S. Cramm

Investigation of a novel material for magnetoelectronics:Co2Cr0.6Fe0.4Al

Heusler compounds are promising candidates for future spintronics device applications. The electronic and magnetic properties of Co2Cr0.6Fe0.4Al, an electron-doped derivative of Co2CrAl, are investigated using circularly polarized synchrotron radiation and photoemission electron microscopy (PEEM). Element specific imaging reveals needle shaped Cr rich phases in a homogeneous bulk of the Heusler compound. The ferromagnetic domain structure is investigated on an element-resolved basis using x-ray magnetic circular dichroism (XMCD) contrast in PEEM. The structure is characterized by micrometre-size domains with a superimposed fine ripple structure; the lateral resolution in these images is about 100 nm. The domains look identical for Co and Fe giving evidence of a ferromagnetic coupling of these elements. No ferromagnetic contrast is observed at the Cr line. Magnetic spectroscopy exploiting XMCD reveals that the lack of magnetic moment, detected in a SQUID magnetometer, is mainly due to the moment of the Cr atom.

Organic photoconductors and C60

Abstract We report experimental results on photoelectron spectroscopy (XPS, UPS) and X-ray absorption near edge spectroscopy (XANES) from poly(3-octylthiophene) (P3OT) and Cu-phthalocyanine (Cu-Pc) in contact with C 60 . The interaction between these organic photoconductors and the C 60 is rather weak. At room temperature an inter-diffusion between C 60 and P3OT is observed, whereas Cu-Pc and C 60 build a stable interface. The spectra reveal the relative positions of the electronic levels close to the optical gap. From this we conclude that an electron transfer to C 60 via some excitonic states of the photoconductor is energetically possible. Cu-Pc exhibits a band-bending induced by C 60 as deduced from the observed shifts in the XPS and UPS spectra and the missing shifts in the XANES spectra.

Reduction of surface magnetism of Co2Cr0.6Fe0.4Al Heusler alloy films

Element specific magnetization has been determined at the surface and in the bulk of Co2Cr0.6Fe0.4Al Heusler alloy films grown on α-Al2O3 and capped by Al, using x-ray magnetic circular dichroism both in transmission and total electron yield. The magnetic moments for Co and Fe are considerably reduced at the upper surface in comparison to their values in the bulk of the film. The large reduction at room temperature of 17% for thick films averaged along the electron escape depth implies an even larger reduction at the topmost layer which is crucial for spin-dependent transport. The surface magnetization decreases additionally with respect to the bulk value with decreasing film thickness below 20nm.

Standing-wave excited soft x-ray photoemission microscopy: Application to Co microdot magnetic arrays

