Competing Magnetic Interactions and the Role of Unpaired 4 f Electrons in Oxygen-Deficient Perovskites Ba 3 R Fe 2 O 7.5 ( R = Y, Dy).

Oxygen-deficient perovskite compounds with the general formula Ba ₃ R Fe ₂ O _7.5 present a good opportunity to study competing magnetic interactions between Fe ³⁺ 3 d cations with and without the involvement of unpaired 4 f electrons on R ³⁺ cations. From analysis of neutron powder diffraction data, complemented by ab initio density functional theory calculations, we determined the magnetic ground states when R ³⁺ = Y ³⁺ (non-magnetic) and Dy ³⁺ (4 f ⁹ ). They both adopt complex long-range ordered antiferromagnetic structures below T _N = 6.6 and 14.5 K, respectively, with the same magnetic space group C _a 2/ c (BNS #15.91). However, the dominant influence of f-electron magnetism is clear in temperature dependence and differences between the size of the ordered moments on the two crystallographically independent Fe sites, one of which is enhanced by R -O-Fe superexchange in the Dy compound, while the other is frustrated by it. The Dy compound also shows evidence for temperature- and field-dependent transitions with hysteresis, indicating a field-induced ferromagnetic component below T _N .