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A. Y. Ramos,
H. C. N. Tolentino,
O. Bunau,
Y. Joly, S. Grenier,
R. A. Souza,
N. M. Souza-Neto,
F. Baudelet,
F. Wilhelm,
A. Rogalev,
O. Proux,
A. Caneiro
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ABSTRACT: The emergence of a ferromagnetic component in $LaMnO_{3}$ with low Cr-for-Mn
substitution has been studied by x-ray absorption spectroscopy and x-ray
magnetic circular dichroism at the Mn and Cr K edges. The local magnetic moment
strength for the Mn and Cr are proportional to each other and follows the
macroscopic magnetization. The net ferromagnetic components of $Cr^{3+}$ and
$Mn^{3+}$ are found antiferromagnetically coupled. Unlike hole doping by La
site substitution, the inclusion of $Cr^{3+}$ ions up to x = 0.15 does not
decrease the Jahn-Teller (JT) distortion and consequently does not
significantly affect the orbital ordering. This demonstrates that the emergence
of the ferromagnetism is not related to JT weakening and likely arises from a
complex orbital mixing.
05/2013;
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ABSTRACT: A formalism for the reflectivity of electromagnetic waves by magnetic materials is presented with an application. It is applied to retrieve the magnetic moment density along the depth of magnetic materials with arbitrary magnetic moment direction using matricial algebra, including roughness between layers. The reflectivity is derived following a classical description with Maxwell equations and a permittivity built from the quantum scattering amplitude. Approximations on the relative power of the Thomson scattering and the magnetic terms are trackable in order to evaluate the validity of the formalism case-by-case, from the optical light regime up to soft and hard X-rays. Eigenwaves are used throughout the whole formalism. In order to illustrate the methodology, we present an application to a W/Fe/W trilayer performed at the Fe L -edge, in the soft X-ray regime.
Journal of Magnetism and Magnetic Materials - J MAGN MAGN MATER. 01/2012; 324:105-112.
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ABSTRACT: The local environment of nickel atoms in Y NiO3 across the pressure- induced
insulator to metal (IM) transition was studied using X-ray absorption
spectroscopy (XAS) supported by ab initio calculations. The monotonic
contraction of the NiO6 units under applied pressure observed up to 13 GPa,
stops in a limited pressure domain around 14 GPa, before resuming above 16 GPa.
In this narrow pressure range, crystallographic modifications basically occur
in the medium/long range, not in the NiO6 octahedron, whereas the evolution of
the near-edge XAS features can be associated to metallization. Ab initio
calculations show that these features are related to medium range order,
provided that the Ni-O-Ni angle enables a proper overlap of the Ni eg and O 2p
orbitals. Metallization is then not directly related to modifications in the
average local geometry of the NiO6 units but more likely to an inter-octahedra
rearrangement. These outcomes provides evidences of the bandwidth driven nature
of the IM transition.
12/2011;
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ABSTRACT: Using X-ray absorption spectroscopy (XAS), we studied the local structure in LaMnO3 under applied pressure across and well above the insulator-to-metal (IM) transition. A hysteretic behavior points to the coexistence of two phases within a large pressure range (7 to 25 GPa). The ambient phase with highly Jahn-Teller (JT) distorted MnO6 octahedra is progressively substituted by a new phase with less-distorted JT MnO6 units. The electronic delocalization leading to the IM transition is finger-printed from the pre-edge XAS structure around 30 GPa. We observed that the phase transition takes place without any significant reduction of the JT distortion. This entails band overlap as the driving mechanism of the IM transition.
EPL (Europhysics Letters) 10/2011; 96(3):36002. · 2.17 Impact Factor
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ABSTRACT: The possibility to investigate complex magnetic profiles throughout an ultrathin magnetic film or an interface by soft x-ray resonant magnetic reflectivity is presented. The determination of in- and out-of-plane magnetic profile is shown to be possible with a subnanometer resolution by measuring the reflectivity over a wide angular range. The technique is applied to a granular magnetic multilayer and to a perpendicular exchange bias coupled system.
Journal of Physics Conference Series 02/2010; 211(1):012015.
