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The role of EXAFS in some condensed matter systems: Challenges and opportunities for
NSLS II
Vince HarrisDepartment of Electrical and Computer Engineering
Northeastern UniversityBoston, MA 02115 USA
Date: Jan. 16, 2008
Many contributions from:• Aria Yang• Scott Calvin• Bruce Ravel• Joe Woicik• Trevor Tyson• Dave Pappas• Many studies have benefited from the beamline
expertise of John Kirkland and Barry Karlin• Maybe as many as 50-75 coauthors to
acknowledgde!
Nanoelectronics
• Similar Moore’s trend is seen in the minimum feature size.
• Gate oxide barriers are below 10nm. 1 nm in some cases.
• How does one study materials properties at these length scales?
• How does synchrotron radiation tools help address these needs?– Phase ID– Atomic fraction
measurements– Bond lengths– Coordination– Phase stability– Cation site distribution and
valency
Other Moore technologies•Minimum feature size of ICs•Magnetic storage media•Digital photography pixel density•CPU speed…
• The study of thin films– NSLS I
• Focused light beamlines versus nonfocus beamlines– Metallization of diamond (Spintronics)
– Crystallization of thin films (phase change)
– Strained semiconductors (Nanoelectronics)
– Buried layers (Nanoelectronics)
– Cation distribution in oxides (Microwave electronics)
– Issues• Sample size• Concentration• Thickness• Beam stability• Dynamics
Scope
Experimental detailsMBE grown Fe (0.8nm)
EXAFS measurements on X23B NSLSSample size is 2 mm x 3 mm
TEY, room temp, std. conditionsSpot size ~250 m x 250 m
FEFF modeling (FEFF3?)Goal
Determine nature of metallizationStructure identification
Structure stability: bcc vs fccBond distances, coordination
PRB Brief Report 52(11) 7890 (1995)
Experimental detailsMBE grown Fe (0.8nm)
EXAFS measurements on X23B NSLSSample size is 2 mm x 3 mm
TEY, room temp, std. conditionsSpot size ~250 m x 250 m
FEFF modeling (FEFF3?)Goal
Determine nature of metallizationStructure identification
Structure stability: bcc vs fccBond distances, coordination
PRB Brief Report 52(11) 7890 (1995)
ResultsFEFF modeling (FEFF 3.11 single scattering)
determines Fe-Fe and Fe-C bonds2-11 Å-1 (k2)
Fe-Fe: 2.51 +/- 0.02 Å (~1% in-plane contraction of lattice)Fe-C: 1.84 +/- 0.02 Å (~2.7 atoms)
(Fe-C 1.98Å (Fe2C); 2.14Å (Fe3C))Growth of single crystal austenite
ChallengesVery thin films (6-8 Å, 4-5 ml)
Small and very valuable substratesSynthetic diamond 2 mm x 3 mm substrates
Beam spot size: 250 m x 250 mIf beam walks, this is a nightmare!
What would be great?In situ measurements during growth
In-plane vs out-of-plane measurements
What is needed?Higher countsSmaller spotsBeam stability
Linearity in detector circuitry, challenging.
PRB Brief Report 52(11) 7890 (1995)
Phys. Rev. B43, 3, 15 Jan. 1991 (Brief Report)
GoalsNature of strain in GeSi epitaxial films
ExperimentalSamples are 34 nm GeSi (31% Ge)
CVD grown filmsIn-plane vs out-of-plane bond distances
and lattice parametersEXAFS using FYSample spinning
X23A2 beam does not walkGood linearity and reproducibilityTypically JW many scans merged
ResultsRaw data with the first neighbor signal superimposed
Bottom image is the fit to the FF dataVery accurate bond distances
Strain is distributed in 2nd and 3rd neighbors, not so much in the NN
•JW often uses 50-75 mm diameter •wafer substrates with wide beam spot
Appl. Phys. Lett., Vol. 73, No. 9, 31 August 1998
Goals and SamplesInvestigate nature of local
distortions in buried layers (InP caps)Important semiconductor materialsSamples are in some cases 5-6 nm
MOCVD processed
ExperimentalFluorescent yield
Spinning samples, wobbleIn-plane and glancing incidence
ResultsRaw data with best fit of NN bonds
FindingsIn-As and Ga-As bonds are measured
to a high accuracyEpitaxial films experience tetragonal
distortions except for x=0.53 sample which had a perfect lattice match to the substrate
NoteworthyLarge substrates
Can this be done on small substrates…probably not
Appl. Phys. Lett., Vol. 73, No. 9, 31 August 1998
Appl. Phys. Lett. 68 (15), 8 April 1996
Experimental detailsIon beam sputtered Fe80B20 Metglass™ films
