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We present here a method for the quantitative prediction of the spectroscopic specular reflectivity line-shape in anisotropic layered media. The method is based on a 4 &times, 4 matrix formalism and on the simulation from the first principles (through density functional theory&mdash, DFT) of the anisotropic absorption cross-section. The approach was used to simulate the reflectivity at the oxygen K-edge of a 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) thin film on Au(111). The effect of film thickness, orientation of the molecules, and grazing incidence angle were considered. The simulation results were compared to the experiment, permitting us to derive information on the film geometry, thickness, and morphology, as well as the electronic structure.

Hybridization of electronic states and orbital symmetry in transition metal oxides are generally considered key ingredients in the description of both their electronic and magnetic properties. In the prototypical case of ${\mathrm{La}}_{0.65}{\mathrm{Sr}}_{0.35}{\mathrm{MnO}}_{3}$ (LSMO), a landmark system for spintronics applications, a description based solely on Mn $3d$ and O $2p$ electronic states is reductive. We thus analyzed elemental and orbital distributions in the LSMO valence band through a comparison between density functional theory calculations and experimental photoelectron spectra in a photon energy range from soft to hard x rays. We reveal a number of hidden contributions, arising specifically from La $5p$, Mn $4s$, and O $2s$ orbitals, considered negligible in previous analyses; our results demonstrate that all these contributions are significant for a correct description of the valence band of LSMO and of transition metal oxides in general.

In this review, the technique of resonant soft X-ray reflectivity in the study of magnetic low-dimensional systems is discussed. This technique is particularly appealing in the study of magnetization at buried interfaces and to discriminate single elemental contributions to magnetism, even when this is ascribed to few atoms. The major fields of application are described, including magnetic proximity effects, thin films of transition metals and related oxides, and exchange-bias systems. The fundamental theoretical background leading to dichroism effects in reflectivity is also briefly outlined.

The physical properties of nanostructured thin films and multilayers depend crucially on their interfaces. The precise knowledge of the interface morphology at the atomic scale is mandatory to reliably understand their macroscopic response. These details become even more critical when dealing with magnetic thin films as the finest structural and/or compositional local changes may affect the magnetic coupling and alter the macroscopic response. Soft X-ray Resonant Magnetic Reflectivity (SXRMR) is a suitable technique in this field, combining specific aspects of x-ray absorption and x-ray scattering processes to simultaneously obtain complementary structural and magnetic information on thin films and multilayers. Here, we present a case study on a MgO/Co/MgO trilayer in which the combined use of SXRMR, x-ray absorption spectroscopy and x-ray magnetic circular dichroism has allowed to reconstruct the elemental and magnetic depth profiles with sub-nanometric resolution and to distinguish fine compositional defects at the Co-on-MgO and MgO-on-Co interfaces.

Angle resolved-Auger-photoelectron coincidence spectroscopy (AR-APECS) has been exploited to investigate the role that electron correlation plays in the exchange-coupling at the ferromagnetic/antiferromagnetic interface of a Fe/CoO bilayer growth on Ag(001). The effective correlation energy U$_{\textrm{eff}}$, usually employed to assess the energy distribution of core-valence-valence Auger spectra, has been experimentally determined for each possible combination of the orbital (e$_g$ or t$_{2g}$) and the spin (majority or minority) of the two valence electrons involved in the Auger decay. Coulomb and exchange interactions have been identified and compared with the result obtained on the Fe/Ag system. The presented analysis reveals in the Fe/CoO interface an enhancement of the Coulomb interaction for the e$_g$ orbital and of the exchange interaction for the t$_{2g}$ orbital with respect to the Fe/Ag case that can be associated to the stronger electron confinement and to the exchange coupling between the two layers, respectively., Comment: 37 pages, 6 figures

We have probed the structural and magnetic properties of a ferromagnetic/organic interface constituted by a polycrystalline Co layer deposited on a fullerene thin film through resonant soft X-ray reflectivity measurements. The fitting analysis of the reflectivity indicates the formation of a sharp interface with limited intermixing and a null remanent magnetization in a & x007E;1 nm thick region of the Co film at the interface with C60. This information contributes to elucidate the role of organic & x2013;inorganic interfaces in the charge and spin transport inside organic spintronic devices.

