Nanocomposite R/sub 2/Fe/sub 14/B//spl alpha/-Fe permanent magnets

We have studied the crystallization, crystal structure, structure morphology, and magnetic properties of R/sub 6/Fe/sub 87/Nb/sub 1/B/sub 6/ with R=Nd, Pr, Dy, Tb and Nd/sub 3.5/Fe/sub 91/Nb/sub 2/B/sub 3.5/ melt-spun ribbons with a microstructure consisting of a mixture of exchange-coupled magnetically hard (R/sub 2/Fe/sub 14/B/sub 1/) and soft (/spl alpha/-Fe) phases. The as-spun ribbons of R-rich composition crystallize in two steps; at first the Y/sub 3/Fe/sub 62/B/sub 14/-type+/spl alpha/-Fe phases are formed for R=Nd, Pr, and Dy and subsequently they transform to 2:14:1 and /spl alpha/-Fe upon heating above 700/spl deg/C. The intermediate phase in the case of Tb/sub 6/Fe/sub 87/Nb/sub 1/B/sub 6/ is of the TbCu/sub 7/-type. Very high remanences up to 145 emu/g with reduced remanences, M/sub r//M/sub s/, up to 0.8 were observed. The coercivity of the samples was found to vary with the R element and the content of the hard phase. The highest room temperature coercivity of 4.5 kOe was obtained in a Nd/sub 4/Tb/sub 2/Fe/sub 36/Nb/sub 2/B/sub 6/ sample. Electron microscopy reveals a grain size of 30-50 nm, which is much larger than the size (10 nm) predicted for optimum coupling.