15 nm thickAnnealed at different temperatures and different times
EXAFS measurements on X23B NSLSSample size is 5 mm x 5 mm
TEY, room temp, std. conditionsSpot size ~1 mm x 1 mm
No FEFF modeling Linear modeling of standards (poor mans PCA)
GoalDetermine phases present
Determine atomic fraction of each Structural stability
Onset of crystallization
Error bars determined by background removal and counting statistics
ResultsMass of sampled data: ~1 nanogram
DSC and DTA have 10-3-10-4 gram sensitivityStandards’ data are fit linearly with the
following adjustable parameters:-atomic fraction of each phase
-Debye-Waller coefficient of the amorphous phaseError bars
Determined by fitting the mean, +/- standard deviationsSystematic variation of atom fractions
until 2 doubles (this is larger)Counting statistics (10 scans of each sample)
result in mean and standard deviationsFindings
Onset of crystallizations is 200K below bulk samplesEffective technique for the accurate measurement of phases
I wish we could have….In situ measurements would allow for the
calculation of activation energies(in situ heating and film growth)
The stability of amorphous phases in small volumes are important for understanding
the limits of phase change media
Appl. Phys. Lett. 68 (15), 8 April 1996
After 8 minutes the Fe is fully crystallized as DO3 (bcc FeSi ordered) phase of ~12-15 nm.
On the other hand, the Cu atoms never grow beyond clusters that are ~ 1nm.
Clusters have an close packed (fcc) symmetry
IEEE TRANSACTIONS ON MAGNETICS. VOL. 29. NO. 6. 1993
ExperimentalMelt spun ribbon samples having excellent soft magnetic properties
Amorphous as spunCrystallized by annealing at
550oC (opt. T) for various timesEXAFS transmission, RT
GoalsDoes the 1% Cu make a difference and how?
EXAFS detailsRibbons: 25 mm thick, 2 mm wide, 20 mm long
Problems with sample uniformityMapping the transmitted beam we find regions of uniform
thickness that are about 300 mm x 1000 mmFe data is easy but of little interest
Cu data is challenging The beam walks a little due to large moment arm
EXAFS analysis is largely examining Fourier transforms
Appt. Phys. Lett. 64 (8), 21 February 1994
FindingsThe presence of Nb limits the nucleation
and growth of the Cu atomsWithout Nb, the Cu is fully crystallized after
2 minutes at temperatureThe Cu atoms and clusters act as
nucleation sites for the DO3 FeSi grains
How to improve measurements…Small spot, more light, stable beam position
In situ heatingSometimes samples are what they are….
0% Nb3% Nb
The role of 3% Nb
R
R*
S
R
R*
S
Pyramidal1
Octahedral9
Tetrahedral2Magnetoplumbite
Octahedral2
Tetrahedral1Spinel
SymmetryNumber of ions p.u.c.
Structure
Pyramidal1
Octahedral9
Tetrahedral2Magnetoplumbite
Octahedral2
Tetrahedral1Spinel
SymmetryNumber of ions p.u.c.
Structure
Magnetoplumbite structure (Ba M type)
Spinel structure0
1
2
3
4
5
6
7
8
0 1 2 3 4 5 6 7 8
Four
ier T
rans
form
Am
plitu
de
Radial Coordinate (Å)
Fe EXAFSFe
3O
4 powder std.
w/ statistical error
Fe(B
)-O
Fourier transformed Fe EXAFS from Fe3O4 standard illustrating pair correlations leading to A- & B-site fingerprinting
Partial Radial Distribution Function
IEEE Trans. Magn. 31, 6, Nov. 1995
GoalsThe goal is to gather information on both the
element specific cation distribution and valencyThis is true for the manganites as well
Magnetic and electronic properties
Experimental detailsSamples are ferrite films prepared by SSP or PLD
Size is typically 3-5 mm on a sideThickness is typically 100-200 nm
EXAFS: FY or TEY
ResultsAs shown the peaks from 2-4 Å experience systematic changes
FEFF 6: Multiple scattering fits without FEFFITFirst determination of cation site distribution in ferrites
(one step better than ME)Findings allow interpretation of M vs y
trends with increasing ZnThese results have provided a methodology to correlate SRO properties to fundamental magnetic properties in the ferrites
What we really need is…Site specific, element specific, cation distribution and valency…Appl. Phys. Lett. 68 (15), 8 April 1996
Other ferrite systems studied by NRL and NEU researchers since 2000
PLD Cu-ferriteAria Yang, et al., Appl. Phys. Lett.,
86, 252510 (2005).