Resonant soft X-ray reflectivity at the carbon K-edge was applied to a trigonal tetracene single crystal. The angular resolved reflectivity was quantitatively simulated describing the tetracene crystal in terms of its dielectric tensor, which was derived from the anisotropic absorption cross section of the single molecule, as calculated by density functional theory. A good agreement was found between the experimental and theoretically predicted reflectivity. This allows us to assess the anisotropic optical constants of the organic material, probed at the carbon K-edge, in relation to the bulk/surface structural and electronic properties of the crystal, through empty energy levels.

Progress in understanding and exploiting the properties of complex oxide heterostructures requires advances in stateof- the-art growth and characterization techniques for these materials. Currently, atomic control of their synthesis is demonstrably inferior to that of their semiconductor counterparts, [ 1 ] despite recent progress for example in creating a high mobility two-dimensional electron gas in semiconducting ZnO [ 2 , 3 ] and in exploring novel interface effects in perovskites. [ 4 ] This superiority in growth control can be attributed in part to the development of quantitative, in situ refl ection high-energy electron diffraction (RHEED) possible in the UHV environment typical of semiconductor fi lm growth. [ 5 ] Oxides with the perovskite structure (chemical formula ABO 3 ) have attracted enormous interest as they display a wide range of exotic properties driven by strong spin-charge-lattice coupling, [ 6 ] such as quantum critical behavior in superconducting cuprates, colossal magnetoresistance in phase-separated manganites, and magnetoelectric coupling in BiFeO 3 . This coupling underlies the strong response of the electronic structure to small distortions of the BO 6 octahedra away from simple cubic coordination, distortions driven either by a Jahn-Teller (J-T) mechanism (removing energy level degeneracy by symmetry reduction) or by octahedral rotations to accommodate imperfect packing within the cubic unit cell that depends on cationic radii. These distortions are signifi cantly infl uenced in a stochastic way by disorder, unavoidably introduced by random A-site doping [ 7 ] or off-stoichiometry (ratio A:B ? 1, or oxygen vacancies) often seen in epitaxial films. In the search for novel electronic properties at interfaces, additional defect mechanisms such as cation intermixing and segregation must be addressed and their precise distribution near the interface known. Ultimately, identifying new electronic properties will depend critically on eliminating these forms of disorder and defects, or precisely controlling them if desired, through atomic design. Here we report evidence of signifi cant improvements in the atomic design of perovskite interfaces made possible by advances in in situ RHEED control of surfaces that exploit the detection of the characteristic octahedral distortions in the surface layer as it is being deposited. This method allows optimization of the phase diagram versus doping of films, and eliminates intermixing and anomalous lattice dilations at interfaces that have previously been observed. Careful structural analysis shows an unusual evolution of octahedral distortions including both J-T type and rotations near the interface not seen in bulk. These new results should be included in electronic structure calculations modeling the properties of real heterointerfaces.

Ge (100) wafers were implanted with 100 keV Mn+ ions with a dose of 2 x 10(16) ions/cm(2) at different temperatures, ranging from 300 to 573 K. The surface morphology of implanted samples, analyzed with scanning electron microscopy and atomic force microscopy measurements, reveals for the 300-463 K implant temperature range the formation of a surface swelled and porous film, containing sponge-like structures. On the contrary, samples implanted in the 513-573 K temperature range present an atomically flat surface, with a roughness less than 1 nm, indicating that crystalline order has been preserved. X-ray photoemission spectroscopy depth profiling measurements indicate the presence of adsorbed oxygen in the porous layer of lower-temperature implanted samples, as well the presence of a large Mn concentration below the expected end of range for impinging ions. Mn and 0 concentrations at anomalously great depths are maximum in the 413 K implanted sample, indicating that the phenomenon of ion beam induced porosity is best favored at a well defined temperature. (C) 2006 Elsevier B.V. All rights reserved.

The magneto-optical Kerr effect is used to study epitaxial Mn x Ge 1− x /Ge (1 0 0) films grown at different substrate temperatures and Mn concentrations. The samples are magnetic at low temperature (5 K) and lose this property as the measuring temperature is raised. Negligible hysteresis is observed for the as-deposited samples. After thermal annealing, the magnetic character is enhanced as evidenced by an increase of saturation and the appearance of coercivity. Films heated in air have larger hysteresis and maintain a magnetic response up to room temperature.

