Rietveld analysis of intensively milled, exchange coupled, nanocrystalline Nd-Fe-B: Microstructure and magnetic properties

Rietveld investigations have been applied on intensively milled and annealed stoichiometric Nd2Fe14B powders to get detailed information on the microstructure. The obtained data (phase ratio, grain sizes) allow a complete understanding of the magnetic behaviour. Due to an incomplete phase transformation, residual iron dominates the magnetic properties for most samples. Therefore remanent values slightly above 0.8 T cannot clearly be attributed to a size controlled enhancement effect of the 2:14:1 phase. Experiments under optimum inert gas conditions, however, led to nanocrystalline Nd2Fe14B powder with residual phases below 2 wt.% and a remanence value of 0.87 T. This result reveals microstructure controlled remanence enhancement on single phase Nd2Fe14B prepared by intensive milling, so far only reported for melt spun material.