Standing-wave excited soft x-ray photoemission microscopy: Application to Co microdot magnetic arrays Alexander X. Gray, 1,2,a Florian Kronast, 3 Christian Papp, 2,4 See-Hun Yang, 5 Stefan Cramm, 6 Ingo P. Krug, 6 Farhad Salmassi, 7 Eric M. Gullikson, 7 Dawn L. Hilken, 7 Erik H. Anderson, 7 Peter Fischer, 2 Hermann A. Durr, 3 Claus M. Schneider, 6 and Charles. S. Fadley 1,2 Department of Physics, University of California, Davis, California 95616, USA Material Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA Helmholtz-Zentrum Berlin, Albert-Einstein-Strase 15, 12489 Berlin, Germany Lehrstuhl fur Physikalische Chemie II, Universitat Erlangen-Nurnberg, Egerlandstrase 3, D-91054 Erlangen, Germany IBM Almaden Research Center, San Jose, California 95120, USA Institute of Solid State Research IFF-9, Research Center Julich, Julich D-52425, Germany Center for X-Ray Optics, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA We demonstrate the addition of depth resolution to the usual two-dimensional images in photoelectron emission microscopy (PEEM), with application to a square array of circular magnetic Co microdots. The method is based on excitation with soft x-ray standing-waves generated by Bragg reflection from a multilayer mirror substrate. Standing wave is moved vertically through sample simply by varying the photon energy around the Bragg condition. Depth-resolved PEEM images were obtained for all of the observed elements. Photoemission intensities as functions of photon energy were compared to x-ray optical calculations in order to quantitatively derive the depth-resolved film structure of the sample. Soft x-ray photoelectron emission microscopy (PEEM) is an established powerful technique, enabling element- specific studies of surfaces and nanostructures through images obtained via core-level excitations of photoelectrons and secondary electrons. PEEM has been used extensively in recent years to study element-specific physical, chemical, structural, and magnetic properties of surfaces, resulting in a plethora of literature revealing interesting surface and nano- structure phenomena. 1–6 Most of these studies involve image formation from low-energy secondary electrons, although a growing number of microscopes can image with an energy- resolved photoelectron or Auger intensity that is thus also element-specific. PEEM images to date are inherently two- dimensional in space, with resolutions in the lateral x and y directions that are now typical 10–20 nm but which promise in the future to be in the few nanometer regime. 7 Very recently, a method has been suggested, by which soft x-ray standing-wave excitation extends the dimensionality of the PEEM, giving this technique depth resolution. 8 This depth resolution is achieved by setting-up an x-ray standing wave (SW) field in the sample by growing it on a synthetic periodic multilayer mirror substrate which in first- order Bragg reflection acts as the SW-generator. 9 The SW can then be moved vertically through the sample in several following ways: by varying the photon energy or the incidence angle through the Bragg condition, or in a previous study of this type, also by growing one layer of the sample as a wedge and looking along the wedge. 8 As the antinodes of the electromagnetic field shift vertically through the sample, they highlight various parts of the sample, introducing depth- selectivity in the photoemission process. Exploiting this depth-selectivity in an element-specific way requires an energy-selection of photoelectrons contributing to the PEEM image so as to image with a given core-electron intensity. In a previous exploratory study, this SW-PEEM technique has been used to determine the depth-resolved properties of a Ag- wedge/Co/Au multilayer nanostructure. 8 It was shown that with a suitable wedge-profile sample, the vertical resolution in the PEEM images approaches - ±3–4 A thus about 1/10 of the SW period, which is in turn equal to the period of the multilayer of 3–4 nm. In this study, we demonstrate the next step in the SWPEEM approach; imaging a lateral array of microstructures with depth resolution, as a prelude to future applications with true nanometer resolution in three dimensions. In contrast with the previous study 8 the approach used here, namely, moving the SW through the structure by means or varying photon energy, is simpler and more versatile because it does not require one to grow the layer of interest in a shape of a wedge. Due to this improvement, it is possible to measure patterned structures which are more technologically relevant, such as arrays of microdots or pillars described in this paper. One of the major advantages of the SW-PEEM technique is the enhanced interface-sensitivity of the measurement due to the possibility of translating the antinodes of the SW vertically through the sample and therefore choosing the “epicenter” of the photoemission. This effect cannot be achieved with other methods, such as, for example, using harder x-rays in order to increase the probing depth. We have investigated a nanostructured system consisting of square arrays of circular magnetic cobalt microdots, n o m i n al ly 4 n m in th ic k ne s s an d 1 µ m in di a m e ter , g r o w n o n a m u l t i l a y e r s u b s t r a t e o f c o n f i g u r a t i o n [23.6 A -Si/ 15.8 A -Mo] X 40, as depicted in cross section in

Interface magnetization of ultrathin epitaxial Co2FeSi(110)/Al2O3 films

Element-specific magnetic properties of ultrathin epitaxial Co2FeSi(110) films were measured using x-ray magnetic circular dichroism (XMCD). The epitaxial Heusler films were grown by RF magnetron sputtering on substrates. The magnetization of thicker films as determined by XMCD is smaller than expected for a half-metallic material. In addition, the magnetization decreases considerably for films thinner than 10 nm. The thickness dependence of the magnetic moment can be described by introducing a certain number of dead layers representing a deficiency of magnetization at the interfaces. Quantitative evaluation results in a dead layer thickness of 0.8 nm at room temperature, consisting of a temperature induced size effect of 0.1 nm and a surface effect of 0.15 nm at the top and 0.55 nm at the bottom interface.

