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 $P{6}_{3}/mmc$). CeCuSi orders ferromagnetically below ${T}_{\text{C}}=15.5\phantom{\rule{0.222222em}{0ex}}\mathrm{K}$ with easy magnetization direction within the basal plane. The ${\mathrm{Ce}}^{3+}$ ions are situated within a triangular lattice with a point group of ${D}_{3d}$. 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 ${T}_{\text{C}}$ 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.