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 APS ESRF SPring-8
CREST project for synchrotron-based Mössbauer spectroscopy (CREST = Core Research for Evolutional Science and Technology) FY 2005 ~ 2010 (~ JPY 400, 000 / 5. 5 years) M. Seto, Kyoto University Project Leader Y. Yoda, JASRI/SPring-8 Methodological improvements on the beam line (@BL 09 XU) T. Mitsui, Japan Atomic Energy Agency Methodological improvements on the beam line (@BL 11 XU) 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)
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 & Co 2 Mn. Sn films) Results on Synchrotron-Radiation-Based Mössbauer Spectroscopy * Measurements in time domains for Fe films and Co 2 Mn. Sn films * Measurements in energy domains using a nuclear Bragg monochromator for Fe films * Measurements in energy domains using a standard absorber for Co 2 Mn. Sn films Conclusion
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 me. V even after monochromatization in general) Special techniques are required to use synchrotron radiation as a source for Mössbauer spectroscopy.
Historical Backgrounds * Proposal for the use of synchrotron radiation as a Mössbauer source S. L. Ruby Journal de Physique, Colloq. 35, C 6 - 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. Hannon Phys. Rev. Lett. 54, 835 (1985) * Observation of time spectra for nuclear forward scattering (NFS) of synchrotron radiation J. B. Hastings, D. P. Siddons, U. van Bürck, R. Hollatz, U. Bergmann Phys. 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. Yoda Phys. Rev. Lett. 102, 217602 (2009)
Conventional Mössbauer spectroscopic setups for thin films and nano-structures on single crystal substrates Transmission geometry Oscillation -rays Proportional counter Sample Radioactive source Scattering geometry (CEMS geometry) Proportional counter -rays Radioactive source - rays 57 Fe 14. 4 ke. V for or 23. 8 ke. V for 119 Sn e- Sample Counts Oscillation * Small sample house full of counter gas Internal conversion electrons Doppler velocity (mm/s) (Energy of -rays) Conversion electrons 7. 3 ke. V for 57 Fe or 19. 4 ke. V for 119 Sn (@ Nagoya Inst. Tech. ) Low-temperature CEMS system ~ 70 K (w/ He+2%CH 4 gas) ~ 30 K (w/ H 2 gas)
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 Beam Sample N Detector S Magnetic field
Test samples and Mössbauer nuclei (1) Fe firms, nano-structures, monatomic layers For 57 Fe Mössbauer spectroscopy -rays 14. 4 ke. V Counts Calculated CEMS spectrum Fe 33 T Doppler velocity (mm/s) (Energy of -rays) 57 Fe nuclear energy levels (2) Co 2 Mn. Sn Heusler alloy films For 119 Sn Mössbauer spectroscopy Calculated CEMS spectra for various hyperfine fields -rays 23. 8 ke. V Co Mn Sn Counts 0 T 5 T 10 T 15 T 119 Sn nuclear energy levels Doppler velocity (mm/s) (Energy of -rays)
Background of the test samples Heusler alloys with an L 21 -type structure * Theoretically predicted to be “half metallic” (or w/ highly spin-polarized conduction electrons) * Promising materials for spin-electronics devices Strategy Preparation of L 21 -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) X (Co etc. ) Y (Mn etc. ) Z (Sn etc. ) Co 2 Mn. Sn (Tc = 811 K, 5 B)
Mössbauer spectra for Co 2 Mn. Sn measured using a radioactive source Bulk Co 2 Mn. Sn alloy prepared by arc-melting (Long range L 21 order parameter = 0. 9) L 21 B 2 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. Gavriliuk et al. , J. Appl. Phys. 77 (1995) 2648. Co Mn Sn
Mössbauer spectra for Co 2 Mn. Sn measured using a radioactive source L 21 Bulk Co 2 Mn. Sn alloy prepared by arc-melting (Long range L 21 order parameter = 0. 9) B 2 Co 2 Mn. Sn(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 Co 2 Mn. Sn!! Co Mn Sn
