Magnon-Phonon-Hybridisation in FeCl2

High resolution inelastic neutron scattering measurements have been performed on the metamagnet FeCl2 using the Riso cold source triple axis. The purpose was to confirm the existence of strong hybridised modes in the antiferromagnetic phase of FeCl2. Constant Q surveys made in the |100| direction without an applied magnetic field established the splitting at the nominal point of intersection Kx=0.17 to be 0.32 meV. The application of a 8.9 KG field parallel to the c axis raised the degeneracy of the magnons enabling both polarisations to be identified. From the observed magnon energy gap at Kx=0 the gyromagnetic ratio g was estimated to be 4.01±0.2 in close agreement with the value 4.14 obtained by resonance experiments. Detailed measurements made in the |100| direction showed that the hybridisation takes place only between the magnons and the low energy TA⊥ (Σ2) phonon in agreement with theory.