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HFI/NQI 2010 (September 14, 2010, Geneva)

HFI/NQI 2010 (September 14, 2010, Geneva). Investigations on Thin Fe Films and Heusler Alloy Films Using Synchrotron-Radiation-Based Mössbauer Spectroscopy. K. Mibu Nagoya Institute of Technology JANAN. Three world centers for synchrotron-based Mössbauer spectroscopy. ESRF. APS.

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HFI/NQI 2010 (September 14, 2010, Geneva)

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  1. HFI/NQI 2010 (September 14, 2010, Geneva) Investigations on Thin Fe Films and Heusler Alloy Films Using Synchrotron-Radiation-Based Mössbauer Spectroscopy K. Mibu Nagoya Institute of Technology JANAN

  2. Three world centers for synchrotron-based Mössbauer spectroscopy ESRF APS SPring-8

  3. CREST project for synchrotron-based Mössbauer spectroscopy (CREST = Core Research for Evolutional Science and Technology) FY 2005 ~ 2010 (~ JPY 400,000,000 / 5.5 years) M. Seto, Kyoto University Project Leader Y. Yoda, JASRI/SPring-8 Methodological improvements on the beam line (@BL09XU) T. Mitsui, Japan Atomic Energy Agency Methodological improvements on the beam line (@BL11XU) S. Kishimoto, High Energy Accelerator Research Institute (KEK) Improvements of detector systems H. Kobayashi, Hyogo Prefectural University Applications to material research (High pressures) K. Mibu, Nagoya Institute of Technology Applications to material research (Thin films & Nano-structures)

  4. Outline - Focusing on the measurements of hyperfine fields for thin films - Introduction *Quick introduction to synchrotron-radiation-based Mössbauer spectroscopy *Conventional Mössbauer spectroscopy for thin films and nano-structures *Background of the test samples (Fe films & Co2MnSn films) Resultson Synchrotron-Radiation-Based Mössbauer Spectroscopy *Measurements in time domains for Fe films and Co2MnSn films *Measurements in energy domains using a nuclear Bragg monochromator for Fe films *Measurements in energy domains using a standard absorber for Co2MnSn films Conclusion

  5. Advantages and disadvantages of synchrotron radiation as a source for Mössbauer spectroscopy Advantages * Small beam size (Around 1 mm without focusing devices,10 mm or less with focusing devices) * Low angular divergence * High intensity * Polarization * Pulse structures * Energy selectivity Disadvantages * Broad energy bandwidth (Wider than 1 meV even after monochromatization in general) Special techniques are required to use synchrotron radiation as a source for Mössbauer spectroscopy.

  6. * Proposal for the use of synchrotron radiation as a Mössbauer sourceS. L. RubyJournal de Physique, Colloq. 35, C6 - 209 (1974)* Conclusive observation of Mössbauer spectra using synchrotron radiation and nuclear Bragg monochromator E. Gerdaw, R. Rüffer, H. Winkler, W. Tolksdorf, C. P. Klages, J. P. HannonPhys. Rev. Lett. 54, 835 (1985)* Observation of time spectra for nuclear forward scattering (NFS) of synchrotron radiationJ. B. Hastings, D. P. Siddons, U. van Bürck, R. Hollatz, U. BergmannPhys. Rev. Lett. 66, 770 (1991)* Development of new method to measure Mössbauer spectra in energy domain using synchrotron radiation M. Seto, R. Masuda, S. Higashitaniguchi, S. Kitao. Y. Kobayashi, C. Inaba, T. Mitsui, Y. YodaPhys. Rev. Lett. 102, 217602 (2009) Historical Backgrounds

  7. Oscillation Oscillation       • -rays Radioactive source Radioactive source Conventional Mössbauer spectroscopic setups for for thin films and nano-structures on single crystal substrates Transmission geometry Proportional counter Counts Sample Doppler velocity (mm/s) (Energy of -rays) Scattering geometry (CEMS geometry) * Small sample house full of counter gas • -rays Proportional counter Internal conversion electrons e- Sample  - rays 14.4 keV for 57Fe or 23.8 keV for 119Sn (@ Nagoya Inst. Tech.) Low-temperature CEMS system Conversion electrons 7.3 keV for 57Fe or 19.4 keV for 119Sn ~ 70 K (w/ He+2%CH4 gas) ~ 30 K (w/ H2 gas)

