title: Large-scale synthesis of single-crystalline iron oxide magnetic nanorings.
abstract: We present an innovative approach to the production of single-crystal iron oxide nanorings employing a solution-based route. Single-crystal hematite (alpha-Fe2O3) nanorings were synthesized using a double anion-assisted hydrothermal method (involving phosphate and sulfate ions), which can be divided into two stages: (1) formation of capsule-shaped alpha-Fe2O3 nanoparticles and (2) preferential dissolution along the long dimension of the elongated nanoparticles (the c axis of alpha-Fe2O3) to form nanorings. The shape of the nanorings is mainly regulated by the adsorption of phosphate ions on faces parallel to c axis of alpha-Fe2O3 during the nanocrystal growth, and the hollow structure is given by the preferential dissolution of the alpha-Fe2O3 along the c axis due to the strong coordination of the sulfate ions. By varying the ratios of phosphate and sulfate ions to ferric ions, we were able to control the size, morphology, and surface architecture to produce a variety of three-dimensional hollow nanostructures. These can then be converted to magnetite (Fe3O4) and maghemite (gamma-Fe2O3) by a reduction or reduction-oxidation process while preserving the same morphology. The structures and magnetic properties of these single-crystal alpha-Fe2O3, Fe3O4, and gamma-Fe2O3 nanorings were characterized by various analytical techniques. Employing off-axis electron holography, we observed the classical single-vortex magnetic state in the thin magnetite nanorings, while the thicker rings displayed an intriguing three-dimensional magnetic configuration. This work provides an easily scaled-up method for preparing tailor-made iron oxide nanorings that could meet the demands of a variety of applications ranging from medicine to magnetoelectronics.
authors.affiliation: Beijing National Laboratory for Molecular Sciences, Peking University, Beijing 100871, China.
authors.forename: Chun-Jiang
authors.initials: CJ
authors.lastname: Jia
authors.forename: Ling-Dong Sun
authors.initials: LD
authors.lastname: Sun
authors.forename: Feng
authors.initials: F
authors.lastname: Luo
authors.forename: Xiao-Dong
authors.initials: XD
authors.lastname: Han
authors.forename: Laura J
authors.initials: LJ
authors.lastname: Heyderman
authors.forename: Zheng-Guang
authors.initials: ZG
authors.lastname: Yan
authors.forename: Chun-Hua
authors.initials: CH
authors.lastname: Yan
authors.forename: Kun
authors.initials: K
authors.lastname: Zheng
authors.forename: Ze
authors.initials: Z
authors.lastname: Zhang
authors.forename: Mikio
authors.initials: M
authors.lastname: Takano
authors.forename: Naoaki
authors.initials: N
authors.lastname: Hayashi
authors.forename: Matthias
authors.initials: M
authors.lastname: Eltschka
authors.forename: Mathias
authors.initials: M
authors.lastname: Kläui
authors.forename: Ulrich
authors.initials: U
authors.lastname: Rüdiger
authors.forename: Takeshi
authors.initials: T
authors.lastname: Kasama
authors.forename: Lionel
authors.initials: L
authors.lastname: Cervera-Gontard
authors.forename: Rafal E
authors.initials: RE
authors.lastname: Dunin-Borkowski
authors.forename: George
authors.initials: G
authors.lastname: Tzvetkov
authors.forename: Jörg
authors.initials: J
authors.lastname: Raabe