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Abstract:
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,LB, 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 A-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.