ATLAD Cu-ferriteAria Yang, et al., J. Appl. Phys.,
accepted, 2008.
PLD and ATLAD Mn-ferriteAria Yang, et al., IEEE Trans. on Magnetics, 42 (10), 2870 (2006);
Aria Yang, et al., IEEE Trans. On Magn., vol. 40, pp. 2802-2805, 2004;
Xu Zuo, et al., Appl. Phys. Lett., 87, 152505 (2005)
BM NP Zn-ferriteS.A. Oliver, V.G. Harris, H. Hamdeh
and J.C. Ho, Appl. Phys. Lett., 76 (19), 2761 (2000).
NP Mn-ferrite (polyol)C. N. Chinnasamy, et al.,J. Appl.
Phys. 101, 09M509 (2007).
NP MnZn-ferrite (reverse micelle)S. Calvin, E.E. Carpenter, B. Ravel, V.G. Harris, S.A. Morrison, Appl. Phys. Lett.,
81(20), 3828 (2002).
NP Co-ferriteK. Giri, E. M. Kirkpatrick, P. Moongkhamklang, S. A.
Majetich, and V.G. Harris, Appl. Phys. Lett. 80, 2341 (2002);
NP Co-ferrite (reverse micelle)Sichu Li, et al., IEEE Trans. On Magn., vol. 37,
No. 4, 2350-2352 (2001); Sichu Li, et al., J. Appl. Phys., 87 (9), 6223 (2000).
PLD Co-ferriteG. Hu, V.G. Harris, and Y. Suzuki, IEEE
Trans. On Magn., vol. 37, No. 4, 2347-2349 (2001); G. Hu, T.K. Nath, C.B. Eom and
V.G. Harris, Y. Suzuki, Phys. Rev. B (Rapid Comm.), 62(2), R779 (2000).
Sc-doped Ba-hexaferrite
A. Yang, Z. Chen, C. Vittoria, and V.G. Harris, J. Appl. Phys.,
accepted, 2008.
PLD Ni-ferriteC.N. Chinnasamy, et al., J. Appl.
Phys., 101, 09M517 (2007).
Synthesis
Boiling NaOH
Washing
FeCl3.6H2O =0.1 MMnCl2.4H2O = 0.05 M
Nano-particles are formed in the general sequence of nucleation,growth; Ostwald ripening; Aggregation/Agglomeration/Sintering/Coalescence (La Mer’s Law)
Experimental
Nanoparticle ferrites: Opportunities and ChallengesTechnologies are varied, from biosensor, cancer remediation therapies, MRI contrast agents, laser printer toner, radar absorbing materials (stealth)
• The small particles are spherical like morphology and the larger particles consist of a mixture of both cubic and spherical particles. • The particle size increased while we decreased the molar concentration of NaOH solution
Fig. TEM micrographs of MnFe2O4 nanoparticles synthesized by using [OH-] molars of (a) 4 M, (b) 2 M, (c) 1 M and (d) 0.425 M.
Magnetism and Microstructure studies
C. N. Chinnasamy, et al., J. Appl. Phys., 101, 09M509 (2007)
0 100 200 300 400 500-5
0
5
10
15
20
25
30
35
40 4 nm 7.5 nm 25 nm 50 nm
M (e
mu/
g)
T (oC)
0 10 20 30 40 50
320
340
360
380
400
TN (
oC
)
Average particle size (nm)
• The Néel temperature is found to increase with particle size • The Néel temperature is significantly higher (400 C) than the bulk MnFe2O4 (300 C). • Contradiction to earlier reports that indicated an increase in TN with reducing particle size and attributed to a finite size scaling.
FIG. Real part of Fourier transform of EXAFS data and best fits of manganese edge, and iron edge, for both 4 nm and 50 nm samples. The data was Fourier transformed with the k range of 2.6-11.5Å-1, and k weight of 3.
