MAGNETIC-PROPERTIES OF NPGA3 AT HIGH-PRESSURES

High-pressure studies on NpGa3 were performed using Np-237 Mossbauer s pectroscopy, resistivity measurements, and x-ray diffraction, up to 9. 2 GPa between 1.3 and 130 K, up to 25 GPa between 1.3 K and room tempe rature, and up to 40 GPa at room temperature, respectively. The cubic AuCu3 crystal structure at ambient pressure is preserved up to 40 GPa. The bulk modulus B-0 is 75(2) GPa with B-0' = 6(2). NpGa3 orders anti ferromagnetically at ambient pressure at T-N = 67 K. The magnetic orde ring temperature increases up to similar to 200 K at 25 GPa. At 51 K a nd at ambient pressure a first-order antiferro- to ferromagnetic (AF-F ) transition occurs with a sudden reduction of the magnetic hyperfine field B-hf by similar to 15% when entering the AF phase. At elevated p ressure this transition is no longer observed. At 4.2 K B-hf, the valu e of the electric quadrupole coupling constant \e(2)qQ\ and the isomer shift S slightly but continuously decrease with reduced volume. Above T-N a negative logarithmic resistivity slope d rho/d In T < 0 is pres ent at ambient pressure and disappears at 3 GPa. The slight decrease o f B-hf at 4.2 K, the variation of e(2)qQ and S with pressure, and the suppression of d rho/d In T at similar to 3 GPa indicate 5f electron d elocalization. This delocalization, however, is much less pronounced t han in band magnets, as, e.g., in NpOs2. A magnetic phase diagram is s uggested that consistently explains the pressure variation of rho, B-h f and T-ord. We discuss the properties of NpGa3 in terms of the Kondo interaction and the Doniach phase diagram, and alternatively, within a model which includes 5f electron delocalization effects. This second model seems to be promising.