K. Starke

Freie Universität Berlin, Berlin, Land Berlin, Germany

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Publications (62)139.59 Total impact

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    ABSTRACT: We present a systematic study of the Rashba-type spin–orbit interaction at the (0001) surfaces of rare-earth metals and their surface monoxides, specifically of Tb metal and the O/Tb, O/Lu and O/Y surfaces. By means of photoemission experiments and ab initio band-structure calculations, we uncover the influence of this interaction on the surface electronic structure. In turn, the dramatic impact of the charge-density distribution of the surface/interface states on the strength of the Rashba-type spin splitting is demonstrated. We discuss the Rashba effect at magnetic and non-magnetic rare-earth surfaces, and compare with cases where it is negligible. The difference between the Rashba effect and magnetic linear dichroism in photoemission is pointed out to help avoid possible confusion in connection with the simultaneous appearance of these two effects at a magnetic surface.
    New Journal of Physics 01/2009; 11(1):013035. · 4.06 Impact Factor
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    ABSTRACT: For epitaxial trilayers of the magnetic rare-earth metals Gd and Tb, exchange coupled through a nonmagnetic Y spacer layer, element-specific hysteresis loops were recorded by the x-ray magneto-optical Kerr effect at the rare-earth M5 thresholds. This allowed us to quantitatively determine the strength of interlayer exchange coupling (IEC). In addition to the expected oscillatory behavior as a function of spacer-layer thickness dY, a temperature-induced sign reversal of IEC was observed for constant dY, arising from magnetization-dependent electron reflectivities at the magnetic interfaces.
    Physical Review Letters 06/2008; 100(22):227203. · 7.94 Impact Factor
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    ABSTRACT: Core level photoemission (PE) spectra from magnetically ordered lanthanide materials exhibit large Magnetic Circular Dichroism (MCD), i.e. the intensities of individual PE multiplet lines depend strongly on the relative orientation between sample magnetization and photon momentum of the incident circularly polarized light. MCD-PE is most pronounced in PE from 4f levels where, in relevant cases, PE multiplet lines are strong for one magnetization direction, either parallel or antiparallel to the photon momentum, and almost absent for the other. The potential for application of MCD-PE as a near-surface magnetometer with atomic-layer resolution is shown for the (0001) surfaces of Gd and Tb, where the magnetization of the topmost atomic layer - by help of the surface corelevel shift - is spectroscopically separated from the sub-surface magnetization. In multicomponent magnetic thin films containing different lanthanide elements, MCD-PE allows to monitor the magnetization in an element specific way. This is demonstrated for the example of the heteromagnetic interface Eu/Gd(0001).
    01/2007: pages 65-84;
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    ABSTRACT: We have investigated the bulk valence-band structure of Tb metal in the ferromagnetic phase by angle-resolved photoelectron spectroscopy and full-potential-linearized-augmented-plane-wave calculations. The experiments were performed at undulator beamline 7.0.1 of the Advanced Light Source using a three-axis rotatable low-temperature goniometer and a display-type photoelectron spectrometer that give access to a large region of momentum space. The results of our calculations, which make use of recent progress in the theoretical description of the magnetic properties of 4f metals, are in remarkably good agreement with experiment. This can be best seen from a comparison of the electronic structure in high-symmetry directions, at critical points, on Fermi contours, and at band crossings with the Fermi level. To our knowledge, the present work represents the most detailed combined experimental and theoretical study of the electronic structure of a magnetic lanthanide metal to date.
    Phys. Rev. B. 01/2007; 76(3).
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    ABSTRACT: We have observed the spin-state interference by measuring the photoelectron spin polarization in the resonant preedge 4d→4f photoemission from magnetized Gd. The photoemission is induced by circularly polarized light which determines one preferential direction of electron spin orientation due to polarization transfer and spin-orbit interaction. Another direction perpendicular to the first one is determined by the target electron spin orientation connected with the target magnetization. We have measured the component of spin polarization perpendicular to those two directions which can only appear due to spin-state interference which implies coherence of the spin states produced by the two mechanisms of the photoelectron spin polarization.
    Physical Review B 01/2006; 74(16). · 3.77 Impact Factor
  • F. Heigl, K. Starke, M. Bode
