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ABSTRACT: We use XMCD at the L2,3 edges of iron to analyze the site-specific magnetic contributions of ions in nanoparticles of maghemite, γ-Fe2O3. Maghemite is the oxidized form of magnetite Fe3O4. We report results of the study of the magnetic order on tetrahedral and octahedral magnetic sublattices in γ-Fe2O3 nanoparticles as a function of the applied magnetic field. Magnetic contributions of Fe3+Oh and Fe3+Td are determined using the Ligand Field Multiplet theory. It is found that when the external field decreases, the magnetic contribution of Fe3+ ions on octahedral sites is significantly reduced by comparison to the contribution of Fe3+ ions on tetrahedral ones. This shows that a higher degree of canting exits on octahedral sites.
Physica Scripta 11/2006; 2005(T115):626. · 1.20 Impact Factor
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ABSTRACT: Studies of magnetoelectric ε- Fe <sub>2</sub> O <sub>3</sub> nanoparticles (30 nm ) in silica by <sup>57</sup> Fe Mössbauer spectrometry (4.2–600 K ,0–6 T ) are reported. They indicate a magnetic/structural transition at ∼130 K , a high-temperature collinear ferrimagnetic spin structure similar to Fe Ga O <sub>3</sub> and Fe Al O <sub>3</sub> , and the Curie temperature T<sub>C</sub> at 585 K . Reduced hyperfine field versus T/T<sub>C</sub> is (S=5/2) Brillouin-like or Langevin-like according to the structural site. The magnetization at 300 K is estimated to 0.25μ<sub>B</sub>/ Fe in accord with published data.
Journal of Applied Physics 10/2005; · 2.17 Impact Factor
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ABSTRACT: Samples of γ-Fe2O3 nano-particles with a mean size of 4.0(3) nm and with different hydration and surfactant degrees were prepared by sol–gel methods. Morphology and structural data were obtained by transmission electron microscopy and x-ray diffraction, whereas the surface effects and hyperfine interactions were analysed mainly by Mössbauer spectroscopy. The relative number of surface iron positions was found to be proportional to the amount of OH− and SO42− groups on the particle surface, which in turn is strictly dependent on the preparation conditions. Strong relaxation processes versus temperature were evidenced in the analysed systems. New criteria for the evaluation of the blocking temperature via Mössbauer measurements are proposed. The results are in good agreement with blocking temperatures obtained by magnetic measurements. Moreover, it was shown that the inter-particle magnetic interactions decrease with the number of iron surface states.
Journal of Physics Condensed Matter 03/2003; 15(10):1797. · 2.55 Impact Factor
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ABSTRACT: Mossbauer absorption measurements on the 84.25 keV gamma ray of 170Yb in YbAl3 have been made from 1.2K to 130K in applied magnetic fields up to 75 kOe. The isomer shift measurements show that Yb has a temperature independent valency of 2.7+or-0.3. An excess hyperfine field, induced by an applied magnetic field, is explained in terms of an ionic orbital hyperfine field generated by a strongly correlated 4f electron band.
Journal of Physics F Metal Physics 01/2001; 8(5):983.
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ABSTRACT: Various properties of spinel iron oxide nanoparticles are reviewed in relation to the
control of the particle size and the control of their dispersion in a variety of matrices. The deep
knowledge of the different synthesis parameters and the interfacial oxide-solution characteristics
allows to prepare a large variety of materials like calibrated nanoparticles and inorganic or hybrid
nanocomposites.
The European Physical Journal Applied Physics 05/2000; 10(03):167 - 172. · 0.77 Impact Factor
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ABSTRACT: The effect of an applied magnetic field on the temperature at the maximum of the zero-field-cooled (ZFC) magnetization, MZFC , is studied using the recently obtained analytic results of Coffey et al (Coffey W T et al 1998 Phys. Rev. Lett. 80 5655) for the prefactor of the Néel relaxation time which allow one to precisely calculate the prefactor in the Néel-Brown model and thus the blocking temperature as a function of the coefficients of the Taylor series expansion of the magnetocrystalline anisotropy. The present calculations indicate that even a precise determination of the prefactor in the Néel-Brown theory, which always predicts a monotonic decrease of the relaxation time with increasing field, is insufficient to explain the effect of an applied magnetic field on the temperature at the maximum of the ZFC magnetization. On the other hand, we find that the non-linear field dependence of the magnetization along with the magnetocrystalline anisotropy appears to be of crucial importance to the existence of this maximum.
