Electrical resistivity rho(T) of the 5f ferromagnet UGa2 was investigated for single-crystal samples as a function of pressure and magnetic field. The Curie temperature monotonously increases from T$_{C}$ = 124 K under quasi-hydrostatic pressure up t
o 154 K at p = 14.2 GPa, after which it turns down steeply and reaches T$_{C}$ = 147 K at p = 15.2 GPa. At 20 GPa the compound is already non-magnetic. This dramatic variation is compatible with exchange interactions mediated by the 5f hybridization with the non-f states. The external pressure first enhances the exchange coupling of the 5f moments, but eventually suppresses the order by washing out the 5f moments. Such a two-band model is adequate for the weakly delocalized 5f states. The spin-disorder resistivity, which produces very high rho-values (300 muOmega.cm) is gradually suppressed by the pressure. In the paramagnetic state, this leads to a crossover from initial negative to positive drho/dT.
In this study we report the results of study of novel ternary $Np_2PtGa_3$ compound. The x-ray-powder diffraction analysis reveals that the compound crystallizes in the orthorhombic CeCu$_2$-type crystal structure (space group Imma) with lattice para
meters $a$ = 0.4409(2) nm, $b$ = 0.7077(3) nm and $c$ = 0.7683(3) nm at room temperature. The measurements of dc magnetization, specific heat and electron transport properties in the temperature range 1.7 - 300 K and in magnetic fields up to 9 T imply that this intermetallic compound belongs to a class of ferromagnetic Kondo systems. The Curie temperature of $T_C sim$ 26 K is determined from the magnetization and specific heat data. An enhanced coefficient of the electronic specific heat of $gamma$ = 180 mJ/(mol at. Np K$^2$) and -lnT dependence of the electrical resistivity indicate the presence of Kondo effect, which can be described in terms of the S = 1 underscreened Kondo-lattice model. The estimated Kondo temperature $T_K sim$ 24 K, Hall mobility of $sim$ 16.8 cm$^2$/Vs and effective mass of $sim$ 83 $m_e$ are consistent with assumption that the heavy-fermion state develops in $Np_2PtGa_3$ at low temperatures. We compare the observed properties of $Np_2PtGa_3$ to that found in $Np_2PtGa_3$ and discuss their difference in regard to change in the exchange interaction between the conduction and localized 5f electrons. We have used the Fermi wave vector $k_F$ to evaluate the Rudermann-Kittel-Kasuya-Yosida (RKKY) exchange. Based on experimental data of the (U, Np)$_2$(Pd, Pt)Ga$_3$ compounds we suggest that the evolution of the magnetic ground states in these actinide compounds can be explained within the RKKY formalism.
