Crystallization of FeB alloys with rare earths to produce hard magnetic materials (invited)

Abstract

Among the first crystallized melt quenched alloys to exhibit interesting hard magnetic properties was (Fe_0.82B_0.18)_0.9Tb_0.05La_0.05. The high coercive force of this alloy results from a mixture of R₆Fe₂₃ and Fe₃B phases with a very fine grained microstructure (∼300 Å). The hard magnetic state only exists over a narrow range of annealing temperatures due to phase decomposition and grain growth at higher temperatures. Recent results on alloys with compositions corresponding to varying ratios of R₆Fe₂₃ and Fe₃B have suggested a new magnetically hard ternary phase near the composition R_0.15Fe_0.783B_0.067. In contrast to earlier alloys, the hard magnetic state of this material remains essentially unchanged over a rather wide range of annealing temperatures up to at least 800 °C. When R is mostly Nd or Pr, isotropic magnets with energy products as high as 13 MGOe can be produced.