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

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


Cou

nts

0 T

5 T

10 T

15 T

Calculated CEMS spectrafor various hyperfine fields


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


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


Detector

PulsedX-rays


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


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



Detector

PulsedX-rays


Energy spectra

Monochromator


Thin film sample

Detector


ResonantX-rays


Energy spectra


+ Doppler shift ）





Detector

Monochromator


57FeBO3

Thin film sample

PulsedX-rays

Oscillation


E neV


+ 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


57FeBO3

Thin film sample

PulsedX-rays

Oscillation


E neV


+ 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


57FeBO3

Thin film sample

PulsedX-rays

Oscillation


E neV


+ Doppler shift)

H



Detector

PulsedX-rays


Energy spectra




Monochromator


Thin film sample

Detector


ResonantX-rays


Energy spectra


+ Doppler shift ）


Detector

Monochromator


57FeBO3

Thin film sample

PulsedX-rays

Oscillation


E neV


+ 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


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



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


Thin film sample

Detector


ResonantX-rays



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


Thin film sample

Detector


ResonantX-rays


-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


Thin film sample

Detector


ResonantX-rays


-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

hfi/nqi 2010 (september 14, 2010, geneva)

Documents

use of synchrotron radiation

mssbauer source

based mssbauer spectroscopyk

films advantages

energy domains

films nanostructurescrest

core research

time domains