Ferromagnetic half-metallicity in YBaCo2O6 and spin-state driven metal–insulator transition

Abstract

Cobaltates have rich spin-states and diverse properties. Using spin-state pictures and first-principles calculations, here we study the electronic structure and magnetism of the mixed-valent double perovskite YBaCo₂O₆. We find that YBaCo₂O₆ is in the formal intermediate-spin (IS) Co³⁺/low-spin (LS) Co⁴⁺ ground state. The hopping of e_g electrons from IS-Co³⁺ to LS-Co⁴⁺via double exchange gives rise to ferromagnetic half-metallicity, which well accounts for recent experiments. The reduction of both magnetization and the Curie temperature by oxygen vacancies is discussed, aided by Monte Carlo simulations. We also explore several other possible spin-states and their interesting electronic/magnetic properties. Moreover, we predict that a volume expansion of more than 3% would tune YBaCo₂O₆ into the high-spin (HS) Co³⁺/LS Co⁴⁺ ferromagnetic state and simultaneously drive a metal–insulator transition. Therefore, spin-states are a useful parameter for tuning the material properties of cobaltates.