Characteristics of magnetic transitions in NpTGa5 with T = Co, Ni, Rh are explained in a unified way with use of a crystalline electric field (CEF) model of localized 5f4 electrons. The model takes a CEF doublet and a singlet as local states, and includes dipolar and quadrupolar intersite interactions in the mean-field theory. Diverse ordering phenomena are derived depending on the magnitude of interaction parameters, which qualitatively reproduce the experimentally observed magnetic behaviors in NpTGa5. The quadrupole degrees of freedom are essential to the diverse magnetic orders. It is argued that NpRhGa5 is close to a multicritical point where quadrupoles and dipoles with different directions are competing to order.
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