Mechanochemically Processed Nd−Fe−Co−Cr−B Nanoparticles with High Coercivity and Reduced Spin Reorientation Transition Temperature

Nd−Fe−B magnets, possessing the highest energy product, are extensively used in cutting‐edge applications, including electrical machines and electrical vehicles. An environmentally benign and cost effective synthesis method of Cr alloyed Nd 2 (Fe,Co) 14 B magnetic nanoparticles using a dry mechanochemical process is reported. The method is solvent free, facile, energy efficient and scalable. The reduction of mixed oxides of Nd, Fe, Co, B and Cr is performed by using Ca. The coercivity ( H C ) of the nanoparticles is found to depend on the dispersant content, with the highest value obtained for Nd 2 (Fe 11.25 Co 2 Cr 0.75 )B with 40 % CaO dispersant. The H C of isolated Nd 2 (Fe 11.25 Co 2 Cr 0.75 )B nanoparticles and nanoparticles embedded in a CaO matrix is found to be 11.5 kOe and 14.4 kOe, respectively, largest values for heavy rare earth free Nd−Fe−B nanoparticles with reasonable saturation and remanent magnetization, regardless of synthesis route. Considering the density of Nd 2 Fe 14 B, an energy product of 14.2 MGOe is obtained for the nanoparticles. The thermal coefficient of remanence and thermal coefficient of coercivity for aligned samples are −0.06 % and −0.29 %, respectively, in the temperature range between 100 K and 400 K. The spin reorientation temperature is found to be ∼30 K less than that of bulk Nd 2 Fe 14 B magnets.