EXAFS data, fitting model and Fourier transform fitting results
•Stoichiometry Constraint:
;
MnMnTetMnOct
FeFeTetFeOct
1
2
MnTetFeTet
MnOctFeOct (Normalized)
MnFe
FeFracFe
Fe
FeOctFracFeOct
Calculated: Mn
MnOctFracMnOct
11
1
3
2
1
3
2
13
3
3
2
3
2
FracFe
FracFeOctFracFe
Fe
MnFeFe
FeOct
Fe
MnFe
Fe
Fe
FeOct
FeFe
FeOctFracMnOct
Guess Parameters:
EXAFS ConstraintsEXAFS Constraints
•Multiple scattering paths treatments
•Element dependence of first coordination shells
•Constraints for Nanoparticles3
3 11
4 16
r r
R R
r: distance to the coordination shell in questionR: radius of the particle
S. Calvin, et. al., Phys. Scr., T 115, 744 (2005)
Average particle sizes (diameter)
4 nm 7.5 nm 25 nm 50 nm Ceramic standard
Lattice parameter (Ǻ)
8.421(2) 8.400(2) 8.387(2) 8.383(2) 8.5
Oxygen parameter
0.388(2) 0.387(2) 0.387(1) 0.385(1) 0.3846
EXAFS R factors
0.0237 0.0267 0.027 0.0199 ---
62.91%±1.84% 57.83±2.09% 55.68±1.74% 51.07±1.93% 20%
Table. Results determined from the EXAFS best fits
Nanoparticle samples have reduced lattice parameters in comparison to the bulk value of 8.5 Ǻ.
Contraction -> Increase of super exchange interaction -> Increase of Néel temperature
(Mn1−Fe)tet[MnFe]octO4
ResultsNeel temperature reflects the
strength of the exchangeNeel temperature increases with
particle size but inversely with quench rate
What is needed in NP research?Insight into nucleation and growth
In situ growth
Automated NP chemical processor
Objective:Objective:Allowing identical samples to be synthesized on demand in synchrotron and the institution, so that the characterization resources of the home institution’s laboratory can be applied immediately to samples synthesized at the beamline.
Scott Calvin, Sarah Lawrence College ([email protected]); Everett E. Carpenter, Virginia Commonwealth University Department of Chemistry
Argonaut Surveyor™ Chemical Processor
A Monochromator
B Incident x-ray detector
C Flow cell
D Lytle detector
B
DC
E F
GH
A
x-ray hutch
E Argonaut processor
F Waste collection
G Control computer
H Beamline computer
Configuration of systemtested at NSLS
Configuration figure adapted from S. Calvin et al., Rev. Sci. Instrum. 76, 016103 (2005).
ten 25-mL reaction vessels temperature controllable from -80°C to 150°C
nine sources of reagents
robotic arm can transfersamples and reagents underinert atmospheres
nylon screws
bore
Kaptonwindow
nylon screws
O-ring
bores (offset to ensure new aliquot displaces old)
Kapton window
Custom flow cell
Flow cell figures adapted from S. Calvin et al., Rev. Sci. Instrum. 76, 016103 (2005).
effR (Å)
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5 0
43
86
129
172
0
43
86
129
172
Time (min.)
Time-resolved mode
Example of time-resolved EXAFS data at the iron edge collected at X23B using the Surveyor™ and custom flow cell. Figure adapted from S. Calvin et al., Rev. Sci. Instrum. 76, 016103 (2005). Details of reaction are given in that reference.
effR (Å)
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5 0
43
86
129
172
0
43
86
129
172
Time (min.)
Time-resolved mode
Example of time-resolved EXAFS data at the iron edge collected at X23B using the Surveyor™ and custom flow cell. Figure adapted from S. Calvin et al., Rev. Sci. Instrum. 76, 016103 (2005). Details of reaction are given in that reference.
Cation filling
Oxygen bonding
Too complicated to be of value (for now)
effR (Å)
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5 0
43
86
129
172
0
43
86
129
172
Time (min.)
Time-resolved mode
Example of time-resolved EXAFS data at the iron edge collected at X23B using the Surveyor™ and custom flow cell. Figure adapted from S. Calvin et al., Rev. Sci. Instrum. 76, 016103 (2005). Details of reaction are given in that reference.
Too slow! Little information to be learned in the area of nucleation. Yes, we can slow the kinetics down some, but then we are not duplicating laboratory conditions.
Cation filling
Oxygen bonding
Too complicated to be of value (for now)
AT-LAD grown MnFe2O4 films
Objective:Objective:Allowing identical samples to be synthesized on demand in synchrotron and the institution, so that the characterization resources of the home institution’s laboratory can be applied immediately to samples synthesized at the beamline.
