Electron magnetic resonance study of transition-metal magnetic nanoclusters embedded in metal oxides

Abstract

Here, we report on the results of an electron magnetic resonance (EMR) study of a series of $\mathrm{Ni}∕\mathrm{Zn}\mathrm{O}$ and $\mathrm{Ni}∕\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ nanocomposites (NCs) to probe the resonance features of ferromagnetic (FM) Ni nanoclusters embedded in metal oxides. Interest in these NCs stems from the fact that they are promising for implementing the nonreciprocal functionality employed in many microwave devices, e.g., circulators. We observe that the EMR spectrum is strongly affected by the metallic FM content and its environment in the NC sample. We report the existence of broad and asymmetric features in the EMR spectra of these NCs. Our temperature dependent EMR data revealed larger linewidth and effective $g$ factor, in the range of 2.1--3.7 (larger than the free electron value of $\ensuremath{\approx}2$), for all samples as temperature is decreased from room temperature to $150\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The line broadening and asymmetry of the EMR features are not intrinsic properties of the metallic nanophase but reflect the local (nonmagnetic or magnetic) environment in which they are embedded. Furthermore, the results of a systematic dependence of the room temperature EMR linewidth and resonant field on the Ni content and the corresponding effective microwave losses measured in previous works show a remarkable correlation. This correlation has been attributed to the dipolar coupling between magnetic nanoparticles in the NCs.