Enhanced ferrimagnetism in auxetic NiFe2O4 in the crossover to the ultrathin lm limit

The competition of charge, spin and orbital degreesof freedom in complex oxides leads to intriguing phys-ical phenomena, including ferromagnetism, ferroelectri-city or multiferroicity [1]. Fertilized by the continuouslyadvancing art of oxide growth, the controlled synthesisof high-quality oxide heterostructures now approaches amonolayer-precision [2]. Designing electronic propertiesin ultrathin oxide lms and interfaces thereby opens uproutes to explore novel nanoelectronic functionalities forapplications.In the context of spin-based electronics, oxides fea-turing both magnetic and insulating properties reveala highly e ective spin lter e ect, where spin-polarizedelectron currents are generated by a spin-dependent tun-nelling process. Up to 100% spin ltering has beendemonstrated in magnetic oxides with low Curie tem-perature T

Generation of Ultrashort and Coherent Synchrotron Radiation Pulses at DELTA

Pump-probe experiments to study ultrafast dynamic phenomena such as electron transfer, lattice vibrations, phase transitions, chemical reactions, or spin dynamics require two short radiation pulses as well as good control of the time delay between them. The first pulse to excite (“pump”) the sample under study is usually a femtosecond laser pulse in the near-visible regime. For the second pulse to analyze (“probe”) the state of the sample as a function of the delay, however, light with shorter and tunable wavelength would be desirable. Conventional synchrotron light sources produce pulses with a typical duration of 30–100 ps (FWHM), given by the electron bunch length in a storage ring, which is not well suited for ultrafast studies. The bunch length can be reduced to a few picoseconds in the so-called low-α mode (e.g., [1]) by lowering the momentum compaction factor α of the storage ring. Radiation pulses in the femtosecond range, however, are obtained more easily by extracting synchrotron light from a small fraction of the electron distribution, rather than the whole bunch, which can be achieved with the laser-based methods described below.

Quasi 2D electronic states with high spin-polarization in centrosymmetric MoS2 bulk crystals.

Time reversal dictates that nonmagnetic, centrosymmetric crystals cannot be spin-polarized as a whole. However, it has been recently shown that the electronic structure in these crystals can in fact show regions of high spin-polarization, as long as it is probed locally in real and in reciprocal space. In this article we present the first observation of this type of compensated polarization in MoS2 bulk crystals. Using spin- and angle-resolved photoemission spectroscopy (ARPES), we directly observed a spin-polarization of more than 65% for distinct valleys in the electronic band structure. By additionally evaluating the probing depth of our method, we find that these valence band states at the point in the Brillouin zone are close to fully polarized for the individual atomic trilayers of MoS2, which is confirmed by our density functional theory calculations. Furthermore, we show that this spin-layer locking leads to the observation of highly spin-polarized bands in ARPES since these states are almost completely confined within two dimensions. Our findings prove that these highly desired properties of MoS2 can be accessed without thinning it down to the monolayer limit.

Fabrication of layered nanostructures by successive electron beam induced deposition with two precursors: protective capping of metallic iron structures.

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.

Time-resolved magnetic imaging in an aberration-corrected, energy-filtered photoemission electron microscope.

We report on the implementation and usage of a synchrotron-based time-resolving operation mode in an aberration-corrected, energy-filtered photoemission electron microscope. The setup consists of a new type of sample holder, which enables fast magnetization reversal of the sample by sub-ns pulses of up to 10 mT. Within the sample holder current pulses are generated by a fast avalanche photo diode and transformed into magnetic fields by means of a microstrip line. For more efficient use of the synchrotron time structure, we developed an electrostatic deflection gating mechanism capable of beam blanking within a few nanoseconds. This allows us to operate the setup in the hybrid bunch mode of the storage ring facility, selecting one or several bright singular light pulses which are temporally well-separated from the normal high-intensity multibunch pulse pattern.