Journal of Physics Condensed Matter 03/2000; 12(13):3077. · 2.55 Impact Factor
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ABSTRACT: We present a microscopic model for nanoparticles, of the maghemite (-Fe2O3) type, and perform classical Monte Carlo simulations of their magnetic properties. On account of Mössbauer spectroscopy and
high-field magnetisation results, we consider a particle as composed of a core and a surface shell of constant thickness.
The magnetic state in the particle is described by the anisotropic classical Dirac-Heisenberg model including exchange and
dipolar interactions and bulk and surface anisotropy. We consider the case of ellipsoidal (or spherical) particles with free
boundaries at the surface. Using a surface shell of constant thickness ( nm) we vary the particle size and study the effect of surface magnetic disorder on the thermal and spatial behaviors of the
net magnetisation of the particle. We study the shift in the surface “critical region” for different surface-to-core ratios
of the exchange coupling constants. It is also shown that the profile of the local magnetisation exhibits strong temperature
dependence, and that surface anisotropy is responsible for the non saturation of the magnetisation at low temperatures.
Physics of Condensed Matter 01/2000; 14(4):681-689. · 1.53 Impact Factor
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ABSTRACT: We study the finite-size and surface effects on the thermal and spatial behaviors of the magnetisation of a small magnetic particle. We consider two systems: 1) A box-shaped isotropic particle of simple cubic structure with either periodic or free boundary conditions. This case is treated analytically using the isotropic model of D-component spin vectors in the limit $D\to \infty$, including the magnetic field. 2) A more realistic particle ($\gamma $-Fe$_{2}$O$_{3}$) of ellipsoidal (or spherical) shape with open boundaries. The magnetic state in this particle is described by the anisotropic classical Dirac-Heisenberg model including exchange and dipolar interactions, and bulk and surface anisotropy. This case is dealt with by the classical Monte Carlo technique. It is shown that in both systems finite-size effects yield a positive contribution to the magnetisation while surface effects render a larger and negative contribution, leading to a net decrease of the magnetisation of the small particle with respect to the bulk system. In the system 2) the difference between the two contributions is enhanced by surface anisotropy. The latter also leads to non saturation of the magnetisation at low temperatures, showing that the magnetic order in the core of the particle is perturbed by the magnetic disorder on the surface. This is confirmed by the profile of the magnetisation. Comment: 6 pages of RevTex including 4 Figures, invited paper to 3rd EuroConference on Magnetic Properties of Fine Nanoparticles, Barcelona, October 99
10/1999;
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ABSTRACT: The models proposed to account for the variation of the blocking temperature of a magnetic particle in an assembly of interacting particles are re-discussed. Experimental results on the thermal variation of the relaxation time are reported in support of the discussion. Except few cases related to a collective state, all data are well explained by the Dormann-Bessais-Fiorani model accounting for the interparticle interactions, in the framework of the superparamagnetic Neel-Brown model. The energy barrier unambiguously increases with increasing interactions, even when they are very weak, in disagreement with the Morup-Tronc model. (C) 1999 Published by Elsevier Science B.V. All rights reserved.
Journal of Magnetism and Magnetic Materials. 07/1999; 202(1):251-267.
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ABSTRACT: The effect of an applied magnetic field on the temperature at the maximum of the ZFC magnetization, $M_{ZFC}$, is studied using the recently obtained analytic results of Coffey et al. (Phys. Rev. Lett. {\bf 80}(1998) 5655) for the prefactor of the N\'{e}el relaxation time which allow one to precisely calculate the prefactor in the N\'{e}el-Brown model and thus the blocking temperature as a function of the coefficients of the Taylor series expansion of the magnetocrystalline anisotropy. The present calculations indicate that even a precise determination of the prefactor in the N\'{e}el-Brown theory, which always predicts a monotonic decrease of the relaxation time with increasing field, is insufficient to explain the effect of an applied magnetic field on the temperature at the maximum of the ZFC magnetization. On the other hand, we find that the non linear field-dependence of the magnetization along with the magnetocrystalline anisotropy appears to be of crucial importance to the existence of this maximum. Comment: 14 LaTex209 pages, 6 EPS figures. To appear in J. Phys.: Condensed Matter
03/1999;
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ABSTRACT: Magnetic relaxation experiments were performed on γ-Fe2O3 nanoparticle assemblies with interparticle interactions of varying strength. Weak waiting-time effects have been detected in an interacting-particle sample. The direct determination of the volume distribution from MRE measurements and the application of our model for interparticle interactions are discussed.
