Ferromagnetic ultrafine alloy powders by hydrogen reduction of mixed halide vapor and its application to magnetic recording

The authors developed a chemical process to prepare ferromagnetic ultrafine alloy powders using the hydrogen reduction of mixed chloride (FeCl2, CoCl2, NiCl2) vapor around 1200K. The product is necklace like chain of primary particles. The size distribution of the primary particle is log-normal and the geometric standard deviation is about 1.5±0.1 regardless of the median diameter of 30-110 nm. A washing treatment by polar solvents reduced the residual chlorine from a few percent to less than 10ppm. The partial pressure of chloride mainly affects the primary particle size of the product. Reaction temperature also affects, but the residence time does not. A greater dilution and a higher temperature favor a smaller primary particle and result in a better magnetic property. Degradation of σsof the product UFP at 60°C and 90%RH is measured and found to be less than 10% in 5 days. The friction factor of the powder layer is 15 to 30% less than that of commercial metallic powder. Samples of floppy disk are made from these powders and evaluated. The overall performance cleared the present standards for high density magnetic recording.