References of "Grandjean, Fernande"
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See detailPhase transitions in LaFeAsO: Structural, magnetic, elastic, and transport properties, heat capacity and Mossbauer spectra
McGuire, Michael A; Christianson, Andrew D; Sefat, Athena S et al

in Physical Review. B (2008), 78(9),

We present results from a detailed experimental investigation of LaFeAsO, the parent material in the series of "FeAs" based oxypnictide superconductors. Upon cooling, this material undergoes a tetragonal ... [more ▼]

We present results from a detailed experimental investigation of LaFeAsO, the parent material in the series of "FeAs" based oxypnictide superconductors. Upon cooling, this material undergoes a tetragonal-orthorhombic crystallographic phase transition at similar to 160 K followed closely by an antiferromagnetic ordering near 145 K. Analysis of these phase transitions using temperature dependent powder x-ray and neutron-diffraction measurements is presented. A magnetic moment of similar to 0.35 mu(B) per iron is derived from Mossbauer spectra in the low-temperature phase. Evidence of the structural transition is observed at temperatures well above the transition temperature (up to near 200 K) in the diffraction data as well as the polycrystalline elastic moduli probed by resonant ultrasound spectroscopy measurements. The effects of the two phase transitions on the transport properties (resistivity, thermal conductivity, Seebeck coefficient, and Hall coefficient), heat capacity, and magnetization of LaFeAsO are also reported, including a dramatic increase in the magnitude of the Hall coefficient below 160 K. The results suggest that the structural distortion leads to a localization of carriers on Fe, producing small local magnetic moments which subsequently order antiferromagnetically upon further cooling. Evidence of strong electron-phonon interactions in the high-temperature tetragonal phase is also observed. [less ▲]

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See detailIron oxide deposits associated with the ectosymbiotic bacteria in the hydrothermal vent shrimp Rimicaris exoculata
Corbari, Laure; Cambon-Bonavita, Marie-Anne; Long, Gary et al

in Biogeosciences (2008), 5

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See detailAntimony-121 Mossbauer spectral study of the Eu14MnSb11 and Yb14MnSb11 Zintl compounds
Hermann, Raphaël ULg; Grandjean, Fernande ULg; Kafle, D. et al

in Inorganic Chemistry (2007), 46(25), 10736-10740

The antimony-121 Mossbauer spectra of the Yb14MnSb11 and Eu14MnSb11 Zintl compounds have been measured between 2 or 5 and 300 K. The resulting three-dimensional arrays of the spectral counts, velocity ... [more ▼]

The antimony-121 Mossbauer spectra of the Yb14MnSb11 and Eu14MnSb11 Zintl compounds have been measured between 2 or 5 and 300 K. The resulting three-dimensional arrays of the spectral counts, velocity, and temperature have been simultaneously fit with a minimum number of free parameters. These fits yield a 0 Kelvin transferred hyperfine field of 2.9(2) T, a Curie temperature of 57(3) K, and a Mossbauer temperature of 182(2) K for Yb14MnSb11; in this case the transferred field arises solely from the ordering of Mn2+. Because Eu14MnSb11 has both Eu2+ and Mn2+ ions that are magnetically ordered, its antimony-121 Mossbauer spectra are more complex and reveal two magnetic transitions, the first at 92(1) K resulting from the ordering of the Mn 2+ ions and the second at 9.5(1.0) K resulting from the ordering of the Eu2+ ions; the corresponding 0 Kelvin transferred hyperfine fields are 1.3(1) and 3.7(1) T. The antimony-121 isomer shifts yield electronic configurations of 5s(1.74)5p(4.28) and 5s(1.74)5p(4.19) for the average antimony anion in Yb14MnSb11 and Eu14MnSb11, respectively. [less ▲]

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See detailA Mossbauer spectral study of the GdCo4-xFexB compounds
Grandjean, Fernande ULg; Hermann, Raphaël ULg; Popiel, E. et al

in Journal of Applied Physics (2007), 101(2),

The iron-57 Mossbauer spectra of the GdCo4-xFexB compounds, where x is 0.10, 0.15, 0.20, 0.25, 1, 2, 2.5, and 2.6, have been measured at room temperature and reveal relatively small iron hyperfine fields ... [more ▼]

