Soft magnetic behaviour and permeability spectra in amorphous and nanocrystalline Fe80.5Nb7B12.5 alloys

The magnetic behaviour and the frequency dependencies of the complex magnetic permeability are studied in a series of heat-treated amorphous and nanocrystalline Fe80.5Nb7B12.5 samples with different volume fractions of the crystalline phase. The magnetic behaviour has been studied by vibrating sample magnetometry (VSM) and by room-temperature hysteresis loop measurements. The spectra of real and imaginary parts of the complex permeability, μ(f)=μ'(f)-jμ''(f), were measured using an impedance analyzer in the frequency range from 0.01 to 10 MHz. The obtained permeability spectra for the investigated nanocrystalline alloys show typical features of a domain wall relaxation dispersion. A marked deterioration of the soft magnetic behaviour takes place in the partially crystallized samples after annealing for 1 h at 803 K. This is reflected by a drastic reduction of permeability and a strong increase of relaxation frequency. An improvement of soft magnetic behaviour characterized by low values of coercivity and relaxation frequency as well as by high permeability is observed after annealing for 1 h at 883 K which resulted in an advanced degree of nanocrystallization in the samples