NANAOCOMPOSITE R(2)FE(14)B/ALPHA-FE PERMANENT-MAGNETS

We have studied the crystallization, crystal structure, structure morphology, and magnetic properties of R(6)Fe(87)Nb(1)B(6) with R=Nd, Pr, Dy, Tb and Nd35F91Nb2B35 melt-spun ribbons with a microstructure consisting of a mixture of exchange-coupled magnetically hard (R(2)Fe(14)B(1)) and soft (alpha-Fe) phases. The as-spun ribbons of R-rich composition crystallize in two steps; at first the Y3Fe62B14-type + alpha-Fe phases are formed for R=Nd, Pr, and Dy and subsequently they transform to 2:14:1 and alpha-Fe upon heating above 700 degrees C. The intermediate phase In the case of Tb6Fe87Nb1B6 is of the TbCu7-type. Very high remanences up to 145 emu/g with reduce remanences, M(r)/M(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 Nd4Tb2Fe86Nb2B6 sample. Electron microscopy reveal a grain size of 30-50 nm, which is much larger than the size (10 nm) predicted for optimum coupling.