The iron-57 Mossbauer spectra of the GdCo4-xFexB compounds, where x is 0.10, 0.15, 0.20, 0.25, 1, 2, 2.5, and 2.6, have been measured at room temperature and reveal relatively small iron hyperfine fields of approximately 12-18 T, relatively large quadrupole interactions of approximately +0.9 and -1 mm/s, and three very different types of spectra for x=0.10 and 0.15, x=0.25, 1, and 2, and x=2.5 and 2.6. The differences result from both the different easy magnetization directions in these compounds and the different cobalt and/or iron occupancies of the crystallographic 2c and 6i sites. The spectra have been fitted by calculating the spectral absorption with the complete iron-57 nuclear excited state Hamiltonian for the iron 2c and 6i sites. The fits have used an asymmetry parameter eta and Euler angles theta and phi that relate the hyperfine field to the iron electric field gradient axes of each crystallographic site in an orientation that is consistent with the structural and magnetic properties of the site. The results of the fits indicate both that the full Hamiltonian approach is required for physically reasonable spectral fits and that the small observed fields result from the presence of large orbital contributions which subtract from the Fermi contact contributions to the magnetic hyperfine fields of the two sites. The iron 2c occupancy obtained from the Mossbauer spectral area has been used to model the compositional dependence of the magnetic anisotropy constant in the GdCo4-xFexB compounds. (c) 2007 American Institute of Physics. [less ▲]

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See detailAntimony-121 Mossbauer spectral study of alpha-Zn4Sb3
Hermann, Raphaël ULg; Grandjean, Fernande ULg; Chen, Tsi*-Chi et al

in Inorganic Chemistry (2007), 46(3), 767-770

The Mossbauer spectra of alpha-Zn4Sb3, a compound that is best formulated as alpha-Zn13Sb10 or (Zn2+)(13)(Sb3-)(6)(Sb-2(4-))(2), have been measured between 5 and 120 K. The resulting six spectra have been ... [more ▼]

The Mossbauer spectra of alpha-Zn4Sb3, a compound that is best formulated as alpha-Zn13Sb10 or (Zn2+)(13)(Sb3-)(6)(Sb-2(4-))(2), have been measured between 5 and 120 K. The resulting six spectra have been simultaneously fit with two components in the ratio of 3:2 corresponding to the Sb3- and Sb2- ions identified in this valence semiconductor. The fits yield temperature independent isomer shifts of -8.17(2) and -9.73(2) mm/s and quadrupole interactions of -4.9(2) and 0 mm/s for the Sb3- and Sb2- ions, respectively; the corresponding Mossbauer temperatures are 197(5) and 207(5) K, temperatures that are lower than the Debye temperature of beta-Zn4Sb3. The isomer shifts correspond to electronic configurations between 5s(2)5p(6) and 5s(1.75)5p(4.01) for the Sb3- ions and between 5s(2)5p(5) and 5s(1.80)5p(3.38) for the Sb2- ions, configurations that are in good agreement with the expected configurations for this valence semiconductor and with the results of band structure calculations. [less ▲]

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See detailFe-57 Mossbauer spectral and muon spin relaxation study of the magnetodynamics of monodispersed gamma-Fe2O3 nanoparticles
Rebbouh, Leila; Hermann, Raphaël ULg; Grandjean, Fernande ULg et al

in Physical Review b (2007), 76(17),

The Mossbauer spectra of monodispersed iron oxide nanoparticles with diameters of 4, 7, 9, and 11 nm have been measured between 4.2 and 315 K and fitted within the formalism for stochastic fluctuations of ... [more ▼]

