[Show abstract][Hide abstract] ABSTRACT: Mn5Ge3(001) thin films grown on Ge(111)-c(2×8) reconstructed surfaces were studied by angle-resolved photoemission using synchrotron radiation in the 14–94 eV photon energy range. The results obtained in the ΓALM plane and in the ΓAHK plane are in agreement with simulations starting with band structure calculations based on the density functional theory. This provides a unique validation of band structure calculations for a proper description of the electronic properties of Mn5Ge3. Only the spectral feature very close to the Fermi level cannot be well explained by the simulation. This departure is discussed in terms of the three-dimensional nature of the sample and of correlation effects.
[Show abstract][Hide abstract] ABSTRACT: We have performed angle- and spin-resolved photoemission measurements on Fe3O4(0 0 1)/MgO(0 0 1). Despite intrinsic electron-lattice (polaron) interaction a Fe t2g band dispersion is observed, as well as a Fermi surface. The states close to the Fermi level show a spin polarization up to −72% as measured with a 4.65 eV laser light. All the data can be simulated starting from GGA+U bulk band-structure calculations and taking into account the polaron and initial-state lifetime broadening involved in the photoemission process. This conciliates the electronic structure of Fe3O4 with a band model and confirms the half-metallic nature of Fe3O4.
[Show abstract][Hide abstract] ABSTRACT: An investigation of the structural, magnetic and electronic properties of ≈ 3 nm thick Mn5Ge3 films epitaxially grown on a Ge(1 1 1)-c(2 × 8) reconstructed surface is reported. High resolution transmission electron microscopy and selected area electron diffraction give evidence of 2.2% in-plane compressive strain between the Mn5Ge3 film and the Ge substrate. Magnetooptical Kerr effect measurements show that the films are ferromagnetic with a Curie temperature of ≈ 325 K. The analysis of Ge 3d core level photoelectron spectra of the Mn5Ge3 films allows determining an upper limit of 76 meV for the Ge 3d5/2 core-hole lifetime broadening. The Ge 3d3/2 core-hole lifetime broadening is found to be 15 meV larger than that of the Ge 3d5/2 core hole, because of the existence of a Coster–Kronig decay channel due to the metallic character of Mn5Ge3.Research Highlights► A Mn5Ge3 film grown on Ge(111) exhibits a 2.2% in-plane compressive strain. ► The Mn5Ge3 film is ferromagnetic (Tc » 325 K). ► The magnetic moment per unit cell is 2.7 at 300 K (7 mB at 13 K). ► The Ge 3d core level of Mn5Ge3 shows a M4-M5V Coster-Kronig transition broadening.
[Show abstract][Hide abstract] ABSTRACT: X-ray magnetic circular dichroism and resonant photoemission at the Fe and Ni L2,3 edges have been used to investigate the electronic structure of NiFe2O4 (NFO) thin films. The results, when compared to those obtained on Fe3O4, indicate that in a 12 nm NFO film the Ni atoms occupy mainly B sites, as in bulk NFO, and that a decrease in the thickness of the film results in a modification of the nickel hybridization.
The European Physical Journal Special Topics 03/2009; 169(1). · 1.80 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The structural, electronic, and magnetic properties of the Mn0.06Ge0.94 diluted magnetic semiconductor, grown at 520 K by molecular-beam epitaxy on Ge(001)2×1, have been investigated. Diluted and highly ordered alloys, containing Mn5Ge3 nanocrystals, were grown. The valence band photoelectron spectrum of Mn0.06Ge0.94 shows a feature located at −4.2 eV below the Fermi level, which is the fingerprint of substitutional Mn atoms in the Ge matrix. Magnetization measurements show the presence of a paramagnetic component due to substitutional Mn atoms and of a ferromagneticlike component due to Mn5Ge3 nanocrystallites. The Mn L2,3 x-ray absorption spectrum of this polyphase film shows no marked multiplet structure, but a bandlike character.
[Show abstract][Hide abstract] ABSTRACT: We have measured the X-ray resonant magnetic scattering at the 2p edge of Mn in diluted magnetic semiconductor Mn0.06Ge0.94/Ge(001)2×1 thin films. Large magnetic effects in the resonant reflectivity were observed over a wide temperature range. The asymmetry ratio of the magnetic scattering increases with increasing temperature, in accordance to the temperature dependence of the magnetization obtained from SQUID measurements. This is attributed to the contribution of ferromagnetic Mn5Ge3 nanoclusters present in the Mn0.06Ge0.94 film.
