Magnetic structure and effects of pressure on U₄PdGa ₁₂

We report on the magnetic structure, thermopower, compressibility, and electrical resistivity under pressure of the recently discovered heavy-fermion antiferromagnet U₄PdGa₁₂. The ordering temperature of the annealed polycrystalline U₄PdGa₁₂ is TN ∼45.5 K. The magnetic structure has been investigated by neutron diffraction. Extra reflections in the powder spectrum appear below ∼46 K, in good agreement with macroscopic measurements. The simplest magnetic structure compatible with neutron-diffraction data is a collinear antiferromagnetic structure described by the propagation vector q= [001]. In this case the ordered uranium moment amounts to 1.01 (6) μ_B and point along the a axis. Thermopower confirms the heavy-fermion features in this uranium intermetallic with positive and high values of Seebeck coefficient. The magnetic transition is clearly identified and induces an upturn below 45.5 K which is followed by a continuous decrease in S (T) at lower temperature. Electrical resistivity measurements of U₄PdGa₁₂ under pressure of up to 13.5 GPa indicate the disappearance of the magnetic ordering temperature T_N around P _c = 10 GPa while compressibility measurements performed by x-ray diffraction up to 25 GPa do not show any structural phase transition related to this magnetic collapse. At higher pressure (p>10 GPa) and at low temperature, a non-Fermi-liquid behavior is observed, with a T^5/3 dependence of the resistivity. This suggests the possible occurrence of a quantum critical point near P_c.