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

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

CREST project for synchrotron-based Mössbauer spectroscopy 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 T. Mitsui, Japan Atomic Energy Agency Methodological improvements on the beam line 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) (CREST = Core Research for Evolutional Science and Technology)

Outline 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 MnSn films) Results on Synchrotron-Radiation-Based Mössbauer Spectroscopy * Measurements in time domains for Fe films and Co 2 MnSn films * Measurements in energy domains using a nuclear Bragg monochromator for Fe films * Measurements in energy domains using a standard absorber for Co 2 MnSn films Conclusion - Focusing on the measurements of hyperfine fields for thin films -

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

Historical Backgrounds * Proposal for the use of synchrotron radiation as a Mössbauer source S. L. Ruby Journal de Physique, Colloq. 35, C (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, (2009)

Conventional Mössbauer spectroscopic setups for for thin films and nano-structures on single crystal substrates Low-temperature CEMS system ~ 70 K (w/ He+2%CH 4 gas) ~ 30 K (w/ H 2 gas) Doppler velocity (mm/s) (Energy of  -rays) Counts Scattering geometry (CEMS geometry) Transmission geometry Oscillation Radioactive source  -rays Sample Proportional counter Internal conversion electrons e-e- Proportional counter Sample Oscillation Radioactive source  -rays * Small sample house full of counter gas  - rays 14.4 keV for 57 Fe or 23.8 keV for 119 Sn   Conversion electrons 7.3 keV for 57 Fe or 19.4 keV for 119 Sn Nagoya Inst. Tech.)

Advantages of synchrotron-radiation-based Mössbauer spectroscopy for the investigations on thin films and nano-structures 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 Magnetic field Sample Detector Cryostat NS Magnet Beam - In comparison with conversion electron Mössbauer spectroscopy (CEMS) using a radioactive source and a proportional gas counter –

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

Background of the test samples Preparation of L2 1 -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) Co 2 MnSn (T c = 811 K,   5  B ) * Theoretically predicted to be “half metallic” (or w/ highly spin-polarized conduction electrons) * Promising materials for spin-electronics devices Heusler alloys with an L2 1 -type structure X (Co etc.) Y (Mn etc.) Z (Sn etc.) Strategy

Mössbauer spectra for Co 2 MnSn measured using a radioactive source Co Mn Sn Bulk Co 2 MnSn alloy prepared by arc-melting (Long range L2 1 order parameter = 0.9) L2 1 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. Gavriliuk et al., J. Appl. Phys. 77 (1995) 2648.

Co 2 MnSn(40.2 nm) film prepared by atomically controlled alternate deposition （ T sub = 500 ℃ ） Mössbauer spectra for Co 2 MnSn measured using a radioactive source Sharp distribution !! Co Mn Sn Bulk Co 2 MnSn alloy prepared by arc-melting (Long range L2 1 order parameter = 0.9) L2 1 B2 Anti -site ☞ Available to investigate magnetic interface effect or size effect of Co 2 MnSn!!

Outline 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 MnSn films) Results on Synchrotron-Radiation-Based Mössbauer Spectroscopy * Measurements in time domains for Fe films and Co 2 MnSn films * Measurements in energy domains using a nuclear Bragg monochromator for Fe films * Measurements in energy domains using a standard absorber for Co 2 MnSn films Conclusion - Focusing on the measurements of hyperfine fields for thin films -

Setups of synchrotron-based Mössbauer spectroscopy Energy spectra Monochromator Standard absorber (eg. CaSnO 3 ) Thin film sample Detector Pulsed X-rays Oscillation Resonant X-rays High-resolution monochromator Energy spectra （ Energy dip in neV order + Doppler shift ） Monochromator Thin film sample Detector Pulsed X-rays High-resolution monochromator  E  meV Time spectra High efficiency Wide element-applicability Difficult distribution analysis For the investigations on thin films High efficiency Limited element-applicability ( 57 Fe only) Easy distribution analysis Medium efficiency Wide element-applicability Easy distribution analysis Detector Monochromator Nuclear Bragg monochromator 57 FeBO 3 Thin film sample Pulsed X-rays Oscillation High-resolution monochromator  E  neV (Super-monochromatization + Doppler shift) H

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

Nuclear resonant time spectra for Fe nanowires  30 nm X-rays (Parallel) X-rays (Perpendicular) 57 Fe wire （ 30 nm wide, 30 nm thick BL11XU, SPring-8 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, (2002)) APS group (e.g., C. L’abbé et al., Phys. Rev. Lett. 93, (2004)) Quantum beat + Dynamical beat