We present simulations of X-ray resonant magnetic reflectivity (XRMR) spectra of the surface magnetic dead layer in La1−x Sr x MnO3 (LSMO) films that take in account the effect of different forms of roughness that can be encountered experimentally. The results demonstrate a method to distinguish between surface (morphological) roughness, and two generic kinds of magnetic roughness at the buried interface between the surface dead layer and the fully magnetic bulk part of the film. We show that the XRMR technique can distinguish between different types of magnetic roughness at the dead layer/bulk interface only if the sample surface is nearly atomically flat (the morphological roughness is one unit cell or less). Furthermore, to distinguish between the two types of magnetic roughness, the simulations show that fitting of XRMR spectra out to very high incidence angles must be performed. In the specific case of LSMO films with a dead layer with average thickness of 4 unit cells, this corresponds to an incidence angle > 50∘.

Summary form only given. The synthesis of optimal materials for spin dependent electronics is the challenging goal of recent studies. Magnetic multilayers are the most popular solution, but a promising alternative is constituted by magnetic semiconductors. One of these is Mn:Ge which shows ferromagnetic properties and magnetoresistance phenomena below room temperature, attributed to Mn/sub x/Ge/sub 1-x/ clusters. In the present work, we investigate the properties of samples obtained by implanting Mn/sup +/ ions on Ge(100) at energy of 100 keV, with a dose of 2 x 10/sup 16/ at./cm/sup 2/. The temperature of the substrate was 300/spl deg/C. We report the results obtained in the as-prepared material, in the material annealed for 1 hour at 400/spl deg/C, and 600/spl deg/C.

International audience; The performance of multilayer optics depends on the quality of the buried interfaces between materials, whose intermixing strongly affects their behavior. We present an experimental method to determine, in a non destructively way, the amount of material intermixing at interfaces of multilayer structures. The reflection mechanism is related to the build up in the multilayer of a standing wave field, whose peaks and the valleys move as a function both of wavelength and of incidence angle. Exploiting this fact it is possible to modulate the electric field inside the multilayer in order to have different parts of the multilayer structure excited at a different extent and in particular the buried interfaces regions. The excitation is directly proportional to the intensity of the electric field and to the concentration of a given element in the sample. The excitation can be detected with different techniques, f.i. electron core level photoemission, fluorescence, luminescence, total electron yield. The flexibility of the experimental apparatus of the BEAR beamline (Elettra Trieste, Italy) allowed us to study some important classes of layered structures in the soft X-ray energy range, using the above mentioned techniques together with the determination of the Bragg conditions through the measurement of the specular reflectivity. We demonstrate the possibility of obtaining quantitative information on the width of the intermixing region, strongly related to the interface roughness, through the comparison with a phenomenological model of the intermixing and a numerical simulation of the standing field inside the multilayer.

We have developed and elaborated a series of Mg/Co-based periodic multilayers to build efficient mirrors for the extreme ultraviolet (EUV) range. For s-polarized light and at 45° of grazing incidence, the reflectivity of as-deposited Mg/Co is 42.6% at 25.1 nm. X-ray emission spectroscopy and nuclear magnetic resonance measurements do not indicate any noticeable interdiffusion at the interfaces between layers. Scanning transmission electronic microscopy images attest the high structural quality of the stack. X-ray reflectivity (XRR) curves in the hard x-ray and EUV domains confirm this description and estimate a weak interfacial roughness (~ 0.5 nm). Taking advantage of the magnetic character of Co, we have performed resonant magnetic reflectivity measurements by scanning the photon energy around the Co L absorption edge for opposite circular polarizations. The magnetization profile of the Co layers within Co/Mg determined with an expected depth resolution of one monolayer confirms the interface abruptness. Scanning electron microscopy images and XRR curves give evidence of the thermal stability of Mg/Co up to 300 °C. From that value, a strong change in the sample morphology due to the delamination of the multilayer from the substrate occurs. This should account for the drastic reflectivity drop observed above this temperature. Starting from Mg/Co, we have inserted a Zr layer at one or at the other interface or at both interfaces to estimate the effect of the introduction of a third material within the period. We have found that Mg/Co/Zr is more efficient (50% of reflectivity) than Mg/Zr/Co and Mg/Zr/Co/Zr (~ 40%). Through time-of-flight secondary ion mass spectrometry depth profiling and NMR measurements, we have assigned this difference to an intermixing process when Co layers are deposited onto Zr layers.

