Competing Magnetic Interactions and the Role of Unpaired 4f Electrons in Oxygen-Deficient Perovskites Ba₃RFe₂O_7.5 (R = Y, Dy)

Oxygen-deficient perovskite compounds with the general formula Ba3RFe2O7.5 present a good opportunity to study competing magnetic interactions between Fe3+ 3d cations with and without the involvement of unpaired 4f electrons on R3+ cations. From analysis of neutron powder diffraction data, complemented by ab initio density functional theory calculations, we determined the magnetic ground states when R3+ = Y3+ (non-magnetic) and Dy3+ (4f9). They both adopt complex long-range ordered antiferromagnetic structures below TN = 6.6 and 14.5 K, respectively, with the same magnetic space group Ca2/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 TN.