Stability of half-metallic antiferromagnet La₂VMnO₆, first-principles calculation study

The double perovskite La₂VMnO₆ with a Fm3m structure was predicted by Pickett et al. as a half-metal (HM) antiferromagnet (AFM) based on first principle calculations, while Androulakis et al.'s experimental result showed that La₂VMnO₆ also exhibits the same structure as a ferrimagnetic (FiM) state. For thoroughly understanding what happens in the double perovskite La₂VMnO₆ with a Fm3m structure, we first calculated the fixed cubic Fm3m structure before recalculating the structure through an optimization process. Our results show that La₂VMnO ₆ is a FiM state when found in the fixed and volume-relaxed cubic Fm3m structure. However, after full relaxation, it becomes a HM-AFM and remains the same state even on-site Coulomb interactions (U) are taken into consideration. This recalculation illustrates that the magnetic state of the Mn ion is sensitive to the crystal field of MnO₆ octahedron, and is the main reason behind the transformation of the magnetic state of La ₂VMnO₆ from a FiM to a AFM state after full relaxation. Because the difference of total energy ΔE^FiM-AFM is small (merely about 57.1 meV/f.u.) and shows up as nearly the same space group in both FiM and AFM states La₂VMnO₆, we conclude that in which state it is mainly determined by the crystal field of the MnO₆ octahedron, i.e. the distortion magnitude of the MnO₆ octahedron, which is related to the process of synthesis. Although Androulakis et al.'s experimental result show that this is a Fm3m/FiM state, we think that it is still possible for a I4/mmm/AFM state to exists, i.e. a HM-AFM La ₂VMnO₆.