Magnetic phase diagram and multiferroicity of Ba3 MnNb2 O9: A spin- 52 triangular lattice antiferromagnet with weak easy-axis anisotropy

We have performed magnetic, electric, thermal, and neutron powder diffraction (NPD) experiments as well as density functional theory (DFT) calculations on Ba3MnNb2O9. All results suggest that Ba3MnNb2O9 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 TN1=3.4 K and TN2=3.0 K. The neutron diffraction measurement and the DFT calculation indicate a 120 spin structure in the ab plane with out-of-plane canting at low temperatures. With increasing magnetic field, the 120 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.