Abstract
Granular systems composed by nanostructured magnetic materials embedded in a non-magnetic matrix present unique physical properties that depend crucially on their nanostructure. In this work, we have studied the structural and magnetic properties of NiZn-ferrite nanoparticles embedded in SiO2, a granular system synthesized by sol-gel processing. Samples with ferrite volumetric fraction x ranging from 6% to 78% were prepared, and characterized by X-ray diffraction, Mössbauer spectroscopy and vibrating sample magnetometry. Our results show the formation of pure stoichiometric NiZn-ferrite in the SiO2 matrix for x < 34%. Above these fraction, our samples presented also small amounts of Fe2O3. Mössbauer spectroscopy revealed the superparamagnetic behaviour of the ferrimagnetic NiZn-ferrite nanoparticles. The combination of different ferrite concentration and heat treatments allowed the obtaintion of samples with saturation magnetization between 1.3 and 68 emu/g and coercivity ranging from 0 to 123 Oe, value which is two orders of magnitude higher than the coercivity of bulk NiZn-ferrite.
Highlights
Granular magnetic solids (GMS) consisting of magnetic nanosized particles embedded in an immiscible insulating or metallic matrix receive enormous attention nowadays due to new magnetic properties presented by these peculiar structures, like high coercivity and giant magnetoresistance, very attractive for technological applications[1]
The hyperfine properties were investigated by Mössbauer spectroscopy (MS) at room temperature (RT) by using a 57Co(Rh) source and transmission geometry and magnetization measurements were obtained by vibrating sample magnetometry (VSM) at 300, 100 and 4.2 K
Samples with x ≤ 16% present strong superparamagnetic behaviour due to their small particles size and volumetric fraction and these spectra were fitted to a broad doublet
Summary
Granular magnetic solids (GMS) consisting of magnetic nanosized particles embedded in an immiscible insulating or metallic matrix receive enormous attention nowadays due to new magnetic properties presented by these peculiar structures, like high coercivity and giant magnetoresistance, very attractive for technological applications[1]. In a granular magnetic solid, two main parameters determine the magnetic properties of the composite material: the average diameter of the particles and the volumetric fraction of the magnetic phase Different systems such as Fe, Fe2O3 and Ni, dispersed in insulating matrixes like silica or alumina present considerable changes in the magnetic properties when compared with their equivalent pure bulk materials[2,3,4,5]. Ni Zn ferrites, technologically important materials, have spinel configuration based on a face-centred cubic lattice of the oxygen ions, with the unit cell consisting of 8 formula units of the type (ZnyFe1-y)[Ni1-yFe1+y]O4, were the metallic cation in ( ) occupy the tetrahedral sites (A) and the metallic cations in [ ] occupy the octahedral sites [B]6 Due to their broad application in the electronic industry, NiZn ferrites have been extensively studied. We have investigated the synthesis, by a sol-gel method, of granular Ni0.5Zn0.5Fe2O4 (NiZn-ferrite) embedded in a silica matrix, and studied the structural and magnetic properties of the obtained material
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