An orbital-based scenario for the magnetic structure of neptunium compounds

Onishi, Hiroaki  ; Hotta, Takashi

We discuss a crucial role of orbital degree of freedom to understand the complex magnetic structure of actinide compounds from a microscopic viewpoint. Typical examples which we pick up here are UTGa and NpTGa, where T denotes a transition metal ion. An orbital degenerate Hubbard model constructed from the - coupling scheme is provided to investigate the magnetic structure of actinide compounds. By analyzing the model Hamiltonian with the use of unbiased numerical technique such as exact diagonalization, we obtain the phase diagram including several kinds of magnetic states. An orbital-based scenario is proposed to understand the change in the magnetic structure from G- to A-type antiferromagntic (AF) phases, experimentally observed in UNiGa and UPtGa. We also discuss the appearance of three kinds of AF phases, C-, A-, and G-type, for NpTGa (T=Fe, Co, and Ni).