-
J. Fujii,
B. R. Salles,
M. Sperl,
S. Ueda,
M. Kobata,
K. Kobayashi,
Y. Yamashita,
P. Torelli,
M. Utz, C. S. Fadley, [......],
I. Di Marco,
O. Eriksson,
P. Thunström,
G. H. Fecher,
S. Ouardi,
H. Stryhanyuk,
E. Ikenaga,
C. H. Back,
G. van der Laan,
G. Panaccione
[show abstract]
[hide abstract]
ABSTRACT: We report high-resolution hard x-ray photoemission spectroscopy results on
(Ga,Mn)As films as a function of Mn doping. Supported by theoretical
calculations we identify, over the entire 1% to 13% Mn doping range, the
electronic character of the states near the top of the valence band.
Magnetization and temperature dependent core-level photoemission spectra reveal
how the delocalized character of the Mn states enables the bulk ferromagnetic
properties of (Ga,Mn)As.
06/2013;
-
A. A. Greer,
A. X. Gray,
S. Kanai,
A. M. Kaiser,
S. Ueda,
Y. Yamashita,
C. Bordel,
G. Palsson,
N. Maejima,
S.-H. Yang,
G. Conti,
K. Kobayashi,
S. Ikeda,
F. Matsukura,
H. Ohno,
C. M. Schneider,
J. B. Kortright,
F. Hellman, C. S. Fadley
Applied Physics Letters 11/2012; 101(20):202402-4. · 3.84 Impact Factor
-
A X Gray,
J Minár,
S Ueda,
P R Stone,
Y Yamashita,
J Fujii,
J Braun,
L Plucinski,
C M Schneider,
G Panaccione,
H Ebert,
O D Dubon,
K Kobayashi, C S Fadley
[show abstract]
[hide abstract]
ABSTRACT: A detailed understanding of the origin of the magnetism in dilute magnetic semiconductors is crucial to their development for applications. Using hard X-ray angle-resolved photoemission (HARPES) at 3.2 keV, we investigate the bulk electronic structure of the prototypical dilute magnetic semiconductor Ga(0.97)Mn(0.03)As, and the reference undoped GaAs. The data are compared to theory based on the coherent potential approximation and fully relativistic one-step-model photoemission calculations including matrix-element effects. Distinct differences are found between angle-resolved, as well as angle-integrated, valence spectra of Ga(0.97)Mn(0.03)As and GaAs, and these are in good agreement with theory. Direct observation of Mn-induced states between the GaAs valence-band maximum and the Fermi level, centred about 400 meV below this level, as well as changes throughout the full valence-level energy range, indicates that ferromagnetism in Ga(1-x)Mn(x)As must be considered to arise from both p-d exchange and double exchange, thus providing a more unifying picture of this controversial material.
Nature Material 10/2012; 11(11):957-62. · 32.84 Impact Factor
-
A. X. Gray,
D. W. Cooke,
P. Krüger,
C. Bordel,
A. M. Kaiser,
S. Moyerman,
E.E. Fullerton,
S. Ueda,
Y. Yamashita,
A. Gloskovskii,
C. M. Schneider,
W. Drube,
K. Kobayashi,
F. Hellman, C. S. Fadley
Physical Review Letters 01/2012; · 7.37 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The evolution of ferromagnetic domains across the temperature-driven antiferromagnetic (AF) to ferromagnetic (FM) phase transition in uncapped and capped epitaxial FeRh thin films was studied by x-ray magnetic circular dichroism and photoemission electron microscopy. The coexistence of the AF and FM phases was evidenced across the broad transition and the different stages of nucleation, growth and coalescence were observed. The FM phase nucleates into single domain islands and the width of the transition of the individual nuclei is sharper than that of the macroscopic transition.
Applied Physics Letters 01/2012; 100:262401. · 3.84 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We report on the stepwise generation of layered nanostructures via electron beam induced deposition (EBID) using organometallic precursor molecules in ultra-high vacuum (UHV). In a first step a metallic iron line structure was produced using iron pentacarbonyl; in a second step this nanostructure was then locally capped with a 2-3 nm thin titanium oxide-containing film fabricated from titanium tetraisopropoxide. The chemical composition of the deposited layers was analyzed by spatially resolved Auger electron spectroscopy. With spatially resolved x-ray absorption spectroscopy at the Fe L₃ edge, it was demonstrated that the thin capping layer prevents the iron structure from oxidation upon exposure to air.