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H L Meyerheim,
J-M Tonnerre,
L Sandratskii,
H C N Tolentino,
M Przybylski,
Y Gabi,
F Yildiz,
X L Fu,
E Bontempi, S Grenier,
J Kirschner
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ABSTRACT: Using soft-x-ray resonant magnetic scattering in combination with first-principles calculations for noncollinear magnetic configurations we present a new model of the magnetism in ultrathin fcc Fe films on Cu(001). We find the presence of blocks with robust magnetic structure, while the relative directions of the moments of different blocks are sensitive to the detailed atomic structure and temperature. The magnetic noncollinearity is directly demonstrated, which has not been possible so far.
Physical Review Letters 12/2009; 103(26):267202. · 7.37 Impact Factor
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ABSTRACT: A user friendly tool allowing the simulation of x-ray absorption near edge structure (XANES) spectra and Resonant X-ray Diffraction (RXD) peak intensity is necessary for many purposes. We present the actual developments of the FDMNES code which realizes this task in a mono-electronic approach. The code uses both the multiple-scattering theory and the finite different method in a fully relativistic frame, including thus the spin-orbit interaction. In diffraction, the resonant and non-resonant, magnetic and non-magnetic components are all included, allowing an easy use by non experts of the code. In the same idea, the automatic analysis of the unit cell (or molecule) symmetry greatly simplifies the user's work. Summation on and energy shift between the different absorption sites are automatically included. Comparison with experiment in order to fit parameters is also possible. The last advances with the self-consistent calculations are discussed. It is shown that within the multiple-scattering theory, using the muffin-tin approximation on the shape of the potential, the improvement is not high for non magnetic situation and limited to the pre-edge region. The improvements coming from the non-muffin-tin corrections are notably higher. More improvement results from self-consistency for the magnetic NdMg system. Various examples in oxides and metal in RXD and XANES are given showing the potentiality of the code. The fit procedure is applied in the magnetite low temperature phase case.
Journal of Physics Conference Series 11/2009; 190(1):012007.
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ABSTRACT: A combination of spectroscopic probes was used to develop a detailed experimental description of the transport and magnetic properties of superlattices composed of the paramagnetic metal CaRuO$_3$ and the antiferromagnetic insulator CaMnO$_3$. The charge carrier density and Ru valence state in the superlattices are not significantly different from those of bulk CaRuO$_3$. The small charge transfer across the interface implied by these observations confirms predictions derived from density functional calculations. However, a ferromagnetic polarization due to canted Mn spins penetrates 3-4 unit cells into CaMnO$_3$, far exceeding the corresponding predictions. The discrepancy may indicate the formation of magnetic polarons at the interface.
04/2009;
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ABSTRACT: The subtle interplay among electronic degrees of freedom (charge and orbital orderings), spin and lattice distortion that conspire at the Verwey transition in magnetite (Fe3O4) is still a matter of controversy. Here, we provide compelling evidence that these electronic orderings are manifested as a continuous phase transition at the temperature where a spin reorientation takes place at around 130 K, i.e., well above TV approximately 121 K. The Verwey transition seems to leave the orbital ordering unaffected whereas the charge ordering development appears to be quenched at this temperature and the temperature dependence below TV is controlled by the lattice distortions. Finally, we show that the orbital ordering does not reach true long range (disorder), and the correlation length along the c-direction is limited to 100 angstroms.
Physical Review Letters 12/2008; 101(22):226401. · 7.37 Impact Factor
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M. Garcia-Fernandez,
V. Scagnoli,
U Staub,
A. M. Mulders,
M. Janousch,
Y Bodenthin,
D. Meister,
B. D. Patterson,
A. Mirone,
Y Tanaka,
T Nakamura, S. Grenier,
Y. Huang,
K. Conder
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ABSTRACT: We have performed non-resonant x-ray diffraction, resonant soft and hard x-ray magnetic diffraction, soft x-ray absorption and x-ray magnetic circular dichroism measurements to clarify the electronic and magnetic states of the Co3+ ions in GdBaCo2O5.5. Our data are consistent with a 3+ Py Co HS state at the pyramidal sites and a 3+ Oc Co LS state at the octahedral sites. The structural distortion, with a doubling of the a axis (2ap x 2ap x 2ap cell), shows alternating elongations and contractions of the pyramids and indicates that the metal-insulator transition is associated with orbital order in the t2g orbitals of the 3+ Py Co HS state. This distortion corresponds to an alternating ordering of xz and yz orbitals along the a and c axes for the 3+ Py Co . The orbital ordering and pyramidal distortion lead to deformation of the octahedra, but the 3+ Oc Co LS state does not allow an orbital order to occur for the 3+ Oc Co ions. The soft x-ray magnetic diffraction results indicate that the magnetic moments are aligned in the ab plane but are not parallel to the crystallographic a or b axes. The orbital order and the doubling of the magnetic unit cell along the c axis support a non-collinear magnetic structure. The x-ray magnetic circular dichroism data indicate that there is a large orbital magnetic contribution to the total ordered Co moment.
