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We present x-ray absorption and x-ray magnetic circular dichroism studies of iron oxide passivated iron nanoparticles. The combination of measurements with different probing depths allows us to determine the oxide composition while the magnetic data reveal the magnetic interactions across the interface between the α-Fe core and the spin canted ferrimagnetic iron oxide shell. These studies shed light on the mechanisms that govern the magnetic properties of core-shell nanoparticles. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have investigated the magnetic depth profile of an epitaxial Fe3O4 thin film grown directly on a semiconducting ZnO substrate by soft x-ray resonant magnetic reflectometry (XRMR) and electron energy loss spectroscopy (EELS). Consistent chemical profiles at the interface between ZnO and Fe3O4 are found from both methods. Valence selective EELS and XRMR reveal independently that the first monolayer of Fe at the interface between ZnO and Fe3O4 contains only Fe3+ ions. Besides this narrow 2.5 Å interface layer, Fe3O4 shows magnetic bulk properties throughout the whole film making highly efficient spin injection in this system feasible. [ABSTRACT FROM AUTHOR]

X-ray magnetic circular dichroism is applied to investigate the magnetic response of cuboidal Fe/Fe oxide core shell nanoparticles. Characteristic differences observed in electron and fluorescence detection modes are observed. Total yield detection, overemphasizing the internal metal oxide interface, is still able to detect local magnetization changes that have too little weight in volumetric detection to be observed there. [ABSTRACT FROM AUTHOR]

Abstract: We combine sensitivity to atomic number, chemical shifts, probing depth, and magnetic order in a field-dependent magnetic circular X-ray dichroism study at the Mn L-edge of the diluted ferromagnetic semiconductor Ga1−xMnxAs and observe different Mn constituents: ferromagnetic Mn with an lineshape and paramagnetic Mn with distinct lineshape. The paramagnetic Mn is assigned to interstitials with surface segregation tendency. [Copyright &y& Elsevier]

We present a study of the changes in the magnetic and electronic properties of small, deposited Fe clusters upon exposure to the graphite surface. The clusters exhibit strong X-ray magnetic circular dichroism (XMCD) at the L3 edge while matrix isolated in a thin Ar film. XMCD and photoemission experiments show that the clusters are driven into a nonmagnetic state by the interaction to graphite. Our results support earlier calculations for adatoms and dimers and extend their validity to larger cluster sizes. They also provide a basis for an understanding of the magnetic properties of carbon encapsulated transition metal particles. [Copyright &y& Elsevier]

Of all the current detection techniques with nanometre resolution, only X-ray microscopy allows imaging of nanoparticles in suspension. Can it also be used to investigate structural dynamics? When studying the response to mechanical stimuli, the challenge lies in its application with a precision comparable with the spatial resolution. In the first shear experiments performed in an X-ray microscope, this has been accomplished by inserting a piezo actuator driven shear cell into the focal plane of a scanning transmission X-ray microscope. Thus shear-induced re-organization of magnetite nanoparticle aggregates could be demonstrated in suspension. As X-ray microscopy proves suitable for studying structural change, new prospects open up in physics at small length scales. [ABSTRACT FROM AUTHOR]

Fe clusters of 105±2 atoms/cluster were mass selectively deposited onto Cu(111) at cryogenic temperatures. XMCD was used to measure temperature and direction dependent magnetization curves. The clusters are superparamagnetic at the lowest temperature measured (10 K). Their magnetization curves are consistent with magnetic moments of ≈2.5μB per atom which are thus enhanced over the bulk values. Within experimental accuracy, the clusters do not present magnetocrystalline anisotropy in the temperature range of 10 K to 60 K. [ABSTRACT FROM AUTHOR]

The dynamics of magnetic vortices in thin Permalloy disks having artificial defects in the form of small holes at different locations within the disk has been investigated by means of frequency-domain spatially resolved ferromagnetic resonance. It is found that the vortex can be effectively captured by such a defect. Consequently the commonly observed gyrotropic vortex motion in an applied microwave field of 1 mT is suppressed. However, if in addition a static magnetic field of at least 4.3 mT is applied, the vortex core is nucleated from the artificial defect and a modified gyrotropic motion starts again. [ABSTRACT FROM AUTHOR]

