Magnetic properties, phase and structural state of Ga-doped τ-MnAl during ball-milling

An increased thermodynamic stability of the ferromagnetic τ-phase in Ga-doped Mn-Al alloys allows to investigate the influence of mechanical stress on structural state and the key magnetic properties of the material. This work focuses on the determination of the structural state and magnetic properties, as well as their correlation, of Mn 55 Al 36 Ga 9 powder during milling. Herein, an ingot with the nominal composition of Mn 55 Al 36 Ga 9 was fabricated via induction melting. It was then homogenized, quenched and annealed in order to obtain a high concentration of the τ-phase, and was subsequently grinded in a low-energy mode using a planetary ball mill for 1, 3 and 6 hours. The chosen milling regime, without an addition of surfactants, resulted in a formation of anisotropic flake-like particles, which primarily consisted of τ-phase. An absence of considerable changes in τ-phase content during the ball-milling shows its’ stability to degradation via the process. However, annealing of the milled Mn 55 Al 36 Ga 9 powder at 700 °C for 20 minutes led to an intensive decomposition of the τ-phase with formation of β-Mn. The coercive force (H c ) reaches 217 kA/m after 6 hours of grinding, while maximum magnetization I max value reaches 356 kA/m after 20 min of annealing following 6 hours of milling. This is due to a decrease in the antiferromagnetic exchange interaction between Mn atoms. Structural studies of the milled and annealed powders were provided by scanning and transmission electron microscopy, atomic-force microscopy and X-Ray diffraction analysis. • The τ-phase content remained constant through the whole milling process. • Milling resulted in formation of high-anisotropy flake-like particles. • The coercivity value (217 kA/m) was obtained after 6 h milling.