Scott Calvin, Sarah Lawrence College ([email protected]); Everett E. Carpenter, Virginia Commonwealth University Department of Chemistry
Properties of AT-LAD grown films
X. Zuo et al., J. Appl. Phys. 97, 10G103 (2005)
1 mTorr 50 mTorr
Percentage of Fe at A sites (calculated)
9% 18%
Percentage of Mn at B sites (i.e. Inversion parameter)
18% (4) 35% (4)
Lattice Parameter 8.570(4) 8.602(2)
Oxygen Parameter 0.392 (1) 0.3948(8)
Debye-Waller factor 0.010(2) 0.010(2)
Inversion parameters
• 4Ms decreased as Inversion parameter (percentage of Mn ions at B sites) increased, which is similar with Cu-ferrite system
2
2.5
3
3.5
4
4.5
5
0.1 0.2 0.3 0.4
4M
s (kG
)
Inversion parameter,
X-ray absorption spectraU4B, NSLS, BNL
• Sample processed above Pox>5 mTorr more oxidized than samples processed less than 2 mTorr.
• Mn3+: 4B• Mn2+: 5B• Fe3+: 5B• over oxidized Mn ions at B site could reduce magnetization
• Could Mn and Fe have different valence at different sites?
DAFS Basics
• Diffraction Anomalous Fine Structure
A Feature Of Resonant Elastic Scattering Observed Above Absorption Edges in the Energy Dependence of :
– Bragg Peaks
• DAFS can be used to isolate the fine structure from a subset of resonant atoms based on their long-range order
XAFS µ(E)
DAFS I(E)
f’(E) + if’’(E)
Experimental Setup
• Do an XAFS-like energy scan with the sample and the detector mounted on a diffractometer– Track the Bragg peak using software or
feedback
• Advantages: Atom specific, site specific
• Bragg reflection chosen: – <222> ~ B sites– <422> ~ A sites– <111> ~ combination of A & B sites
(for self-consistency check)– EXAFS Fluorescence (for self-
consistency check)
1mT 50mT
Mn@A +2.18(5) +2.31(7)
Mn@B +3.38(8) +3.42(9)
Mn avg. 2.39 2.70
Fe@A +3.35(8) +2.40(9)
Fe@B +2.92(9) +3.10(4)
Fe avg. 2.9597 2.979
Table. Oxidation charge of Mn and Fe at specific site for both ATLAD MnFe2O4 films
Mn edge, 1mT Mn edge, 50 mT
1mT 50mT Standard
Mn@A-O 2.06(1) 1.96(3) 1.98
Mn@B-O 1.98(3) 2.02(3) 2.05
Fe@A-O 1.96(2) 1.93(4) 1.98
Fe@B-O 2.02(3) 2.02(5) 2.05
0
0.5
1
1.5
2
2.5
0 1 2 3 4 5 6
DataFit
F. T
. Am
plit
ude
(a
. u.)
Radial Distance (Angstrom)
(a) (422)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 1 2 3 4 5 6
DataFit
F. T
. Am
plit
ude
(a
. u.
)
Radial Distance (Angstrom)
(b) (222)
Figure. The EXAFS FT data and the fit to the theoretical standards of the Mn (422) and (222) K edge of the 1 mTorr sample.
Table. First oxygen neighbor bond distance of Mn and Fe at specific site for both ATLAD MnFe2O4 films
1mT 50mT Bulk
SEM-EDX measured stoichiometry, Mn: Fe ratio
1:2 7:13 1:2
4Ms (G)
(20oC)
4547 2251 50001
4Ms (G)
(4K)
7212 4321 70001
Predicted 4Ms (G) (0K) 7447 6837
Net Charge (Mn and Fe ions)
+8.31 +8.64 +8
J. Smit and H. P. J. Wijn, Ferrites, John Wiley & Sons, Philips Technical Library, 157 (1959)
These parameters, and nothing short, are needed as input for first principles and MF theories
This is underway500
520
540
560
580
600
0 0.2 0.4 0.6 0.8
Nee
l Tem
pera
ture
(K
)
Inversion parameter,
What are the central issues in NSLS-II EXAFS beamlines
• Sensitivity to sub nanometer thickness is absolutely essential
• These thin layers might be buried• The available surface area may be 4-6 mm2
• Dynamics are essential– Film growth– NP synthesis– Magnetic fields– Electric fields– Lasers– What else…..?
