“Spin bag” model proposed for room-temperature ferromagnetism in Sr₃YCo₄O_10+δ

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 Sr₃YCo₄O_10+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.

Schematic of the different perovskite layers present in the room-temperature ferromagnet Sr₃YCo₄O_10+δ(0.5 < δ< 1.0). Panel (a) shows an oxygen-rich layer with no oxygen vacancies, while (b) and (c) demonstrate the oxygen-deficient layers with ordered oxygen vacancies. Each circled CoO₆ octahedron of the R layer contains a low-spin Co³⁺ ion, and forms a ferromagnetic spin bag with the surrounding four adjacent CoO₆ octahedra containing high-spin Co³⁺ ions. Reproduced with permission from Chem. Mater. 24 (2012), DOI: 10.1021/cm302007q; p. 3117. © 2012 American Chemical Society.

The researchers find that the last two cases give rise to ferromagnetism with a Co magnetic moment close to that measured experimentally (~0.25 µ_B/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.