Mössbauer and magnetic study of Co x Fe3−x O4 nanoparticles

Magnetic nanoparticles of cobalt ferrites Co x Fe3−x O4 (x = 1 or 2) have been obtained either by mechanical milling or thermal treatment of pre-prepared layered double hydroxide carbonate x-LDH–CO3. Mechanical milling of the 1-LDH–CO3 leads to the large-scale preparation of nearly spherical nanoparticles of CoFe2O4, the size of which (5 to 20 nm) is controlled by the treatment time. Core-shell structure with surface spin-canting has been considered for the nanoparticles formed to explain the observed hysteresis loop shift (from ZFC–FC) in the magnetic properties. Annealing treatment of the 2-LDH–CO3 below 673 K results in the formation of nearly spherical pure Co2FeO4 nanoparticles. At 673 K and above, the LDH decomposition leads to the formation of a mixture of both spinels phases Co2FeO4 and CoFe2O4, the amount of the latter increases with annealing temperature. Unusually high magnetic hardness characterized by a 22 kOe coercive field at 1.8 K has been observed, which reflects the high intrinsic anisotropy for Co2FeO4

Scanning transmission electron microscopy study of the evolution of needle-like nanostructures in CoFe2O4 and nife 2O4 silica nanocomposite aerogels

Magnetic nanocomposite materials consisting of 10 wt % CoFe 2O4 or NiFe2O4 nanoparticles in a silica aerogel matrix have been synthesized by the sol?gel method. A 100-kV aberration-corrected scanning transmission electron microscope (STEM) has been used to study these materials, and bright field and high angle annular dark field images show that after heat treatment at both 450 and 900 °C, they contain needle-like nanostructures Ì?1 nm in width and 10 nm in length. High resolution STEM images show that the needle-like nanostructures have a layered internal structure with typical interlayer spacings of 0.33 ± 0.02 nm. Electron energy loss spectroscopy using a 0.13-nm diameter probe gives information on the composition of these nanostructures. The results presented here for samples heat treated at 450 °C are consistent with needle-like nanostructures arising from Co and Ni silicate hydroxides which are separate from the also present Fe-containing phase of ferrihydrite nanoparticles. Samples heat treated at 900 °C have previously been shown to contain round ferrite nanoparticles ?8 nm in diameter. The results presented here are consistent with the needle-like nanostructures being transformed into ferrite-like phases after heat treatment at 900 °C, and the needle-like nanostructures are often found attached to round ferrite nanoparticles. © 2011 American Chemical Society

Giant Magnetic Hardness in the Synthetic Mineral Ferrimagnet K2Co3II(OH)(2)(SO4)(3)(H2O)(2)

Wepresent the synthesis, single-crystal X-ray (173 K) and powder neutron (2-30 K) structures and its thermal, optical and magnetic properties of K 2CoII3(OH)2(SO4) 2(H2O)2. It is a ferrimagnet (TC= 29.7 K) constructed of Co3(OH)2 diamond chains connected by sulfate and it displays hysteresis loops ranging from being soft with nearly zero coercivity between 29 and 10 K to very hard reaching coercive field exceeding 70 kOe at 1.8 K. This dramatic change is associated with the changes in domain shape due to the strong exchange anisotropy. Considerable frequency dependence of the acsusceptibilities is observed in the ordered state.Measurements on a single crystal have established the magnetic axes to be a-axis (easy), b-axis (intermediate), and c-axis (hard). © 2010 American Chemical Society