Spin fluctuations in Ce_1-xLa_xB₆ and Ce₃Pd₂₀Si₆

Heavy-fermion metals exhibit a plethora of low-temperature ordering phenomena, among them the so-called hidden-order phases that in contrast to conventional magnetic order are invisible to standard neutron diffraction. The simple-cubic hidden-order compounds CeB₆ and Ce₃Pd₂₀Si₆ are famous for an elusive phase attributed to the antiferroquadrupolar ordering of Ce 4f moments. In the ground state, these systems also develop a more usual antiferromagnetic (AFM) order. In our recent inelastic neutron scattering experiments, we investigated the spin dynamics in these related compounds, including their dependence on magnetic field and La doping. In CeB₆, we discovered an intense ferromagnetic (FM) low-energy collective mode that dominates the magnetic excitation spectrum of CeB₆, thus placing CeB₆ much closer to a FM instability than could be anticipated. This propensity of CeB6 to ferromagnetism may account for much of its unexplained behavior, such as the existence of a pronounced electron spin resonance, and should lead to a substantial revision of existing theories that have so far largely neglected the role of FM interactions. On the other hand, the absence of such zone-center fluctuations in Ce₃Pd₂₀Si₆ demonstrates that it they are unrelated to the magnetically hidden multipolar order parameter. Instead, the paramagnetic state of Ce₃Pd₂₀Si₆ is characterized by a diffuse quasielastic scattering pattern with a broad momentum-space structure peaked near the (111) wave vector, which represents the corner of the unfolded Brillouin zone. Its symmetry suggests that it stems from the simple-cubic Ce sublattice occupying the 8c Wyckoff site, whereas the crystallographically inequivalent 4a site remains magnetically silent in this material.