K8.1
Nuclear Resonant Scattering of Synchrotron Radiation
O. Leupold
European Synchrotron Radiation Facility (ESRF), Grenoble, France
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Since its observation in 1985 [1] nuclear resonant scattering (NRS) of synchrotron radiation has become an excellent tool to study hyperfine interactions as well as dynamical effects in solids. It has proven to be a complementary method to Mössbauer spectroscopy. NRS combines the advantages of both local probe experiments and scattering techniques and gives valuable information on magnetic structures in solids. The effective thickness of the sample and the hyperfine fields determine the modulation of the time behaviour - dynamical beats and quantum beats, respectively - of the reemitted photons following the nuclear excitation. Due to the polarization of the synchrotron radiation beam the quantum beat structure gives more information on the direction of the hyperfine field than could be obtained by classical Mössbauer spectroscopy. For a review on the method and recent results see, e.g. [2].
NRS experiments benefit from the high beam quality of 3rd generation synchrotron radiation sources, as the small beam size and divergence. This allows one to use focusing or collimating optics for extreme sample conditions. Here we report mainly on results from the NRS beam line ID18 at the ESRF in Grenoble, France [3].
First we will give a short introduction to the NRS method and to the underlying beamline layout. Then we will focus on selected applications of NRS in magnetism, which especially benefit from the outstanding properties of synchrotron radiation.
- High pressure studies in large external magnetic fields and at variable temperatures utilizing a cryomagnetic system.
- Magnetism on surfaces and multilayer systems studied in grazing incidence geometry - both time differential and time integral, again at cryogenic temperatures and in high external magnetic fields.
[1] E. Gerdau et al., Phys. Rev. Lett. 54 (1985) 835.
[2] E. Gerdau and H. de Waard, eds., Hyperfine Interactions 123/124 (1999).
[3] R. Rueffer, A.I. Chumakov, Hyperfine Interactions 97/98 (1996) 589.
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K8.2
Synchrotron Mössbauer Reflectometry for Investigation of Hyperfine Interactions in Periodical Multilayers with Nanometer Resolution
M.A. Andreeva
Department of Physics, M.V. Lomonosov Moscow State University, Moscow, Russia
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Highly collimated synchrotron radiation allows to perform the angular resolved nuclear resonance measurements and to use the advantages of coherent interaction such as diffraction and total reflection for structure investigations. The time spectra of the nuclear resonance Bragg reflectivity from periodical multilayers are very sensitive to the hyperfine field distributions across bilayer depth [1-2]. We supposed that the oscillation structure of the radiation field generated inside a periodical film at the Bragg condition as well as the penetration depth leads to the enhancement of contributions from definite depths to the reflectivity spectra [2]. Than the reflectivity spectra are changed with the angle variation in vicinity of the Bragg angle (as it follows from the standing wave theory). We have made the depth-selective experimental investigation of [57Fe/Cr]*n, [57Fe/V]*n and [57Fe/Co]*n multilayers at ESRF [4-6]. The reflectivity time spectra shown variations with the angle only in the cases when the reflectivity was relatively large [4], but the enhancement of the contribution from the central part of resonant layers in the reflectivity spectra was observed for all investigated samples. The theory of nuclear resonance reflectivity, which takes into account variations of the anisotropy of scattering amplitude with depth and polarization mixing, is based on the generalized Parratt equations. For a qualitative analysis of the structure sensitivity we used a kinematical limit of the reflectivity theory. Such approximation allows to distinguish the dynamical and kinematical effects in the reflectivity spectrum formation.
[1] T.S. Toellner, et al., Phys. Rev. Lett. 74 (1995) 3475.
[2] M.A. Andreeva, et al., Hyperfine Interactions 126 (2000) 343.
[3] M.A. Andreeva. JETP Lett. 69 (1999) 863.
[4] M.A. Andreeva, et al., The Physics of Metals and Metallography 91, suppl.1 (2001) 22.
[5] B. Lindgren, et al., Hyperfine interactions (2002) accepted.
[6] B. Kalska, et al., Hyperfine interactions (2002) accepted.
