Structural And Magnetic Characterization Of Single-Phase Ni0.5Zn0.5Fe2O4 Nanoparticles Dispersed In SiO2 Matrix Synthesized By Sol-Gel Processing

G. POZO LÓPEZ; A.M. CONDÓ; S.P. SILVETTI; M. DEL C. AGUIRRE; S.E. URRETA

Mérida

Congreso; XI Congreso Interamericano de Microscopía - CIASEM 2011; 2011

Comité Interamericano De Sociedades De Microscopía

NiZn ferrites are versatile and technologically important magnetic materials, combining chemical stability with a high-Curie temperature, low magnetic coercivity, high resistivity values, and little eddy current loss in high frequency operations [1, 2]. Ferrite properties are known to be strongly influenced by composition, microstructure, and so by the synthesis methodology. Various processing routes to obtain nanoscale ferrites are reported in the literature such as mechanical alloying, sol–gel, co-precipitation, hydrothermal, combustion and citrate methods. The sol-gel technique is widely preferred, as large quantities of final product, with very pure and homogeneous nanostructures, are obtained at a low cost [3, 4]. Nevertheless, in preparing sol-gel derived NiZn ferrite/SiO2 nanocomposites some difficulties, as the appearance of different phases apart from the single ferrite spinel phase, remain unsolved [5, 6]. In this work, we focus on the synthesis of NiZn ferrite/silica (Ni0.5Zn0.5Fe2O4)x/(SiO2)1-x (x= 5, 20 and 50 wt.%) nanocomposites by a sol-gel process using tetraethylorthosilicate (TEOS) and metallic nitrates as precursors. These xerogels were further heat treated for 1 hour at 1273 K, in air atmosphere, to promote the precursor transformation into NiZn ferrite. The influence of the NiZn ferrite concentration on the structure, morphology and magnetic behaviour of the obtained nanocomposites is studied. The sample microstructures were characterized by X-ray diffraction and transmission electron microscopy using a Philips CM200 UT microscope. Samples for TEM were prepared by dispersing the powder in ethanol and depositing a drop on a holey carbon grid. RT magnetization curves were measured in a vibrating sample magnetometer and temperature dependence of magnetization was studied in a Quantum Design SQUID magnetometer. For the concentrations analysed, the resulting powders are made up of single phase NiZn ferrite nanocrystals, with fcc structure and a lattice constant close to the tabulated value, dispersed in an amorphous silica matrix; however, TEM studies revealed different microstructures. Samples SG5TT and SG20TT consist of nearly spherical, well-dispersed NiZn ferrite nanoparticles, embedded in the silica amorphous matrix, as illustrated in Figs. 1 and 2, respectively. In sample SG20TT the NiZn ferrite particles are smaller, more numerous and they are closer to each other than in sample SG5TT. A broader particle size distribution is found in sample SG5TT, (26 ± 10) nm, in comparison with sample SG20TT, (11 ± 3) nm. For sample SG50TT, two families of NiZn ferrite particles are observed: small round nanoparticles with a diameter of (10 ± 3) nm embedded in the amorphous silica matrix and large (30 nm to 70 nm) non-spherical particles, forming agglomerates outside the amorphous matrix (Fig. 3). Fig. 4 is a typical HRTEM image of sample SG20TT, showing a crystalline particle of about 10 nm embedded in the amorphous silica and in the inset the fast Fourier transform (FFT), corresponding to the NiZn ferrite zone axis. The different magnetic behaviours exhibited by the samples are well explained considering these microstructures. References [1] A.C.F.M. Costa et al., J. Magn. Magn. Mater. 256 (2003) 174. [2] K.H. Wu et al., Mater. Res. Bull. 40 (2005) 239. [3] A.S. Albuquerque et al., Mater. Res.-Ibero-Am. J. 2 (1999) 235. [4] C. Caizer, M. Popovici, C. Savii, Acta Mater. 51 (2003) 3607. [5] M. Stefanescu et al., Mater. Chem. Phys. 113 (2009) 342. [6] S. Ponce-Castañeda et al., J. Sol-Gel Sci. Technol. 25 (2002) 37.