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

Very unusual transitions from ferromagnetic CaCu 3 Ge 4 O 12 to antiferromagnetic CaCu 3 Ti 4 O 12 to ferromagnetic CaCu 3 Sn 4 O 12 are observed in solid solutions of the A -site ordered perovskites. The effective magnetic interaction between Cu 2+ 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.