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    ABSTRACT: The temperature range of helical antiferromagnetic (AFM) order in epitaxially grown Tb metal films on W(110) was studied via ac susceptibility using the magneto-optical Kerr effect. The temperature range of the AFM phase was found to get wider with increasing annealing temperature, reaching a maximum width of 17 K when the film is annealed at Tan=1200 K. This shows that the AFM phase is stabilized in epitaxial films as compared to bulk Tb metal. Annealing-induced changes of the surface topography of the films were monitored by scanning tunnel microscope and low-eneregy electron diffraction, revealing a statistical surface roughness for Tan<600 K and the formation of large terraces separated by monatomic steps for 600 K<Tan<800 K. Annealing above ≈800 K results in step bunching, followed by a breakup of the film for Tan>1000 K. Both the extension of the AFM phase in the film and the annealing-induced changes of the surface topography can be explained by assuming the presence of a reconstructed Tb monolayer at the film-substrate interface that serves as an effective substrate on which a slightly strained Tb film lattice is stabilized upon annealing.
    Physical Review B 07/2005; 72(3). · 3.77 Impact Factor
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    ABSTRACT: The epitaxial system Sm/Co(0001) was studied for Sm coverages up to 1 monolayer (ML) on top of ultrathin Co/W(110) epitaxial films. Two ordered phases were found for 1/3 and 1 ML Sm, respectively. The valence state of Sm was determined by means of photoemission and magnetic properties were measured by magneto-optical Kerr effect. We find that 1 ML Sm causes a strong increase of the coercivity with respect to that of the underlying 10 ML Co film. Element-specific hysteresis loops, measured by using resonant soft x-ray reflectivity, show the same magnetic behaviour for the two elements. Comment: 4 pages, 5 figures. Accepted version, minor changes: discussion of soft x-ray reflectivity spectra included, references added
    Physical Review B 06/2005; · 3.77 Impact Factor
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    ABSTRACT: Lanthanide metals are a particular class of magnetic materials in which the magnetic moments are carried mainly by the localized electrons of the 4f shell. They are frequently found in technically relevant systems, to achieve, e.g., high magnetic anisotropy. Magneto-optical methods in the X-ray range are well suited to study complex magnetic materials in an element-specific way. In this work, we report on recent progress on the quantitative determination of magneto-optical constants of several lanthanides in the soft X-ray region and we show some examples of applications of magneto-optics to hard-magnetic interfaces and exchange-coupled layered structures containing lanthanide elements.
    Applied Physics A 01/2005; 80(5):1021-1027. · 1.55 Impact Factor
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    ABSTRACT: This version is available at the following Publisher URL: http://prb.aps.org
    01/2005;
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    ABSTRACT: Second-harmonic (SH) generation at the ferromagnetic Gd(0001) surface was investigated with 35-fs laser pulses and photon energies between 1.44 and 1.68 eV. In pump-probe mode the SH field was measured during the first 3 ps, and the time evolution, including contributions from coherent phonons and magnons, was analyzed. All data reveal strong changes with wavelength, which correlate with the SH magnetic contrast and can be well explained by the Gd(0001) exchange-split surface state. In SH interferometry an energy splitting of 13 meV was found that is attributed to a frequency modulation of the surface state by coherent phonons of approximately 3 THz.
    Journal of the Optical Society of America B 01/2005; 22(1). · 2.21 Impact Factor
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    ABSTRACT: Excitation of the Gd ( 0001 ) surface state by 800 nm femtosecond laser pulses initiates transient coherent surface and bulk vibrations. These are investigated by time-resolved second harmonic and linear reflectivity probing simultaneously the motion of the surface layer and its coupling to bulk modes localized near the surface. Initially, the surface vibration frequency of 2.8 THz lies ˜18% below the frequency detected in linear reflectivity. Within the first 3 ps the surface vibration frequency is found to increase while the bulk one decreases. Both types of vibrations persist only during nonequilibrium between electron and lattice temperature and are damped within the electron-phonon relaxation time. We conclude that the observed frequency shifts cannot be accounted for by anharmonic effects but originate from time-dependent changes of the potential energy at and near the surface due to nonequilibrium electron distributions.
    Physical Review B 06/2004; · 3.77 Impact Factor
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    ABSTRACT: We give experimental and theoretical evidence of the Rashba effect at the magnetic rare-earth metal surface Gd(0001). The Rashba effect is substantially enhanced and the Rashba parameter changes its sign when a metal-oxide surface layer is formed. The experimental observations are quantitatively described by ab initio calculations that give a detailed account of the near-surface charge density gradients causing the Rashba effect. Since the sign of the Rashba splitting depends on the magnetization direction, the findings open up new opportunities for the study of surface and interface magnetism. Comment: 4 Figs
    03/2004;
