The influence of quadrupolar ordering on magnetocaloric effect in rare-earth borides

Abstract

One of the alternative technology to vapor compression cooling is the magnetocaloric effect (MCE) that is a solid-state cooling application, in which heating or cooling occurs in magnetic materials when applying a magnetic field. The feature of MCE is an energy-efficient, noise-free, and environmentally friendly. For practical applications, it is essential to find candidates that are a giant magnetocaloric effect to achieve cooling efficiency. Recently, there are several attempts to give rise to the giant magnetic entropy change from multiple instabilities due to a strong correlation among the magnetic, electronic, and structure. It can be a promising strategy for exploring new candidates. The highly degenerated f-shells in the rare-earth boride system show a variety of coupling phenomena between the local degree of freedom: spin, orbitals, and lattice at least. It leads to various and complex phase diagrams in magnetic, electronic, and structural properties of the 4f-electron system. For example, the cooperative Jahn-Teller effect or the quadrupolar strain effect can be understood when the formation of a new shape of anisotropy in 4f-shells and the lower symmetry by the strain-mode in rare-earth sites is strongly coupled to the lattice degree of freedom. Another one is that the spin and orbital degree of freedom is strongly coupled. It gives rise to the complex magnetic structures such as a multi-k structure, a spin re-orientation, and a non-collinear magnetic structure by the strong spin-orbit coupling. Thus, it is definitely required to investigate MCE in rare-earth boride system in presence of multiple instabilities in order to explore the potential candidates.