As-prepared, single-crystalline bismuth ferrite nanoparticles show strong size-dependent magnetic properties that correlate with: (a) increased suppression of the known spiral spin structure (period length of approximately 62 nm) with decreasing nanoparticle size and (b) uncompensated spins and strain anisotropies at the surface. Zero-field-cooled and field-cooled magnetization curves exhibit spin-glass freezing behavior due to a complex interplay between finite size effects, interparticle interactions, and a random distribution of anisotropy axes in our nanoparticle assemblies.

No abstract

The magnetic spinel ferrites, MFe 2 O 4 (wherein 'M' = a divalent metal ion such as but not limited to Mn, Co, Zn, and Ni), represent a unique class of magnetic materials in which the rational introduction of different 'M's can yield correspondingly unique and interesting magnetic behaviors. Herein we present a generalized hydrothermal method for the synthesis of single-crystalline ferrite nanoparticles with 'M' = Mg, Fe, Co, Ni, Cu, and Zn), which can be systematically and efficaciously produced simply by changing the metal precursor. Our protocol can moreover lead to reproducible size control by judicious selection of various surfactants. As such, we have probed the effects of both (i) size and (ii) chemical composition upon the magnetic properties of these nanomaterials using complementary magnetometry and Mössbauer spectroscopy techniques. The structure of the samples was confirmed by atomic PDF analysis of X-ray and electron powder diffraction data as a function of particle size. These materials retain the bulk spinel structure to the smallest size (i.e. 3 nm). In addition, we have explored the catalytic potential of our ferrites as both (a) magnetically recoverable photocatalysts and (b) biological catalysts, and noted that many of our asprepared ferrite systems evinced intrinsically higher activities as compared with their iron oxide counterparts.

We investigate the size- and composition-dependent ac magnetic permeability of superparamagnetic iron oxide nanocrystals for radio frequency (RF) applications. The nanocrystals are obtained through high-temperature decomposition synthesis, and their stoichiometry is determined by Mössbauer spectroscopy. Two sets of oxides are studied: (a) as-synthesized magnetite-rich and (b) aged maghemite nanocrystals. All nanocrystalline samples are confirmed to be in the superparamagnetic state at room temperature by SQUID magnetometry. Through the one-turn inductor method, the ac magnetic properties of the nanocrystalline oxides are characterized. In magnetite-rich iron oxide nanocrystals, size-dependent magnetic permeability is not observed, while maghemite iron oxide nanocrystals show clear size dependence. The inductance, resistance, and quality factor of hand-wound inductors with a superparamagnetic composite core are measured. The superparamagnetic nanocrystals are successfully embedded into hand-wound inductors to function as inductor cores.

Ferritins are ubiquitous iron storage and detoxification proteins distributed throughout the plant and animal kingdoms. Mammalian ferritins oxidize and accumulate iron as a ferrihydrite mineral within a shell-like protein cavity. Iron deposition utilizes both O(2) and H(2)O(2) as oxidants for Fe(2+) where oxidation can occur either at protein ferroxidase centers or directly on the surface of the growing mineral core. The present study was undertaken to determine whether the nature of the mineral core formed depends on the protein ferroxidase center versus mineral surface mechanism and on H(2)O(2) versus O(2) as the oxidant. The data reveal that similar cores are produced in all instances, suggesting that the structure of the core is thermodynamically, not kinetically controlled. Cores averaging 500 Fe/protein shell and diameter approximately 2.6 nm were prepared and exhibited superparamagnetic blocking temperatures of 19 and 22 K for the H(2)O(2) and O(2) oxidized samples, respectively. The observed blocking temperatures are consistent with the unexpectedly large effective anisotropy constant K(eff)=312 kJ/m(3) recently reported for ferrihydrite nanoparticles formed in reverse micelles [E.L. Duarte, R. Itri, E. Lima Jr., M.S. Batista, T.S. Berquó and G.F. Goya, Large Magnetic Anisotropy in ferrihydrite nanoparticles synthesized from reverse micelles, Nanotechnology 17 (2006) 5549-5555.]. All ferritin samples exhibited two magnetic phases present in nearly equal amounts and ascribed to iron spins at the surface and in the interior of the nanoparticle. At 4.2 K, the surface spins exhibit hyperfine fields, H(hf), of 436 and 445 kOe for the H(2)O(2) and O(2) samples, respectively. As expected, the spins in the interior of the core exhibit larger H(hf) values, i.e. 478 and 486 kOe for the H(2)O(2) and O(2) samples, respectively. The slightly smaller hyperfine field distribution DH(hf) for both surface (78 kOe vs. 92 kOe) and interior spins (45 kOe vs. 54 kOe) of the O(2) sample compared to the H(2)O(2) samples implies that the former is somewhat more crystalline.

