Intergranular exchange interaction in nanocrystalline hard magnetic rare earth-iron-boron-based melt-spun alloy ribbons

Rapidly solidified nanocrystalline RE(Fe/Co) B (RE = Nd, Pr) alloys with enhanced hard magnetic properties were obtained by melt spinning. The effects of processing, composition, phase constitution, microstructure and temperature on the exchange interaction between nanocrystallites were studied by analysing the magnetization and demagnetization curves, characteristic loops and Henkel plots. Uniform single phase behaviour was observed for single phase and nanocomposite alloys with fine nano-grains, produced by optimum melt spin processing. It was shown, by constructing Henkel plots, that decreasing the hard phase grain size enhances the exchange coupling. Also, introducing and increasing the volume fraction of soft phase crystallites within the 2/14/1 nanocrystalline matrix, by progressively reducing the RE:(Fe/Co) ratio, also increased the strength of the exchange coupling, which was manifested by increased height of the positive peak in the Henkel plot. At elevated temperatures, the exchange coupling was further enhanced due to an increased exchange length (L-ex = (A/K)(1/2)), in spite of the fact that both the anisotropy constant K and the exchange constant A decrease with increasing temperature. Analysis of the microstructure parameters in the modified Brown equation, based on a nucleation mechanism of domain reversal and coercivity, indicate that Co substitution for Fe and introduction of soft alpha-(FeCo) nanocrystallites not only reduce the stray field effects associated with grain boundary roughness but also enhance the exchange coupling between neighbouring nanograins.