Easy-plane ferromagnetic ordering and crystal-field ground state in the Kondo lattice CeCuSi

We report the successful growth of CeCuSi single crystals using a metallic flux method and the physical properties using structural, magnetic, electrical transport, optical, and heat capacity measurements. CeCuSi crystallizes in a hexagonal-bar shape, and single crystal x-ray diffraction confirms the ZrBeSi-type structure (space group P63/mmc). CeCuSi orders ferromagnetically below TC = 15.5 K with easy magnetization direction within the basal plane. The Ce3+ ions are situated within a triangular lattice with a point group of D3d . We perform a detailed crystalline electric field (CEF) analysis of the anisotropic magnetic susceptibility, the Schottky anomaly in heat capacity, and the Raman-active excitations. The results indicate a ground-state doublet with magnetic moment primarily in the basal plane, and a ferromagnetic interaction along both directions. The exponential behavior in resistivity and in heat capacity below TC can also be well explained by the ferromagnetic magnon model. We found that CeCuSi does not exhibit the CEF hard-axis ordering observed in many ferromagnetic Kondo lattice compounds. Our CEF analysis suggests that the exchange interactions along both axes are ferromagnetic, potentially explaining the absence of hard-axis ordering.