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Polycrystalline ferrites Ni 0.5- x Cu 0.12 Zn 0.4 Co x Fe 1.98 O 4- δ with different cobalt-substituted contents ( x  = 0, 0.003, 0.006, 0.009, 0.012, 0.015) are synthetized via the solid-state reaction method. The magnetocrystalline anisotropy constant is calculated based on the law of approach to saturation. As the amount of cobalt substitution increases, the absolute value of the magnetocrystalline anisotropy constant first decreases and then rises due to the positive compensation by the Co 2+ ions. The value of the magnetocrystalline anisotropy constant tends to be zero for the samples with cobalt substitution contents in the range of 0.003–0.006. Meanwhile, when the cobalt substitution content is 0.003, the NiCuZn ferrite shows minimum ferromagnetic resonance linewidth (9.87 kA/m) and low dielectric loss tangent (tg δ ε , 1.1 × 10 −4 ) at 9.4 GHz as well as a high magnetization ( M s , 406 kA/m) and high squareness ratio ( B r / B s , 0.83). Furthermore, the linewidth contribution is identified by the anisotropy and porosity linewidth broadening model based on the spin-wave approach, and the results show that porosity linewidth broadening is the predominant effect. [ABSTRACT FROM AUTHOR]

• HfO 2 additive could improve microstructure and enhance DC resistivity ρ. • P h is reflected by the area of hysteresis loops (B - H). • P e is affected by the combined action of microstructure and DC resistivity ρ. Core losses in manganese-zinc (MnZn) ferrites is an important factor governing the performances of power conversion devices such as power converters and switching mode power supplies. With the development trend of miniaturization, lightweight, and high frequency for power supply, it is worth emphasizing that the temperature characteristics of ferrite cores are closely related to energy transfer efficiency and reliability of power conversion devices and components. Herein, the effect of high valence ion additive HfO 2 on the microstructure and electromagnetic performances of MnZn ferrites was systematically discussed, as well as physical mechanism of temperature dependence of core losses. The variation of hysteresis loss has been correlated with the hysteresis loop area (B-H), and the effect of the ratio of square of average grain size to resistivity D 2/ ρ on eddy current loss has been taken into consideration. Remarkably, eddy current loss does not monotonically decrease with the increase of HfO 2 content via enhancing resistivity ρ , as well with the variation of average grain size D. The state-of-the-art MnZn ferrites with excellent temperature characteristics of core losses exhibits great potential in power conversion devices operating in a wide temperature range. [ABSTRACT FROM AUTHOR]