Progress with insulating nanocomposites based on ferrite plating of Sm2Fe17N3 micropowders

Abstract Typical exchange spring magnets are composed of two phases, a rare-earth hard magnetic material and a metallic soft magnetic material, whose magnetization value is higher than those of hard magnetic phases, such as α-Fe, Fe-B, or Fe-Co. A novel high-electrical-resistance composite magnet was fabricated by consolidating micron-sized ferromagnetic nitride Sm 2 Fe 17 N 3 powder particles coated with a continuous nano-sized soft magnetic ferrite oxide layer, which suppressed intergrain conductivity but sustained the magnetic exchange interactions between grains [J. Appl. Phys., 103 (7) , 07E129-1-3, 2008]. A non-magnetic resin or ceramic insulator may suffice for the coated layer but gives rise to high electrical resistance of the magnet with large deterioration of magnetization. The soft magnetic ferrite oxide layer not only suppressed intergrain conductivity, but also only slightly reduced the magnetization of the magnet. At present, the only exchange spring magnet having a combination of a soft magnetic oxide and hard magnetic nitride is the ferrite/Sm 2 Fe 17 N 3 composite magnet. Sm 2 Fe 17 N 3 powder particles with a size of 2 μm were coated with an “iron ferrite” layer with a grain size of about 10 nm by ferrite plating, which is an aqueous process, following which the samples were consolidated by the explosive consolidation (EC) technique. The coercivity and rectangularity of the ferrite/Sm 2 Fe 17 N 3 composite magnet decreased slightly compared to those of a Sm 2 Fe 17 N 3 magnet. The fully dense ferrite/Sm 2 Fe 17 N 3 magnet exhibited a resistivity of about 4000 μΩ cm, which is ten times larger than that of a fully dense Sm 2 Fe 17 N 3 magnet. Thus, the soft magnetic ferrite layer in the composite magnet maintained the magnetic exchange coupling between ferromagnetic Sm 2 Fe 17 N 3 grains and simultaneously suppressed intergrain electrical coupling to increase the resistivity. This decreased the eddy current loss and improved the high-frequency characteristics of the composite magnets. Therefore, ferrite/Sm 2 Fe 17 N 3 composites are promising materials for magnets that are operated at high frequencies in advanced applications, such as electric vehicle magnets.