Study on itinerant ferromagnetism in rocksalt-type neodymium monoxide epitaxial thin film and heterostructure

概要

In this chapter, the electrical and magnetic properties expected for neodymium monoxide NdO were introduced. Among the rare earth (RE) oxides, the most stable compound is the rare earth sesquioxides RE2O3, which have trivalent rare-earth ions. RE2O3 are mainly paramagnetic insulators and electrically and magnetically inactive. In contrast, rare-earth monoxides REO with unusual divalent rare-earth ions show various magnetism and electrical conductivity. For example, in europium (Eu) oxides, Eu2O3 is a paramagnetic insulator, while EuO is a ferromagnetic insulator originating from localized 4f electrons.

About 40 years ago, REO were synthesized by reacting RE metals and their sesquioxides under high temperature and high pressure. However, it was not easy to obtain high-quality REO samples by this method, and the physical properties have scarcely been investigated. Recently, our group has succeeded in synthesizing REO as a thin film. In this method, high-quality REO samples are obtained by structural stabilization. Then, various physical properties of REOs have been revealed by the measurements of the thin films.

In this study, I focused on neodymium monoxide NdO. NdO is expected to have magnetism derived from the 4f electrons and electrical conductivity derived from the 5d electrons. As for the magnetism, there are two magnetic interactions between the 4f electrons in NdO. One is a ferromagnetic direct exchange interaction between the nearest neighbor neodymium ions, and the other is an antiferromagnetic superexchange interaction between the next nearest neighbor neodymium ions. In addition to these interactions, there is a possible contribution from the RKKY interaction mediated by the 5d electrons. Among these three interactions, the ferromagnetic interaction intensifies with decreasing interionic distance, but the antiferromagnetic interaction is almost independent of distance. So, the ferromagnetic interaction dominates in NdO with a small oxide ion, and NdO is expected to show ferromagnetism. The purpose of this study is the synthesis of NdO in the form of high-quality epitaxial thin film and the investigation of its electrical and physical properties. Application of NdO for a ferromagnetic heterostructure is also demonstrated.

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