Thin Solid Films 01/2008; 517(1):428-429. · 1.87 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Mn0.06Ge0.94 samples have been grown by molecular-beam epitaxy on Ge(0 0 1)2 × 1. High-resolution transmission electron microscopy shows the coexistence of an ordered diluted Mn0.06Ge0.94 film and of nanoscopic crystallites, which were identified as Mn5Ge3 by electron diffraction. The magnetic properties of the Mn0.06Ge0.94 samples show a superposition of a paramagnetic behavior, due to the interaction of Mn atoms diluted in the Ge host, and a ferromagnetic behavior attributed to the Mn5Ge3 crystallites dispersed into the films. The Mn L2,3 X-ray absorption spectra of the Mn0.06Ge0.94 films exhibit a lineshape typical of metallic Mn, with considerably reduced multiplet structure.
[Show abstract][Hide abstract] ABSTRACT: We investigated the structural and magnetic properties of MnxGe1−x, with 0.02⩽x⩽0.1. The MnxGe1−x samples were grown on Ge(001)2×1 by molecular beam epitaxy, at a substrate temperature of 520K. The samples were characterized in situ by reflection high-energy electron diffraction and ex situ by high-resolution transmission electron microscopy, energy dispersive X-ray reflectivity and magneto-optical Kerr effect. From microscopy images we evidenced on all samples the presence of Mn5Ge3 nanocrystallites in addition to the MnxGe1−x diluted magnetic semiconductor with an estimated Mn concentration x≈1.5%. The size and the density of the Mn5Ge3 precipitates were found to increase with increasing the Mn concentration x. Magnetic analysis showed ferromagnetism with a Curie temperature of 280K for all samples.
[Show abstract][Hide abstract] ABSTRACT: We report on resonant inelastic X-ray scattering measurements on Na0.7CoO2 and on the corresponding hydrated compound Na0.35CoO2 · yH2O at the Co K-edge. The spectra exhibit a single excitation at 10 eV energy transfer which resonates at the energy of the Co K white line. The experimental results are analyzed within the linear muffin-tin orbital approach and compared to the calculated electronic density of states and optical conductivity spectra. The poor agreement between calculations and experiment is attributed to effective screening effects from electrons in the Fermi sea. The 10 eV feature can be tentatively attributed to a charge transfer excited state, or more likely to a damped plasmon mode.
Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 05/2006; 246(1):165-169. · 1.19 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have investigated the Raman–Auger crossover of the Mn 2p non-radiative decays in bulk MnO. For the highest kinetic energy transition in the Mn 2p3p3p decay, Raman behaviour is observed up to ≈3.4eV above the resonance energy (maximum of the L3 absorption). In the case of the 2p3p3d and 2p3d3d transitions a similar situation occurs up to ≈5eV above the resonance energy. We compare these results to those obtained on a thin MnO layer deposited on Cu, on bulk Mn and on La0.7Sr0.3MnO3. The results are discussed in terms of the screening dynamics of the Mn 2p core hole and of the metallic/insulating character of the material.
Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 01/2006; 246(1):184-188. · 1.19 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Resonant inelastic soft X-ray scattering is a developing technique well suited to the study of correlation effects in complex materials. We briefly describe its potential for materials science and illustrate its sensitivity with a study of elementary excitations at Cu sites in YBa2Cu3O7−δ as a function of oxygen content. High resolution measurements at the Cu 2p edge reveal structure that, with the help of theoretical models, can be interpreted as due to d–d excitations of the type 3d (x2−y2) to 3d (3z2−r2), Zhang–Rice singlets, and changes in the Cu spin configuration.
Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 01/2006; · 1.19 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Resonant inelastic x-ray scattering is well suited to studying pre-edge features in x-ray absorption spectra. Here it is applied to the Ce L3 edge in CeNi, CeNi2, and CeNi7. A relatively strong feature is observed corresponding to 4 f2 final states. We show that its presence is not the consequence of 2p → 4f quadrupole transitions but rather to final state interactions with conduction electrons and Ni 3d states.
[Show abstract][Hide abstract] ABSTRACT: We present resonant X-ray emission spectroscopy data (RXES) taken at the L3 edge of Ce in CeO2. Its relation to the L3 X-ray absorption spectrum is discussed and reference is also made to results for ionic and metallic Ce compounds. Emphasis is placed on the help that the resonant inelastic X-ray scattering technique can provide in understanding X-ray absorption spectra, in particular, as concerns the validity of an impurity Anderson model versus a band structure approach. The former has proved to be very successful in the interpretation of 3d → 4f → 3d RXES but we confirm that solid-state effects have to be taken into account where transitions to extended 5d conduction states are concerned. On the other hand, we show that pre-edge structure is most likely attributable to f-electron–core-hole interactions in the final state rather than to band structure effects.
