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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. - PowerPoint PPT Presentation
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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-8APS
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 UniversityProject Leader
Y. Yoda, JASRI/SPring-8Methodological improvements on the beam line (@BL09XU)
T. Mitsui, Japan Atomic Energy AgencyMethodological improvements on the beam line (@BL11XU)
S. Kishimoto, High Energy Accelerator Research Institute (KEK)Improvements of detector systems
H. Kobayashi, Hyogo Prefectural UniversityApplications to material research (High pressures)
K. Mibu, Nagoya Institute of TechnologyApplications 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 & Co2MnSn films)
Results on 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
- 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, 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. 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 for thin films and nano-structures on single crystal substrates
Low-temperature CEMS system~ 70 K (w/ He+2%CH4 gas)~ 30 K (w/ H2 gas)
Doppler velocity (mm/s)(Energy of -rays)
Cou
nts
Scattering geometry (CEMS geometry)
Transmission geometry
Oscillation
Radioactivesource
-raysSample
Proportional counter
Internal conversion electronse-
Proportionalcounter
Sample
Oscillation
Radioactivesource
-rays
* Small sample house full of counter gas
- rays
14.4 keV for 57Fe or
23.8 keV for 119Sn
Conversion electrons
7.3 keV for 57Feor
19.4 keV for 119Sn
(@ Nagoya Inst. Tech.)
Advantages of synchrotron-radiation-based Mössbauer spectroscopyfor 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
N S
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) Co2MnSn Heusler alloy films
For 57Fe Mössbauer spectroscopy
For 119Sn Mössbauer spectroscopy
-rays 23.8 keV
2
1I
2
3I
119Sn nuclear energy levels
-rays
14.4 keV
2
1I
2
3I
57Fe nuclear energy levels
CoMnSn
-15 -10 -5 0 5 10 15
Velocity (mm/s)
A
bsor
ptio
n
Doppler velocity (mm/s)(Energy of -rays)
Cou
nts
0 T
5 T
10 T
15 T
Calculated CEMS spectrafor various hyperfine fields
Doppler velocity (mm/s)(Energy of -rays)
Cou
nts
Calculated CEMS spectrum
33 T
Fe
Background of the test samples
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)
* Theoretically predicted to be “half metallic” (or w/ highly spin-polarized conduction electrons)
* Promising materials for spin-electronics devices
Heusler alloys with an L21-type structure
X (Co etc.)
Y (Mn etc.)
Z (Sn etc.)
Strategy
S N
N S
電子流
D↓ D↑
F
D↓ D↑
F
Mössbauer spectra for Co2MnSn measured using a radioactive source
CoMnSn
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. Gavriliuk et al., J. Appl. Phys. 77 (1995) 2648.
Co2MnSn(40.2 nm) film prepared by atomically controlled alternate deposition( Tsub = 500 ℃ )
Mössbauer spectra for Co2MnSn measured using a radioactive source
Sharp distribution !!
CoMnSn
Bulk Co2MnSn alloy prepared by arc-melting (Long range L21 order parameter = 0.9)
L21
B2
Anti-site
☞ Available to investigate magnetic interface effect or size effect of Co2MnSn!!
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 & Co2MnSn films)
Results on 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
- Focusing on the measurements of hyperfine fields for thin films -
Setups of synchrotron-based Mössbauer spectroscopy
Energy spectra
Monochromator
Standard absorber(eg. CaSnO3 )
Thin film sample
Detector
PulsedX-rays Oscillation
ResonantX-rays
High-resolutionmonochromator
Energy spectra
( Energy dip in neV order
+ Doppler shift )
Monochromator Thin film sample
Detector
PulsedX-rays
High-resolutionmonochromator E meV Time spectra
High efficiencyWide element-applicabilityDifficult distribution analysis
For the investigations on thin films
High efficiencyLimited element-applicability (57Fe only)Easy distribution analysis
Medium efficiencyWide element-applicabilityEasy distribution analysis
Detector
Monochromator
Nuclear Braggmonochromator
57FeBO3
Thin film sample
PulsedX-rays
Oscillation
High-resolutionmonochromator
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
57Fe nuclei
14.4 keV
100 ns
Resonantly scatteredX-rays
(“delayed” signal)
PulsedX-rays
Monochromator Thin film sample
Detector
PulsedX-rays
High-resolutionmonochromator 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)
57Fe 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, 237201 (2002)) APS group (e.g., C. L’abbé et al., Phys. Rev. Lett. 93, 037201 (2004))
Quantum beat+
Dynamical beat
119Sn 23.87 keV
Time spectrum
5 hrs.
