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The magnetic response of Ni-Zn ferrites at microwave frequencies has been recently investigated by means of resonance techniques, by several authors. In this chapter, we present a review of recent results obtained on the resonant microwave absorption (electron paramagnetic resonance, EPR, and ferromagnetic resonance, FMR) in the X-band (9.5 GHz), of polycrystalline Ni-Zn ferrites (ZnxNi1-xFe2O4) for several temperature ranges. We begin at high temperatures in the paramagnetic state (T > TC, where TC is the Curie point); as temperature decreases, the onset of magnetic ordering is investigated, with its effects on the main FMR parameters. When experiments are carefully carried out, magnetic transitions can be detected as critical points in plots of the thermal behavior of the resonance line width. We investigate also the behavior of nonresonant properties by means of the low-field microwave absorption (LFMA). This absorption, which occurs at applied fields of the same order of magnitude than the anisotropy field, HK, of the sample, is providing valuable information concerning the magnetization processes. LFMA is typically measured in the -1 kOe TC since it depends on the magnetization processes in the ordered phase. For the 200 K < T < TC temperature range, a direct comparison of the anisotropy field calculated from LFMA and a calculation by using results of a direct measurement of HK on a ferrite single crystal. A very good agreement is obtained, thus confirming that LFMA is strongly dependent of the total anisotropy (magnetocrystalline, magnetoelastic and shape anisotropies) of the sample. We use as well a novel nonresonant microwave absorption technique known as magnetically modulated microwave absorption spectroscopy, MAMMAS. This technique is particularly well adapted to detect phase transitions of many types, as it is based on the change of microwave absorption regime during a change of crystalline, magnetic or electronic structure. MAMMAS is briefly described and applied to Ni-Zn ferrites.

The Temperature Behavior of Resonant and Non-resonant Microwave Absorption in Ni-Zn Ferrites