Magnetically frustrated dynamics on the Cairo pentagonal lattice

We have synthesized a large high quality 2.35 g single crystal of Bi2Fe4O9, a quasi two-dimensional Cairo pentagonal lattice, a new motif in geometrical frustration. Bi2Fe4O9 shows AF order for T < 245 K while θCW = -1670 K, indicating strong frustration. Above Tn an unusual emergent state exists. In order to understand the emergent state is imperative that ground state is well characterised. Previous studies on a smaller single crystal and powder samples by other groups found spin waves in the energy range 5-100 meV with one model indicating a strong need for anisotropy while the other does not. Two of the spins couple strongly (24 meV) and the others less strong (3-6 meV), with all couplings found to be AFM. We have independently investigated the low-energy spin wave excitations at 10 K with high energy resolution using CAMEA and EIGER at PSI, and have uncovered significant differences to previous work. In particular we observe a clear double spin gap. We now wish to extend our inelastic neutron scattering studies to access higher energy spin waves across S(Q, ω) in the ordered phase and the disordered phases. This will improve the understanding of the Hamiltonian of this system in order and address the effect of geometrical frustration in this unique pentagonal system. MERLIN is ideally suited to probe the subtle effects found in this compound.