PREPARATION AND MAGNETIC ANISOTROPY OF NANOCRYSTALLINE Fe0.13(CoxNi1-x)0.87FINE FIBERS

Small diameter ferromagnetic metal fibers (Fe, Ni, Co and their alloys fibers) with the anisotropic characteristics are attractive as fillers in polymer--matrix composites, advanced electromagnetic interference (EMI) shielding and wide--band microwave absorbing materials. Because lectromagnetic radiations with high frequencies only penetrate the near surface region of an electrical conductor, the composite material containing metal fibers with a small diameter is more effective than that with a large diameter. The filler of magnetic metal fibers with a diameter of 1 μm or less is therefore required technologically. Although iron fine fibers have been produced and used in several technological fields owing to a low cost, these iron fibers with a high specific surface area are generally not chemically stable due to easily oxidizing in an ambient atmosphere, which lowers their performance. The alloying can improve the anti--oxidation properties of ferromagnetic metal fibers and enhance their magnetic characteristics. In the present work, the nanocrystalline Fe_0.13(Co_xNi_1-x)_0.87(x=0.20, 0.30, 0.50, 0.80) fine fibers were prepared by the organic--gel thermal reduction process using citric acid and metal salts as the raw materials. The structure and morphology of the gel precursors and the fibers derived from these gel precursors in the thermal reduction process were characterized by FTIR, XRD and SEM. The magnetic properties for as--prepared alloy fibers were examined using vibrating sample magnetometer (VSM). The diameters of alloy fibers are in the range of 0.3 to 2 μm and these consist of grains with the size of about 34 nm. The experimental data show that the aligned nanocrystalline Fe_0.13(Co_xNi_1-x)_0.87 fibers exhibit an obvious magnetic anisotropy. This magnetic anisotropy is mainly effected by the magnetocrystalline anisotropy, shape anisotropy and magnetostatic interaction. The magnetizing ease axis for the nanocrystalline fiber is parallel to the fiber axis whilst the hard axis is perpendicular to the fiber axis. The nanocrystalline Fe_0.13(Co_0.50Ni_0.50)_0.87 fibers have a very high remanence ratio of 0.48.