ANALYSIS OF THE COMPLEX PERMEABILITY OF NiCuZn FERRITES UP TO 1 GHz WITH REGARD TO Cu CONTENT AND SINTERING TEMPERATURE

The frequency dependence of the permeability of Ni0.60-xCuxZn0.40Fe1.98O4 ferrites (x = 0, 0.10, 0.20, 0.30) with regard to the Cu concentration and heat treatment was investigated. Single-phase magnetic ceramics were prepared and characterised in terms of their reversible complex permeability in the 100 kHz to 1 GHz frequency range. The complex permeability spectra were analysed into three elementary magnetisation processes with changing weight factors as the Cu content or sintering temperature, T(SINT), increased. In addition to the spin rotation relaxation at 150-400 MHz, the contribution from reversible domain wall bowing rising at 20-80 MHz was identified. For higher Cu contents or T(SINT), the latter mechanism was suppressed to the benefit of reversible wall displacement in the range below 1 MHz. For domain wall motion, system dynamics theory and least-squares curve fitting of complex permeability spectra were used to distinguish between the high-frequency underdamped resonance (domain wall bowing) and the lower frequency overdamped relaxation dispersions (domain wall displacement). This distinction originated in the microstructural variance between the pinned low-energy domain walls in individual small grains and the cooperative rigid high-energy walls in large grains, respectively. The relevant transition in the morphology of NiCuZn ferrites, induced by Cu substitution or by high T(SINT), was identified using SEM and experimental results of dilatometry and density measurements. The densification process was reflected in the increased initial permeability. Moreover, on the basis of the recorded B-H loops, a maximum induction B(max) was derived which followed the density variation of the sintered specimens, contrary to the coercive field HC which was significantly decreased when the magnetic domain walls appeared.