Magnetic phase transition induced ferroelectric polarization in BaFeF4 with room-temperature weak ferromagnetism

The ${\mathrm{Ba}M\mathrm{F}}₄$ $(M=\mathrm{Fe}, \mathrm{Co}, \mathrm{Ni}, \mathrm{and} \mathrm{Mn})$ family consists of typical multiferroic materials, having antiferromagnetism at around liquid nitrogen temperature. In this paper, polycrystalline ${\mathrm{BaFeF}}₄$ has been prepared by solid state reaction. The slight deficiency of Fe leads to the coexistence of valence states of + 2 and + 3, facilitating the electrons to hop between the neighboring ${\mathrm{Fe}}²⁺$ and ${\mathrm{Fe}}³⁺$ ions through the middle ${\mathrm{F}}^{\ensuremath{-}}$ ion, leading to the strong double exchange interaction with weak ferromagnetism above room temperature. A bifurcation at $\ensuremath{\sim}170\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ between the zero-field-cooled and field-cooled temperature-dependent magnetization curves indicates the onset of two-dimensional antiferromagnetism, which is completed at $\ensuremath{\sim}125\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ with the sudden drop of magnetization. Despite being a type-I multiferroic, its magnetoelectricity (ME) can be evidenced by the pyroelectric current, which shows a peak starting at $\ensuremath{\sim}170\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and finishing at $\ensuremath{\sim}125\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The saturated ferroelectric polarization change of $\ensuremath{\sim}34$ $\ensuremath{\mu}\mathrm{C}/{\mathrm{m}}²$ is observed, which is switchable by the reversed poling electric field, and decreases to $\ensuremath{\sim}30$ $\ensuremath{\mu}\mathrm{C}/{\mathrm{m}}²$ under a magnetic field of 90 kOe. This ME can be qualitatively reproduced by first-principles calculations. Our results represent substantial progress to search for high-temperature multiferroics in ferroelectric fluorides.