Pressure-induced ferroelectric reentrance in a molecular magnet*

Multiferroics have attracted tremendous research interests with their rich physics and potential in constructing next-generation multifunctional devices. Metal-organic framework molecular magnets have been a recent focus in single phase multiferroics investigations. These materials are usually bonded by intermolecular forces and their magnetic moments interact through H-bond super-exchange couplings. As a result, pressure induced lattice changes would play a significant role in exploring for complex phase transitions in these materials. We investigated the crystal structure, magnetism and ferroelectricity changes of a molecular magnet (NH₄)₂FeCl₅ H₂O under pressure. Interesting magnetic transitions correlated with ferroelectricity were observed at systematically increased pressures. The strong couplings between magnetic phase, ferroelectricity and the crystal structure are identified that originate from the reorientation of ammonia under pressure. The interplay of spin, charge and lattice under pressure in this molecular magnet system brings new insights into the study of multiferroics and new routes in designing multi-functional materials.