Giant caloric effects in charge–spin–lattice coupled transition-metal oxides

Abstract

The caloric effects of solids can provide us with highly efficient and environmentally friendly energy systems. Exploring novel caloric materials is challenging but critically important in developing future technologies. Typical solid caloric effects are magnetocaloric, electrocaloric, and barocaloric effects induced respectively by magnetic fields, electric fields, and pressure, and materials showing large caloric responses through the effects attract lots of recent attention. In this perspective article, novel transition-metal oxides that show giant caloric effects are highlighted. The compounds are NdCu₃Fe₄O₁₂ and BiCu₃Cr₄O₁₂ containing unusually high valence states of transition-metal ions and show charge transitions to relieve the electronic instabilities. The charge, spin, and lattice degrees of freedom in the compounds are strongly correlated and the primarily induced charge transitions cause unusual first-order magnetic phase transitions that provide significant latent heat. Importantly, the large latent heat can be utilized through the caloric effects in multiple ways. The details of the giant caloric effects in the charge–spin–lattice coupled transition-metal oxides are summarized and the mechanism of the effects is discussed.