Nanocomposite R)2)Fe)14)B/alpha -Fe permanentmagnets

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 Nd)3.5)Fe )91)Nb)2)B)3.5) 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 Y)3)Fe)62)B)14)-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 deg C. The intermediate phase in the case of Tb)6)Fe)87)Nb)1)B)6) is of the TbCu)7)-type. Very high remanences up to 145 emu/g with reduced 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 Nd)4)Tb)2)Fe )36)Nb)2)B)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