Microstructural and Mössbauer properties of low temperature synthesized Ni-Cd-Al ferrite nanoparticles

We report the influence of Al3+ doping on the microstructural and Mössbauer properties of ferrite nanoparticles of basic composition Ni0.2Cd0.3Fe2.5 - xAlxO4 (0.0 ≤ x ≤ 0.5) prepared through simple sol-gel method. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray, transmission electron microscopy (TEM), Fourier transformation infrared (FTIR), and Mössbauer spectroscopy techniques were used to investigate the structural, chemical, and Mössbauer properties of the grown nanoparticles. XRD results confirm that all the samples are single-phase cubic spinel in structure excluding the presence of any secondary phase corresponding to any structure. SEM micrographs show the synthesized nanoparticles are agglomerated but spherical in shape. The average crystallite size of the grown nanoparticles was calculated through Scherrer formula and confirmed by TEM and was found between 2 and 8 nm (± 1). FTIR results show the presence of two vibrational bands corresponding to tetrahedral and octahedral sites. Mössbauer spectroscopy shows that all the samples exhibit superparamagnetism, and the quadrupole interaction increases with the substitution of Al3+ ions

The influence of ruthenium on the magnetic properties of gamma-Fe2o3(maghemite)studied by Mossbauer spectrscopy.

Ruthenium-doped gamma-Fe2O3 has been synthesized and examined by x-ray powder diffraction, XANES, EXAFS and by Fe-57 Mossbauer spectroscopy. Ruthenium K-edge x-ray absorption spectroscopy shows that ruthenium adopts a fully occupied octahedral site in the spinel related gamma-Fe2O3 structure as Ru4+. The Fe-57 Mossbauer spectra recorded in the presence of a longitudinal magnetic field of 6 T confirmed the octahedral coordination of the tetravalent ions and canting angles for the Fe3+ ions were determined as 24degrees for those in octahedral sites and 33degrees for those in tetrahedral sites. The Fe-57 Mossbauer spectra recorded in situ from ruthenium-doped gamma-Fe2O3 showed parameters typical of maghemite up to 600 K but with a magnetic hyperfine field distribution suggesting an inhomogeneous distribution of ruthenium within particles of varied size around about 15 nm. At 700 K a phase transition from gamma-Fe2O3 to alpha-Fe2O3 was observed and further studies showed the ruthenium-doped alpha-Fe2O3 to have a Morin transition temperature of about 400 K

Grain size effect on the phase transformation temperature of nanostructured CuFe2O4

We report a large decrease in tetragonal to cubic phase transformation temperature when grain size of bulk CuFe2O4 is reduced by mechanical ball milling. The change in phase transformation temperature was inferred from in situ high temperature conductivity and x-ray diffraction measurements. The decrease in conductivity with grain size suggests that ball milling has not induced any oxygen vacancy while the role of cation distribution in the observed decrease in phase transformation temperature is ruled out from in-field Fe-57 Mossbauer and extended x-ray absorption fine structure measurements. The reduction in the phase transformation temperature is attributed to the stability of structures with higher crystal symmetry at lower grain sizes due to negative pressure effect. (C) 2011 American Institute of Physics. doi: 10.1063/1.3493244

Synthesis, structure and properties of a semivalent iron oxoborate, Fe₂OBO₃

Transition metal oxoborates are of interest for magnetic and optical properties. Crystals of many MII2MIIIO2BO3 and MIIMIIIOBO3 materials can be grown from borate fluxes. In the FeII–FeIII–B–O system, flux growth results in Fe3O2BO3 crystals, but solid state reaction at higher temperatures has yielded Fe2OBO3 as a polycrystalline powder. This has been characterised by synchrotron and neutron diffraction, electron microscopy, Mossbauer spectroscopy, and conductivity and magnetic measurements. Two notable transitions occur, a broad semiconductor– semiconductor change accompanied by a structural transition at 317 K, and L-type ferrimagnetic order below a Curie temperature of 155 K. An average (Fe2+)0.5(Fe3+)0.5 valence is observed at the two crystallographically distinct sites in Fe2OBO3, indicating that charge ordering occurs

Electrostatically driven charge-ordering in Fe₂OBO₃

Charge-ordering is an important phenomenon in conducting metal oxides: it leads to metal-insulator transitions in manganite perovskites (which show `colossal' magnetoresistances), and the Verwey transition in magnetite (in which the material becomes insulating at low temperatures when the conduction electrons freeze into a regular array). Charge-ordered `stripes' are found in some manganites and copper oxide superconductors; in the latter case, dynamic fluctuations of the stripes have been proposed as a mechanism of high-temperature superconductivity. But an important unresolved issue is whether the charge ordering in oxides is driven by electrostatic repulsions between the charges (Wigner crystallization), or by the strains arising from electron-lattice interactions (such as Jahn-Teller distortions) involving different localized electronic states. Here we report measurements on iron oxoborate, Fe2OBO3, that support the electrostatic repulsion charge-ordering mechanism: the system adopts a charge-ordered state below 317 K, in which Fe2+ and Fe3+ ions are equally distributed over structurally distinct Fe sites. In contrast, the isostructural manganese oxoborate, Mn2OBO3, has been previously shown to undergo charge-ordering through Jahn-Teller distortions. We therefore conclude that both mechanisms occur within the same structural arrangement