08/2008;
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ABSTRACT: The magnetic profile across the interface of a perpendicular exchange coupled [NiO/CoO]3/Pt-Co/Pt(111) system is investigated. The magneto-optic Kerr effect reveals a strong coupling between the antiferromagnetic (AFM) oxide and the ferromagnetic (FM) Pt-Co layer, by an increasing coercivity and a rotation of the easy magnetization axis of the FM layer along the AFM spins. Soft x-ray resonant magnetic reflectivity is used to probe the spatial distribution of the out-of-plane magnetization inside the oxide above its ordering temperature. It extends over 1 nm and exhibits a change of sign.
Physical Review Letters 04/2008; 100(15):157202. · 7.37 Impact Factor
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ABSTRACT: The question of how bulk electronic order is terminated at a surface is an intriguing one, and one with possible practical implications--for example in nanoscaled systems that may be characterized by their surface behaviour. One example of such order is orbital order, and in principle it should be possible to probe the termination of this order with surface X-ray scattering. Here, we report the first observation of the scattering arising from the termination of bulk orbital order at the surface of a crystal--so-called 'orbital truncation rods'. The measurements, carried out on a cleaved perovskite, La(0.5)Sr(1.5)MnO(4), reveal that whereas the crystallographic surface is atomically smooth, the orbital 'surface', which is observed through the atomic displacements caused by the orbital order, is much rougher, with a typical scale of the surface roughness of approximately 7 degrees A. Interestingly, the temperature dependence of this scattering shows evidence of a surface-induced second-order transition.
Nature Material 01/2008; 6(12):972-6. · 32.84 Impact Factor
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S Grenier,
K J Thomas,
J P Hill,
U Staub,
Y Bodenthin,
M García-Fernández,
V Scagnoli,
V Kiryukhin,
S-W Cheong,
B G Kim,
J M Tonnerre
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ABSTRACT: We report the observation of superstructures associated with the oxygen 2p states in two prototypical manganites using x-ray diffraction at the oxygen K edge. In the stripe order system Bi0.31Ca0.69MnO3, hole-doped O states are orbitally ordered, at the same propagation vector as the Mn orbital ordering, but no oxygen charge stripes are found at this periodicity. In La7/8Sr1/8MnO3, we observe a 2p charge ordering described by alternating hole-poor and hole-rich MnO planes that is consistent with some of the recent predictions.
Physical Review Letters 12/2007; 99(20):206403. · 7.37 Impact Factor
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ABSTRACT: We report the first observation of `orbital truncation rods' -- the
scattering arising from the termination of bulk orbital order at the surface of
a crystal. The x-ray measurements, performed on a cleaved, single-layered
perovskite, La0.5Sr1.5MnO4, reveal that while the crystallographic surface is
atomically smooth, the orbital `surface' is much rougher, with an r.m.s.
deviation from the average `surface' of ~0.7nm. The temperature dependence of
this scattering shows evidence of a surface-induced second order transition.
06/2007;
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ABSTRACT: We have studied nanostructured samples of InAs/InP(001) by means of Grazing Incidence Diffraction Anomalous Fine Structure. The samples, grown by molecular beam epitaxy, show a periodic corrugation on the surface giving rise to an array of self-assembled quantum wires after deposition of 2.5 monolayers of InAs. We measured the (440) and (420) GIDAFS spectra, at the As K-edge, at incidence and outgoing angles close to the substrate's critical angle. We analysed the anomalous diffraction lineshapes vs. the energy, as well as the oscillatory part of the signal in the extended region above the edge and obtained, for the first time, information about composition and strain inside the quantum wires and close to the interface. Our results suggest possible interfaces.