A monoenergetic positronium (Ps) beam of 0-60-eV energy and an angular width of + 5' is created by charge-exchange collisions of a slow positron beam passing through an Ar gas cell. The Ps beam is directed at a LiF(100) crystal, and reflected Ps atoms are detected at an annihilation target. With angles of incidence of 50' to 60' we observe a specularly reflected beam with a maximum reflected fraction R =(30~5)% at a Ps energy of 7 eV. At higher energies (20-60 eV) the reflectivity (R 0.5% at 60 eV) can be ascribed principally to a short Ps mean free path X (0.75+0.15) A, and to a lesser extent a Ps inner potential of about 4 eV. PACS numbers: 36.10.Dr, 61.14.Hg, 71.60.+z, 79.20.Rf Scattering experiments employing various subatomic and atomic particles have proven to be valuable in the study of surfaces. The most widely used technique is low-energy electron diffraction (LEED), particularly since computations are now successful in accounting for the multiple scattering of electrons from the inner atomic layers. ' Neutral molecules and atoms like H2 and He interact mainly with surface atoms, but the few meV beams typically used in diffraction experiments are not energetic enough to probe small-scale surface structure that does not affect the long-range part of the van der Waals surface potential.On the other hand, the large He mass is especially suited to the study of inelastic effects. Positronium (Ps), the bound state of a positron and an electron, should interact more intimately with a surface than He because of its much larger energy for a given wavelength.Furthermore, at energies above its 6.8-eV binding energy Ps is expected to break up with high probability upon scattering from a solid, and unlike electron scattering, Ps collisions should be confined to the outermost surface layers. Thus low-energy Ps diffraction (LEPD) could be a unique probe of ordered surfaces, combining some of the best characteristics of both LEED and He diffraction. The most important experimental question is whether the elastic scattering of energetic Ps is sufficiently intense to make LEPD possible with Ps beams presently available.In this Letter we describe the first measurement of the angle-resolved Ps reflection from a solid surface. We find that for LiF(100) there is a significant reflection probability for energies up to 60 eV and that LEPD studies should indeed be feasible.Our Ps beam is produced by positrons undergoing charge-exchange collisions with Ar atoms. If we consider only events in which the Ar ion and the Ps atom are formed in their ground states, the Ps will be essentially monoenergetic because of the negligible recoil energy of the heavy ion. Since an energy of 15.7 eV is required to remove an electron from Ar, the threshold energy for positrons to form Ps is Eth=15.7 -6.8=8.9 eV. Ps formed in excited states will also be present. Using the calculated formation probability of 2S Ps and the expected large total collisional cross section of Ps excited states, we estimate that the excited-state contamination of our be...

Comparative studies are presented of iron oxide nanoparticles in the 7-15 nm average diameter range ball milled in hexane in the presence of oleic acid. Transmission electron microscopy identified spherical particles of decreasing size as milling time and/or surfactant concentration increased. Micromagnetic characterization via Mössbauer spectroscopy at room temperature yielded broadened magnetic spectroscopic signatures, while macromagnetic characterization via vibrating sample magnetometry of 7-8 nm diameter particles showed largely superparamagnetic behavior at room temperature and hysteretic at 2 K. Zero-field and field-cooled magnetization curves exhibited a broad maximum at ∼215 K indicating the presence of strong interparticle magnetic interactions. The specific absorption rates of ferrofluids based on these nanoparticle preparations were measured in order to test their efficacies as hyperthermia agents.

Thin films with a stoichiometry of CeFeOx were conformally deposited on high-surface-area γ-Al2O3 by Atomic Layer Deposition (ALD). X-ray diffraction (XRD) patterns, High-Resolution Transmission Electron Microscopy (HRTEM) images, Raman spectra,...