CEMS spectrum
(w/ source)
External field
Cryostat
N S
Magnet
119Sn nuclei
Life time 18 ns
23.87 keV
PulsedX-rays
23.87 keV
Co2MnSn ( 40.2 nm ) w/ 119Sn 50%
(Prep. by atomically controlled alternate deposition)
Nuclear resonant time spectrum for a “thick” Co2MnSn film
Monochromator
sample
Detector
PulsedX-rays
High-resolutionmonochromator E meV Time spectra
ResonantlyScattered X-rays
x 4
Cr
Cr Cr Cr Cr
Cr
Cr Cr Cr Cr
Co Mn Sn& Co Mn Sn& Co
Cr
Cr Cr Cr Cr
Cr
Cr Cr Cr Cr
Mn Sn& Co Mn Sn& Co Mn Sn&
Cr
Cr Cr Cr Cr
Cr
Cr Cr Cr Cr
Co Mn Sn& Co
Cr
Cr Cr Cr Cr
Cr
Cr Cr Cr Cr
Mn Sn& Co Mn Sn&
x 6
x 6x 12@ 400oC
119Sn total 1.2 nm ( ~ 6 atomic layers )
“Ultra-thin” Co2MnSn films for Mössbauer measurements
Co Mn Sn& Co Mn Sn& Co
Mn Sn& Co Mn Sn& Co Mn Sn&
Co Mn Sn& Co
Mn Sn& Co Mn Sn&
119Sn total 1.2 nm ( ~ 6 atomic layers )
Nuclear resonant time spectrum for “ultra-thin” Co2MnSn films
Measurement time ~ 3 hrs./ each
Smaller hyperfine fields
Setups of synchrotron-based Mössbauer spectroscopy
Monochromator Thin film sample
Detector
PulsedX-rays
High-resolutionmonochromator E meV Time spectra
Energy spectra
Monochromator
Standard absorber(eg. CaSnO3 )
Thin film sample
Detector
PulsedX-rays Oscillation
ResonantX-rays
High-resolutionmonochromator
Energy spectra
( Energy dip in neV order
+ Doppler shift )
High efficiencyWide element-applicabilityDifficult distribution analysis
For the investigations on thin films
High efficiencyLimited element-applicability (57Fe only)Easy distribution analysis
Medium efficiencyWide element-applicabilityEasy distribution analysis
Detector
Monochromator
Nuclear Braggmonochromator
57FeBO3
Thin film sample
PulsedX-rays
Oscillation
High-resolutionmonochromator
E neV
(Super-monochromatization
+ Doppler shift)
H
57FeBO3
single crystal
Monochromatized incident beam
ΔE > meVSuper-monochromatized
beamΔE ~ 10 neV
Ee1
Eg
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 57Fe)
+ Doppler shift with oscillating the 57FeBO3 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 57FeBO3 single crystal for intense super-monochromatized beams
2. Realization of energetically-modulated monochromatized beams at a fixed angle and position
Detector
Monochromator
Nuclear Braggmonochromator
57FeBO3
Thin film sample
PulsedX-rays
Oscillation
High-resolutionmonochromator
E neV
(Super-monochromatization
+ Doppler shift)
H
Measurements of 57Fe monolayer using a radioactive source
0 10 20 30 40
0.0
0.1
Hyperfine field (T)
Dis
trib
utio
n
MgO(001)/Cr(1.0 nm)/ 56Fe(10.0 nm)/ 57Fe(0.2 nm)/Cr(1.0 nm)
Cr
57Fe
MgO(001)
56Fe
CEMS w/ a radioactive source(46 mCi (1.7 GBq), 7 days, RT)
Effect = 0.36%
Measurements of 57Fe monolayer using nuclear Bragg monochnomator
MgO(001)/Cr(1.0 nm)/ 56Fe(10.0 nm)/ 57Fe(0.2 nm)/Cr(1.0 nm)
Cr
57Fe
MgO(001)
56Fe
CEMS w/ a radioactive source(46 mCi (1.7 GBq), 7 days, RT)
Effect = 0.36%
Synchrotron w/ nuclear Bragg monochnomator
(Incident angle ~ 1.6o, Measurement ~ 3 hrs)
