Coupled magnetic and ferroelectric states in the distorted honeycomb system Fe4Ta2O9

We report on the magnetic, thermodynamic, dielectric, and pyroelectric measurements on the hitherto unreported Fe4Ta2O9 . This system is seen to exhibit a series of magnetic transitions, many of which are coupled to the emergence of ferroelectric order, making Fe4Ta2O9 the only genuine multiferroic in its material class. We suggest that the observed properties arise as a consequence of an effective reduction in the dimensionality of the magnetic lattice, with the magnetically active Fe2+ ions preferentially occupying a quasi-two-dimensional buckled honeycomb structure. The low-temperature H-T phase diagram of Fe4Ta2O9 reveals a rich variety of coupled magnetic and ferroelectric phases similar to that observed in the distorted kagome systems.