A-site magnetism in A-site-ordered perovskite-structure oxides

Abstract

A-site-ordered perovskite-structure oxides AA'3B4O12 with Cu2+ ions at the A' site are highlighted. Some new compounds were synthesized under high-pressure and high-temperature conditions. With nonmagnetic ions at the B site, only the Cu2+ spins at the A' site contribute to the magnetic properties, providing an intriguing magnetism system that has a cubic S=1/2 spin sublattice. When the A-site ion is nonmagnetic, either ferromagnetism (CaCu3Ge4O12 and CaCu3Sn4O12) or antiferromagnetism (CaCu3Ti4O12) of Cu2+ spins arises. The ferromagnetic Cu-Cu direct-exchange interaction and the antiferromagnetic Cu-O-Ti-O-Cu superexchange interaction are mutually competitive, and a perfect balance of the interactions in a solid-solution sample causes unusual instability resulting in a magnetic tri-critical point. Introducing magnetic lanthanide (Ln) ions into the A site was expected to produce A-A' magnetic interaction, and in both the antiferromagnetic (Ln2/3Cu3Ti4O12) and ferromagnetic [LnCu3(Ge3/4Ga1/4)4O12] compounds the A'-site Cu2+ spins are ordered but the magnetic interactions between the A- and A'-site magnetic sublattices are very weak. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.