  In order to understand a crucial role of orbital degree of freedom in the
magnetic structure of recently synthesized neptunium compounds NpTGa_5 (T=Fe,
Co, and Ni), we propose to discuss the magnetic phase of an effective
two-orbital model, which has been constructed based on a j-j coupling scheme to
explain the magnetic structure of uranium compounds UTGa_5. By analyzing the
model with the use of numerical technique such as exact diagonalization, we
obtain the phase diagram including several kinds of magnetic states. An
orbital-based scenario is discussed to understand the change in the magnetic
structure among C-, A-, and G-type antiferromagnetic phases, experimentally
observed in NpFeGa_5, NpCoGa_5, and NpNiGa_5.
