Mean-field model for magnetic orders in NpTGa₅ with T = Co, Ni, or Rh

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.