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

Abstract

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 Nd₂ (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 Nd₂ (Fe_11.25 Co₂ Cr_0.75 )B with 40 % CaO dispersant. The H_C of isolated Nd₂ (Fe_11.25 Co₂ 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 Nd₂ Fe₁₄ 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 Nd₂ Fe₁₄ B magnets.

Keywords: coercivities; magnetic properties; maximum energy products; mechanochemistry; nanoparticles; rare earths; spin reorientation transition temperatures.