title: Aging investigation of cobalt ferrite nanoparticles in low pH magnetic fluid.
abstract: In this study, we report on how surface-passivated and nonpassivated cobalt ferrite nanoparticles (8 nm diameter), suspended as ionic magnetic fluids and aged under low pH conditions, revealed different behavior as far as the time evolution of the iron/cobalt cation distribution, crystal quality, coercivity, and saturation magnetization are concerned. Different techniques were used to perform a detailed study regarding the chemical stability, structural stability, and surface and magnetic properties of the suspended nanoparticles as a function of the aging time. Properties of surface-passivated and nonpassivated nanoparticles were investigated by transmission electron microscopy, X-ray diffraction, atomic absorption spectrometry, magnetic measurements, Raman spectroscopy, and Mössbauer spectroscopy. Our data showed that the employed nanoparticle surface passivation process, besides the formation of an iron-rich surface layer, modifies the nanoparticle core as well, improving the crystal quality while modifying the Fe/Co cation distribution and the nanoparticle dissolution rate profile. Magnetic data showed that the saturation magnetization increases for surface-passivated nanoparticles in comparison to the nonpassivated ones, though coercivity decreases after passivation. These two observations were associated to changes in the cation distribution among the available tetrahedral and octahedral sites.
authors: Instituto de Física, Universidade de Brasília, Brasília DF 70910-900, Brazil. soler@unb.br
authors: Maria A G
authors: MA
authors: Soler
authors: Emilia C D
authors: EC
authors: Lima
authors: Sebastião W
authors: SW
authors: da Silva
authors: Tiago F O
authors: TF
authors: Melo
authors: Angela C M
authors: AC
authors: Pimenta
authors: João P
authors: JP
authors: Sinnecker
authors: Ricardo B
authors: RB
authors: Azevedo
authors: Vijayendra K
authors: VK
authors: Garg
authors: Aderbal C
authors: AC
authors: Oliveira
authors: Miguel A
authors: MA
authors: Novak
authors: Paulo C
authors: PC
authors: Morais
mesh_terms: D003035:Cobalt / Q000737:chemistry*; D005290:Ferric Compounds / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D008280:Magnetics; D046529:Microscopy, Electron, Transmission; D053758:Nanoparticles / Q000737:chemistry*; D012996:Solutions / Q000737:chemistry; D013499:Surface Properties; D013997:Time Factors; D014961:X-Ray Diffraction
chemical_list: D005290:Ferric Compounds; D012996:Solutions; C001215:ferrite; D003035:Cobalt