Expanded Heisenberg Hamiltonians from a Mn/Bi DFT+U study on hexagonal antiferromagnet CaMn2Bi2: excitations and strain-controlled magnetic anisotropy switching

The manganese pnictide CaMn ₂ Bi ₂ exhibits narrow-gap antiferromagnetism with Mn atoms arranged in a puckered honeycomb structure, and is currently a promising candidate for ultra-fast light control of AFM states. In this paper, we perform a detailed study of the magnetic properties of CaMn ₂ Bi ₂ using density functional theory (DFT) combined with the Hubbard U correction and spin-orbit coupling, which accurately describe the magnetic configurations. In DFT+U approach, we apply an on-site U not only to Mn-3d orbitals but also to Bi-6p ones to improve the description of Mn–Bi hybridization and the small SOC-driven gap. We show that a standard Heisenberg spin model is insufficient to describe these magnetic excitations, and an extended model accurately describes these using local on-site magnetization terms, linked to the Néel vector and inspired by Hubbard-model physics. We further investigate the role of the spin-orbit coupling, and find that the magnetic anisotropy of CaMn ₂ Bi ₂ shows an easy plane, with the preferred magnetization direction being exchanged between axes in the plane by applying small strain values. This strain-tunable magnetization, driven by the interplay between spin-orbit interactions and lattice distortions, highlights the potential for controlling magnetic states in Mn-pnictides for future applications in spintronic and magneto-optical devices.