A unitary picture of the structural properties of Mn(x)Ge(1-x) diluted alloys fabricated by either ion implantation or molecular beam epitaxy (MBE), at various growth temperatures (from 80 to about 623 K) and few per cent concentrations, is proposed. Analysis is based on synchrotron radiation x-ray absorption spectroscopy at the Mn K edge. When the growth temperature exceeds 330 K, the MBE samples show a high number of precipitated ferromagnetic nanoparticles, mainly Mn(5)Ge(3), nucleated from the previous occupation of interstitial tetrahedral sites. Efficient substitution is observed in the case of MBE samples made by alternate layers of GeMn alloys grown at T ≤ 433 K and undoped Ge thick layers. Similar good dilution properties are obtained by implanting Mn ions at low temperatures (80 K). Possible precursors to preferential mechanisms in the alloy formation are discussed on the basis of the present comparative study.

The performance of manganite-based magnetic tunnel junctions (MTJs) has suffered from reduced magnetization present at the junction interfaces that is ultimately responsible for the spin polarization of injected currents; this behavior has been attributed to a magnetic "dead layer" that typically extends a few unit cells into the manganite. X-ray magnetic scattering in resonant conditions (XRMS) is one of the most innovative and effective techniques to extract surface or interfacial magnetization profiles with subnanometer resolution, and has only recently been applied to oxide heterostructures. Here we present our approach to characterizing the surface and interfacial magnetization of such heterostructures using the XRMS technique, conducted at the BEAR beamline (Elettra synchrotron, Trieste). Measurements were carried out in specular reflectivity geometry, switching the left/right elliptical polarization of light as well the magnetization direction in the scattering plane. Spectra were collected across the Mn L2,3 edge for at least four different grazing angles in order to better analyse the interference phenomena. The resulting reflectivity spectra have been carefully fit to obtain the magnetization profiles, minimizing the number of free parameters as much as possible. Optical constants of the samples (real and imaginary part of the refractive index) in the interested frequency range are obtained through absorption measurements in two magnetization states and subsequent Kramers-Kronig transformation, allowing quantitative fits of the magnetization profile at different temperatures. We apply this method to the study of air-exposed surfaces of epitaxial La2/3Sr1/3MnO3 (001) films grown on SrTiO3 (001) substrates., 11 pages + 3 figures; accepted to JMMM (2009)

Ge substrates of (100) orientation were irradiated with 100 keV Mn ions (2 x 10(16) at./CM2) at a temperature of 190 degrees C. Pronounced swelling of the irradiated material up to 150 nm, associated with the formation of a Mn-doped amorphous porous Ge layer, is investigated by scanning electron microscopy and X-ray photoelectron depth profiling. Magneto-optical Kerr effect hysteresis loops have been used to characterize the magnetic properties of the obtained sample. The details of the hysteresis loops reveal that the ferromagnetic property of the sample up to similar to 20 K and the subsequent strong non-linear paramagnetic response must be ascribed to Mn atoms dispersed in the amorphous porous implanted layer. (c) 2007 Elsevier B.V. All rights reserved.

Magneto-optical Kerr effect hysteresis loops at various wavelengths in the visible/near-infrared range have been used to characterize the magnetic properties of alloys obtained by implanting Mn ions at fixed energy in a Ge matrix. The details of the hysteresis loops reveal the presence of multiple magnetic contributions. They may be attributed to the inhomogeneous distribution of the magnetic atoms and, in particular, to the known coexistence of diluted Mn in the Ge matrix and metallic Mn-rich nanoparticles embedded in it [Phys. Rev. B 73, 195207(2006)]., 2 pages, 2 figures. Proceeding of the International Conference on Magnetism. Kyoto, August 20-25 2006

