High electrical resistance composite magnets of Sm2Fe17N3 powders coated with ferrite layer for high frequency applications

This paper describes a novel type of high electrical resistance composite magnet. They were made by compacting or consolidating Sm-Fe-N powders coated with a continuous ferrite layer, which suppresses intergrain conductivity but sustains magnetic exchange interactions among grains. Sm-Fe-N powders (2 mu m in size) were coated with an "iron ferrite" (an intermediate between magnetite and maghemite) layer by ferrite plating, an aqueous process. They were compacted at 100 MPa to form a ferrite/SmFeN composite magnet (note that row Sm-Fe-N powders cannot be consolidated strongly enough to form magnets), and then consolidated to 92-94 vol % by the explosive consolidation technique. Coercivity and rectangularity of the compacting composite magnet decreased slightl by 2.0% and 1.4%, respectively, when compared to those of Sm-Fe-N powder compact. We also estimated the resistivity of fully dense ferrite/Sm-Fe-N magnet to be about 4000 mu Omega cm, ten times higher than the estimated value of a fully dense Sm-Fe-N magnet. Thus, the ferrite layer in our composite magnet retains magnetic exchange coupling among Sm-Fe-N grains, and yet suppresses intergrain electrical coupling to increase resistivity. This will decrease eddy current loss and improve high frequency characteristics of composite magnets. (C) 2008 American Institute of Physics.