Fe-Al 2 O 3 Nanocomposite: Synthesis and Magnetic Properties
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2 A. SANTOS,' W. A. A. MACEDO,' J. D. ARDISSON,1 A. D. C. VIEGAS, J. E. SCHMIDT Nuclear - CDTN, Tecnologia da 'Laborat6rio de Fisica Aplicada, Centro de Desenvolvimento Brazil. Horizonte, Belo 30123-970 / MCT, CNEN 2 Instituto de Fisica, Universidade Federal do Rio Grande do Sul, 91501 Porto Alegre, Brazil.
ABSTRACT We have investigated the synthesis and the structural and magnetic properties of Fe nanoparticles embedded in A120 3 matrix, a granular nanocomposite prepared by sol-gel processing. Samples with volumetric Fe content ranging from 20 to 62% were obtained starting from Al nitrate and Fe sulfate as precursors and the preparation method results initially in a mixture of Fe- and Al oxides. The conversion of Fe oxides into metallic Fe was done by calcination at 8000 C followed of reduction at 6000 C for 2 hours, in H2 atmosphere. Our results indicated the following phases in the amorphous alumina2 matrix, after reduction:3 oa-Fe, the main phase, a- and y-Fe2 03 , Fe 30 4 and some interstitial Fe ' and substitutional Fe + atoms in the A12 0 3 lattice. For the investigated system, Fe reduction rate was very sensitive to the sample porosity and, with the applied method, we have obtained reductions of the Fe atoms into metallic Fe ranging from 45 to 68%, preserving the mean diameter of the oa-Fe nanoparticles between 55 and 80 nm. VSM measurements at room temperature resulted in coercivity between 450 and 630 Oe and saturation magnetization between 40 and 110 emu/g. Magneto-transport measurements in samples with 25% metallic Fe (and 51% total Fe) reveal AR/R close to 2% at room temperature. INTRODUCTION The study of granular magnetic materials, composites of magnetic nanoparticles embedded in a nonmagnetic matrix, has attracted much attention in the last years since granular solids have been found to present interesting properties like giant magnetoresistance [1-3]. Several methods have been used to prepare magnetic granular Fe - ceramic systems. Sputtering [4], evaporation [5], ball milling [6] and different chemical routes are applied. Solgel method is one chemical route that present the advantages of high reproducibility and easy scale up from bench scale to the processing of high quantities. Sol-gel processing was applied, for example, to obtain Fe-Si0 2 [7], Ni-SiO 2 [8] and Fe-A120 3 [9] nanocomposites. In ref. [7], the applied reduction at 3500 C resulted in low reduction rate for Fe atoms. For Ni-Si0 2, reduction in H2 at a temperature range of 600 - 9000 C [8] allowed the complete reduction of Ni oxides to metallic Ni. In a previous work [9], we have investigated the sol-gel synthesis and the structural and magnetic properties of Fe nanoparticles homogeneously dispersed in alumina, within a range of low concentration of Fe (I - 2.5 vol.%). For these samples, by applying H2 -reduction at temperatures ranging from 600 to 10000 C, we have obtained conversion rates as high as 74% and a maximal coercivity of 819 Oe. Granular Fe-A120 3 powder prepared by ball milling within a broad Fe concentration range have
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