Neutron scattering study of magnetic ordering and excitations in the ternary rare-earth diborocarbide Ce11 B2 C2

Neutron scattering experiments have been performed on the ternary rare-earth diborocarbide Ce11 B2 C2. The powder-diffraction experiment confirms formation of a long-range magnetic order at TN =7.3 K, where a sinusoidally modulated structure is realized with the modulation vector q= [0.167 (3), 0.167 (3), 0.114 (3)]. Inelastic excitation spectra in the paramagnetic phase comprise significantly broad quasielastic and inelastic peaks centered at ω 0, 8, and 65 meV. Crystalline-electric-field (CEF) analysis satisfactorily reproduces the observed spectra, confirming their CEF origin. The broadness of the quasielastic peak indicates strong spin fluctuations due to coupling between localized 4f spins and conduction electrons in the paramagnetic phase. A prominent feature is suppression of the quasielastic fluctuations, and concomitant growth of a sharp inelastic peak in a low-energy region below TN. This suggests dissociation of the conduction and localized 4f electrons on ordering, and contrasts the presently observed incommensurate phase with spin-density-wave order frequently seen in heavy fermion compounds, such as Cex La1-x Ru2 Si2.