The effect of annealing temperature on MnAl alloy powder synthesized by electrodeposition method was investigated. Single L10-MnAl (τ-phase) with nearly stoichiometric composition was obtained by electrodeposition method. In annealed at 400°C sample coercivity of 12.3 kOe was observed. And by annealing at 450–550°C, metamagnetic behavior was observed. In these samples, the main phase was τ-phase although weak peaks of low-temperature Al-rich γ2 phase appeared. The order parameter S of the τ-phase exhibited a peak value with variation of the annealing temperature. The peak value was 0.90 at an annealing temperature of 500°C, where the metamagnetic behavior was most clearly observed. This indicates that the nearly perfectly ordered τ-phase shows metamagnetism and the Mn atoms occupying the Al sites play an important role in the metamagnetism of the τ-phase.

Manganese (Mn)-based strong magnets have long been a challenge because their 3 d half-filled nature, owing to the close proximity of Mn atoms, results in antiferromagnetic ordering. Among various Mn magnetic materials, L1 0 -MnAl (τ-phase) has received much attention since it shows ferromagnetism at a high Curie temperature despite the very short Mn–Mn distance. However, because of the difficult synthesis of the stoichiometric and perfectly ordered τ-phase, its intrinsic magnetic properties and mechanism are unclear. Here, we show the first observation of antiferromagnetism, having sixfold magnetic superstructure along the c-axis, in stoichiometric and chemically ordered τ-phase. Moreover, we found that super-exchange interaction between Mn atoms via p -electrons of Al atoms causes antiferromagnetism in τ-phase. The ferromagnetism in the conventional Mn-rich τ-phase results from the suppression of the super-exchange interaction due to the substitution the excess Mn atoms for the Al atoms. The current study of Mn-based magnetic materials mainly focuses on the lattice constant engineering based on the simple Beth-Slater picture of direct exchange. These findings present effective ways to obtain high magnetization without antiferromagnetic ordering.