Investigations on low energy product of MnAl magnets through recoil curves

Abstract

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 (Ms), 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 (Ta = 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 Ta range of 350 °C–450 °C. Ribbons annealed at Ta = 450 °C are made up of a homogeneous Mn-rich τ-phase. Ribbons exhibiting high Ms (maximum ~125 emu g−1) show low coercivity (Hc, 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 (Mr/Ms, Mr 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.