NANOCOMPOSITE R(2)FE(14)B/ALPHA-FE AND SM-2(FE-GA)(17)C-X/ALPHA-FE MAGNETS

We have studied the crystallization, crystal structure, microstructure, and magnetic properties of melt-spun ribbons of R(6)(Fe-Nb)(88)B-6 with R=Nd, Pr, Dy, Tb and Nd-3.5(Fe-Nb)(93)B-3.5 consisting of a mixture of exchange-coupled magnetically hard R(2)Fe(14)B and soft alpha-Fe phases. The as-spun ribbons of R-rich composition crystallize in two steps; for R=Nd,Pr, and Dy at first the Y3Fe62B14-type + alpha-Fe phases and subsequently they transform to 2:14:1 and alpha-Fe upon heating above 700 degrees C. The intermediate phase in the case of Tb-6(Fe-Nb)(88)B-6 is of the TbCu7-type. Very high remanences up to 145 emu/g with reduced remanences ranging from 0.6 to 0.7 were observed. The coercivity of the samples was found to vary with the R element and the hard phase content. The highest room temperature coercivity of 4.5 kOe was obtained in a Nd4Tb2(Fe-Nb)(88)B-6 sample. The observed reversible demagnetization curves are characteristic of exchange-spring magnets. A two-phase microstructure was also obtained in Sm-Fe-Ga-C ribbons consisting of exchange-coupled 2:17 and alpha-Fe phases after annealing in the temperature range of 700-800 degrees C. The highest coercive force (12.8 kOe) was obtained in samples annealed at 800 degrees C. Annealing at 700 degrees C led to finer grains with lower coercivities (5 kOe) but higher remanences.