Symmetry analysis of the ferroic transitions in the coupled honeycomb system (Fe, Co, Mn)₄Ta₂O₉

Full text: Exotic phenomena such as spin liquid, spin-orbital entities, magnetic order induced multiferroicity (type ii) or quantum criticality have recently triggered extensive research on the ground state properties of frustrated magnetic systems. The ground states of these compounds are determined by the coupling of the spin to the orbital, charge and lattice degrees of freedom. One of the extensively investigated lattices is the honeycomb lattice due to the development of the Kitaev model for quantum spin liquids [1-2]. In this work, we are interested in the coupled honeycomb system M₄A₂O₉ (M=Fe, Co and Mn and A=Nb, Ta). All members have two crystallographically distinct M sites, which are in the distorted octahedral oxygen cages. These cages form edge-shared coplanar and corner-shared buckled honeycombs respectively which are interconnected in the perpendicular direction leading to competing exchange paths. The M=Co and Mn members were magnetoelectrics, whereas Fe₂Ta₂O₉ was reported to exhibit both magnetoelectric and (type ii) multiferroic phases depending on the temperature [3-4]. Magnetoelectrics and multiferroics are technically highly relevant with a variety of applications such as MRAMs and field sensors. However, the coupling mechanism is very complicated [5]. Furthermore, due to the group properties of the symmetry analysis methods such as representation analysis and magnetic space groups, the magnetic structure of the Nb counterpart Co₄Nb₂O₉ is controversially discussed. It is therefore apparent that the above discussed diversities of the properties are determined by the magnetic structure and the closely related electronic structure. These can be elucidated by investigating the structure and dynamics of these compounds, which will help to understand the emergence of different ground states and the diverse phase transitions in this family of materials In this work, we systematically investigate the magnetic and electronic structure of the (Fe, Co, Mn)₄Ta₂O₉ system. We combined several different techniques of neutron powder diffraction, inelastic neutron scattering, heat capacity, electronic band structure calculations and spin wave modeling based on linear spin wave theory. (author)