Arrays of FePt particles (diameter 7 nm) with mean interparticle distances of 60 nm are prepared by a micellar technique on Si substrates. The phase transition of these magnetic particles towards the chemically ordered L10 phase is tracked for 350 kV He+ ion irradiated samples and compared to a nonirradiated reference. Due to the large separation of the magnetically decoupled particles the array can be safely annealed without any agglomeration as usually observed for more densely packed colloidal FePt nanoparticles. The He+ ion exposure yields a significant reduction of the ordering temperature by more than 100 K. [ABSTRACT FROM AUTHOR]

The possibility to quantitatively analyze total electron yield (TEY) spectra as a measure of the x-ray absorption cross section from supported magnetic nanoparticles is explored by a combination of experiments and Monte Carlo simulations. Unless the particles are very small, it is found that saturation effects are important and can lead to a severe underestimation of the magnetic moments, when the magneto-optical sum rules are straightforwardly applied to experimental data. However, unlike for planar samples the possibility to examine and correct the saturation effects by varying the x-ray angle of incidence is not given, the absorption profiles are largely unaffected in their shape. The Monte Carlo simulations, appropriately conducted for the material under investigation, provide an estimate of the saturation effects occurring in nanoparticle TEY data. [ABSTRACT FROM AUTHOR]

We use x-ray absorption spectroscopy as well as its linear and circular magnetic dichroisms to characterize relevant interactions and energy scales in the surface intermetallic CePt5/Pt(111). The experiments provide insight into crystal field splitting, effective paramagnetic moments, their Kondo screening and mutual interactions, and thus into many aspects which typically determine the low-temperature behavior of correlated rare-earth compounds. Exploiting the tunability of Ce valence through the thickness-dependent epitaxial strain at the CePt5/Pt(111) interface, we are able to systematically investigate the impact of hybridization strength on these interactions. Considerable Kondo screening is indeed observed at all CePt5 thicknesses, and found to be strongest in case of strongest hybridization. While the magnetic response is commensurate with an impurity Kondo scale of TK ≳ 10² K for specimen temperatures T≳30 K, this is no longer the case at lower temperature. Its detailed study by x-ray circular magnetic dichroism (XMCD) at one specific thickness of CePt5 reveals an anomaly of the susceptibility at T* ≈25 K instead, which we tentatively associate with the onset of lattice coherence. At lowest temperature we observe paramagnetic saturation with a small Ce 4f saturation magnetization. Within the framework of itinerant 4f electrons, saturation is due to a field-induced Lifshitz transition involving a very heavy band with correspondingly small degeneracy temperature of TF≈7 K. This small energy scale results in the persistence of Curie-Weiss behavior across the entire range of experimentally accessible temperatures (T≳2 K). Our work highlights the potential of magnetic circular dichroism studies in particular for Kondo and heavy-fermion materials, which so far has remained largely unexplored. [ABSTRACT FROM AUTHOR]

X-ray absorption spectroscopy methods are presented as a useful tool to determine local structure, composition and magnetic moments as well as to estimate the effective anisotropy of substrate supported self-assembled arrays of wet-chemically synthesized FePt nanoparticles. A compositional inhomogeneity within the nanoparticles yields reduced magnetic moments with respect to the corresponding bulk material and may also hinder the formation of the chemically ordered L10 phase in FePt nanoparticles. The latter is indicated by a reduced effective anisotropy, which is one order of magnitude smaller than expected from the known value of the corresponding bulk material.As a new approach, measurements of the x-ray absorption near-edge structure of Fe-oxide nanoparticles in dispersion are presented and ageing effects are discussed on the basis of multiplet calculations. [ABSTRACT FROM AUTHOR]

The surface termination of CePt5/Pt(111) is determined experimentally by LEED-IV. In accordance with recent theoretical predictions, a dense Pt terminated surface is being found. Whereas the CePt5 volume lattice comprises Pt kagome layers, additional Pt atoms occupy the associated hole positions at the surface. This finding provides a natural explanation for the remarkable inertness of the CePt5 intermetallic. Implications of the structural relaxations determined by LEED-IV analysis are discussed with regard to observations by scanning tunneling microscopy and electron spectroscopies. [ABSTRACT FROM AUTHOR]

