Large Manipulation of Ferrimagnetic Curie Temperature by A-Site Chemical Substitution in ACu₃Fe₂Re₂O₁₂ (A = Na, Ca, and La) Half Metals

CaCu₃Fe₂Re₂O₁₂ and LaCu₃Fe₂Re₂O₁₂ quadruple perovskite oxides are well known for their high ferrimagnetic Curie temperatures and half-metallic electronic structures. By A-site chemical substitution with lower valence state Na⁺, an isostructural compound NaCu₃Fe₂Re₂O₁₂ with both A- and B-site ordered quadruple perovskite structures in Pn-3 symmetry was prepared using high-pressure and high-temperature techniques. The X-ray absorption study demonstrates the valence states to be Cu²⁺, Fe³⁺, and Re^5.5+. A ferrimagnetic phase transition is found to take place at the Curie temperature T_C ≈ 240 K, which is much less than that observed in A = Ca (560 K) and La (710 K) analogues. NaCu₃Fe₂Re₂O₁₂ possesses a larger saturated magnetic moment up to 9.4 μ_B/f.u. as well as a remarkably reduced coercive field less than 10 Oe at 2 K. Theoretical calculations suggest that NaCu₃Fe₂Re₂O₁₂ displays a half-metallic electronic band structure with complete spin polarization of conduction electrons in the minority-spin bands. The magnetic properties and electronic structures of the ACu₃Fe₂Re₂O₁₂ family are compared and discussed.