Journal of Magnetism and Magnetic Materials - J MAGN MAGN MATER. 01/1999; 196:64-66.
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ABSTRACT: Dispersions of γ-Fe2O3 particles in a
polymer were investigated by Mössbauer spectroscopy with a
longitudinal magnetic field of 60 kOe. Spin canting effects as a
function of the mean particle size varying from 3 to 10 nm, the
interparticle distance, and the temperature are reported.
Hyperfine Interactions 11/1998; 112:97-100. · 0.21 Impact Factor
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ABSTRACT: A new model for the FC and ZFC magnetization processes for fine
particle systems is described that uses the two-level model for the
thermal relaxation. An approximate method for the resolution of the
master equation for a temperature or field varying process is developed.
The volume and easy axis distributions as well as the temperature
dependencies of the intrinsic magnetization and anisotropy are taken
into account. In the low applied field limit and for randomly oriented
systems, an Onsager-type attempt shows that dipole interactions can
account for the dependence of the FC and ZFC magnetization curves on the
concentration and sample shape. Numerical results are in good agreement
with experimental data obtained for various concentration
γ-Fe<sub>2</sub>O<sub>3</sub> nanoparticle systems
IEEE Transactions on Magnetics 08/1998; · 1.36 Impact Factor
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ABSTRACT: Dispersions of γ-Fe2O3 particles in a polymer were investigated by Mössbauer spectroscopy with a longitudinal magnetic field of 60 kOe. Spin canting
effects as a function of the mean particle size varying from 3 to 10 nm, the interparticle distance, and the temperature are
reported.
Hyperfine Interactions 04/1998; 112(1):97-100. · 0.21 Impact Factor
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MMM-Intermag Conference, 1998. Abstracts., The 7th Joint; 02/1998
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ABSTRACT: The dynamical properties of γ-Fe2O3 nanoparticles dispersed in a polymer have been investigated by means of AC susceptibility measurements in a large frequency range (10
Journal of Magnetism and Magnetic Materials - J MAGN MAGN MATER. 01/1998; 183(3).
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Philosophical Magazine B. 10/1997; 76(4):457-462.
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ABSTRACT: The mean particle size of magnetite precipitated in aqueous solution can be adjusted and stabilized against ripening over a large nanometric range (1.5 to 12.5 nm) by controlling the pH and the ionic strength imposed by a non-complexing salt in the precipitation medium. The higher the pH and the ionic strength are, the smaller the particle size is. An explanation for this phenomenon is based on the lowering of the oxide-solution interracial tension due to the surface electrostatic charge increase. A critical pH value corresponding to the saturation of the interface is defined and calculated. When the precipitation is effected above this critical pH value, the spontaneous decrease in surface area by ripening is avoided and, for a given ionic strength, the particle size depends only on the acidity. This model correlates well with the experimental results. This provides the first experimental example of thermodynamic stabilization of oxide nanoparticles.
MRS Proceedings. 12/1995; 432.
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ABSTRACT: -Fe2O3 nanoparticles with varying state of dispersion in a polymer have been investigated by Mssbauer spectroscopy, static magnetic measurements at low applied field, and alternative susceptibility measurements over a large range of frequencies (210–2–104 Hz). The dynamical behaviour was characterized through the variation of the blocking temperature with the characteristic time of the measurement. The Mssbauer blocking temperature was determined according to a procedure described. For quasi-isolated particles an Arrhenius law is demonstrated. Effects of interparticle interactions in concentrated and aggregated systems are satisfactorily explained by the previous model. Dependence of the superparamagnetic susceptibility on the experimental conditions interpreted using the Lorentz or Onsager fields is mentioned.
Hyperfine Interactions 11/1995; 95(1):129-148. · 0.21 Impact Factor
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ABSTRACT: γ-Fe<sub>2</sub>O<sub>3</sub> small particles (⩽10 nm) with various dispersion rates were studied by static magnetic measurements at low and medium applied fields. The results raise various questions concerning the determination of the non-relaxing magnetic moment and of the particle volume, the role of the particle surface and interactions
IEEE Transactions on Magnetics 04/1994; 30(2):1098-1100. · 1.36 Impact Factor
