Metastable Oxides with Magnetic Functionalities

Abstract

Inorganic solid-state materials comprise several types of polymorphs with different structures and properties. Among these, only one of the polymorphs is stable, whereas the others are metastable at ambient temperature and pressure. Sometimes, metastable phases exhibit more curious structures and properties and excellent functionalities than those of the stable phase. Such a phenomenon has been observed in magnetic properties of oxides as well. This paper presents some metastable oxides that manifest magnetic properties and functionalities never observed in their stable counterparts; ferrimagnetism of spinel-type ZnFe₂O₄ with high magnetization and a high magnetic-phase transition temperature; ferromagnetism observed in metastable Eu²⁺-based perovskite-type oxides, such as EuTiO₃, EuZrO₃, and EuHfO₃; and ferromagnetism of the amorphous oxides containing a large amount of Eu²⁺ ions. Amorphous oxides clearly exhibiting ferromagnetic transition are very rare.