SrFe_1−xMoxO_2+δ: parasitic ferromagnetism in an infinite-layer iron oxide with defect structures induced by interlayer oxygen

The recent discovered compound SrFeO₂ is an infinite-layer-structure iron oxide with unusual square-planar coordination of Fe²⁺ ions. In this study, SrFe_1−xMo_xO_2+δ (x < 0.12) is obtained by crystal transformation from SrFe_1−xMo_xO_3−δ perovskite via low-temperature (≤380 °C) topotactic reduction. The parasitic ferromagnetism of the compound and its relationship to the defect structures are investigated. It is found that substitution of high-valent Mo⁶⁺ for Fe²⁺ results in excess oxygen anions O²⁻ inserted at the interlayer sites for charge compensation, which further causes large atomic displacements along the c-axis. Due to the robust but flexible Fe-O-Fe framework, the samples are well crystallized within the ab-plane, but are significantly poorer crystallized along the c-axis. Defect structures including local lattice distortions and edge dislocations responsible for the lowered crystallinity are observed by high resolution transmission electron microscopy. Both the magnetic measurements and electron spin resonance spectra provide the evidence of a parasitic ferromagnetism (FM). The week FM interaction originated from the imperfect antiferromagnetic (AFM) ordering could be ascribed to the introduction of uncompensated magnetic moments due to substitution of Mo⁶⁺ (S = 0) for Fe²⁺ (S = 2) and the canted/frustrated spins resulted from defect structures. (paper)