The Mossbauer spectra of monodispersed iron oxide nanoparticles with diameters of 4, 7, 9, and 11 nm have been measured between 4.2 and 315 K and fitted within the formalism for stochastic fluctuations of the hyperfine Hamiltonian. In this model, the hyperfine field is assumed to relax between the six +/- x, +/- y, and +/- z directions in space with a distribution of relaxation rates that is temperature dependent. Muon spin relaxation measurements have been carried out on the 9 nm particles between 4.2 and 295 K. Both techniques reveal three regimes in the magnetic dynamics of these nanoparticles. In the low-temperature regime, between 4.2 and similar to 30 K, the nanoparticle magnetic moments are blocked and a spin-glass-like state is observed with nearly static hyperfine fields, as is indicated by the well resolved magnetic Mossbauer spectra and the slow exponential decay of the muon asymmetry functions. In the high-temperature regime, above similar to 125 K, the nanoparticle magnetic moments and, hence, the hyperfine fields, relax rapidly and a typical thermally activated superparamagnetic behavior is observed, as is indicated by the Mossbauer doublet line shape and the muon asymmetry functions that are unquestionably characteristic of monodispersed nanoparticles. In the intermediate regime between similar to 30 and 125 K, the Mossbauer spectra are the superposition of broad sextets and doublets and the muon asymmetry functions have been fitted with a sum of two terms, one relaxing term similar to that observed at and above 125 K and one term characteristic of static local fields. Hence, in this intermediate regime, the sample is magnetically inhomogeneous and composed of nanoparticles rapidly and slowly relaxing as a result of interparticle interactions. The magnetic anisotropy constants determined from both the Mossbauer spectral and magnetic susceptibility results decrease by a factor similar to 4 with increasing diameter from 4 to 22 nm and increase linearly with the percentage of iron(III) ions present at the surface of the nanoparticles. The interparticle interaction energy is estimated to be between 89 and 212 K from the temperature dependence of the magnetic hyperfine field measured on the 9 nm nanoparticles. [less ▲]

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See detailA Mossbauer spectral study of the YCo4-xFexB compounds
Long, Gary J; Hermann, Raphaël ULg; Grandjean, Fernande ULg et al

in Journal of Physics. Condensed Matter : An Institute of Physics Journal (2006), 18(48), 10765-10773

The Mossbauer spectra of the YCo4-xFexB compounds, where x is 1, 2 and 3, have been measured at 78 and 295 K and reveal relatively small iron hyperfine fields of about 15 T, relatively large quadrupole ... [more ▼]

The Mossbauer spectra of the YCo4-xFexB compounds, where x is 1, 2 and 3, have been measured at 78 and 295 K and reveal relatively small iron hyperfine fields of about 15 T, relatively large quadrupole interactions of about +1.0 and -1.0 mm s(-1) and very different spectra for YCo3FeB and YCo2Fe2B as compared with YCoFe3B. As a consequence, the spectra have been fitted with a full Hamiltonian by adjusting the hyperfine parameters of the iron 2c and 6i sites found in these compounds. The fits have used an asymmetry parameter, eta and Euler angles, theta and phi, that relate the hyperfine field to the iron electric field gradient axes of each iron site in an orientation that is consistent with the structural and magnetic properties of the site. The fits indicate that the relative areas of the spectral components assigned to the two iron 2c and 6i sites are in agreement with the iron occupancy of the sites as determined by neutron diffraction. The results of the fits also indicate both that the full Hamiltonian approach is required for physically reasonable spectral fits and that the small observed fields result from the presence of large orbital contributions which subtract from the Fermi contact contributions to the magnetic hyperfine fields of the two sites. [less ▲]

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See detailThe dynamics of the guests in filled germanium clathrates
Hermann, Raphaël ULg; Grandjean, Fernande ULg; Keppens, V. et al

in Mater. Res. Soc. Symp. Proc. Vol. 886 (2006)

In the filled gallium-germanium clathrates, R8Ga16Ge30, where R is Ba, Sr, or Eu, the guests are located in two large cages and are weakly bound to the crystalline clathrate framework. The caged guests ... [more ▼]

In the filled gallium-germanium clathrates, R8Ga16Ge30, where R is Ba, Sr, or Eu, the guests are located in two large cages and are weakly bound to the crystalline clathrate framework. The caged guests exhibit a localized "rattling" vibrational mode that provides an efficient mechanism for reducing the thermal conductivity. Inelastic neutron scattering and nuclear inelastic scattering measurements have yielded the phonon density of states in R8Ga16Ge30; the line width of the localized vibrational modes is found to be an important parameter in determining the lattice thermal conductivity. Neutron diffraction studies on R8Ga16Ge30 have shown that the guests in the larger cage are located off-center, and it was proposed that their jumping about the four off-center locations is responsible for the observed glass-like thermal conductivity at temperatures below 10 K. The detection of such slow guest motion is challenging because the typical time and energy scales involved are ca. 4 ns and 1 mu eV, respectively. We have studied the slow europium tunneling dynamics in Eu4Sr4Ga16Ge30 by both Mossbauer and microwave absorption spectroscopy. [less ▲]