Nanotechnology 11/2011; 22(47):475304. · 3.98 Impact Factor
-
J Fujii,
M Sperl,
S Ueda,
K Kobayashi,
Y Yamashita,
M Kobata,
P Torelli,
F Borgatti,
M Utz, C S Fadley,
A X Gray,
G Monaco,
C H Back,
G van der Laan,
G Panaccione
[show abstract]
[hide abstract]
ABSTRACT: We report x-ray photoemission spectroscopy results on (Ga,Mn)As films as a function of both temperature and Mn doping. Analysis of Mn 2p core level spectra reveals the presence of a distinct electronic screening channel in the bulk, hitherto undetected in more surface sensitive analysis. Comparison with model calculations identifies the character of the Mn 3d electronic states and clarifies the role, and the difference between surface and bulk, of hybridization in mediating the ferromagnetic coupling in (Ga,Mn)As.
Physical Review Letters 10/2011; 107(18):187203. · 7.37 Impact Factor
-
A M Kaiser,
A X Gray,
G Conti,
J Son,
A Greer,
A Perona,
A Rattanachata,
A Y Saw,
A Bostwick,
S Yang,
S-H Yang,
E M Gullikson,
J B Kortright,
S Stemmer, C S Fadley
[show abstract]
[hide abstract]
ABSTRACT: Standing-wave-excited photoemission is used to study a SrTiO3/LaNiO3 superlattice. Rocking curves of core-level and valence band spectra are used to derive layer-resolved spectral functions, revealing a suppression of electronic states near the Fermi level in the multilayer as compared to bulk LaNiO3. Further analysis shows that the suppression of these states is not homogeneously distributed over the LaNiO3 layers but is more pronounced near the interfaces. Possible origins of this effect and its relationship to a previously observed metal-insulator-transition in ultrathin LaNiO3 films are discussed.
Physical Review Letters 09/2011; 107(11):116402. · 7.37 Impact Factor
-
A X Gray,
C Papp,
S Ueda,
B Balke,
Y Yamashita,
L Plucinski,
J Minár,
J Braun,
E R Ylvisaker,
C M Schneider,
W E Pickett,
H Ebert,
K Kobayashi, C S Fadley
[show abstract]
[hide abstract]
ABSTRACT: Traditional ultraviolet/soft X-ray angle-resolved photoemission spectroscopy (ARPES) may in some cases be too strongly influenced by surface effects to be a useful probe of bulk electronic structure. Going to hard X-ray photon energies and thus larger electron inelastic mean-free paths should provide a more accurate picture of bulk electronic structure. We present experimental data for hard X-ray ARPES (HARPES) at energies of 3.2 and 6.0 keV. The systems discussed are W, as a model transition-metal system to illustrate basic principles, and GaAs, as a technologically-relevant material to illustrate the potential broad applicability of this new technique. We have investigated the effects of photon wave vector on wave vector conservation, and assessed methods for the removal of phonon-associated smearing of features and photoelectron diffraction effects. The experimental results are compared to free-electron final-state model calculations and to more precise one-step photoemission theory including matrix element effects.
Nature Material 08/2011; 10(10):759-64. · 32.84 Impact Factor
-
Phys. Status Solidi RRL. 01/2011; 1:1--3.
-
[show abstract]
[hide abstract]
ABSTRACT: Cr1-xAlx exhibits semiconducting behavior for x=0.15–0.26. This Letter uses hard x-ray photoemission spectroscopy and density functional theory to further understand the semiconducting behavior. Photoemission measurements of an epitaxial Cr0.80Al0.20 thin film show several features in the valence band region, including a gap at the Fermi energy (EF) for which the valence band edge is 95±14 meV below EF. Theory agrees well with the valence band measurements, and shows an incomplete gap at EF due to the hole band at M shifting almost below EF.
