Room temperature multiferroic BaMnF4 films

The fluorides contain much stronger ionic bonds with F-, instead of the much stronger covalent bonds with O2-, which may break the empirical rule of the contradicting requirements of d-shell electrons in oxide perovskites. However, the magnetic ordering temperatures of fluorides are much lower than room temperature, which strongly limits their practical applications. In this work, BaMnF4 films have been successfully prepared on (0 0 0 1) Al2O3 substrate by pulsed laser deposition. The X-ray diffraction pattern confirms the (0 1 0) orientation of the films, and Mn ions are in +2 valence state from X-ray photoelectron spectroscopy. The clear observation of amplitude and phase hysteresis loops by piezoresponse force microscopy confirms the ferroelectricity at room temperature. Magnetization measurements show the hysteresis loop clearly, together with the bifurcation at 300 K between zero field cooling and field cooling temperature dependent magnetization cures, indicating the Curie temperature higher than room temperature. Furthermore, exchange bias effect is observed below 200 K, and its strength increases drastically below 50 K, due to the magnetization pinned by the spontaneous polarization. The improved ferromagnetism at room temperature in BaMnF4 films might be due to the epitaxial strain from the lattice mismatch between the film and substrate, which provides an efficient strategy to induce room temperature multiferroicity in fluorides.