Preparation, magnetic properties and microstructure of lean rare-earth permanent magnetic materials

J. Bernardi, T.Schrefl, J. Fidler, T. Rijks, Kort, V. Archambault, D. Pere, S. David, D. Givord, J.F. Sullivan, P. Smith, J.M.D. Coey, U. Czernik and M. Grönefeld

J.M.M.M., 219 (2000) 186-198.

Nanocrystalline, lean rare-earth composite alloys around the nominal composition Nd3.25Tb1Fe72.75Co5B18 were prepared by various techniques, such  as melt-spinning, melt-extraction, splat cooling and mechanically alloying. The Tb and Co content have been found to be crucial in these alloys for achieving high coercivities, up to 500 kA/m. A typical value for the remanence is 1.05¯1.10 T. No crystallographic texture was observed, thus, the remanence enhancement was obtained by the spring magnet behavior. The best results have been found when using amorphous precursors and fairly high heating rates during the annealing treatment in an infrared furnace. The amount of Tb could be reduced which yields a lower coercivity but a higher remanence was obtained. TEM and Mössbauer analysis was carried out to determine the volume fraction of soft and hard phases. Optimized magnets contained typically a homogeneous nanocrystalline microstructure of about 50 vol% hard magnetic phase and about 50 vol% soft magnetic α-Fe plus (Fe,Co)3B with a narrow grain size distribution. The addition of Si or Nb and Cu improved the microstructure and the magnetic properties. Bonded magnets produced from optimized lean rare-earth magnetic powders show an improved corrosion resistance compared to magnets with higher rare-earth content.

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Feb. 13, 2001