Probing emergent phenomena in molecular multiferroic (NH4)2FeCl5· H2O*

Research into functional molecular magnets has been attract much attention because this class of material allows manipulating the coupling between magnetism and other intriguing properties, e.g., photo-activity, conductivity, superconductivity, and electric polarization with potential for many technological applications. Recently, (NH4)2[FeCl5(H2O)] is discovered to be a type-II molecular multiferroic exhibiting extraordinary coupled magnetic and ferroelectric properties. Applying modest magnetic field induces successive magnetic transitions with distinct magnetoelectric (ME) coupling and the underlying mechanism is not fully understood yet. In this talk, we present neutron diffraction studies that reveal microscopic details of phase competition in applied magnetic field. The comprehensive inelastic neutron scattering study further reveals that the magnetic interactions in (NH₄)₂[FeCl₅(H₂O)] are much more robust than the electric polarization in response to delicate reorganizations of the electronic degrees of freedom in an applied magnetic field. These findings provide unique information towards a deep understanding of the nature of the magnetoelectric effect and the enhanced magnetic frustration that led to the complex B vs. T and P vs. T phase diagrams in this compound.