Anomalous thermal expansion and strong damping of the thermal conductivity of NdMnO3 and TbMnO3 due to 4f crystal field excitations

We present measurements of the thermal conductivity kappa and the thermal expansion alpha of NdMnO3 and TbMnO3. In both compounds, a splitting of the 4f multiplet of the R3 ion causes Schottky contributions to alpha. In TbMnO3 this contribution arises from a crystal field splitting, while in NdMnO3 it is due to the Nd Mn exchange coupling. Another consequence of this coupling is a strongly enhanced canting of the Mn moments. The thermal conductivity is greatly suppressed in both compounds. In TbMnO3, the main scattering process is resonant scattering of phonons between different energy levels of the 4f multiplets, whereas the complex 3d magnetism of the Mn ions is of minor importance. In NdMnO3, resonant phonon scattering on the 4f levels dominates at low temperature, while scattering on magnetic excitations becomes important at higher temperatur