Severe tuning of permanent magnet properties in gas-atomized MnAl powder by controlled nanostructuring and phase transformation

Abstract Isotropic nanocrystalline MnAl particles have been synthesized by gas-atomization with permanent magnet properties tailored through a rapid-milling method followed by annealing at reduced temperatures. Unprecedented short milling times of 90 and 270 s have been used in the milling process. The study has allowed establishing a correlation between morphology, microstructure and magnetic properties, which has resulted in the possibility of tuning magnetization and coercivity by nanostructuring and phase transformation in a controlled manner. The choice of milling media (steel and tungsten carbide) with dissimilar densities determines the impact energy during the milling process and influences morphological and microstructural characteristics. As a consequence an increase in coercivity above 55% that of the optimized starting material while maintaining remanence has been achieved (steel media) by comparison with an extremely high-coercive powder (4.8 kOe) at expenses of magnetization (tungsten carbide). This has been possible by reduction of the crystalline size to the nanometer scale, inducement of efficient microstrain in the grains and coexistence of the ferromagnetic τ-MnAl phase, responsible of the magnetization, and the non-magnetic β-phase, playing a fundamental role in coercivity development. A common characteristic of both milling processes is the possibility of reducing the annealing temperature needed for achievement of optimized permanent magnet properties in about 75 °C, by comparison with the starting material. This decreased temperature, in combination with the extremely short processing times, makes of this route a promising one to be used in the production of isotropic nanocrystalline MnAl powder with tailored properties.