The structural, electronic, and magnetic properties of a ${\mathrm{Mn}}_{x}{\mathrm{Ge}}_{1\ensuremath{-}x}$ alloy prepared through room-temperature ion implantation ($100\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$, $2\ifmmode\times\else\texttimes\fi{}{10}^{16}\phantom{\rule{0.3em}{0ex}}\text{ions}∕{\mathrm{cm}}^{2}$) and subsequent $400\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ annealing have been investigated with several experimental techniques. The as-implanted sample shows a quasi-Gaussian Mn concentration depth profile with a projected range (peak Mn concentration $x\ensuremath{\simeq}12\phantom{\rule{0.3em}{0ex}}\mathrm{at.}∕%$) at $55\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ and end of range at $140\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The structural investigation shows that the overall implanted Ge layer is amorphous. In particular, up to a depth of $60\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, the implanted layer is also porous and oxidized, whereas the deepest implanted region $(60\char21{}140\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ is purely composed of amorphous Ge with Mn atoms diluted in it. This sample manifests magnetic hysteresis up to $20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and a strong nonlinear S-shaped magnetic response up to $150\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Upon annealing at $400\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, the top porous layer remains essentially amorphous, whereas partial reconstruction into Ge nanocrystals ($\ensuremath{\sim}10\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ in diameter) occurs in the $60\char21{}140\text{\ensuremath{-}}\mathrm{nm}$-deep implanted region. Part of the Mn atoms, mainly belonging to the top porous layer, further diffuses toward the surface and forms chemical bonds with O contaminants, becoming magnetically inactive. The other Mn atoms, mainly in the region between 60 and $140\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ from the surface, remain trapped in the residual amorphous matrix or in the Ge nanocrystals, whereas formation of Mn-Ge extrinsic phases (like ${\mathrm{Mn}}_{11}{\mathrm{Ge}}_{8}$ and ${\mathrm{Mn}}_{5}{\mathrm{Ge}}_{3}$) is excluded. The magnetic response of the annealed sample originates from the existence of a soft and a harder magnetic component, assigned to the dilution of Mn atoms in residual amorphous Ge and Ge nanocrystals, respectively. The hard component, attributable to a ${\mathrm{Mn}}_{x}{\mathrm{Ge}}_{1\ensuremath{-}x}$ diluted magnetic semiconductor in nanocrystalline form, manifests magnetic hysteresis up to above $250\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.

The structural and electronic properties of MnxGe1-x alloys (x

We performed structural and electrical investigations on high temperature Fe implanted InP in order to correlate the lattice position of the implanted atoms after annealing treatments to their electrical activation as compensating deep traps. The overall results demonstrate that the Fe2+ deep trap activation properties are strictly connected to the annealing evolution of the lattice location of the Fe atoms in substitutional sites, which in turn is controlled by the background doping density in the substrates. © 2007 American Institute of Physics.

X-ray diffraction, transmission electron microscopy, extended x-ray absorption fine structure spectroscopy, and x-ray photoemission spectroscopy have been used to study the structural properties of MnxGe1-x ion implanted alloys (x similar or equal to 0.04) at 240 and 270 degrees C substrate temperatures. Between 40% and 50% of the Mn atoms are found to occupy substitutional sites. No interstitial Mn atoms are found. The Mn-Ge coordination distance is 2.50(2) angstrom. Moreover, in the subsurface implanted layer [up to 32(2) nm depth] all the Mn atoms are effectively diluted in the Ge matrix, reaching a peak doping concentration of 7 +/- 1%. (c) 2006 American Institute of Physics.

In this paper we present structural and electrical investigations on high temperature Fe-implanted InP. The aim of the work is to relate the lattice position of the implanted atoms after annealing treatments (from 300 to 600 degrees C) with their electrical activation as compensating deep traps and to draw a comprehensive picture of the activation mechanisms. The overall results demonstrate that the electrical behavior and the Fe2+ deep trap activation properties are strictly connected to the annealing evolution of the implant-induced damage and to the escape process of the Fe atoms from substitutional sites, which in turn is controlled by the background doping density in the substrates. (c) 2006 American Institute of Physics.

We have studied the electrical activation of the Fe2+3+ trap in Fe-implanted InP by means of capacitance-voltage and deep level transient spectroscopy analyses. Five deep traps have been identified and we have characterized the concentration and depth distribution of the Fe2+3+ deep trap, located at EC -0.66 eV. The InP substrate background doping, i.e., the Fermi-level position, plays a crucial role in the Fe activation process by setting an upper limit to the amount of Fe centers electrically activated as deep acceptor traps. © 2005 American Institute of Physics.

An ion implanted Mn–Ge(100) alloy has been fabricated with a 30nm subsurface layer of disorderly diluted Mn atoms, which settle all in substitutional lattice sites after 400°C annealing in vacuum. The Mn L2,3 x-ray absorption spectroscopy of the disordered Mn implanted alloy is featureless, while the one taken on the alloy with complete substitutional Mn dilution shows a Mn2+ multiplet structure.

Single GaInP layers, grown by MOVPE lattice matched to GaAs are examined. The role of the implantation temperature and dose in determining the crystal damage, and the recovery of this damage by the following high temperature annealing treatments is investigated by RBS-channeling measurements. The influence of implant and annealing conditions on the redistribution properties of the Fe atoms throughout the crystal is studied by means of SIMS Fe depth profiling. High-resolution X-ray diffraction measurements are employed for structural characterization. The electrical properties related to Fe implantation are studied by current-voltage measurements on mesa devices. Deep level properties are investigated by DLTS-PICTS measurements.