  8. Advantages of synchrotron-radiation-based Mössbauer spectroscopy for the investigations on thin films and nano-structures - In comparison with conversion electron Mössbauer spectroscopy (CEMS) using a radioactive source and a proportional gas counter – 1. Easier to measure at low temperatures, in magnetic fields, with electric fields, and under other special sample conditions 2. Possible to detect in-plane magnetic anisotropy using the polarization characters 3. Not necessary to maintain radioactive sources, and applicable to all the Mössbauer nuclei in principle ☞Large potential needs from the field of material science But ・・・　Difficult to get sufficient number of probe nuclei in the narrow beam path in comparison with the case of bulk powder or single crystal samples ☞Required to measure in an oblique incidence (or grazing angle) geometry ☞Sometimes necessary to enrich probe nuclei in the sample appropriately Cryostat Sample Detector Beam N S Magnet Magnetic field

  9. -rays 14.4 keV -rays 23.8 keV        Co Mn       Sn Test samples and Mössbauer nuclei (1) Fe firms, nano-structures, monatomic layers For 57Fe Mössbauer spectroscopy Calculated CEMS spectrum Counts 33 T  Doppler velocity (mm/s) (Energy of -rays) 57Fe nuclear energy levels 0 T Calculated CEMS spectra for various hyperfine fields (2) Co2MnSn Heusler alloy films For 119Sn Mössbauer spectroscopy 5 T 10 T 15 T Counts Fe 119Sn nuclear energy levels Doppler velocity (mm/s) (Energy of -rays)

  10. X (Co etc.) Y (Mn etc.) Z (Snetc.) Background of the test samples Heusler alloys with an L21-type structure * Theoretically predicted to be “half metallic” (or w/ highly spin-polarized conduction electrons) * Promising materials for spin-electronics devices Strategy Preparation of L21-type alloy films by atomically controlled alternate deposition * Control of local crystallographic structures * Control of interfacial atomic species * Stabilization of non-equilibrium phases (Top view) Co2MnSn (Tc = 811 K,  5 B)

  11. Co Mn Sn Mössbauer spectra for Co2MnSn measured using a radioactive source Bulk Co2MnSn alloy prepared by arc-melting (Long range L21 order parameter= 0.9) L21 B2 Anti -site Similar spectra: J. M. Williams et al., J. Phys. C, 1 (1968) 473, etc. R. A. Dunlap et al., Can. J. Phys. 59 (1981) 1577, etc. A. G. Gavriliuket al., J. Appl. Phys. 77 (1995) 2648.

  12. Co Mn Sn Mössbauer spectra for Co2MnSn measured using a radioactive source Bulk Co2MnSn alloy prepared by arc-melting (Long range L21 order parameter= 0.9) L21 B2 Co2MnSn(40.2 nm) film prepared by atomically controlled alternate deposition （Tsub = 500℃ ） Anti -site Sharp distribution !! ☞Available to investigate magnetic interface effect or size effect of Co2MnSn!!

  13. Outline - Focusing on the measurements of hyperfine fields for thin films - Introduction *Quick introduction to synchrotron-radiation-based Mössbauer spectroscopy *Conventional Mössbauer spectroscopy for thin films and nano-structures *Background of the test samples (Fe films & Co2MnSn films) Resultson Synchrotron-Radiation-Based Mössbauer Spectroscopy *Measurements in time domains for Fe films and Co2MnSn films *Measurements in energy domains using a nuclear Bragg monochromator for Fe films *Measurements in energy domains using a standard absorber for Co2MnSn films Conclusion

  14. Setups of synchrotron-based Mössbauer spectroscopy For the investigations on thin films Detector High-resolution monochromator E  meV Time spectra High efficiency Wide element-applicability Difficult distribution analysis Thin film sample Monochromator Pulsed X-rays Detector E  neV Nuclear Bragg monochromator 57FeBO3 High-resolution monochromator Energy spectra High efficiency Limited element-applicability (57Fe only) Easy distribution analysis Thin film sample H Monochromator Pulsed X-rays Oscillation (Super-monochromatization + Doppler shift) Energy spectra Detector Medium efficiency Wide element-applicability Easy distribution analysis Standard absorber (eg. CaSnO3 ) High-resolution monochromator Resonant X-rays Thin film sample Monochromator Pulsed X-rays Oscillation （Energy dip in neV order + Doppler shift）

  15.      Intensity (log. Scale) Time Measurements of nuclear resonant time spectra Detector High-resolution monochromator E  meV Time spectra Dependence of the “quantum beat” patterns on the direction of magnetic hyperfine field (rel. to the beam polarization) Thin film sample Monochromator Pulsed X-rays 14.4 keV 14.4 keV Pulsed X-rays Resonantly scattered X-rays (“delayed” signal)  100 ns 57Fe nuclei * Interference beat frequency => Inversely proportional to the hyperfine field * Interference beat pattern => Dependent on the direction of hyperfine field Cited from R. Röhlsberger et al., J. Magn. Magn. Mater. 282, 329 (2004).