Physical Review Letters 01/2010; 105:236404. · 7.37 Impact Factor
-
B. C. Sell,
S. B. Ritchey,
S.-H. Yang,
S. S. P. Parkin,
M. Watanabe,
B. S. Mun,
L. Plucinski,
N. Mannella,
A. Nambu,
J. Guo,
M. W. West,
F. Salmassi,
J. B. Kortright, C. S. Fadley
[show abstract]
[hide abstract]
ABSTRACT: We demonstrate that a standing-wave/wedge (swedge) method for probing buried solid-solid interfaces can be carried out using soft x-ray emission spectroscopy (XES) and resonant inelastic x-ray scattering (RIXS). For the particular case of an Al2O3/Fe/Cr structure of relevance to giant magnetoresistance, measurements of Fe L and Cr L total intensities and Fe L magnetic circular dichroism are used to derive for both the top and bottom Fe interfaces the depth profile of composition and the depth dependence of the atom-specific Fe contribution to magnetization. Using XES and RIXS in this method, as compared to photoelectron spectroscopy in prior work, permits studying more deeply buried interfaces, and suggests future applications to a wide variety of magnetic and nonmagnetic nanostructures.
Journal of Applied Physics 04/2008; 103(8):083515-083515-8. · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We discuss a newly developed x-ray standing-wave∕wedge (swedge) method for probing the composition, magnetization, and electronic densities of states in buried interfaces and layers in spintronic nanostructures. In work based on photoemission, this method has permitted determining concentration and magnetization profiles through giant magnetoresistive (GMR) and magnetic tunnel junctions structures, as well as individual layer densities of states near the Fermi level in a tunnel junction. Using x-ray emission and resonant inelastic x-ray scattering for detection has permitted probing deeper layers and interfaces in a GMR structure. Various future applications of this method in nanomagnetism are suggested, including using more energetic hard x-ray standing waves so as to probe more deeply below a surface and standing-wave excitation in spectromicroscopy to provide depth sensitivity.
Journal of Applied Physics 03/2008; 103(7):07C519-07C519-6. · 2.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We have studied the temperature dependence of W(110) soft x-ray angle-resolved photoemission spectra excited at photon energies of 260 and 870 eV and between 300 and 780 K. The experimental results have been compared to both a free-electron final-state model and theoretical one-step model calculations of the photocurrent. At 300 K, clear band dispersions can be observed in the data. The temperature dependence of the data can be analyzed qualitatively in terms of a direct-transition band-dispersion regime (“UPS” limit) versus a nondirect-transition density-of-states regime (“XPS” limit). The ratio between direct and nondirect transitions is estimated from a Debye-Waller factor, which for example at h=870 eV predicts 70% direct transitions at 300 K, and 41% at 780 K, and these values qualitatively describe our data. Beyond this, the state-of-the-art one-step theoretical calculations reproduce well the band dispersions and matrix element effects in the measured spectra at room temperature. However, simulating the temperature dependence is more complicated, and including phonon effects via complex phase shifts accounts for the suppression of existing direct-transition features, but does not reproduce new, density-of-states-related background intensity which shows up in higher-temperature experimental spectra. Finally, we also discuss the implications of this work for future experiments on other materials and at even higher photon energies up to 10 keV.
Physical Review B (Condensed Matter and Materials Physics). 01/2008; 78(3):035108-8.
-
F. Offi,
P. Torelli,
M. Sacchi,
P. Lacovig,
A. Fondacaro,
G. Paolicelli,
S. Huotari,
G. Monaco, C. S. Fadley,
J. F. Mitchell,
G. Stefani,
G. Panaccione
[show abstract]
[hide abstract]
ABSTRACT: We have performed hard-x-ray photoemission experiments at photon energy close to 6 keV on the ferromagnetic bilayered manganite La1.2Sr1.8Mn2O7 across its transition temperature TC. Differences in both core level and valence band spectra have been observed as a function of temperature: We correlate the energy shift of the O 1s peak with a charge-transfer associated with the structural change taking place at the transition. A prominent shoulder on the low binding energy side of the Mn 2p core level that is observed below TC is tentatively ascribed to increased nonlocal screening in the ordered ferromagnetic phase.
Phys. Rev. B. 01/2007; 75(1).
