Exploring the pathways for enhancing the hard magnetic properties of binary Al-55at.%Mn Heusler alloy through mechanical alloying

Abstract

This work reports enhancing the hard magnetic properties of a binary ferromagnetic Heusler alloy based on Mn-Al system through mechanical milling. We report the processing induced evolution of magnetic properties for two sets of alloys, both having the same stoichiometric composition of Al-55at.%Mn with one of them containing high temperature ε phase while the other consisting of only metastable ferromagnetic τ phase. No effect of milling on the magnetic properties of ε phase could be detected due to its nonmagnetic nature. However, subsequent annealing at 350 °C for 30 min after milling results in structural change and exhibits magnetic response. The phase transitions were found to depend on prior milling history. The highest saturation magnetization and coercivity were obtained for 4 h milled sample that is annealed at 350 °C for 30 min with values of 23 emu/g and 5.2 KOe, respectively. In the case of samples with initial microstructure consisting of grains of only metastable τ phase, no decomposition could be observed when milled for a period up to 9 h. Additionally, it was observed that after 3 h of milling, the saturation magnetization value reduces to 24 emu/g and coercivity increases to 5.2 KOe from an initial values of 116 emu/g and 0.98KOe respectively. Further milling causes a decrease in both the values. Annealing of the 3 h milled powder at 350 °C for 30 min, resulted in a slight decrease in coercivity (Hc = 4.9 KOe) while a significant increase in saturation magnetization (34 emu/g) value could be observed. Experimental results suggest that magnetization reversal is domain nucleation controlled and that the nonmagnetic phases (β + γ2) present can act as the pinning sites.