Investigation of crystal-field and spin-wave excitations in a honeycomb magnet BiErGeO5

We have investigated the magnetic properties of a two-dimensional distorted honeycomb magnet BiErGeO5 using various thermodynamic and μSR experiments. Er^3+ is a Kramers ion and expected to form ground state doublets at low temperature. In zero-field, heat capacity shows two broad peaks at T* ≃ 1.1 K and T** ≃ 8.3 K and a sharp anomaly at around TN ≃ 0.4 K indicative of a magnetic ordering. With increasing field, the broad peaks at T* and T** move towards high temperatures and then overlap with each other in high fields. These observations suggest that the two peaks at T* and T** are related to the CEF excitations of Er^3+ ions. Surprisingly, the temperature dependent zero-field muSR relaxation rate λ shows a plateau below 10 K, without any feature at TN ≃ 0.4 K. These contrasting results from heat capacity and zero-field muSR require further investigations. The low energy CEF levels significantly influence the ground state and hence the thermodynamic properties. In order to confirm the ground state and to model the magnetization and heat capacity data, we need to have an estimate of the CEF levels. Therefore, we propose to do inelastic neutron scattering (INS) experiments in zero-field in MERLIN and OSIRIS spectrometer. In MERLIN, we want to measure high-energy CEF in the temperature range of 5- 300 K. OSIRIS spectrometer will be used to evaluate low-energy CEF, short range correlations, and spin wave excitations below 5 K.