Weak ferromagnetism and short range polar order in NaMnF₃ thin films

The orthorhombically distorted perovskite fluoride NaMnF₃ (Pnma space group) has been predicted to have a polar instability at low temperatures if a = c distortion is imposed. Epitaxial NaMnF₃ thin films were grown on cubic SrTiO₃ (100) substrates via molecular beam epitaxy (MBE) to test such prediction. Thin film structural quality as a function of the substrate temperature and film thickness was investigated using X-ray diffraction (XRD), in-situ reflection high-energy electron diffraction (RHEED), and atomic force microscopy (AFM). The best films were smooth and single phase grown with four different twin domains. In-plane magnetization measurements revealed antiferromagnetic ordering with weak ferromagnetism below the Néel temperature T_N = 66 K. For the dielectric studies, NaMnF₃ films were grown on 30 nm SrRuO₃ (100) layer used as bottom electrode grown via pulsed laser deposition. The complex permittivity as a function of frequency indicated a strong Debye-like relaxation times. A power-law divergence of characteristic relaxation time revealed an order-disorder phase transition at 8 K. The slow relaxation dynamic indicated the formation of super-dipoles (superparaelectric moments) that extend over several unit cells, similar to polar nanoregions of relaxor ferroelectrics.