Ferromagnetic fluctuations in YbNi₄P₂ measured by inelastic neutron scattering

YbNi₄P₂ is one of the very few heavy-fermion systems which allow the study of ferromagnetic quantum criticality. The Curie temperature T_C=0.17 K can be suppressed by substituting arsenic on the phosphorus site, without changing the ferromagnetic nature of the ordered state. The ordered moment, even of the unsubstituted compound, is only around 0.05 μ_B, which hinders elastic neutron scattering experiments. To gain microscopic insight into the nature of the interactions, we have studied the magnetic excitations of polycrystalline YbNi₄P₂ by time-of-flight neutron spectroscopy. For momentum transfers larger than about 0.6 Å⁻¹ we find a quasi-elastic response whose width at low temperatures is limited by the Kondo effect. In contrast, the low-energy magnetic response is distinctly different for Q approaching zero: At low temperatures, but still in the paramagnetic phase, susceptibility and lifetime of the spin fluctuations are strongly enhanced, indicating the proximity of ferromagnetism