\textbf{Ferroelectricity and Persistent Photocurrent in NaMnF}$_{\mathrm{\mathbf{3}}}$\textbf{ Thin Film}

Abstract: While studies of complex oxides have yielded many fascinating phenomena based on electron correlation effects, there are strong indications that complex fluorides may have analogous, or even enhanced properties. NaMnF$_{\mathrm{3}}$ is one such example, where multiferroic characters and magnetoelectric coupling have recently been predicted theoretically. Thin films of NaMnF$_{\mathrm{3}}$ with 50 nm thickness were grown on SrTiO$_{\mathrm{3}}$ substrates via molecular beam epitaxy. Ferroelectric properties of NaMnF$_{\mathrm{3}}$ were studied by piezoelectric force microscopy. Stable ferroelectric switching were obtained at room temperature by biased scanning probe. In addition, the sample also exhibits an interesting coupling between its out-of-plane polarization and in-plane electric field applied. At low temperatures, photocurrent was observed under 400 nm laser illumination. Amplitude and direction of such photocurrent can be hysterically controlled by external biases. This phenomenon likely results from the combined effects of built-in field in ferroelectrically polarized NaMnF$_{\mathrm{3}}$ thin film and the photocarriers generated in SrTiO$_{\mathrm{3}}$ substrate. These findings indicate great potential of complex fluorides in applications such as ferroelectric capacitors, switches, and memory devices.