-
N. Mannella,
S. -H. Yang,
B. S. Mun,
F. J. Garcia de Abajo,
A. W. Kay,
B. C. Sell,
M. Watanabe,
H. Ohldag,
E Arenholz,
A T Young,
Z. Hussain,
M. A. Van Hove, C S Fadley
[show abstract]
[hide abstract]
ABSTRACT: We present experimental and theoretical results for the variation of the O 1s intensity from a NiO(001) surface as the excitation energy is varied through the Ni 2p1/2,3/2 absorption resonances, and as the incidence angle of the radiation is varied from grazing to larger values. For grazing incidence, a strong multi-atom resonant photoemission (MARPE) effect is seen on the O 1s intensity as the Ni 2p resonances are crossed, but its magnitude decreases rapidly as the incidence angle is increased. Resonant x-ray optical (RXRO) calculations are found to predict these effects very well, although the experimental effects are found to decrease at higher incidence angles faster than those in theory. The potential influence of photoelectron diffraction effects on such measurements are also considered, including experimental data with azimuthal-angle variation and corresponding multiple-scattering-diffraction calculations, but we conclude that they do not vary beyond what is expected on the basis of the change in photoelectron kinetic energy. Varying from linear polarization to circular polarization is found to enhance these effects in NiO considerably, although the reasons are not clear. We also discuss the relationship of these measurements to other related interatomic resonance experiments and theoretical developments, and make some suggestions for future studies in this area. Comment: Phys. Rev. B, in press
09/2006;
-
[show abstract]
[hide abstract]
ABSTRACT: We have measured the valence-band densities of states (DOSs) in buried CoFeB and CoFe layers in a magnetic tunnel junction using a novel extension of a recently developed standing wave/wedge soft x-ray photoemission technique. The CoFe DOS at the Fermi level is substantially enhanced when the CoFe thickness is reduced from 25 to 15 Å. This enhancement, which we suggest is due to the amorphous character of the CoFe when in thickness and results in a spin-polarized peak in the DOS of primarily Co origin, can be directly correlated to marked improvements in magnetic transport and switching properties. This technique for studying buried-layer DOSs should also be applicable to other multilayer nanostructures.
Journal of Physics Condensed Matter 04/2006; 18(19):L259. · 2.55 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Using photoemission intensities and a detection system employed by many groups in the electron spectroscopy community as an example, we have quantitatively characterized and corrected detector non-linearity effects over the full dynamic range of the system. Non-linearity effects are found to be important whenever measuring relative peak intensities accurately is important, even in the low-countrate regime. This includes, for example, performing quantitative analyses for surface contaminants or sample bulk stoichiometries, where the peak intensities involved can differ by one or two orders of magnitude, and thus could occupy a significant portion of the detector dynamic range. Two successful procedures for correcting non-linearity effects are presented. The first one yields directly the detector efficiency by measuring a flat-background reference intensity as a function of incident x-ray flux, while the second one determines the detector response from a least-squares analysis of broad-scan survey spectra at different incident x-ray fluxes. Although we have used one spectrometer and detection system as an example, these methodologies should be useful for many other cases.
01/2005;
-
[show abstract]
[hide abstract]
ABSTRACT: The temperature dependence of the O K-edge pre-edge structure in the x-ray absorption spectra of the perovskites La(1-x)A(x)MnO(3), (A = Ca, Sr; x = 0.3, 0.4) reveals a correlation between the disappearance of the splitting in the pre-edge region and the presence of Jahn-Teller distortions. The different magnitudes of the distortions for different compounds is proposed to explain some dissimilarity in the line shape of the spectra taken above the Curie temperature. Comment: To appear in Phys. Rev. B, 5 pages, 3 figures
01/2005;
-
[show abstract]
[hide abstract]
ABSTRACT: A high-speed, one-dimensional detector array for electrons and UV/X-ray photons has been developed. The detector is capable of handling the high countrates encountered in at third generation synchrotron radiation sources and is free from nonlinearity problems present in charge coupled device (CCD) detectors. Electrons are counted by a configuration of microchannel plates, an array of charge collection electrodes, and custom-designed integrated circuits (IC) assembled on a ceramic hybrid. The charges are collected on 768 strips with a 48 μm pitch that are wire-bonded to 6 pairs of signal processing ICs. Each front-end IC has 128 channels of amplifiers (peaking time 25 ns) and discriminators. The pulse-pair resolution is 50 ns leading to a maximum linear countrate/channel of 2 MHz. The second, custom-designed IC features 24-b buffered counters and a serial link for the transfer of commands and data. A possible deadtime-less readout of all channels in 150 μs opens the door to time resolved experiments. The complete detector system includes the high-voltage power supply, a field programmable gate array (FPGA)-based data acquisition system, and supporting software. Special care has been taken to insure reliable operation in an ultra-high vacuum environment. The detector architecture and design is described and measured performance characteristics such as spatial resolution and count-rate linearity are presented.
IEEE Transactions on Nuclear Science 11/2004; · 1.45 Impact Factor
