Permeability–frequency spectra of Nanoperm alloys under different heat treatment conditions

Abstract

The dependence of the products of initial permeability ( μ i) and relaxation frequency of domain wall motion ( f 0) on different annealing temperatures was studied in Nanoperm alloys by using an impedance analyzer. It is shown that in addition to the domain wall pinning strength, the domain thickness and the resistivity have important influences on permeability–frequency spectra. Below the optimum annealing temperature, μ i f 0 decreases with increasing annealing temperature, which indicates that increase of annealing temperature leads to smaller resistivity, due to higher volume fraction of nanocrystalline phase, and larger domain thickness, due to stronger ferromagnetic coupling between grains. Beyond the optimum, μ i f 0 rapidly increases. This shows that the overannealing makes the domain thickness decrease rapidly, which results from drastic grain growth and the interruption of ferromagnetic coupling between grains. In actual application, the frequency range of application of nanocrystalline soft magnetic alloys may be expanded by sacrificing partial permeability by controlling heat treatment conditions.