Temperature dependence of broadband losses in Fe-Co

We investigate the temperature dependence of the energy loss W(f) of 0.10 and 0.20 mm thick Fe-Co-V sheets (Vacoflux and Vacodur) in the range -50 °C ≤ T ≤ 155 °C. The measurements, performed from DC to f = 5 kHz on ring samples and Epstein strips, show that W(f) passes through a minimum value around room temperature at all tested polarization values (1.0 ≤ Jp ≤ 1.9 T). The largest effect occurs under quasi-static regime and declines with frequency, depending on the sheet thickness and the ensuing role of the dynamic loss. The somewhat abnormal increase of the quasi-static loss Whyst with temperature, which contrasts with a concurrent decrease of the magnetocrystalline anisotropy constant, is interpreted in terms of temperature-dependent internal stresses and their change with T. The stresses are assumed to descend from the different thermal expansion coefficients of the ordered and disordered structural phases, a conclusion made plausible by the highly magnetostrictive properties of the material, dwelling in a low anisotropy environment. The AC properties are treated by adapting the loss decomposition to the inception and development of a non-uniform induction profile across the sheet thickness (skin effect) at high frequencies. The classical loss component is calculated via the numerical solution of the Maxwell’s diffusion equation, where the magnetic constitutive equation of the material is identified with the normal magnetization curve. It turns out that the so-found Wclass(f) and the resulting excess loss Wexc(f) are moderately dependent on temperature and W(f) eventually tends towards a slow monotonical decrease with T at the highest frequencies. The large advantage brought about by a reduced sheet thickness (0.10 mm vs. 0.20 mm) in the kHz range, caused by a correspondingly reduced Wclass(f) and Wexc(f) figures, is highlighted