Element-specific magnetic moments of epitaxially grown Fe3Si in DO3 crystal symmetry were analyzed by means of x-ray absorption spectroscopy and its associated magnetic circular dichroism. To detect the weak magnetization induced at the Si sites, measurements were performed at both the Si Ê edge and the Si L3 2 edges. By band structure calculations based on either the SPR-KKR method or FLAPW with GGA, the spectroscopic features could be reproduced and provide an insight to the underlying physics. In addition, comparison of the experimental data to calculated spectra made it possible for us to estimate the induced effective spin and orbital magnetic moment of Si in our sample, i.e., &mgr;effs = (--0.011 ± 0.005) &mgr;B and &mgr;I = (--0.003 ± 0.003) &mgr;B, respectively. The sign and the order of magnitude of the tiny orbital magnetic moment has been confirmed by application of the magneto-optical sum rule. [ABSTRACT FROM AUTHOR]

The low-temperature electronic properties of rare-earth intermetallics are substantially influenced by the symmetry and magnitude of the crystal electric field. The direct spectroscopic analysis of crystal-field splitting can be challenging, especially in low-dimensional systems, because it requires both high spectral resolution and pronounced sensitivity. We demonstrate the eligibility of electronic Raman spectroscopy for this purpose by the direct determination of the 4ƒ level splitting in ultrathin ordered CePt5 films down to ≈ 1.5 nm thickness on Pt(111). Crystal-field excitations of Ce 4ƒ electrons give rise to Raman peaks at energy losses up to ≈ 25 meV. Three distinct peaks occur which we attribute to inequivalent Ce sites, located (i) at the interface to the substrate, (ii) next to the Pt-terminated film surface, and (iii) in the CePt5 layers in between. The well-resolved Raman signatures allow us to identify a reduced crystal-field splitting at the interface and an enhancement at the surface, highlighting its strong dependence on the local atomic environment. [ABSTRACT FROM AUTHOR]

Recently it has been shown that surface magnetic doping of topological insulators induces backscattering of Dirac states which are usually protected by time-reversal symmetry [Sessi et al., Nat. Commun. 5, 5349 (2014)]. Here we report on quasiparticle interference measurements where, by improved Fermi level tuning, strongly focused interference patterns on surface Mn-doped Bi2Te3 could be directly observed by means of scanning tunneling microscopy at 4 K. Ab initio and model calculations reveal that their mesoscopic coherence relies on two prerequisites: (i) a hexagonal Fermi surface with large parallel segments (nesting) and (ii) magnetic dopants which couple to a high-spin state. Indeed, x-ray magnetic circular dichroism shows superparamagnetism even at very dilute Mn concentrations. Our findings provide evidence of strongly anisotropic Dirac-fermion-mediated interactions and demonstrate how spin information can be transmitted over long distances, allowing the design of experiments and devices based on coherent quantum effects in topological insulators. [ABSTRACT FROM AUTHOR]

Soft x-ray linear and circular dichroism (XLD, XMCD) experiments at the Ce M4,5 edges are being used to determine the energy scales characterizing the Ce 4f degrees of freedom in the ultrathin ordered surface intermetallic CeAgx/Ag(111). We find that all relevant interactions, i.e., Kondo scattering, crystal field splitting and magnetic exchange coupling occur on small scales. Our study demonstrates the usefulness of combining x-ray absorption experiments probing the temperature dependence of both linear and circular dichroism owing to their strong sensitivity for anisotropies in both charge distribution and paramagnetic response, respectively. [ABSTRACT FROM AUTHOR]

We present an extensive study of the structure, composition, electronic, and magnetic properties of Ce-Pt surface intermetallic phases on Pt(111) as a function of their thickness. The sequence of structural phases appearing in low-energy electron diffraction may invariably be attributed to a single underlying intermetallic atomic lattice. Findings from both microscopic and spectroscopic methods prove compatible with CePt5 formation when their characteristic probing depth is adequately taken into account. The intermetallic film thickness serves as an effective tuning parameter which brings about characteristic variations of the cerium valence and related properties. Soft x-ray absorption (XAS) and magnetic circular dichroism (XMCD) prove well suited to trace the changing Ce valence and to assess relevant aspects of Kondo physics in the CePt5 surface intermetallic. We find characteristic Kondo scales of the order of 10² K and evidence for considerable magnetic Kondo screening of the local Ce 4f moments. CePt5/Pt(111) and related systems therefore appear to be promising candidates for further studies of low-dimensional Kondo lattices at surfaces. [ABSTRACT FROM AUTHOR]