Electromagnetism and magnetization in granular two-phase nanocomposites: A comparative microwave study

Abstract

Cold-pressed powder compacts in our experiments were prepared from commercial nanopowders of ZnO, Ni, Co and γ-Fe2O3. A systematic study of the room temperature effective permeability tensor of composite samples made of these nanophases is performed and provides a signature for the nonreciprocity of wave propagation in these nanostructures. Our measurements which cover a broad range of frequency in the microwave region provide a wealth of information leading to a much better understanding of the electromagnetic wave transport in nanogranular materials throughout this frequency range. We report our observations on the frequency and composition dependences of the permeability tensor components of a large set of nanocomposites (NCs) at different magnetic fields. It is found that mixing Ni nanoparticles with ZnO nanoparticles results in a smaller linewidth of the gyromagnetic resonance and an increased coercivity compared to a sample consisting solely of Ni nanoparticles. On the contrary, mixing of Co nanoparticles with ZnO nanoparticles resulted in the disappearance of the off-diagonal component of the permeability tensor and an increase in coercivity. Deviations of the saturation magnetization of Ni and Co in the Ni∕ZnO and Co∕ZnO NCs from bulklike values were observed. It is believed that the different microwave magnetic behaviors of the Ni∕ZnO and Co∕ZnO NCs are related to the difference in magnetic anisotropy of the Ni and Co particles. It is argued that surface and boundaries in the samples can play a significant role in the microwave magnetic response of these nanostructures. These NCs are promising for implementing the nonreciprocal functionality employed in many microwave devices, including isolators and circulators.