Twinning defects in MnAl permanent magnets: A density functional theory investigation

Abstract

We have investigated the impact of twinning defects on magnetic properties of ferromagnetic 'r-MnAl permanent magnets by employing density functional theory based simulations. The energy difference between twinned and pristine structures clearly suggests that twining defects can readily form, particularly during high temperature synthesis of MnAl. Moreover, ferromagnetic coupling between twin variants across boundary is energetically more stable as compared to antiferromagnetic coupling with zero net magnetization. Importantly, our non-collinear spin calculations reveal that aligning each twin variant with its respective bulk easy axis is energetically unfavorable. Rather, the easy axis of magnetization within twin region is directed parallel to the twin boundary (TB). This leads to a noticeable reduction (- 40%) in magnetic anisotropy energy within TB region. In light of these results, we discuss the impact of twinning on coercivity mechanism: TB is more likely to facilitate the nucleation of reverse domain instead of acting as pinning center, leading to deterioration of both remanence and coercivity.