Ab-initio calculation of Exchange Interactions in YMnO3

Hexagonal manganites with the composition RMnO3 undergo several magnetic phase transitions due to frustration effects between the manganese centres, which are located on a triangular lattice. Additional local magnetic moments of the metal R give rise to a rich variety of different magnetic phases, especially in the low-temperature region. In the parent compound YMnO3 only the manganese centres can be magnetic, thus it allows the investigation of the Mn-Mn coupling strength. All-electron density-functional calculations are carried out for several relative spin arrangements of nearest and next nearest neighbouring Mn sites; the standard spin-polarised LSDA and an extension to non-collinear magnetism are employed. In order to extract the coupling constants a Heisenberg model is fitted to the total energy differences between the investigated structures.