Study of NiFe/SiO 2 Nanocomposites
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Study of NiFe/SiO 2 Nanocomposites S. Hui 1, Y.D. Zhang1, T. D. Xiao 1, Mingzhong Wu2, Shihui Ge 2, W. A. Hines2, J. I. Budnick2, M. J. Yacaman3, and H. E. Troiani 4 1 Inframat Corporation, 74 Batterson Park Road, Farmington, CT 06032 2 Physics Department and IMS, University of Connecticut, Storrs, CT 06269 3 Department of Chemical Engineering, University of Texas, Austin, TX 78712 4 CNM and Texas Materials Institute, University of Texas, Austin, TX 78712 ABSTRACT (Ni 75Fe 25)v/(SiO2)1-v nanocomposites with v =0.5, 0.7, and 1.0, where 75 denotes the atomic percent of Ni in the Ni-Fe alloy phase and v denotes the volume fraction of the magnetic constituent in the composite, were synthesized using a wet chemical approach. The x-ray diffraction and TEM experiments show that the synthetic NiFe/SiO2 is a two-phase composite system in that an amorphous insulating SiO2 layer coats each Ni-Fe particle. The Ni-Fe particle is in a fcc Ni-Fe alloy state. Its size can be controlled over a rather large range between 5 nm to 70 nm by adjusting the reaction parameters. Particular attention was paid to reduce the chemical reaction temperature so as to insure the smallness of the particle size. Meanwhile, measurements of the saturation magnetization indicated that the higher the heat treatment temperature, the more complete the chemical reaction to form the Ni-Fe alloys from precursor materials. INTRODUCTION Magnetic nanocrystalline solids offer attractive properties for various applications such as the active component of ferrofluids [1], recording tapes [2], biomedical materials [3], separation techniques [4], as well as permanent magnets and soft magnetic materials [5-6]. Furthermore, nanocompositing opened new opportunities to develop novel magnetic materials [7]. Such materials provide great possibilities for the atomic engineering of materials with specific magnetic properties. Thin film nanocomposites with significantly improved high frequency properties have been developed based on the exchange coupling mechanism [8-9]. By coating magnetic nanoparticles with a second insulating phase, the following improvements could be achieved. Since the distribution of the two phases is homogenous on a nanometer scale, the aggregation of magnetic nanoparticles themselves is greatly limited. In this case, the magnetic materials can retain the nature of the nanocrystals. Therefore, the interaction of the magnetic particles could be adjusted and studied because the magnetic cores are kept at a well-defined distance. Coating the metallic magnetic nanoparticles prevents oxidation since the particles are extremely active and pyrophoric at ambient conditions. Coating with an insulating phase can improve the electrical resistivity of the magnetic materials. This maintains low eddy current losses for soft magnetic materials in high frequency applications. Coating also hinders the diffusion or the grain growth of metallic particles during the formation or sintering of the nanoparticles. A wide variety of routes have been employed to synthesize magn
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