Magnetoresistance and de Haas-van Alphen effect in U₃As₄ and U₃P₄

We measured the transverse magnetoresistance and de Haas-van Alphen (dHvA) oscillation in the ferromagnetic compounds U₃As₄ and U₃P₄. The magnet oresistance in both compounds increases as a function of Hⁿ (n ≃ 2) in a wide angular region, indicating that U₃As₄ and U₃P₄ are compensated metals. As for U₃P₄. the angular dependence of the magnetoresistance shows clearly dips for particular field directions. This implies the existence of a multiply-connected Fermi surface which favors open orbits along the 〈110〉 and 〈112〉 directions. We also detected several dHvA branches in U₃As₄ and U₃P₄. Most of them are due to the closed Fermi surfaces, which are explained by recent results of fully relativistic spin-polarized LMTO band calculations. The cyclotron effective mass is large, ranging from 5 to 70 m₀, and consistent with the electronic specific heat coefficient of 83mJ/K²·mol in U₃As₄ and 90 mj/K²·mol in U₃P₄.