Study of the Switching Field in Amorphous and Nanocrystalline FeCoMoB Microwire

We have studied the frequency dependence of switching field in a wide range of frequencies in amorphous and nanocrystalline microwires with nominal composition Fe₄₀Co₃₈Mo₄B₁₈. Samples were heat treated for 1 h at different temperatures in a wide temperature range 20-600°C. Three regions in the frequency dependence of the switching field were identified. Drop of switching field at low frequencies up to 50 Hz is explained in term of structural relaxation. Above 50 Hz the magnetoelastic contribution of the switching field is dominant. The magnetoelastic contribution of the switching field can be fitted by the power law (H _sw ^¿ ~ f^1/n), giving exponent n equal 2 for frequency below 1000 Hz for all studied samples. Above 1000 Hz, the switching field reflects the structure of microwire being highly frequency dependent in as-cast sample and sample annealed at 450°C (where the microwire is quite inhomogeneous) while its frequency dependence is very weak for other annealing temperatures. Moreover, power exponent n gives non-physical values (~ 100) in this range.