Effects of Grain Size on Magnetic and Dielectric Properties of Ferrites at Millimeter-Wave Frequencies

Constitutive material properties of microsize and nanosize hexagonal ferrites have been studied. A quasi-optical guided-wave spectrometer together with three backward-wave oscillators as the sources of radiation were used to characterize the samples in the broad frequency range from 30 to 120 GHz. The constitutive material properties have been found to be dependent on the internal morphology of the particles in the samples. X-ray diffraction studies have been carried out to study the structure of the particles. A model to estimate the ferromagnetic resonance frequency of the samples with different particle structures has been proposed. The ferromagnetic resonance for nanosize powders can only be observed if the particle size is about the size of a single domain. The resonance intensity is much weaker for the nanosize powder compared with the microsize powder. For the nanosize powders, the resonance frequency peaks shift to lower frequencies.