  16.  30 nm X-rays (Perpendicular) X-rays (Parallel) Nuclear resonant time spectra for Fe nanowires @ BL11XU, SPring-8 57Fe wire （30 nm wide, 30 nm thick） Quantum beat + Dynamical beat Excellent works on the determination of magnetization direction in thin film samples have been published by: ESRF group (e.g., R. Röhlsberger et al., Phys. Rev. Lett. 89, 237201 (2002)) APS group (e.g., C. L’abbé et al.,Phys. Rev. Lett. 93, 037201 (2004))

  17. Nuclear resonant time spectrum for a “thick” Co2MnSn film Detector Cryostat High-resolution monochromator 119Sn 23.87 keV E  meV Time spectra sample N S Magnet Monochromator Pulsed X-rays Life time  18 ns External field Resonantly Scattered X-rays Time spectrum Pulsed X-rays 5 hrs.       23.87 keV 23.87 keV Co2MnSn （40.2 nm） w/ 119Sn 50% (Prep. by atomically controlled alternate deposition) 119Sn nuclei CEMS spectrum (w/ source)

  18. “Ultra-thin” Co2MnSn films for Mössbauer measurements Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Co Co Mn Mn Mn Mn Mn Mn Mn Mn & & & & & & & & Sn Sn Sn Sn Sn Sn Sn Sn Co Co @ 400oC Co Co x 12 x 6 Co Co x 6 Cr x 4 Cr Cr Cr Cr 119Sn total 1.2 nm （~ 6atomic layers）

  19. Nuclear resonant time spectrum for “ultra-thin” Co2MnSn films Measurement time~3hrs./ each Mn Mn Mn Mn Mn Mn Mn Mn & & & & & & & & Sn Sn Sn Sn Sn Sn Sn Sn Co Co Co Co Co Co Co 119Sn total 1.2 nm （~ 6atomic layers） Co Smaller hyperfine fields

  20. Setups of synchrotron-based Mössbauer spectroscopy For the investigations on thin films Detector High-resolution monochromator E  meV Time spectra High efficiency Wide element-applicability Difficult distribution analysis Thin film sample Monochromator Pulsed X-rays Detector E  neV Nuclear Bragg monochromator 57FeBO3 High-resolution monochromator Energy spectra High efficiency Limited element-applicability (57Fe only) Easy distribution analysis Thin film sample H Monochromator Pulsed X-rays Oscillation (Super-monochromatization + Doppler shift) Energy spectra Detector Medium efficiency Wide element-applicability Easy distribution analysis Standard absorber (eg. CaSnO3 ) High-resolution monochromator Resonant X-rays Thin film sample Monochromator Pulsed X-rays Oscillation （Energy dip in neV order + Doppler shift）

  21. 57FeBO3 single crystal Single-line Mössbauer source using nuclear Bragg reflection Use of electronically forbidden but nuclear allowed Bragg reflection from an antiferromagnetic single crystal kept near the Néel temperature Monochromatized incident beam ΔE > meV Ee1 (Available only for 57Fe) Super-monochromatized beam ΔE ~ 10 neV 14.4keV Eg + Doppler shift with oscillating the 57FeBO3 or the sample  Spectrum in energy domain G. V. Smirnov et al., JETP Lett. 43, 352(1986). A. I. Chumakov et al., Phys. Rev. B 41, 9545(1990). G. V. Smirnov et al., Phys. Rev. B 55, 5811(1997). G. V. Smirnov, Hyperfine Interactions 125, 91(2000).

