Soft magnetic properties of high-temperature nanocrystalline alloys: Permeability and magnetoimpedance

The technological applicability of FeCoNbBCu alloys is suggested in terms of measurements of room temperature magnetoimpedance and temperature dependence of magnetic permeability μr. Results for the Fe78-xCoxNb6B15Cu1 alloy series show that room temperature soft magnetic properties are enhanced in the lowest Co containing alloy (μr∼10 500 and magnetoimpedance ratio ∼60% at 1 MHz). However, permeability exhibits a smoother thermal dependence in the alloys with medium and high Co content. A tradeoff between magnetic softness and its thermal stability reveals the alloy with 39 at. % Co as the most suitable composition among those studied, characterized by a temperature coefficient of ∼ 0.02%/K from room temperature up to 900 K. This value is 1 order of magnitude smaller than those observed for FeSiBCuNb (FINEMET-type) alloys and Mn ferrites and extended over a much wider temperature range than in these materials.