Cobalt-Induced Performance Instabilities of Mn–Zn Ferrite Cores

Stability and reliability of transformers and inductors in power applications strongly depend on the quality of the employed ferrite cores. Given the critical role of cores, Mn-Zn ferrites' sensitivity to their magnetic history and environmental storage conditions is investigated. To this end, Co-doped and Co-free Mn-Zn ferrites are prepared and primarily characterized in terms of structural, microstructural, and magnetic properties. The produced cores are subjected to endurance tests of technical interest comprising the response to a saturating magnetic bias and longtime storage under elevated temperature or humidity levels. With the exception of the damp heat test, prior imposition of a strong field or thermal aging process are found to induce significant instabilities to the power loss density of the Co-containing Mn-Zn ferrite cores in particular. Depending on the excitation, power loss may notably increase in the low- or high-frequency regime, whereas the effects can be either reversed with time or time independent. The proposed underlying mechanisms are based on the magnetically and thermally induced reorientation of Co
²⁺
cations, which affects magnetocrystalline anisotropy and domain wall kinetics. Studies of different stress tests are supported by analysis of complex permeability spectra and electrical characteristics.