Nanocrystalline NiZn Ferrite/SiO2 Composites Synthesized by Different Methods

G. POZO LÓPEZ; M. DEL C. AGUIRRE; S. P. SILVETTI

San Carlos de Bariloche

Workshop; Pan-American Advanced Studies Institute (PASI) on Nano and Biotechnology; 2006

Nanomaterials represent a novel class of materials with physical and chemical properties that are different from those of the bulk material. Many researchers have been active in preparing granular systems in which magnetic nanocrystalline particles are embedded in a non-magnetic matrix, resulting in mean inter-particle distances that improve the magnetic properties as compared with the bulk magnetic materials, obtained by conventional process. The use of silica as a ceramic matrix provides good thermal stability, fairly good chemical inertness and large surface areas (Wu et al. J. Magn. Magn. Mater., 269, 150. (2004)) (Stefanescu et al., Acta Mater., 54, 1249. (2006)). In general, the major methods for the preparation of magnetic-silica nanocomposites are: mechanical milling, sol?gel methods, wet impregnation methods and subsequent heat treatments and flame synthesis. (Alcalá and Real, Solid State Ionics (2006) article in press). Much interest has been focused on NiZn ferrites due to the good correlation between their magnetic properties and technologic applications. NiZn ferrites have wide application prospect in the field of radio frequency electronic devices due to the high permeability in combination with giant resistivity. In this work, we synthesized NiZn ferrites-SiO2 nanocomposites by different methods: mechanical alloying, sol-gel, wet impregnation of nitrates in powders of amorphous silica and wet impregnation of NiZn ferrite synthesized by mechanical milling, in powders of amorphous silica. The composites obtained were annealed in air atmosphere at 1000°C. The resulting heterostructures were characterized by their structure and magnetic properties. The phase identification of the powders were performed using X-ray diffraction (XRD). Rietveld analysis was applied to the spectra and the average crystal size and lattice deformation were calculated according to the Scherrer and Stokes-Wilson equations, respectively. The magnetic measurements at room temperature were performed in a Vibrating Sample Magnetometer, while low temperature magnetization and hysteresis loops measurements were made in a SQUID magnetometer. The heat treatments of the diverse samples, promoted different phase selections. In the case of the samples prepared by Sol-Gel and wet impregnation with previously milled precursors, the composites consist in NiZn ferrite embedded in an amorphous silica matrix. Instead, for the sample prepared by wet impregnation method of nitrates, small quantities of other phases are present, hematite and willemite. The samples present also different magnetic behavior. These results are interpreted considering the different magnetic phases obtained, their crystal sizes and their mutual magnetic interactions.