Journal of Electron Spectroscopy and Related Phenomena 05/2004; 136(s 1–2):179–183. · 1.71 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have measured resonant inelastic x-ray scattering excited at the L2,3 edges of Ca and Sc, detecting the filling of the 2p core hole by an inner-shell electron. The spectral shape is strongly influenced by the interaction in the final state of the 3s-13dn+1 and 3p-23dn+2 configurations as shown by comparing atomic multiplet calculations performed without and with configuration interaction. We show that this interaction causes a strong redistribution of the spectral weight over a wide energy range.
Physical Review B 01/2003; 67(11). · 3.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The impact of resonant scattering of polarized X-rays is illustrated
taking the gadolinium 4d edge energy range as an example. It is shown
that: (i) the study of the reflection of linearly polarized light by a
magnetized Gd sample gives information on the electronic states of the
system, similar to that given by magnetic circular dichroism in
absorption; (ii) the energy-loss peaks corresponding to spin-flip
transitions in the 4d --> 4f excitation exhibit a strong circular
Surface Review and Letters 01/2002; 9(02):977-981. · 0.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The electronic structure of the Ni3Fe alloy, in the ordered and disordered phases, has been investigated by soft X-ray emission and X-ray photoelectron spectroscopy. The densities of states deduced from these two types of experiment are compared with those of the pure elements and related to the current discussion on transition metal alloys, with emphasis on the order-disorder transition. It is found that transition effects predicted by density of states calculations are not confirmed by the experimental spectra.
Journal of Physics F Metal Physics 01/2001; 6(5):899.
[Show abstract][Hide abstract] ABSTRACT: Calculations of the L alpha soft X-ray emission spectra of Cu-Ni alloys using the muffin-tin form of the coherent potential approximation (KKR-CPA) are presented. The transition matrix elements are determined exactly within the KKR-CPA, and lifetime broadening is included. The results of new experiments on the L alpha spectra are also reported and combined with X-ray photoemission measurements of the core state binding energies, so that the relative positions of the Cu and Ni emission bands may be determined. Good agreement between theory and experiment is found, confirming the KKR-CPA description of the local densities of states in Cu-Ni alloys. Striking non rigid-band behaviour is observed. It is suggested that soft X-ray studies of the type the authors describe should be a useful probe of alloy potential functions.
Journal of Physics F Metal Physics 01/2001; 9(8):1719.
[Show abstract][Hide abstract] ABSTRACT: The valence of gold clusters grown on various alkali halide (100) surfaces have been investigated by X-ray photoelectron spectrometry. For clusters with diameters below about 40 AA, i.e. containing less than approximately 103 atoms, no matter what substrate, a band narrowing and a shift to higher binding energies relative to the bulk is observed. These modifications provide evidence for an infinite-finite system transition.
Journal of Physics F Metal Physics 11/2000; 10(5):1025.
[Show abstract][Hide abstract] ABSTRACT: The 4d valence states in silver deposited on a Si(111) surface in a layer-after-layer fashion have been investigated by X-ray photoelectron and Auger spectroscopy. The valence band photoelectron spectrum of one Ag(111) plane is found to be 13% narrower and 0.3 eV more tightly bound than that of bulk silver. Such a narrowing compares well with the 15% reduction predicted by a tight-binding calculation for Cu(111) thin films. The narrowing of the Ag valence states with decreasing substrate coverage is also evident from the study of the AgM4,5-N4,5N4,5 Auger spectra, whose shape evolves from that of the bulk metal spectrum to that given by an atomic model. This result can be explained on the grounds of a recent theoretical model for Auger core-valence-valence transitions.
Journal of Physics C Solid State Physics 11/2000; 14(18):2539.
[Show abstract][Hide abstract] ABSTRACT: We describe resonant inelastic X-ray scattering (RIXS) experiments and magnetic circular dichroism (MCD) in X-ray fluorescence performed in the 3–5 keV range. The examples chosen are X-ray fluorescence MCD of FeRh and RIXS experiments performed at the L3 edge of Ce. FeRh is antiferromagnetic at room temperature but has a transition to the ferromagnetic state above 400 K. The Rh MCD signal is confronted to an augmented spherical wave calculation. The experiment confirms the predicted spin polarization of the Rh 4d valence states. The RIXS measurements on Ce compounds and intermetallics address the problem of mixed valency especially in systems where degeneracy with the Fermi level remains small. Examples are taken from the 2p→(4f5d)+1 followed by 3d→2p RIXS for a highly ionic compound CeF3 and for almost γ-like CeCuSi.
Journal of Electron Spectroscopy and Related Phenomena 10/2000; · 1.71 Impact Factor