Microwave absorption in Ni-Zn ferrites through the Curie transition

Microwave absorption measurements were carried out on Ni0.35Zn0.65Fe2O4 polycrystalline ferrites, at a constant frequency of 9.4 GHz (X-Band) and do magnetic fields (H-dc) in the 0-0.8 T range. The measuring temperature was varied from 300 to 500 K. A clear evolution. from ferromagnetic resonance (FMR) to paramagnetic resonance (EPR) was observed as a function of temperature which is related with the passage through the Curie point (similar to430 K), as observed by thermal variations of magnetic permeability. In addition, a low field (H-dc < 0.06 T) absorption signal was observed with the same phase as the FMR absorption. However, in contrast with FMR, this signal exhibited hysteresis by cycling the do field and disappears with temperature (similar to350 K) before the Curie point is reached. The assignment of this signal as due to non-resonant microwave absorption processes by the partially magnetized sample is discussed. (C) 2003 Elsevier B.V. All rights reserved.