EPL (Europhysics Letters) 01/2007; 57(4):499. · 2.17 Impact Factor
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ABSTRACT: We report on the experimental characterization of orbital ordering and the associated lattice distortions in highly doped Bi1-xCaxMnO3 . Resonant x-ray diffraction was used at the MnL -edge for the direct observation of the ordered localized states, and at the MnK -edge for the sensitivity to the distortions of the manganese-oxygen octahedra. The orbital ordering on Mn atoms was directly observed at x=0.69 ; the analysis and the numerical simulations of the K -edge spectra allow us to characterize the pattern of the distorted octahedra at x=(4)/(5) . These observations support the Wigner-crystal-type model at both dopings; the bi-stripe model is ruled out at x=0.69 .
Physical Review B - PHYS REV B. 01/2007; 75(8).
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L Lu,
G Chabot-Couture,
X Zhao,
J N Hancock,
N Kaneko,
O P Vajk,
G Yu, S Grenier,
Y J Kim,
D Casa,
T Gog,
M Greven
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ABSTRACT: We report a Cu -edge resonant inelastic x-ray scattering (RIXS) study of charge-transfer excitations in the 2-8 eV range in the structurally simple compound HgBa(2)CuO(4+delta) at optimal doping (T(c)=96.5 K). The spectra exhibit a significant dependence on the incident photon energy which we carefully utilize to resolve a multiplet of weakly dispersive (<0.5 eV) electron-hole excitations, including a mode at 2 eV. The observation of this 2 eV excitation suggests the existence of a remnant charge-transfer gap deep in the superconducting phase. Quite generally, our results, which include additional data for the Mott insulator La(2)CuO(4), demonstrate the importance of exploring the incident photon-energy dependence of the RIXS cross section.
Physical Review Letters 11/2005; 95(21):217003. · 7.37 Impact Factor
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S Grenier,
J P Hill,
V Kiryukhin,
W Ku,
Y-J Kim,
K J Thomas,
S-W Cheong,
Y Tokura,
Y Tomioka,
D Casa,
T Gog
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ABSTRACT: We report a study of electronic excitations in manganites exhibiting a range of ground states, using resonant inelastic x-ray scattering (RIXS) at the Mn K edge. Excitations with temperature dependent changes correlated with the magnetism were observed as high as 10 eV. By calculating Wannier functions, and finite-q response functions, we associate this dependence with intersite d-d excitations. The calculated dynamical structure factor is found to be similar to the RIXS spectra.
Physical Review Letters 03/2005; 94(4):047203. · 7.37 Impact Factor
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L Lu,
X Zhao,
G. Chabot-Couture,
J. N. Hancock,
N. Kaneko,
O. P. Vajk,
G Yu, S. Grenier,
Y J Kim,
D. Casa,
T. Gog,
M. Greven
[show abstract]
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ABSTRACT: We report a Cu $K$-edge resonant inelastic x-ray scattering (RIXS) study of charge-transfer excitations in the 2-8 eV range in the structurally simple compound HgBa$_2$CuO$_{4+\delta}$ at optimal doping ($T_{\rm c} = 96.5 $ K). The spectra exhibit a significant dependence on the incident photon energy which we carefully utilize to resolve a multiplet of weakly-dispersive ($ < 0.5$ eV) electron-hole excitations, including a mode at 2 eV. The observation of this 2 eV excitation suggests the existence of a charge-transfer pseudogap deep in the superconducting phase. Quite generally, our data demonstrate the importance of exploring the incident photon energy dependence of the RIXS cross section. Comment: 5 pages, 3 figures
01/2005;
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L Lu,
G Chabot-Couture,
X Zhao,
JN Hancock,
N Kaneko,
OP Vajk,
G Yu, S Grenier,
YJ Kim,
D Casa,
T Gog,
M Greven
Physical Review Letters 01/2005; 95. · 7.37 Impact Factor