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    ABSTRACT: The magneto-induced fraction of the optical second harmonic reflected from Gd(0001) surfaces was studied in pump–probe experiments as a function of temperature. The magnetic contrast in thermal equilibrium and at 1ps pump–probe delay show a similar temperature dependence, but at 1ps delay the magnetic contrast is reduced to 30% due to ultrafast demagnetization. Compared to lattice cooling the magnetization recovery is significantly slower within the first 150ps and, therefore, does not obey the typical M(T) behavior in this time range. This is attributed to the localized character of the 4f electrons carrying the large magnetic moment in Gd.
    Journal of Magnetism and Magnetic Materials 01/2004; 272:1001-1002. · 1.83 Impact Factor
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    ABSTRACT: Coherent spin dynamics in the THz domain coupled to a coherent phonon is observed in the time-resolved second harmonic response of the Gd(0001) ferromagnetic metal surface. An LO phonon of 2.9 THz is excited by a transient charge displacement at the surface caused by resonant absorption of a fs laser pulse in the exchange-split surface state. This lattice vibration modulates the interlayer distance inducing a coherent variation of the exchange interaction between spins in adjacent layers. The resulting magnetization dynamics is considered as optical magnon wave packets coupled to the phonon.
    Physical Review Letters 12/2003; 91(22):227403. · 7.94 Impact Factor
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    Thin Solid Films 08/2003; 437(s 1–2):309. · 1.60 Impact Factor
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    ABSTRACT: The second harmonic response of ferromagnetic Gd(0001) surfaces following optical excitation at 750 - 820 nm was studied by pump-probe measurements. As a function of time delay oscillations at a frequency of 3 THz were observed in both the second harmonic fields which are even and odd with respect to magnetization reversal. The periodicity in the even field is attributed to a coherent optical phonon, which corresponds to a modulation of the Gd interlayer distance between the surface layer and the bulk. The proposed excitation mechanism of coherent phonons at metal surfaces goes beyond established models and includes the different lifetimes of the two exchange split surface states. As a consequence of the varying interlayer distance we propose a modulation of the exchange coupling along the surface normal, which is corroborated by the observation of an oscillating odd field. This phenomenon is attributed to a modulation of the effective spin polarization of the 5d exchange split surface states, which is strongly coupled to the localized 4f spin moments. Thus, we propose a coherent optical magnon driven by the coherent phonon excitation.
    03/2003;
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    ABSTRACT: O3 X-ray emission (XE) upon 5p3/2 electron-impact ionization of La(0 0 0 1) and Sm(0 0 0 1) metal surfaces exhibits up to 30% spectral intensity from the topmost atomic surface layer. The energy separation of surface and bulk XE peaks is explained by the surface core-level shift of La and the surface valence transition of Sm. While the bulk XE spectra are in reasonable agreement with theoretical sd partial density of states (DOS), the experimental access to the partial surface-layer DOS opens new opportunities in studying surface phenomena in lanthanide metals and compounds.
    Surface Science. 02/2003; 526(s 1–2):L137–L142.
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    ABSTRACT: Using circularly polarized soft X-rays, we show that large magneto-optical effects are observed in specular reflection geometry both at the N4,5 (4d→4f) and M4,5 (3d→4f) thresholds of magnetic lanthanide elements. The reflected photocurrents can reach micro-amps and change – in first approximation – proportional to the 4f magnetization of the sample. These XMOKE effects allow an element-specific analysis of the sample magnetization and are unaffected by thermal lattice-expansion effects. XMOKE carries substantial potential for the analysis of magnetic nanoscale materials, with possible applications to element-specific microscopy in reflection geometry.
    Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 01/2003; 199:313–317. · 1.27 Impact Factor
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    ABSTRACT: The remanent 4f spin structure of the epitaxial surface monoxide O(1×1)/Gd(0001) has been investigated using magnetic dichroism in core-level photoemission (MDPE). Here we report the observation of a substantial dichroic signal of the chemically shifted Gd-4f PE line. It demonstrates the presence of a non-vanishing net magnetization of the surface monoxide layer at cryogenic temperatures.
    Thin Solid Films 01/2003; 428(1):98-101. · 1.60 Impact Factor
  • Thin Solid Films 01/2003; 437(1):309-309. · 1.60 Impact Factor

Publication Stats

189 Citations
44 Downloads
139.59 Total Impact Points

Institutions

  • 1993–2009
    • Freie Universität Berlin
      • • Institute of Experimental Physics
      • • Department of Physics
      Berlin, Land Berlin, Germany
  • 1992
    • Max Planck Institute for Plasma Physics
      • Max Planck Institute for Plasma Physics, Greifswald
      Arching, Bavaria, Germany