Anisotropic lattice dynamics and intermediate-phase magnetism in delafossite CuFeO2

Hyperfine interactions and Fe-specific lattice dynamics in CuFeO2 were investigated by nuclear resonance scattering methods and compared to ab initio lattice dynamics calculations. Using nuclear forward scattering the collinear spin structure at temperatures below about 11 K could be confirmed, whereas the nuclear forward scattering results in the intermediate temperature range between about 11 K and 14 K are incompatible with the assumption of a sinusoidal distribution of spins parallel to the c axis of CuFeO2. The critical behavior of the average hyperfine field at the phase transition at about 14 K further supports a three-dimensional model for the magnetism in this compound. Moreover, using nuclear inelastic scattering by the 57Fe Mössbauer resonance, Fe-specific lattice dynamics are found to be strongly anisotropic with stiffer bonds in the ab plane of the crystal. The powder averaged, Fe partial density of phonon states can be well modeled using ab initio calculations and low-energy phonons are found to deviate from classical Debye-like behavior, indicating spin-phonon coupling in this compound. Besides, the theoretical phonon spectrum exhibits typical characteristics for delafossite-type material.