Electromagnetic and magnetic properties of multicomponent metal oxides heterostructures: Nanometer versus micrometer-sized particles

摘要

We have measured the composition and frequency-dependent complex effective permittivities and permeabilities in zero applied field of a series of ZnO and ferrimagnetic γ-Fe_(2) O_(3) composites prepared by powder pressing. The overall features of the room temperature electromagnetic properties of these diluted magnetic semiconductor composites exhibit a strong dependence on the powder size of the starting materials. For instance, electromagnetic spectroscopy over the frequency range (300 MHz-10 GHz) shows that composites made of nanoparticles (N-type samples) display a strong increase of the real and imaginary parts of the permeability compared to composites made of micron-sized particles (M-type samples). The observed dielectric behavior as a function of composition is manifestly at odds with the predictions from the simple property-averaging continuum model of Bruggeman. Additionally, a gyromagnetic resonance in the gigahertz region of frequency has been established for N-type samples which is not observable in M-type samples. Examination of the dynamics of the magnetization distribution in N-type samples shows that the usual Landau-Lifshitz-Gilbert (LLG) equation can represent satisfactorily the gyromagnetic resonance line. Two important features of the data are the slight increase of the resonance frequency and the more important decrease of the width at half height of the gyromagnetic resonance line as the content of the magnetic phase is increased. It appears also that the value of the damping constant, characterizing the dynamics of magnetization, extracted from the fit of the gyromagnetic resonance line is consistent with previous experimental determinations. We attribute the remaining deviations in the fit and the discrepancies in the damping constant estimates namely to two approximations in our approach. First, the mean-field model considered here neglects composition fluctuations. Another source of the corrections are those due to the polydispersity of the nanoparticles. In contrast to the permittivity results, the comparison of the experimental values of the effective permeability, as a function of composition, with the analytical model combining the LLG and Bruggeman equations shows a good agreement. Given that the volume fraction of the organic binder has an effect on the shape of the gyromagnetic resonance line, we investigate also how this parameter affects the characteristics of the resonance mode. The analysis of the hysteretic behavior of these multiphase granular materials at room temperature indicates that the coercivity and the saturation magnetization normalized to the content of Fe_(2)O_(3) in the sample is strongly dependent on particle size, but remain practically constant over the entire Fe_(2)O_(3) volume fraction range investigated. Furthermore, the reduced remanence ratio is found much smaller than the Stoner and Wohlfarth's prediction concerning randomly distributed single domain particles without interaction. Possible origins for this difference have been analyzed. The suggestion, through Chen et al.'s analysis C. Chen, O. Kitakami, and Y. Shimada, J. Appl. Phys. 84, 2184 (1988), that the surface anisotropy is responsible for the coercivity behavior is quantitatively consistent with the experimental data concerning N-type samples.