Unusual Ferromagnetic-to-Antiferromagnetic-to-Ferromagnetic Transitions in Cu²⁺ (S=1⁄2) Cubic Spin Lattice of A-Site Ordered Perovskites

Abstract

Very unusual transitions from ferromagnetic CaCu₃Ge₄O₁₂ to antiferromagnetic CaCu₃Ti₄O₁₂ to ferromagnetic CaCu₃Sn₄O₁₂ are observed in solid solutions of the A-site ordered perovskites. The effective magnetic interaction between Cu²⁺ spins changes gradually and systematically from ferromagnetic to antiferromagnetic to ferromagnetic in the solid solution. At intermediate compositions the perfect balance of the competing interactions causes unusual instability. The ferromagnetic-to-antiferromagnetic switching is different from the known change by the bond angles mechanism of the superexchange interaction and has not been observed previously in any ferromagnetic-antiferromagnetic compounds. The present solid solutions provide a unique system where either ferromagnetism or antiferromagnetim arises within the simple A-site cubic S=1⁄2 spin sublattice.