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See detailDirect experimental evidence for atomic tunneling of europium in crystalline Eu8Ga16Ge30
Hermann, Raphaël ULg; Keppens, Veerle; Bonville, Pierre et al

in Physical Review Letters (2006), 97(1),

Mossbauer-effect and microwave absorption experimental evidence unambiguously demonstrates the presence of slow, similar to 450 MHz, tunneling of magnetic europium between four equivalent sites in ... [more ▼]

Mossbauer-effect and microwave absorption experimental evidence unambiguously demonstrates the presence of slow, similar to 450 MHz, tunneling of magnetic europium between four equivalent sites in Eu8Ga16Ge30, a stoichiometric clathrate. Remarkably, six of the eight europium atoms, or 11% of the constituents in this solid, tunnel between these four sites separated by 0.55 angstrom. The off centering of the atoms or ions in crystalline clathrates appears to be a promising route for producing Rabi oscillators in solid-state materials. [less ▲]

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See detailMagnetic and electronic properties of Eu4Sr4Ga16Ge30
Woods, G. T.; Marti; Beekman, M. et al

in Physical Review b (2006), 73(17),

Magnetization, static and ac magnetic susceptibility, nuclear forward scattering, and electrical resistivity measurements have been performed on polycrystalline Eu4Sr4Ga16Ge30, a type I clathrate that has ... [more ▼]

Magnetization, static and ac magnetic susceptibility, nuclear forward scattering, and electrical resistivity measurements have been performed on polycrystalline Eu4Sr4Ga16Ge30, a type I clathrate that has divalent strontium and europium ions encapsulated within a Ga-Ge framework. These data are compared with those of type I clathrates Eu8Ga16Ge30 and Eu6Sr2Ga16Ge30. The ferromagnetic ordering of these Eu-containing clathrates is substantially altered by the incorporation of strontium, as compared to Eu8Ga16Ge30. Ferromagnetism, accompanied by a relatively large negative magnetoresistance, is observed below 15 and 20 K in Eu4Sr4Ga16Ge30 and Eu6Sr2Ga16Ge30, respectively. An effective magnetic moment of 7.83 mu(B) per Eu ion is observed above 30 K for Eu4Sr4Ga16Ge30, a moment which is close to the free-ion moment of 7.94 mu(B) per europium(II) ion. [less ▲]

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See detailA structural, infrared, and Mossbauer spectral study of rosemaryite, NaMnFe3+Al(PO4)(3)
Hatert, Frédéric ULg; Hermann, R. P.; Fransolet, André-Mathieu ULg et al

in European Journal of Mineralogy (2006), 18(6, NOV-DEC), 775-785

Rosemaryite, ideally NaMnFe(3+)AI(PO4)(3), has been collected in the Buranga pegmatite, Rwanda. A single-crystal structure refinement was performed to R-1 = 4.01 %, in the P2(1)/n space group, with a = 12 ... [more ▼]

Rosemaryite, ideally NaMnFe(3+)AI(PO4)(3), has been collected in the Buranga pegmatite, Rwanda. A single-crystal structure refinement was performed to R-1 = 4.01 %, in the P2(1)/n space group, with a = 12.001(2), b = 12.396(1), c = 6.329(1) angstrom, beta 114.48(1)degrees, Vol. = 856.9(2) angstrom(3), Z = 4. The crystal structure and cation distributions are similar to those of ferrorosemaryite, NaFe2+Fe3+Al(PO4)(3), and qingheiite, Na2MnMgAl(PO4)(3), but aluminium predominantly occurs in the M(2a) site, not in the M(2b) site as observed in ferrowyllieite, Na2Fe22+Al(PO4)(3). The topologies of the X(1a) and X(1b) crystallographic sites are identical to those found in ferrorosemaryite, and correspond to a distorted octahedron and to a distorted cube, respectively. The [7+1]-coordinated X(2) site is a very distorted gable disphenoid, similar to the A(2)' site of the alluaudite structure. Mossbauer spectra have been obtained from 4.2 to 295 K, and fitted with a model including two Fe3+ and two Fe2+ doublets. The Fe2+ component corresponding to 2/3 of the Fe2+ spectral area and having a smaller quadrupole splitting of 2.63 mm/s at 15 K, is assigned to the Fe2+ on the M(2a) site, and the Fe2+ component with the larger quadrupole splitting of 3.17 mm/s at 15 K, is assigned to the Fe2+ on the M(1) site. Fe3+ is located only at the M(2a) and M(2b) sites, and the Fe3+ component corresponding to 3/4 of the Fe3+ and exhibiting the larger quadrupole spitting of 0.77 mm/s at 15 K, is most likely associated with Fe3+ on the M(2b) site. The infrared spectrum of rosemaryite shows absorption bands at 3450 and 1624 cm(-1), bands that arise from the vibrational modes of H2O and confirm the presence of water in the channels of the wyllieite structure. A comparison of both the Mossbauer spectra and structural data of rosemaryite with those of other phosphates of the alluaudite and wyllieite groups, is also presented. [less ▲]

