Loss and Permeability Dependence on Temperature in Soft Ferrites

Wideband energy loss and permeability behavior of Mn-Zn and Ni-Zn ferrite ring cores has been investigated between 2 and 50 mT up to 140degC. The measurements have been performed by a fluxmetric method from direct current (dc) to 10 MHz and by a transmission line method from a few 10⁵ Hz to 1 GHz. While magnetic softening upon heating from room temperature always occurs at low frequencies, mixed behavior is observed, depending on the polarization value, on approaching the megahertz range. The loss versus frequency curves at different temperatures tend to coalesce towards the microwave regime. The overall loss and permeability properties are interpreted recognizing the separate roles of domain wall (DW) and rotational processes and their frequency dependence. Weakening of the magnetocrystalline anisotropy with temperature leads to reduced DW dissipation, while affecting the spectral distribution of the damped spin precessional frequencies. Eddy current mechanisms are not involved in such phenomena. Dissipation effects by DWs and rotations are prevalent in the lower and upper range of frequencies, respectively. This feature is quantitatively interpreted generalizing concepts and methods of the statistical theory of losses.