Low energy excitations in multiferroic hexa-YMnO3*

In spite of many years of research, hexa-manganites continue to surprise us with their unusual magnetic and lattice properties which includes multiferroicity (antiferromagnetic + ferroelectric). The triangular arrangement of magnetic Mn ions gives rise to a highly frustrated magnetic interactions and allows coupling to the phonons. The recent discovery of a Gamma- point hybrid magnon-lattice mode in h-YMnO3 by polarized inelastic neutron scattering (Pailhes et al, PRB 79, 134409 (2009)) rises a question about the identity of the coupled phonons. To answer that question, we have combine infrared and Raman spectra with symmetry analysis of magnons and phonons.