Microstructure and Coercivity of Rare Earth Magnets

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High coercivity of 23 kOe was obtained on melt-spun Nd_<15>Fe_<77>B_8 ribbons with 40-100 m in thichness. The samples which showed high coercivity contained a new metastable boundary phase with 20-80 nm in width Nd_2Fe_<14>B grains were surrounded with this phase and they formed smooth and coherent interface structures. These microstructures indicated a strong pinning effect on the motion of the domain wall. The new phase was identified by analysis of electron diffraction patterns as a tetragonal structure with lattice constants of a=0.512 nm, and c=0.801 nm. By heat treatment over than 760 K, it was decomposed into fcc-Nd and a small amount of NdFe_4B_4. Interface structures between Nd_2Fe_<14>B and grain boundaries became incohent and also coercivity decreased.The vertical sections of Sm-Co-Cu-Fe quaternary and Sm-Co-Cu-Fe -Zr quinary phase diagrams were determined in composition range from 6 to 16 at%Sm. The 2/17 phase has hexagonal TbCu_7, hexagonal Th_2Ni_<17> and rhombohedral Th_2Zn_<17> type structures. Only alloys with TbCu_7 phase at high temperature have superior magnetic properties with high coercivity. The Sm content with 2/17 single phase of TbCu_7 type structure shifts to the Sm-poor side with addition of Zr.