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 & Co 2 Mn. Sn films) Results on Synchrotron-Radiation-Based Mössbauer Spectroscopy * Measurements in time domains for Fe films and Co 2 Mn. Sn films * Measurements in energy domains using a nuclear Bragg monochromator for Fe films * Measurements in energy domains using a standard absorber for Co 2 Mn. Sn films Conclusion
Setups of synchrotron-based Mössbauer spectroscopy For the investigations on thin films High-resolution monochromator Pulsed X-rays Monochromator Detector E me. V Time spectra High efficiency Wide element-applicability Difficult distribution analysis Thin film sample Detector High-resolution monochromator Pulsed X-rays Monochromator High-resolution monochromator Nuclear Bragg E ne. V monochromator 57 Fe. BO 3 H Thin film sample Oscillation (Super-monochromatization + Doppler shift) Detector Standard absorber (eg. Ca. Sn. O 3 ) Resonant X-rays Pulsed X-rays Monochromator Thin film sample Oscillation （Energy dip in ne. V order + Doppler shift） Energy spectra High efficiency Limited element-applicability (57 Fe only) Easy distribution analysis Energy spectra Medium efficiency Wide element-applicability Easy distribution analysis
Measurements of nuclear resonant time spectra High-resolution monochromator Pulsed X-rays Detector E me. V Time spectra Thin film sample Monochromator 14. 4 ke. V Dependence of the “quantum beat” patterns on the direction of magnetic hyperfine field (rel. to the beam polarization) Pulsed X-rays 57 Fe nuclei Resonantly scattered X-rays (“delayed” signal) 100 ns * 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. Mater. 282, 329 (2004). Time Intensity (log. Scale) 14. 4 ke. V
Nuclear resonant time spectra for Fe nanowires 30 nm @ BL 11 XU, SPring-8 X-rays (Perpendicular) 57 Fe wire （30 nm wide, 30 nm thick） X-rays (Parallel) 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))
Nuclear resonant time spectrum for a “thick” Co 2 Mn. Sn film High-resolution monochromator E me. V 119 Sn Monochromator Time spectra sample N Pulsed X-rays Detector Cryostat S 23. 87 ke. V Magnet External field Pulsed X-rays 23. 87 ke. V Co 2 Mn. Sn （40. 2 nm） w/ 119 Sn 50% (Prep. by atomically controlled alternate deposition) Life time 18 ns Resonantly Scattered X-rays 23. 87 ke. V 119 Sn nuclei CEMS spectrum (w/ source) Time spectrum 5 hrs.
“Ultra-thin” Co 2 Mn. Sn films for Mössbauer measurements Cr Cr Cr Co Mn & Sn Co @ 400 o. C Mn & Sn Co Cr x 6 Cr Cr Cr Mn & Sn Co Mn & Sn Cr x 4 Cr Cr 119 Sn Cr Cr Cr Co Mn & Sn Co Cr x 12 Cr Cr total 1. 2 nm （~ 6 atomic layers） Cr Cr Cr Mn & Sn Co Mn & Sn Cr x 6 Cr Cr
Nuclear resonant time spectrum for “ultra-thin” Co 2 Mn. Sn films Measurement time ~ 3 hrs. / each Co Mn & Sn Co Mn & Sn Co 119 Sn Mn & Sn Co Mn & Sn total 1. 2 nm （~ 6 atomic layers） Smaller hyperfine fields
Setups of synchrotron-based Mössbauer spectroscopy For the investigations on thin films High-resolution monochromator Pulsed X-rays Monochromator Detector E me. V Time spectra High efficiency Wide element-applicability Difficult distribution analysis Thin film sample Detector High-resolution monochromator Pulsed X-rays Monochromator High-resolution monochromator Nuclear Bragg E ne. V monochromator 57 Fe. BO 3 H Thin film sample Oscillation (Super-monochromatization + Doppler shift) Detector Standard absorber (eg. Ca. Sn. O 3 ) Resonant X-rays Pulsed X-rays Monochromator Thin film sample Oscillation （Energy dip in ne. V order + Doppler shift） Energy spectra High efficiency Limited element-applicability (57 Fe only) Easy distribution analysis Energy spectra Medium efficiency Wide element-applicability Easy distribution analysis
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 > me. V (Available only for 57 Fe) Ee 1 14. 4 ke. V Super-monochromatized beam ΔE ~ 10 ne. V Eg 57 Fe. BO 3 single crystal + Doppler shift with oscillating the 57 Fe. BO 3 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).
