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 ∼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.
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.
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, and Mössbauer spectra demonstrated that 2 nm-thick films exhibited a perovskite structure after reduction at 1073 K but converted to a fluorite phase upon oxidation at 1073 K. The transition between the fluorite and perovskite structures was reversible for at least five oxidation and reduction cycles. Coulometric titration at 1073 K showed that reduction of the fluorite phase occurred in two steps, one at a P(O2) of 10−15 atm and a second at a P(O2) of 10−8 atm. X-ray Photoelectron Spectra (XPS) demonstrated that Ce has +3 valence in the perovskite phase and +4 valence in the fluorite phase, while Fe is mixed +2 and +3 valence in the reduced perovskite phase and +3 valence in the fluorite phase. The CeFeOx thin films were found to retain high surface area and remain conformal to the γ-Al2O3 support upon redox cycling suggesting that they may be useful in applications ranging from catalysis to spintronics.
The magnetic spinel ferrites, MFe₂O₄ (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, respectively, 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 as-prepared ferrite systems evinced intrinsically higher activities as compared with their iron oxide analogues.
The physical properties of
in vitro
iron-reconstituted and genetically engineered human heteropolymer ferritins were investigated. High-angle annular dark-field scanning transmission electron ...microscopy (HAADF-STEM), electron energy-loss spectroscopy (EELS), and
57
Fe Mössbauer spectroscopy were employed to ascertain (1) the microstructural, electronic, and micromagnetic properties of the nanosized iron cores, and (2) the effect of the H and L ferritin subunit ratios on these properties. Mössbauer spectroscopic signatures indicate that all iron within the core is in the high spin ferric state. Variable temperature Mössbauer spectroscopy for H-rich (H
21
/L
3
) and L-rich (H
2
/L
22
) ferritins reconstituted at 1000
57
Fe/protein indicates superparamagnetic behavior with blocking temperatures of 19 K and 28 K, while HAADF-STEM measurements give average core diameters of (3.7 ± 0.6) nm and (5.9 ± 1.0) nm, respectively. Most significantly, H-rich proteins reveal elongated, dumbbell, and crescent-shaped cores, while L-rich proteins present spherical cores, pointing to a correlation between core shape and protein shell composition. Assuming an attempt time for spin reversal of
τ
0
= 10
−11
s, the Néel-Brown formula for spin-relaxation time predicts effective magnetic anisotropy energy densities of 6.83 × 10
4
J m
−3
and 2.75 × 10
4
J m
−3
for H-rich and L-rich proteins, respectively, due to differences in surface and shape contributions to magnetic anisotropy in the two heteropolymers. The observed differences in shape, size, and effective magnetic anisotropies of the derived biomineral cores are discussed in terms of the iron nucleation sites within the interior surface of the heteropolymer shells for H-rich and L-rich proteins. Overall, our results imply that site-directed nucleation and core growth within the protein cavity play a determinant role in the resulting core morphology. Our findings have relevance to iron biomineralization processes in nature and the growth of designer's magnetic nanoparticles within recombinant apoferritin nano-templates for nanotechnology.
The physical properties of
in vitro
iron-reconstituted and genetically engineered human heteropolymer ferritins were investigated.
Magnetic and Mössbauer characterization of single crystalline, sub-micron sized Bi2Fe4O9 cubes has been performed using SQUID magnetometry and transmission Mössbauer spectroscopy in the temperature ...range of 4.2 K ≤ T ≤ 300 K. A broad magnetic phase transition from the paramagnetic to the anti-ferromagnetic state is observed below 250 K, with the Mössbauer spectra exhibiting a superposition of magnetic, collapsed and quadrupolar spectra in the transition region of 200 K < T < 245 K. Room temperature Mössbauer spectra obtained in transmission geometry are identical to those recorded in back-scattering geometry via conversion electron Mössbauer spectroscopy, indicating the absence of strain at the surface. A small hysteresis loop is observed in SQUID measurements at 5 K, attributable to the presence of weak-ferromagnetism arising from the canting of Fe3+ ion sublattices in the antiferromagnetic matrix.
•Sub-micron sized Bi2Fe4O9 cubes were analyzed by Mössbauer spectrometry.•The transition temperature of the magnetic phase is about 240–250 K.•The surface sites give identical Mössbauer signatures as interior sites.
Thin films with a stoichiometry of CeFeO
x
were conformally deposited on high-surface-area γ-Al
2
O
3
by Atomic Layer Deposition (ALD). X-ray diffraction (XRD) patterns, High-Resolution Transmission ...Electron Microscopy (HRTEM) images, Raman spectra, and Mössbauer spectra demonstrated that 2 nm-thick films exhibited a perovskite structure after reduction at 1073 K but converted to a fluorite phase upon oxidation at 1073 K. The transition between the fluorite and perovskite structures was reversible for at least five oxidation and reduction cycles. Coulometric titration at 1073 K showed that reduction of the fluorite phase occurred in two steps, one at a
P
(O
2
) of 10
−15
atm and a second at a
P
(O
2
) of 10
−8
atm. X-ray Photoelectron Spectra (XPS) demonstrated that Ce has +3 valence in the perovskite phase and +4 valence in the fluorite phase, while Fe is mixed +2 and +3 valence in the reduced perovskite phase and +3 valence in the fluorite phase. The CeFeO
x
thin films were found to retain high surface area and remain conformal to the γ-Al
2
O
3
support upon redox cycling suggesting that they may be useful in applications ranging from catalysis to spintronics.
Highly stable CeFeO
x
thin films that can undergo a reversible fluorite to perovskite phase transition were synthesized.
Thin films with a stoichiometry of CeFeO x were conformally deposited on high-surface-area γ-Al 2 O 3 by Atomic Layer Deposition (ALD). X-ray diffraction (XRD) patterns, High-Resolution Transmission ...Electron Microscopy (HRTEM) images, Raman spectra, and Mössbauer spectra demonstrated that 2 nm-thick films exhibited a perovskite structure after reduction at 1073 K but converted to a fluorite phase upon oxidation at 1073 K. The transition between the fluorite and perovskite structures was reversible for at least five oxidation and reduction cycles. Coulometric titration at 1073 K showed that reduction of the fluorite phase occurred in two steps, one at a P (O 2 ) of 10 −15 atm and a second at a P (O 2 ) of 10 −8 atm. X-ray Photoelectron Spectra (XPS) demonstrated that Ce has +3 valence in the perovskite phase and +4 valence in the fluorite phase, while Fe is mixed +2 and +3 valence in the reduced perovskite phase and +3 valence in the fluorite phase. The CeFeO x thin films were found to retain high surface area and remain conformal to the γ-Al 2 O 3 support upon redox cycling suggesting that they may be useful in applications ranging from catalysis to spintronics.
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
2O
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
2O
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 ∼
2.6 nm were prepared and exhibited superparamagnetic blocking temperatures of 19 and 22 K for the H
2O
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
2O
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
2O
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
2O
2 samples implies that the former is somewhat more crystalline.
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