Cobalt-free permanent magnet materials based on iron-rare-earth alloys (invited)

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Abstract
The magnetic properties of rapidly quenched FeRM alloys where R=La,Y,Pr,Nd,Gd and M=B,Si,Al,Ga,Ge have been investigated over a wide range of chemical compositions. The samples are generally magnetically soft in the as-quenched state. Magnetic hardening is produced by annealing the samples around 700 °C. The best properties have been obtained in samples containing Pr and Nd together with B and Si. An energy product of 13 MGOe and a coercive field of 15 kOe have been obtained in a Fe76Pr16B5Si3 sample. The higher Fe content samples appear to be more promising with a potential energy product of 49 MGOe. Thermomagnetic data show that a structural transformation takes place upon heating the samples to 700 °C. The Curie temperature of the as-quenched phase is around 160 °C while that of the new phase is around 320 °C. Transmission electron microscope studies show fine precipitates (∼100 Å) dispersed in a matrix of different chemical composition. X-ray and electron diffraction data indicate that the precipitates have the Fe21R3B tetragonal structure. The high anisotropy of this phase together with its fine size and distribution give rise to the observed high coercive fields.