Abstract
We present an extensive study of the ferromagnetic heavy-fermion compound U4Ru7Ge6. Measurements of electrical resistivity, specific heat, and magnetic properties show that U4Ru7Ge6 orders ferromagnetically at ambient pressure with a Curie temperature T-C = 6.8 +/- 0.3 K.
The low-temperature magnetic behavior of this soft ferromagnet is dominated by the excitation of gapless spin-wave modes. Our results on the transport properties of U(4)Ru(7)Ge(6 )under pressures up to 2.49 GPa suggest that U4Ru7Ge6 has a putative ferromagnetic quantum critical point (QCP) at P-c approximate to 1.7 +/- 0.02 GPa.
In the ordered phase, ferromagnetic magnons scatter the conduction electrons and give rise to a well-defined power law temperature dependence in the resistivity. The coefficient of this term is related to the spin-wave stiffness, and measurements of the very low temperature resistivity show the behavior of this quantity as the ferromagnetic QCP is approached.
We find that the spin-wave stiffness decreases with increasing pressure, implying that the transition to the nonmagnetic Fermi liquid state is driven by the softening of the magnons. The observed quantum critical behavior of the magnetic stiffness is consistent with the influence of disorder in our system.
At quantum criticality (P = P-c approximate to 1.7 +/- 0.02 GPa), the resistivity shows the behavior expected for an itinerant metallic system near a ferromagnetic QCP.