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K8.3
Diffusion Studies in Ordered Alloys: Revive Old Methods with New Ideas and New Possibilities.
G. Vogl, B. Sepiol, M. Kaisermayr and M. Sladecek
Institut für Materialphysik der Universität Wien, Wien, Austria
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We report on investigations of atom diffusion in three- and two-dimensional ordered systems with Mössbauer spectroscopy at synchrotrons [1,2]. We compare the results with results from various neutron scattering methods [3-5]. By way of the examples we shall demonstrate the advantages and drawbacks of these complementary approaches. We shall finally give an oulook to future chances for studying diffusion making use of the coherence of the new synchrotron sources [6].
[1] G.Vogl and B.Sepiol, Diffusion in Crystalline Materials , in: Nuclear Resonant Scattering of Synchrotron Radiation, eds. E.Gerdau and H.de Waard, Baltzer Science Publ. (1999), p.595.
G.Vogl and M.Hartmann, Diffusion Studies with synchrotron radiation, J.Phys.: Condens.Matter 13 (2001) 7763.
[2] M.Sladecek, B.Sepiol, M. Kaisermayr, J.Korecki, B.Handke, H. Thiess, O.Leupold, R.Rueffer, G.Vogl, Surface Sci. (2002), in print.
[3] M.Kaisermayr, C.Pappas, B.Sepiol, G.Vogl, Probing Jump Diffusion in Crystalline Solids with Spin-Echo Spectroscopy, Phys.Rev.Lett. 87 (2001) 175901.
[4] M.Kaisermayr, J.Combet, B.Frick, B.Sepiol, G.Vogl, The Elmentary Jump in Intermetallic Alloys, ILL Scientific Highlights 2000, 46 (2001).
[5] M.Kaisermayr, J.Combet, H.Ipser, H.Schicketanz, B.Sepiol, G.Vogl, Nickel diffusion in B2-NiGa studied with quasielastic neutron scattering, Phys.Rev. B 61 (2000) 12038; CoGa, Phys.Rev. B 63 (2001) 54303.
[6] G.Grübel and G. Vogl, Probing Diffusion in the Time Domain, to appear in Synchrotron Radiation News (2002).
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O8.1
Nuclear Inelastic Scattering of Synchrotron Radiation in Oxides with Colossal Magnetoresistance
A. Rykov1, K. Nomura1, Ts. Sawada1, T. Mitsui2, Y. Yoda3, Y. Kobayashi4 and M. Seto4
1) School of Engineering, The University of Tokyo, Tokyo, Japan
2) Japan Atomic Energy Research Institute , Hyogo, Japan
3) Japan Synchrotron Radiation Research Institute, Hyogo, Japan
4) Research Reactor Institute, Kyoto University, Osaka, Japan
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The oxygen deficient (Sr,Ca)(Fe,Co)O3-d phases are precursors of metallic conductors with large magnetoresistance (MR). The metallic conductivity in these systems is achieved by pressurizing them in oxygen at 600o C and 15 MPa. In these perovskites, large MR was observed for lightly oxygen deficient compositions, and for Mo-derivative Sr2FeMoO6. Resonant nuclear inelastic scattering (NIS) spectra of (Sr,Ca)(Fe,Co)O3-d and Sr2FeMoO6 are measured with the energy resolution of 3.5meV by detecting the 6.3 keV Fe Ka X-rays following after Mössbauer effect on 57Fe transition excited by the monochromatized 14.41 keV synchrotron radiation. Both series of oxides are examined near the metal-to-insulator transition. The changes in phonon density of states (DOS) are obtained. Soft phonon peak arises at ~8 meV in NIS spectra of strongly oxygen deficient (Sr,Ca)(Fe,Co)O3-d. (d near 0.5). With decreasing oxygen deficiency the soft peak disappears. This peak is found to be correlated with the oxygen ordering into brownmillerite structure. Lamb-Mössbauer factors of these oxides were calculated on the base of Debye Model. The correlations between the MR properties and the changes in phonon DOS are to be analyzed.
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