Transition-metal perovskite oxides are used in many devices ranging from piezoelectric transducers to thermoelectric coolers and novel magnetic memories. Despite their abundance, many questions remain about fundamental magnetic ordering in these systems. Researchers Jerry Bettis Jr. and Myung-Hwan Whangbo at North Carolina State University, in collaboration with Hongjun Xiang at Fudan University in China, have now used density functional theory (DFT) to explore the origin of the room-temperature ferromagnetism in Sr3YCo4O10+δ(0.5 < δ < 1.0) (SYCO). They propose a novel kind of ferromagnetism in this oxygen-deficient perovskite, namely, the formation of ferromagnetic “spin bags.”
Reporting their results in the August 28 issue of Chemistry of Materials (DOI: 10.1021/cm302007q; p. 3117), the researchers explain the origin of ferromagnetism in SYCO, whose structure comprises two kinds of perovskite layers that alternate along the c-axis direction. Oxygen vacancies are absent in the oxygen-rich perovskite layers (R layers), but ordered oxygen vacancies are present in the oxygen-deficient perovskite layers (D1 and D2 layers). The research group carried out DFT calculations using a supercell of stacked D1–R–D2–R layers for three situations. A first case has both the crystal structure and the G-type antiferromagnetic structure kept frozen, while a second case has the crystal structure frozen but the magnetic structure relaxed. The third case has both the crystal and magnetic structures relaxed.
The researchers find that the last two cases give rise to ferromagnetism with a Co magnetic moment close to that measured experimentally (∼0.25 μB/Co). Moreover, they find that the Co3+ ions in the R layers form isolated ferromagnetic “spin bags,” with each bag consisting of a low-spin Co3+ ion surrounded by four high-spin Co3+ions in every R layer. They hypothesize that the formation of ferromagnetic spin bags stabilizes the ferromagnetism of SYCO, and are currently investigating other possible conditions for spin bag formation. This curious and novel concept of spin bag formation provides a fresh way of thinking about ferromagnetism in transition-metal oxides.