title: High-Temperature Magnetism as a Probe for Structural and Compositional Uniformity in Ligand-Capped Magnetite Nanoparticles.
abstract: To investigate magnetostructural relationships in colloidal magnetite (Fe3O4) nanoparticles (NPs) at high temperature (300-900 K), we measured the temperature dependence of magnetization (M) of oleate-capped magnetite NPs ca. 20 nm in size. Magnetometry revealed an unusual irreversible high-temperature dependence ofMfor these NPs, with dip and loop features observed during heating-cooling cycles. Detailed characterizations of as-synthesized and annealed Fe3O4NPs as well as reference ligand-free Fe3O4NPs indicate that both types of features inM(T) are related to thermal decomposition of the capping ligands. The ligand decomposition upon the initial heating induces a reduction of Fe3+to Fe2+and the associated dip inM, leading to more structurally and compositionally uniform magnetite NPs. Having lost the protective ligands, the NPs continually sinter during subsequent heating cycles, resulting in divergentMcurves featuring loops. The increase inMwith sintering proceeds not only through elimination of a magnetically dead layer on the particle surface, as a result of a decrease in specific surface area with increasing size, but also through an uncommonly invoked effect resulting from a significant change in Fe3+/Fe2+ratio with heat treatment. The interpretation of irreversible features inM(T) indicates that reversibleM(T) behavior, conversely, can be expected only for ligand-free, structurally and compositionally uniform magnetite NPs, suggesting a general applicability of high-temperatureM(T) measurements as an analytical method for probing the structure and composition of magnetic nanomaterials.
authors.affiliation: International Iberian Nanotechnology Laboratory , Braga 4715-330, Portugal.
authors.forename: Yury V
authors.initials: YV
authors.lastname: Kolen'ko
authors.affiliation: International Iberian Nanotechnology Laboratory , Braga 4715-330, Portugal.
authors.forename: Manuel
authors.initials: M
authors.lastname: Bañobre-López
authors.affiliation: International Iberian Nanotechnology Laboratory , Braga 4715-330, Portugal.
authors.forename: Carlos
authors.initials: C
authors.lastname: Rodríguez-Abreu
authors.affiliation: International Iberian Nanotechnology Laboratory , Braga 4715-330, Portugal.
authors.forename: Enrique
authors.initials: E
authors.lastname: Carbó-Argibay
authors.affiliation: International Iberian Nanotechnology Laboratory , Braga 4715-330, Portugal.
authors.forename: Francis Leonard
authors.initials: FL
authors.lastname: Deepak
authors.affiliation: International Iberian Nanotechnology Laboratory , Braga 4715-330, Portugal.
authors.forename: Dmitri Y
authors.initials: DY
authors.lastname: Petrovykh
authors.affiliation: Center of Physics, University of Minho , Braga 4710-057, Portugal.
authors.forename: M Fátima
authors.initials: MF
authors.lastname: Cerqueira
authors.affiliation: Department of Chemistry, University of California, Davis , Davis California 95616, United States.
authors.forename: Saeed
authors.initials: S
authors.lastname: Kamali
authors.affiliation: Department of Chemistry, University of California, Davis , Davis California 95616, United States.
authors.forename: Kirill
authors.initials: K
authors.lastname: Kovnir
authors.affiliation: National University of Science and Technology "MISIS" , Moscow, Russia.
authors.forename: Dmitry V
authors.initials: DV
authors.lastname: Shtansky
authors.affiliation: Laboratoire CRISMAT, UMR 6508, CNRS-ENSICAEN , Caen 14050, France.
authors.forename: Oleg I
authors.initials: OI
authors.lastname: Lebedev
authors.affiliation: International Iberian Nanotechnology Laboratory , Braga 4715-330, Portugal.
authors.forename: Jose
authors.initials: J
authors.lastname: Rivas