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The electromagnetic (EM) properties of shielded gyromagnetic spheres are characterized with the aid of an electrodynamic model formulated for the transverse electric (TE) modes of an isotropic sphere, in a similar way as dielectric resonators are commonly analyzed. Broadband characteristics of geometric and energy filling factors are presented. A new measure of the magnetic loss tangent is introduced which is positive despite the intrinsic permeability of the gyromagnetic medium being negative, where existing definitions yield a non-physical negative value. The sample size dependence of those quantities is also assessed. The conclusions from the analysis are applied to investigate and de-embed ferromagnetic linewidth errors which can occur in broadband measurements of magnetic garnet spheres. Magnetic anisotropy fields and the gyromagnetic $g$ -factor are also estimated from the measurement data. Experiments were performed in a two-port subwavelength cavity, covering a frequency range of up to 28 GHz. Additionally, two-port rectangular resonant cavity linewidth measurements at a few modes have been conducted. Good agreement between the two methods is found if the systematic error is accounted for. [ABSTRACT FROM AUTHOR]

We report a comparative study of magnetic and magnetoabsorption properties of bulk and nanoparticles of Ga2-xFe_xO3 (x\approx 1.25) compounds synthesized using sol-gel method. The average size of Ga2-xFe_xO3 nanoparticles is found to be 100 nm. We observe an enhancement in the ferrimagnetic transition temperature (TC=354\ K) of Ga2-xFe_xO3 nanoparticles compared to TC=298\ K of the bulk Ga2-xFe_xO3. The bulk and nanoparticles of Ga2-xFe_xO3 exhibit a large coercive field (HC) of HC= 0.95 and 0.8 T at T= 10 K, respectively and we observe that the HC decreases dramatically with increasing temperature. Magnetoabsorption was studied with a LC resonance technique and we found a close correlation between the shift in the resonance frequency due to applied magnetic field and the coercive field measured using dc magnetization measurements. We discuss the possible origins of the observed effects in both bulk and nanoparticles of Ga2-xFe_xO3 compounds. [ABSTRACT FROM AUTHOR]

Ferrite nanoparticles are in the last years a matter of strong interest due to the fact that nanoscale materials possess size-dependent optical, electronic, magnetic, thermal, mechanical, and chemical properties that are comparable to or superior to those of bulk material counterparts, as well as its potential applications in sensors or microwave devices. Nickel ferrites, which are well-known technological materials in various electronic devices, were prepared in the form of nanoparticles by a modified sol-gel technique employing coconut oil, and then annealed at different temperatures in the 400° C-1200 ° C range. This route of preparation has been revealed to be one efficient and cheap technique to obain high-quality nickel ferrite nanosized powder. Sample particle sizes obtained with X-ray diffraction data and Scherrer's formula lie in 13-138 nm, with increased size with annealing temperature. Magnetic field-induced microwave absorption in nanoscale ferrites is a recent an active area of research, in order to characterize and explore potential novel applications. In the present work, microwave magnetoabsorption data of the annealed nickel ferrite nanoparticles are presented. These data have been obtained with a system developped in our laboratory, based on a network analyzer that operates in the frequency range 0-8.5 GHz. We can observe a resonance peak with applied fields of up to 400 mT. The results are interpreted on the basis of ferromagnetic resonance theory. [ABSTRACT FROM AUTHOR]