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 (FeCl 2 , CoCl 2 , NiCl 2 ) 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 σ s of 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.