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

TL;DR

This study reports the growth and comprehensive characterization of CeCuSi single crystals, revealing ferromagnetic order below 15.5 K with a ground state doublet and predominantly in-plane magnetic moments, challenging typical CEF hard-axis ordering in similar compounds.

Contribution

The paper provides the first detailed CEF analysis and magnetic characterization of CeCuSi, demonstrating ferromagnetic interactions along both axes and absence of hard-axis ordering in a Kondo lattice compound.

Findings

CeCuSi orders ferromagnetically below 15.5 K.

Ground state is a doublet with in-plane magnetic moment.

Ferromagnetic interactions are present along both axes.

Abstract

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 $P6_{3}/mmc$). CeCuSi orders ferromagnetically below $T_\textrm{C}=15.5$ K with easy magnetization direction within the basal plane. The 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 $T_\textrm{C}$ 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 (FM-KL) compounds. Our CEF analysis suggests that the exchange interactions along both axes are ferromagnetic, potentially explaining the absence of hard-axis ordering.