Velocity (mm/s)
Cou
nts
Detector
Monochromator
Nuclear Braggmonochromator
57FeBO3
Thin film sample
PulsedX-rays
Oscillation
High-resolutionmonochromator
E neV
(Super-monochromatization
+ Doppler shift)
H
Setups of synchrotron-based Mössbauer spectroscopy
Monochromator Thin film sample
Detector
PulsedX-rays
High-resolutionmonochromator E meV Time spectra
Energy spectra
High efficiencyWide element-applicabilityDifficult distribution analysis
For the investigations on thin films
High efficiencyLimited element-applicability (57Fe only)Easy distribution analysis
Monochromator
Standard absorber(eg. CaSnO3 )
Thin film sample
Detector
PulsedX-rays Oscillation
ResonantX-rays
High-resolutionmonochromator
Energy spectra
( Energy dip in neV order
+ Doppler shift )
Medium efficiencyWide element-applicabilityEasy distribution analysis
Detector
Monochromator
Nuclear Braggmonochromator
57FeBO3
Thin film sample
PulsedX-rays
Oscillation
High-resolutionmonochromator
E neV
(Super-monochromatization
+ Doppler shift)
H
New method for SR Mössbauer spectroscopy in energy domain
Transmitter Scatterer
Detector
Cou
nts
Relative velocity M. Seto, et al., Phys. Rev. Lett. 102, 217602 (2009) * The transmitter can be a sample and the scatterer can be a standard absorber, or vice versa
Transmitter Scatterer
Detector
Cou
nts
Relative velocity
New method for SR Mössbauer spectroscopy in energy domain
M. Seto, et al., Phys. Rev. Lett. 102, 217602 (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
Transmitter Scatterer
Detector
Cou
nts
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, 217602 (2009)
Monochromator
Standard absorber(eg. CaSnO3 )
Thin film sample
Detector
PulsedX-rays Oscillation
ResonantX-rays
High-resolutionmonochromator
New method for SR Mössbauer spectroscopy in energy domain
Setups for thin films
Cr
Cr Cr Cr Cr
Cr
Cr Cr Cr Cr
Co Mn Sn& Co
x 12or
x 24
SR Mössbauer spectrum of a ultra-thin Co2MnSn film in energy domain
Time spectraEnergy spectrum
( @ 300 K )
35 hrs.
119Sn total 1.2 nm
(~ 6 atomic layer)or
2.4 nm(~ 12 atomic layer)
~ 3 hrs.
Monochromator
Standard absorber(eg. CaSnO3 )
Thin film sample
Detector
PulsedX-rays Oscillation
ResonantX-rays
High-resolutionmonochromator
-15 -10 -5 0 5 10 1535,500
36,000
36,500
37,000
37,500
38,000
38,500
Velocity (mm/s)
C
ou
nts
Cr
Cr Cr Cr Cr
Cr
Cr Cr Cr Cr
Co Mn Sn& Co
x 12or
x 24
SR Mössbauer spectrum of a ultra-thin Co2MnSn film in energy domain
Time spectraEnergy spectrum
( @ 20 K )
34 hrs.
119Sn total 1.2 nm
(~ 6 atomic layer)or
2.4 nm(~ 12 atomic layer)
~ 3 hrs.
Monochromator
Standard absorber(eg. CaSnO3 )
Thin film sample
Detector
PulsedX-rays Oscillation
ResonantX-rays
High-resolutionmonochromator
-15 -10 -5 0 5 10 15
20,200
20,400
20,600
20,800
21,000
21,200
21,400
21,600
21,800
Velocity (mm/s)
C
ou
nts
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
Thank you
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