  22. Strategy for the use of nuclear Bragg monochromator for the studies on thin films and nano-structures Detector E  neV Nuclear Bragg monochromator 57FeBO3 High-resolution monochromator Thin film sample H Key techniques: T. Mitsui, M. Seto, R. Masuda, Jpn. J. Appl. Phys. 46 L930 (2007) 1. Use of a top-quality 57FeBO3 single crystal for intense super-monochromatized beams 2. Realization of energetically-modulated monochromatized beams at a fixed angle and position Monochromator Pulsed X-rays Oscillation (Super-monochromatization + Doppler shift)

  23. Measurements of57Fe monolayer using a radioactive source MgO(001)/Cr(1.0 nm)/ 56Fe(10.0 nm)/57Fe(0.2 nm)/Cr(1.0 nm) 57Fe 56Fe Cr MgO(001) CEMS w/ a radioactive source (46 mCi (1.7 GBq), 7 days, RT) Effect = 0.36%

  24. Measurements of57Fe monolayer using nuclear Bragg monochnomator MgO(001)/Cr(1.0 nm)/ 56Fe(10.0 nm)/57Fe(0.2 nm)/Cr(1.0 nm) 57Fe Detector 56Fe Cr E  neV Nuclear Bragg monochromator 57FeBO3 MgO(001) High-resolution monochromator CEMS w/ a radioactive source (46 mCi (1.7 GBq), 7 days, RT) Effect = 0.36% Synchrotronw/ nuclear Bragg monochnomator (Incident angle ~ 1.6o, Measurement ~ 3 hrs) Thin film sample H Monochromator Pulsed X-rays Oscillation (Super-monochromatization + Doppler shift) Counts Velocity (mm/s)

  25. Setups of synchrotron-based Mössbauer spectroscopy For the investigations on thin films Detector High-resolution monochromator E  meV Time spectra High efficiency Wide element-applicability Difficult distribution analysis Thin film sample Monochromator Pulsed X-rays Detector E  neV Nuclear Bragg monochromator 57FeBO3 High-resolution monochromator Energy spectra High efficiency Limited element-applicability (57Fe only) Easy distribution analysis Thin film sample H Monochromator Pulsed X-rays Oscillation (Super-monochromatization + Doppler shift) Energy spectra Detector Medium efficiency Wide element-applicability Easy distribution analysis Standard absorber (eg. CaSnO3 ) High-resolution monochromator Resonant X-rays Thin film sample Monochromator Pulsed X-rays Oscillation （Energy dip in neV order + Doppler shift）

  26. New method for SR Mössbauer spectroscopy in energy domain Detector Transmitter Scatterer Counts M. Seto, et al., Phys. Rev. Lett. 102, 217602 (2009) Relative velocity * The transmitter can be a sample and the scatterer can be a standard absorber, or vice versa

  27. New method for SR Mössbauer spectroscopy in energy domain Detector Transmitter Scatterer Counts M. Seto, et al., Phys. Rev. Lett. 102, 217602 (2009) Relative velocity * The transmitter can be a sample and the scatterer can be a standard absorber, or vice versa

  28. New method for SR Mössbauer spectroscopy in energy domain Detector Transmitter Scatterer Counts M. Seto, et al., Phys. Rev. Lett. 102, 217602 (2009) Relative velocity * The transmitter can be a sample and the scatterer can be a standard absorber, or vice versa

  29. New method for SR Mössbauer spectroscopy in energy domain Setups for thin films Detector Standard absorber (eg. CaSnO3 ) High-resolution monochromator Resonant X-rays Thin film sample Monochromator Pulsed X-rays Oscillation

  30. SR Mössbauer spectrum of a ultra-thin Co2MnSn film in energy domain Cr Cr Cr Cr Time spectra Energy spectrum （@ 300 K） Cr Cr ~3 hrs. Cr Cr Cr Cr Co Mn & Sn Co 35 hrs. x 12 or x 24 119Sn total 1.2 nm (~ 6atomic layer) or 2.4 nm (~ 12atomic layer) Detector Standard absorber (eg. CaSnO3 ) High-resolution monochromator Resonant X-rays Thin film sample Monochromator Pulsed X-rays Oscillation

  31. SR Mössbauer spectrum of a ultra-thin Co2MnSn film in energy domain Cr Cr Cr Cr Time spectra Energy spectrum （@ 20 K） Cr Cr ~3 hrs. Cr Cr Cr Cr Co Mn & Sn Co 34 hrs. x 12 or x 24 119Sn total 1.2 nm (~ 6atomic layer) or 2.4 nm (~ 12atomic layer) Detector Standard absorber (eg. CaSnO3 ) High-resolution monochromator Resonant X-rays Thin film sample Monochromator Pulsed X-rays Oscillation Still necessary to be improvedfor thin films

  32. Conclusion Synchrotron-radiation-based Mössbauer spectroscopy in energy domain for the studies on thin films and nano-structures From the development and demonstration phase to the application phase for material researches

  33. Thank you

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