Nanocomposite R2Fe14B/α-Fe and Sm2 (Fe-Ga)17Cx/α-Fe magnets

We have studied the crystallization, crystal structure, microstructure, and magnetic properties of melt-spun ribbons of R₆(Fe-Nb)₈₈B₆ with RNd, Pr, Dy, Tb and Nd_3.5(Fe-Nb)₉₃B_3.5 consisting of a mixture of exchange-coupled magnetically hard R₂Fe₁₄B and soft α-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 + α-Fe phases and subsequently they transform to 2:14:1 and α-Fe upon heating above 700 °C. The intermediate phase in the case of Tb₆(Fe-Nb)₈₈B₆ 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)₈₈B₆ 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 α-Fe phases after annealing in the temperature range of 700–800 °C. The highest coercive force (12.8 kOe) was obtained in samples annealed at 800 °C. Annealing at 700 °C led to finer grains with lower coercivities (5kOe) but higher remanences.