Anisotropic lattice dynamics and intermediate-phase magnetism in delafossiteCuFeO2

Abstract

Hyperfine interactions and Fe-specific lattice dynamics in ${\mathrm{CuFeO}}_{2}$ 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 ${\mathrm{CuFeO}}_{2}$. 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 ${}^{57}\mathrm{Fe}$ M\ossbauer 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.