  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. One
of the oldest and structurally simplest hidden-order compounds, CeB6, became
famous for an elusive phase that was attributed to the antiferroquadrupolar
ordering of cerium-4f moments. In its ground state, CeB6 also develops a more
usual antiferromagnetic (AFM) order. Hence, its essential low-temperature
physics was always considered to be solely governed by AFM interactions between
the dipolar and multipolar Ce moments. Here we overturn this established
perspective by uncovering an intense ferromagnetic (FM) low-energy collective
mode that dominates the magnetic excitation spectrum of CeB6. Our inelastic
neutron-scattering data reveal that the intensity of this FM excitation by far
exceeds that of conventional spin-wave magnons emanating from the AFM wave
vectors, 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.
