Investigations on low energy product of MnAl magnets through recoil curves

Ferromagnetic τ-MnAl is being investigated as a potential candidate for the development of rare-earth-free permanent magnets. Maximum energy product (BH)_max is reported to be much lower than the expectations based on saturation magnetization (M _s), and magnetocrystalline anisotropy (K). We have reinvestigated the MnAl alloy to shed light on low (BH)_max. We made ribbons of Mn₅₄Al₄₆ alloy and annealed them at different temperatures. As-quenched ribbons (ASQ) are mainly made from non-magnetic ϵ-MnAl and a weakly magnetic phase. Low-temperature annealing (T _a = 300 °C) causes rapid improvement in magnetic properties without significant changes in ϵ-phase, suggesting that the weakly magnetic phase is metastable. Magnetic properties remain similar until transformation of ϵ- to τ-phase starts in T _a range of 350 °C-450 °C. Ribbons annealed at T _a = 450 °C are made up of a homogeneous Mn-rich τ-phase. Ribbons exhibiting high M _s (maximum ∼125 emu g^-1) show low coercivity (H _c, maximum ∼2.04 kOe), or vice versa. Highest (BH)_max obtained in the present study is 1.42 MGOe. It is considerably low due to a poor squareness (M _r/M _s, M _r is the remanence) of hysteresis curve in second quadrant. Based on detailed structural and magnetic characterization, we show that the metastability of ferromagnetic MnAl phase along with the presence of Mn-Mn antiferromagnetic coupling is responsible for low (BH)_max. Recoil measurements suggest that the ribbons are made from grains with a large distribution of K. This is due to the formation of τ-phase in a wide composition range (Mn ∼ 50 to 60 at.%) with the highest thermal stability ∼Mn₅₅Al₄₅. The wide distribution of K results in a low hysteresis squareness. Improvements in (BH)_max require minimization of Mn-Mn antiferromagnetic interactions, narrowing of anisotropy distribution, and breakdown in intergranular exchange. The improvements in (BH)_max seems difficult unless the addition of a third element is considered.