Preparation and magnetic studies of acicular Fe-Co alloy particles

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Abstract
Acicular α-FeOOH particles with particle length of about 0.2 μm and axial ratio of about 8 were used as the starting material to prepare acicular Fe-Co alloy particles. They were made by adsorbing Co(OH)2 on the surface of α-FeOOH particles, followed by dehydration, annealing, and reduction. In order to prevent sintering at high temperatures, the Co-α-FeOOH particles were first dehydrated at 300 °C and then coated with a thin layer of silica before high-temperature treatments. Furthermore, after reduction, the metallic powder was immersed in toluene to avoid oxidation. The particle structure of Fe-Co was determined from x-ray diffraction. It has been found that, up to 50 at. % Co, all the Fe-Co particles have the same bcc structure and similar lattice constant. The magnetic properties of Fe-Co alloy particles with different Co concentrations were investigated using a vibrating sample magnetometer. The specific saturation magnetizations σs are about 30% below those of bulk materials. The maximum value of σs at room temperature is obtained with 42 at. % Co. The coercivity that mainly originates from shape anisotropy is as high as 1250 Oe for the particles with 33 at. % Co. Furthermore, σs drops sharply with Si content while Hc peaks at 0.6% Si. The coercivities of the particles also depend on the crystallite size. Hc shows a maximum when the crystallite size is about 300 Å.

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