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See detailIron(III) species dispersed in porous silica through sol-gel chemistry: nature, size, and catalytic activity
Heinrichs, Benoît ULg; Rebbouh, Leila; Geus, John W. et al

Conference (2006)

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See detailCrystal chemistry of the hydrothermally synthesized Na-2(Mn1-xFex2+)(2)Fe3+(PO4)(3) alluaudite-type solid solution
Hatert, Frédéric ULg; Rebbouh, Leila; Hermann, Raphaël P. et al

in American Mineralogist (2005), 90(4), 653-662

Several compounds of the Na-2(Mn1-xFex2x)(2)Fe3+(PO4)(3) solid solition have been hydrothermally synthesized at 400 degrees C and 1 kbar; pure alluaudite-like compounds have been obtained for x = 0.00, 0 ... [more ▼]

Several compounds of the Na-2(Mn1-xFex2x)(2)Fe3+(PO4)(3) solid solition have been hydrothermally synthesized at 400 degrees C and 1 kbar; pure alluaudite-like compounds have been obtained for x = 0.00, 0.25, 0.50, 0.75, and 1.00. Rietveld refinements of the powder X-ray diffraction patterns indicate the presence of Na+ at the Al and A2' sites, Mn2+ and Fe2+ at the M1 site, and Mn2+, Fe2+, and Fe3+ at the M2 site. The presence of small amounts of Na+ at the M1 site and Mn2+ at the Al site indicates a partially disordered distribution of these cations. An excellent linear correlation has been established between the M1-M2 distance and the energy of the infrared band attributed to the M2+-O vibrations. The Mossbauer spectra, measured between 85 and 295 K, were analyzed in terms of a model which includes the next-nearest neighbor interactions at the M2 and M1 crystallographic sites. Fe2+ and Fe3+ isomer shifts are typical of the alluaudite structure and exhibit the expected second-order Doppler shift. The derived iron vibrating masses and Mossbauer lattice temperatures are within the range of values expected for iron cations in all octahedral environment. The Fe2+ and Fe3+ quadrupole splittings are typical of the alluaudite structure, and the temperature dependence of the Fe2+ quadrupole splitting was fit with the Ingalls model, which yielded a ground state orbital splitting of ca. 460 to 735 cm(-1) for the Fe2+ sites. The isomer shifts and quadrupole splittings of Fe2+ at the M1 site are larger than those of Fe2+ at M2, indicating that the M1 site is both larger and more distorted than the M2 site. [less ▲]

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See detailEinstein oscillators that impede thermal transport
Hermann, Raphaël ULg; Grandjean, Fernande ULg; Long, G. J.

in American Journal of Physics (2005), 73(2), 110-118

The Einstein model of a solid usually lacks a clear illustration in introductory solid-state physics courses because most solids are much better described by the Debye model. Filled antimony skutterudites ... [more ▼]

The Einstein model of a solid usually lacks a clear illustration in introductory solid-state physics courses because most solids are much better described by the Debye model. Filled antimony skutterudites, materials that have recently attracted much attention because of their potential for thermoelectric applications, provide a canonical illustration of the Einstein model. The filling atoms are loosely bound in the atomic cage formed by their neighbors, and hence their description as independent harmonic oscillators is adequate. Simple models for the heat capacity and thermal conductivity of a solid are introduced, with emphasis on the density of vibrational states. These models are used in conjunction with experimental results obtained from heat capacity and inelastic neutron scattering measurements to demonstrate the applicability of the concept of the Einstein oscillator to the filling guests in antimony skutterudites. The importance of these Einstein oscillators for impeding thermal transport is discussed and some simple problems involving the heat capacity, thermal conductivity, and inelastic neutron scattering are proposed. (C) 2005 American Association of Physics Teachers. [less ▲]

