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

TL;DR

This paper presents a mean-field model based on crystalline electric field theory to explain diverse magnetic orders in NpTGa5 compounds with T=Co, Ni, Rh, emphasizing the role of quadrupole interactions.

Contribution

It introduces a unified CEF-based mean-field model incorporating dipolar and quadrupolar interactions to reproduce magnetic behaviors in NpTGa5 compounds.

Findings

Diverse magnetic orders are qualitatively reproduced.

Quadrupole degrees of freedom are crucial for magnetic diversity.

NpRhGa5 is near a multicritical point with competing quadrupole and dipole orders.

Abstract

Characteristics of magnetic transitions in NpTGa$_5$ with T=Co, Ni, Rh are explained in a unified way with use of a crystalline electric field (CEF) model of localized 5$f^4$ 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 NpTGa$_5$. The quadrupole degrees of freedom are essential to the diverse magnetic orders. It is argued that NpRhGa$_5$ is close to a multicritical point where quadrupoles and dipoles with different directions are competing to order.