Magnetic-field-induced detwinning of orientational domains in altermagnet $α$-MnTe: A single-crystal neutron diffraction study

Abstract: Altermagnets combine compensated magnetic order with momentum-dependent spin splitting, which makes them promising for future spintronic and magnonic applications, but their macroscopic responses can be strongly reduced by domain averaging. In hexagonal $α$-MnTe, three symmetry-related in-plane Néel-vector orientations and their time-reversal counterparts form six magnetic domain states, complicating both magnetic structure determination and measurements of the chiral magnon response. Using single-crystal neutron diffraction, we show that an in-plane field applied along the $[-1,1,0]$ direction produces a common magnetic domain orientation that saturates near $μ<em>0H=2$ T. A constrained refinement of 61 nuclear and magnetic reflections collected at 2 K and 2 T yields $\mathbf{M}\parallel[1,1,0]$, an ordered moment of $4.62(8)~μ</em>\mathrm{B}$ per Mn, and an orientational domain population at $r_A:r_B:r_C=0.97:0.03(5):0$ that is compatible with a nearly complete alignment of orientational domains. Furthermore, an abnormal behaviour in the temperature dependence of the selected reflections is observed in 60-88 K, its microscopic origin is discussed within the context of possible intrinsic piezomagnetic or magnetoelastic effects. Because unpolarized diffraction neither distinguishes domain repopulation from continuous Néel-vector rotation nor resolves time-reversal antiphase domains, the field-prepared state is a near-single-orientation state rather than a truly single-domain state. This non-contact domain control provides a practical complement to strain and field-cooling approaches for preparing well-defined magnetic states in bulk $α$-MnTe.