P. Bencok

Magnetism of (Zn,Co)O thin films probed by x-ray absorption spectroscopies

We report on the electronic and magnetic properties of Co-doped ZnO thin films investigated by x-ray absorption spectroscopies and element selective magnetometry. For a low Co concentration (around 5%), we evidence a paramagnetic phase clearly correlated to Co2+ ions substituted to Zn in the ZnO matrix. For higher Co concentrations (around 25%), we demonstrate the coexistence of both paramagnetic and ferromagnetic phases. The use of advanced element and orbital selective techniques allows us through the distinct spectral signature of Co in ionic or metallic states to assign the ferromagnetic phase to the presence of Co in a metallic state as a consequence of Co metal clustering in our films.

Enhancing Magnetic Ordering in Cr-Doped Bi2Se3 Using High-TC Ferrimagnetic Insulator

We report a study of enhancing the magnetic ordering in a model magnetically doped topological insulator (TI), Bi(2-x)Cr(x)Se(3), via the proximity effect using a high-TC ferrimagnetic insulator Y(3)Fe(5)O(12). The FMI provides the TI with a source of exchange interaction yet without removing the nontrivial surface state. By performing the elemental specific X-ray magnetic circular dichroism (XMCD) measurements, we have unequivocally observed an enhanced TC of 50 K in this magnetically doped TI/FMI heterostructure. We have also found a larger (6.6 nm at 30 K) but faster decreasing (by 80% from 30 to 50 K) penetration depth compared to that of diluted ferromagnetic semiconductors (DMSs), which could indicate a novel mechanism for the interaction between FMIs and the nontrivial TIs surface.

X-ray ferromagnetic resonance spectroscopy

We present a method to measure continuous-wave ferromagnetic resonance (FMR) spectra based on the core-level absorption of circularly polarized x rays. The technique is demonstrated by using a monochromatic x-ray beam incident on an yttrium–iron–garnet sample excited by a microwave field at 2.47 GHz. FMR spectra are obtained by monitoring the x-ray absorption intensity at the photon energy corresponding to the maximum of the magnetic circular dichroism effect at the iron L2,3 edges as a function of applied magnetic field. The x-ray FMR signal is shown to be energy dependent, which makes the technique element sensitive and opens up new possibilities to perform element-resolved FMR in magnetic alloys and multilayers.

Spin and orbital moments of nanoscale Fe3O4 epitaxial thin film on MgO/GaAs(100)

Nanoscale Fe3O4 epitaxial thin film has been synthesized on MgO/GaAs(100) spintronic heterostructure, and studied with X-ray magnetic circular dichroism. We have observed a total magnetic moment (ml+s) of (3.32 ± 0.1)μB/f.u., retaining 83% of the bulk value. Unquenched orbital moment (ml) of (0.47 ± 0.05)μB/f.u. has been confirmed by carefully applying the sum rule. The results offer direct experimental evidence of the bulk-like total magnetic moment and a large orbital moment in the nanoscale fully epitaxial Fe3O4/MgO/GaAs(100) heterostructure, which is significant for spintronics applications.

Resonant Soft X-Ray Scattering Investigation of Orbital and Magnetic Ordering in La0.5Sr1.5MnO4.

We report resonant x-ray scattering data of the orbital and magnetic ordering at low temperatures at the Mn L-2,L-3 edges in La0.5Sr1.5MnO4. The orderings display complex energy features close to t ...

Element-resolved x-ray ferrimagnetic and ferromagnetic resonance spectroscopy

We report on the measurement of element-specific magnetic resonance spectra at gigahertz frequencies using x-ray magnetic circular dichroism (XMCD). We investigate the ferrimagnetic precession of Gd and Fe ions in Gd-substituted yttrium iron garnet, showing that the resonant field and linewidth of Gd precisely coincide with Fe up to the nonlinear regime of parametric excitations. The opposite sign of the Gd x-ray magnetic resonance signal with respect to Fe is consistent with dynamic antiferromagnetic alignment of the two ionic species. Further, we investigate a bilayer metal film, Ni 80Fe20(5 nm)/Ni(50 nm), where the coupled resonance modes of Ni and Ni80Fe20 are separately resolved, revealing shifts in the resonance fields of individual layers but no mutual driving effects. Energy-dependent dynamic XMCD measurements are introduced, combining x-ray absorption and magnetic resonance spectroscopies.

Soft x-ray coherent scattering: Instrument and methods at ESRF ID08

An experimental setup has been developed to perform soft x-ray coherent scattering at beamline ID08 of the European Synchrotron Radiation Facility. An intense coherent beam was obtained by filtering the primary beam with the monochromator and a circular pinhole. A pinhole holder with motorized translations was installed inside the UHV chamber of the diffractometer. The scattered intensity was recorded in reflection geometry with a back-illuminated charge coupled device camera. As a demonstration we report experimental results of resonant magnetic scattering using coherent beam. The degree of coherence is evaluated, and it is shown that, while the vertical coherence is much higher than the horizontal one at the source, the situation is reversed at the diffractometer. The intensity of the coherent beam is also discussed.

Absence of local magnetic moments in Ru and Rh impurities and clusters on Ag(100) and Pt(997)

6 pages, 5 figures.-- PACS nrs.: 75.20.Hr; 78.20.Ls; 78.70.Dm.-- ArXiv pre-print available at: http://arxiv.org/abs/0708.3975

Cobalt nanoclusters on metal-supported Xe monolayers: Influence of the substrate on cluster formation kinetics and magnetism

Department Chemie und Biochemie, Ludwig-Maximilians-Universit¨at Mu¨nchen, 81377 Mu¨nchen, Germany(Dated: September 1, 2009)The growth dynamics of submonolayer coverages of Cobalt during buffer layer assisted growthon Ag(111) and Pt(111) substrates is investigated by variable temperature scanning tunneling mi-croscopy in the temperature range between 80 and 150 Kelvin. It is found that attractive cluster-substrate interactions can govern the cluster formation on the Xe buffer layer, if the Xe layer issufficiently thin. The interpretation of the microscopy results are supported by x-ray magneticcircular dichroism which monitors the effect of cluster-substrate interactions on the formation ofmagnetic moments and magnetic anisotropy of Co nanocluster during the different stages of growth.Ab-initiocalculations show that the cluster magnetism is controlled by the interface anisotropy, lead-ing to perpendicular magnetization for Co on Pt(111). Limits of and new potential for nanoclusterfabrication by buffer layer assisted growth are discussed.

Critical Reexamination of Resonant Soft X-Ray Bragg Forbidden Reflections in Magnetite

Magnetite, Fe{sub 3}O{sub 4}, displays a highly complex low-temperature crystal structure that may be charge and orbitally ordered. Many of the recent experimental claims of such ordering rely on resonant soft x-ray diffraction at the oxygen K and iron L edges. We have reexamined this system and undertaken soft x-ray diffraction experiments on a high-quality single crystal. Contrary to previous claims in the literature, we show that the intensity observed at the Bragg forbidden (001/2){sub c} reflection can be explained purely in terms of the low-temperature structural displacements around the resonant atoms. This does not necessarily mean that magnetite is not charge or orbitally ordered but rather that the present sensitivity of resonant soft x-ray experiments does not allow conclusive demonstration of such ordering.