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See detailNeutron and nuclear inelastic scattering study of the Einstein oscillators in Ba-, Sr-, and Eu-filled germanium clathrates
Hermann, Raphaël ULg; Schweika, W.; Leupold, O. et al

in Physical Review b (2005), 72(17),

Inelastic neutron scattering measurements on Ba8Ga16Ge30 and Sr8Ga16Ge30 indicate the presence of soft local modes with Einstein oscillator energies of 4.9 +/- 0.1 and 4.4 +/- 0.1 meV, modes which are ... [more ▼]

Inelastic neutron scattering measurements on Ba8Ga16Ge30 and Sr8Ga16Ge30 indicate the presence of soft local modes with Einstein oscillator energies of 4.9 +/- 0.1 and 4.4 +/- 0.1 meV, modes which are associated with the lowest "rattling" mode of the Ba(B) and Sr(B) guests, respectively. Nuclear inelastic scattering measurements on Eu8Ga16Ge30 yield Einstein oscillator energies of between 3.0 +/- 0.3 and 7.5 +/- 1.2 meV for the different modes of the Eu guests. Further, the resonant character of the measurements on Eu8Ga16Ge30 reveals, without question, that neither of the Eu guests exhibit any vibrational modes above 9 meV. [less ▲]

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See detailStrongly decoupled europium and iron vibrational modes in filled skutterudites
Long, G. J.; Hermann, Raphaël ULg; Grandjean, Fernande ULg et al

in Physical Review b (2005), 71(14),

The europium partial vibrational density of states (DOS) in EuFe4Sb12 and the iron partial vibrational DOS in both EuFe4Sb12 and (CeFe4Sb12)-Fe-57 have been obtained by nuclear inelastic scattering. The ... [more ▼]

The europium partial vibrational density of states (DOS) in EuFe4Sb12 and the iron partial vibrational DOS in both EuFe4Sb12 and (CeFe4Sb12)-Fe-57 have been obtained by nuclear inelastic scattering. The results reveal the strong independence of the iron and rare-earth vibrational modes. The cage filling europium only participates significantly to the low-energy local vibrational modes. The force constants have been obtained from the measured probability of nuclear absorption. The energies of the peaks in the europium and iron DOS are in excellent agreement with the calculated DOS in LaFe4Sb12. The results indicate that nuclear inelastic scattering is the technique of choice for the study of the localized vibrational modes in thermoelectric "phonon glass" materials. [less ▲]

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See detailA Mossbauer spectral study of some iron nitride-based nanocomposites prepared by ball milling
Grandjean, Fernande ULg; Hermann, Raphaël ULg; Long, G. J. et al

in Journal of Magnetism & Magnetic Materials (2005), 292

Powder mixtures of (FeyN)(x) and (Al2O3)(1-x) or (SiO2)(1-x) with x = 0.2 and 0.6 and y = 3.8 have been ball milled for 4, 8, 16, 32, 64, and 128 h and their magnetic properties and Mossbauer spectra have ... [more ▼]

Powder mixtures of (FeyN)(x) and (Al2O3)(1-x) or (SiO2)(1-x) with x = 0.2 and 0.6 and y = 3.8 have been ball milled for 4, 8, 16, 32, 64, and 128 h and their magnetic properties and Mossbauer spectra have been measured. The 5 and 295 K saturation magnetizations decrease with increasing milling time as a result of decreasing particle sizes. Fits with the Langevin function of the field dependence of the magnetization yield particle sizes that are slightly smaller than those determined from a Scherrer analysis of the broadening of the X-ray diffraction peaks. The Mossbauer spectra have been fit with a distribution of hyperfine fields between 0 and 40 T and the peaks in the distribution assigned to the different iron nitride phases present in the nanocomposites. This analysis of the Mossbauer spectra indicates that the gamma'-Fe4N phase present in FeyN is transformed into epsilon-Fe3+xN and alpha-iron by the milling process. The weighted average field decreases and the weighted average isomer shift increases with milling time, as expected for the simultaneous effects of size reduction, the above phase transformation, and mixing with the oxides. A peak in the hyperfine field distribution at 4 T indicates the presence of small superparamagnetic particles, a presence which is also confirmed by the temperature dependence of the Mossbauer spectra and the failure of the magnetization to saturate even at applied fields of 5.5T. (c) 2004 Elsevier B.V. All rights reserved. [less ▲]

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