Insights into the giant magnetocaloric effect in BiCu3Cr4O12 with charge–spin–lattice coupling

The novel magnetocaloric effect of BiCu₃Cr₄O₁₂ is comprehensively investigated, revealing the intricate interplay of charge–spin–lattice coupling. Using first-principles calculations, this study examines charge disproportionation and phonon properties, uncovering the fundamental mechanisms behind structural and electronic instability, as well as strong correlations in the electronic, lattice, and magnetic interactions of the material. Magnetic exchange coupling constants are systematically derived for two distinct crystallographic phases: the low-temperature C2/m phase and the high-temperature Im phase. By incorporating the classical Heisenberg model and a mixed-phase model, this work explores the mechanism of the first-order magnetic phase transition coupled with structural transformation. The giant isothermal magnetic entropy changes are successfully reproduced using Maxwell relations for the total magnetization of mixed phases, demonstrating the accuracy of our models and methods. Additionally, applying the Clausius–Clapeyron equation to the isothermal entropy change highlights the critical role of lattice, electronic, and magnetic contributions in the magnetocaloric effect. These findings illustrate how strong charge, spin, and lattice correlations in BiCu₃Cr₄O₁₂ significantly enhance the isothermal entropy change compared to the isothermal magnetic entropy change with only the magnetic contribution. This study not only deepens the understanding of magnetocaloric materials but also offers valuable insights for developing energy-efficient refrigeration technologies.