119 Sn keV Time spectrum 5 hrs. CEMS spectrum (w/ source) External field Cryostat NS Magnet 119 Sn nuclei Life time  18 ns keV Pulsed X-rays   keV Co 2 MnSn （ 40.2 nm ） w/ 119 Sn 50% (Prep. by atomically controlled alternate deposition) Nuclear resonant time spectrum for a “thick” Co 2 MnSn film Monochromator sample Detector Pulsed X-rays High-resolution monochromator  E  meV Time spectra Resonantly Scattered X-rays

x 4 Cr Co MnSn & Co MnSn & Co Cr MnSn & Co MnSn & Co MnSn & Cr Co MnSn & Co Cr MnSn & Co MnSn & x 6 x 400 o C 119 Sn total 1.2 nm （ ~ 6 atomic layers ） “Ultra-thin” Co 2 MnSn films for Mössbauer measurements

Co MnSn & Co MnSn & Co MnSn & Co Mn Sn & Co MnSn & Co MnSn & Co Mn Sn & Co MnSn & 119 Sn total 1.2 nm （ ~ 6 atomic layers ） Nuclear resonant time spectrum for “ultra-thin” Co 2 MnSn films Measurement time ~ 3 hrs./ each Smaller hyperfine fields

Setups of synchrotron-based Mössbauer spectroscopy Monochromator Thin film sample Detector Pulsed X-rays High-resolution monochromator  E  meV Time spectra Energy spectra Monochromator Standard absorber (eg. CaSnO 3 ) Thin film sample Detector Pulsed X-rays Oscillation Resonant X-rays High-resolution monochromator Energy spectra （ Energy dip in neV order + Doppler shift ） High efficiency Wide element-applicability Difficult distribution analysis For the investigations on thin films High efficiency Limited element-applicability ( 57 Fe only) Easy distribution analysis Medium efficiency Wide element-applicability Easy distribution analysis Detector Monochromator Nuclear Bragg monochromator 57 FeBO 3 Thin film sample Pulsed X-rays Oscillation High-resolution monochromator  E  neV (Super-monochromatization + Doppler shift) H

57 FeBO 3 single crystal Monochromatized incident beam ΔE > meV Super-monochromatized beam ΔE ~ 10 neV E e1 EgEg 14.4keV 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). 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 (Available only for 57 Fe) + Doppler shift with oscillating the 57 FeBO 3 or the sample  Spectrum in energy domain

Strategy for the use of nuclear Bragg monochromator for the studies on thin films and nano-structures Key techniques: T. Mitsui, M. Seto, R. Masuda, Jpn. J. Appl. Phys. 46 L930 (2007) 1. Use of a top-quality 57 FeBO 3 single crystal for intense super-monochromatized beams 2. Realization of energetically-modulated monochromatized beams at a fixed angle and position Detector Monochromator Nuclear Bragg monochromator 57 FeBO 3 Thin film sample Pulsed X-rays Oscillation High-resolution monochromator  E  neV (Super-monochromatization + Doppler shift) H

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

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

Setups of synchrotron-based Mössbauer spectroscopy Monochromator Thin film sample Detector Pulsed X-rays High-resolution monochromator  E  meV Time spectra Energy spectra High efficiency Wide element-applicability Difficult distribution analysis For the investigations on thin films High efficiency Limited element-applicability ( 57 Fe only) Easy distribution analysis Monochromator Standard absorber (eg. CaSnO 3 ) Thin film sample Detector Pulsed X-rays Oscillation Resonant X-rays High-resolution monochromator Energy spectra （ Energy dip in neV order + Doppler shift ） Medium efficiency Wide element-applicability Easy distribution analysis Detector Monochromator Nuclear Bragg monochromator 57 FeBO 3 Thin film sample Pulsed X-rays Oscillation High-resolution monochromator  E  neV (Super-monochromatization + Doppler shift) H

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

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

Monochromator Standard absorber (eg. CaSnO 3 ) Thin film sample Detector Pulsed X-rays Oscillation Resonant X-rays High-resolution monochromator New method for SR Mössbauer spectroscopy in energy domain Setups for thin films

Cr Co MnSn & Co x 12 or x 24 SR Mössbauer spectrum of a ultra-thin Co 2 MnSn film in energy domain Time spectra Energy spectrum 300 K ） 35 hrs. 119 Sn total 1.2 nm (~ 6 atomic layer) or 2.4 nm (~ 12 atomic layer) ~ 3 hrs. Monochromator Standard absorber (eg. CaSnO 3 ) Thin film sample Detector Pulsed X-rays Oscillation Resonant X-rays High-resolution monochromator

Cr Co MnSn & Co x 12 or x 24 SR Mössbauer spectrum of a ultra-thin Co 2 MnSn film in energy domain Time spectra Energy spectrum 20 K ） 34 hrs. 119 Sn total 1.2 nm (~ 6 atomic layer) or 2.4 nm (~ 12 atomic layer) ~ 3 hrs. Monochromator Standard absorber (eg. CaSnO 3 ) Thin film sample Detector Pulsed X-rays Oscillation Resonant X-rays High-resolution monochromator Still necessary to be improved for thin films

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

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