_3.1Si_13.4+xB_9.1 (x = 0; 1.1 and 1.6) have been investigated. Variations of the coercivity, H_c, with the geometry (d/D ratio, d –diameter of the metallic nucleus, D-total diameter, d/D ranging from 0.1 to 0.9) are attributed mainly to the internal stress acting on the metallic nucleus owing to the glass coating. Annealing temperature (300–700 °C) dependencies of H_c exhibit some peculiar differences with respect to that of the classical FINEMET ribbon, especially in the temperature range of 400–500 °C (very beginning of the nucleation of the nanocrystallization process) and at 650–700 °C (magnetic hardening owing to the iron borides segregation) depending in both cases on ratio d/D. With decreasing of d/D ratio the transition soft-hard magnetic character takes place in a narrower annealing temperature range with a rectangular hysteresis loop of H_c≈2400 A/m. The evolution of H_c with the applied magnetic field amplitude, H_o (<2500 A/m) for the as-prepared and treated at low temperature samples results to be quite linear. This linear behaviour of H_c (H_o) allows to assign a nucleation of domain walls mechanism mainly for the magnetization process connecting with a magnetic bistable behaviour, while the non-linear H_c (H_o) curves of samples with x = 1.6 treated above 650 °C and 700 °C suggest that different mechanism takes place.