Grain Size Dependent Magnetic Properties of Nanocrystalline Sm [BE] Co [BD][BJ] /Cu
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Grain Size Dependent Magnetic Properties of Nanocrystalline Sm2 Co17 /Cu L. Bessais1;2 , C. Dj´ega-Mariadassou1 , J. Zhang1 , V. Lalanne1 , and A. Percheron-Gu´egan1 1 LCMTR, UPR209, CNRS, 2/8 rue Henri Dunant, B.P. 28 F-94320 Thiais, France. 2 IUFM de Cr´eteil, F-94861 Bonneuil sur Marne, France. ABSTRACT The evolution of both micro structural and magnetic properties of the Sm 2 Co17 Cu powder, is studied as a function of soft co-milling time. The average grain size in the range 20 - 50 nm was determined by transmission electron microscopy coupled with x-ray diffraction using the Rietveld method. The particle shape and chemical distribution were investigated by elemental mapping, using wavelength dispersive x-ray analysis with electron microprobe analysis. The coercivity evolution shows that an optimum value of 6 kOe is obtained after 5 h co-milling. The microstructure analysis indicates that both materials are well mixed in nanometer scale. This technique appears as a potential route to synthesize nanocrystalline Sm 2 Co17 isolated by non-magnetic metal Cu. INTRODUCTION Tremendous interest in the binary rare-earth transition metal (RE-TM) alloys exists because of their superb magnetic properties [1]. Most commercial high strength magnets are made from either the binary compound SmCo5 or the compound Sm2 Co17 . New core-shell structure as Sm2 Co17 /Cu presents a great interest in the magnetic recording field. High-density storage material requires small magnetically independent particles above their superparamagnetic critical diameter (lower than 20 nm). Thus the powder coating process seems to be an appropriate way to obtain isolated ferromagnetic particles. Magnetic nanostructures often exhibit distinctly different behavior than their bulk counterparts [2]. The ideal medium for high-density magnetic recording with low noise consists of magnetically isolated nanoscale grains [3,4]. For such small grain high magnetocrystalline anisotropy is needed to avoid the magnetization fluctuation caused by thermal effect and demagnetizing field. In the last few years, Sm-Co system has received considerable attention due to the high anisotropy [5,6]. In order to magnetically isolate the grains different solutions have been proposed such as physical separation of the grains [7] and segregation of a nonmagnetic phase at the grain boundaries [8]. Current studies have investigated Sm-Co particles prepared by annealing Sm-Co thin film on different substrates and under layers. This study deals with the grain size dependence of magnetic properties of nanocrystalline Sm2 Co17 particle soft co-milled with Cu nanoparticules in order to get potential candidates for magnetic recording. The requirements for such goal are monodomain magnetically isolated grains of 20 to about 50 nm with magnetization 0 Ms 0.5 T and coercivity HC 5kOe. The special choice of Sm2 Co17 results from its semi-hard magnetic character. EXPERIMENTAL DETAILS The intermetallic Sm2 Co17 was prepared by mechanical alloying technique. High-purity powders Sm (99.99%
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