Strategy for the use of nuclear Bragg monochromator for the studies on thin films and nano-structures Detector High-resolution monochromator Pulsed X-rays Monochromator Nuclear Bragg E ne. V monochromator 57 Fe. BO 3 H Thin film sample Oscillation (Super-monochromatization + Doppler shift) Key techniques: T. Mitsui, M. Seto, R. Masuda, Jpn. J. Appl. Phys. 46 L 930 (2007) 1. Use of a top-quality 57 Fe. BO 3 single crystal for intense super-monochromatized beams 2. Realization of energetically-modulated monochromatized beams at a fixed angle and position
Measurements of 57 Fe monolayer using a radioactive source Mg. O(001)/Cr(1. 0 nm)/ 56 Fe(10. 0 nm)/ 57 Fe(0. 2 nm)/Cr(1. 0 nm) 56 Fe 57 Fe Cr Mg. O(001) CEMS w/ a radioactive source (46 m. Ci (1. 7 GBq), 7 days, RT) Effect = 0. 36%
Measurements of 57 Fe monolayer using nuclear Bragg monochnomator Mg. O(001)/Cr(1. 0 nm)/ 56 Fe(10. 0 nm)/ 57 Fe(0. 2 nm)/Cr(1. 0 nm) High-resolution monochromator Cr Mg. O(001) Pulsed X-rays Monochromator CEMS w/ a radioactive source (46 m. Ci (1. 7 GBq), 7 days, RT) Effect = 0. 36% Nuclear Bragg E ne. V monochromator 57 Fe. BO 3 H Thin film sample Oscillation (Super-monochromatization + Doppler shift) Synchrotron w/ nuclear Bragg monochnomator (Incident angle ~ 1. 6 o, Measurement ~ 3 hrs) Counts 56 Fe 57 Fe Detector Velocity (mm/s)
Setups of synchrotron-based Mössbauer spectroscopy For the investigations on thin films High-resolution monochromator Pulsed X-rays Monochromator Detector E me. V Time spectra High efficiency Wide element-applicability Difficult distribution analysis Thin film sample Detector High-resolution monochromator Pulsed X-rays Monochromator High-resolution monochromator Nuclear Bragg E ne. V monochromator 57 Fe. BO 3 H Thin film sample Oscillation (Super-monochromatization + Doppler shift) Detector Standard absorber (eg. Ca. Sn. O 3 ) Resonant X-rays Pulsed X-rays Monochromator Thin film sample Oscillation （Energy dip in ne. V order + Doppler shift） Energy spectra High efficiency Limited element-applicability (57 Fe only) Easy distribution analysis Energy spectra Medium efficiency Wide element-applicability Easy distribution analysis
New method for SR Mössbauer spectroscopy in energy domain Detector Scatterer Counts Transmitter 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
New method for SR Mössbauer spectroscopy in energy domain Detector Scatterer Counts Transmitter 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
New method for SR Mössbauer spectroscopy in energy domain Detector Scatterer Counts Transmitter 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
New method for SR Mössbauer spectroscopy in energy domain Setups for thin films Detector High-resolution monochromator Standard absorber (eg. Ca. Sn. O 3 ) Resonant X-rays Pulsed X-rays Thin film sample Monochromator Oscillation
SR Mössbauer spectrum of a ultra-thin Co 2 Mn. Sn film in energy domain Cr Cr Cr Co Mn & Sn Co Cr Cr x 12 Cr or Cr x 24 Cr total 1. 2 nm (~ 6 atomic layer) or 2. 4 nm (~ 12 atomic layer) Time spectra Energy spectrum ~ 3 hrs. （@ 300 K） 35 hrs. Detector 119 Sn High-resolution monochromator Standard absorber (eg. Ca. Sn. O 3 ) Resonant X-rays Pulsed X-rays Thin film sample Monochromator Oscillation
SR Mössbauer spectrum of a ultra-thin Co 2 Mn. Sn film in energy domain Cr Cr Cr Co Mn & Sn Co Cr Cr x 12 Cr or Cr x 24 Cr total 1. 2 nm (~ 6 atomic layer) or 2. 4 nm (~ 12 atomic layer) Time spectra Energy spectrum ~ 3 hrs. （@ 20 K） 34 hrs. Detector 119 Sn High-resolution monochromator Standard absorber (eg. Ca. Sn. O 3 ) Resonant X-rays Pulsed X-rays Thin film sample Monochromator Oscillation Still necessary to be improved for thin films
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
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