Thermal Conductivity and Magnon-Phonon Resonant Interaction in Antiferromagnetic FeCl2

The thermal conductivities $K_{\perp }$ and $K_{\parallel }$ in the $c$ plane and along the $c$ axis, respectively, have been measured in Fe${\mathrm{Cl}}_2$ between 1.2 and 80 °K. Both $K_{\perp }$ and $K_{\parallel }$ are found to exhibit, in a wide temperature range, an unusual behavior which reveals the presence of a strong spin-phonon coupling. The experimental results in the antiferromagnetic phase are interpreted on the basis of the magnon-phonon resonant interaction arising from single-ion magnetostriction. It is shown that the thermal conductivity of the resulting magnetoelastic modes can reasonably account for the experimental results: In particular, the anomalous behaviors of $K_{\perp }$ and $K_{\parallel }$ between 5 and 17 °K are explained by the effect of a large magnon scattering on the mixed modes. This large scattering leads to a negligible contribution of the spin waves to heat transport. From the $K_{\perp }$ results, the magnetoelastic coupling constant $G_{44}$ is found to be 3.5 meV.