Magnetic phase diagram and multiferroicity ofBa3MnNb2O9: A spin-52triangular …

We have performed magnetic, electric, thermal, and neutron powder diffraction (NPD) experiments as well as density functional theory (DFT) calculations on ${\mathrm{Ba}}_{3}{\mathrm{MnNb}}_{2}{\mathrm{O}}_{9}$. All results suggest that ${\mathrm{Ba}}_{3}{\mathrm{MnNb}}_{2}{\mathrm{O}}_{9}$ is a spin-5/2 triangular lattice antiferromagnet (TLAF) with weak easy-axis anisotropy. At zero field, we observed a narrow two-step transition at ${T}_{\mathrm{N}1}=3.4$ K and ${T}_{\mathrm{N}2}=3.0$ K. The neutron diffraction measurement and the DFT calculation indicate a ${120}^{\ensuremath{\circ}}$ spin structure in the $ab$ plane with out-of-plane canting at low temperatures. With increasing magnetic field, the ${120}^{\ensuremath{\circ}}$ spin structure evolves into up-up-down ($uud$) and oblique phases showing successive magnetic phase transitions, which fits well to the theoretical prediction for the 2D Heisenberg TLAF with classical spins. Multiferroicity is observed when the spins are not collinear but suppressed in the $uud$ and oblique phases.