Sintered Magnetically Hard Materials: Present and Future

Abstract

Sintering is an essential step for producing many permanent magnets, from the low-cost high-tonnage hard ferrites to the low-tonnage high-properties rare earth-cobalt magnets. Shaping those magnets from powders through a sintering step is not only a matter of densification to a solid and precise part, it must often retain, improve, or confer the essential magnetic property, i.e. coercivity. Present understanding of the relationship between microstructure and hard magnetic properties is briefly covered, with attention focused mainly on the particle size law. The paper reviews the technologies involved in the three main grades of sintered permanent magnets, Alnicos, ferrites, and rare earth-cobalt magnets; except for the first quality, grain growth must be avoided or controlled during sintering. Possible directions for future development of magnetically hard materials are discussed. PM/0252