Master's Defense: Emma Ynill Lenander

Magnetic frustration on the Cairo pentagonal lattice; Bi2Fe4O9. 

The quasi two-dimensional Bismuth Ferrite (Bi2Fe4O9) has a Cairo pentagonal lattice with predominantly antiferromagnetic interactions, leading to geometrical frustration in a fairly unexplored geometry. The material is studied with inelastic neutron scattering to measure the magnetic dynamics (spin waves) in the energy transfer range 0-45 meV. The spin waves have been modelled with software SpinW, by simulating numerically the excitations by fitting to the experimental data. In the data, a double spin gap is observed, which with the model is found to be single-site anisotropy of both iron sites, one being planar and the other axial. There is good agreement between the spin wave model and the experimental measurements. Additional unknown signals were observed in the data and are speculated to be phonons. These were not included in the modelling. Besides the spin waves, the experimental data also includes new interesting physics. A continuum of scattering located above the acoustic-like mode is emerging from the antiferromagnetic Bragg positions. This is scattering that cannot be captured by linear spin wave theory, and thus the general theory of long range magnon physics, and will need new theoretical developments.