# Presentation: 484901

## The role of EXAFS in some condensed matter systems: Challenges and opportunities for NSLS II

- Vince Harris
- Department of Electrical and Computer Engineering
- Northeastern University
- Boston, MA 02115 USA
- _[harris@ece.neu.edu](mailto:harris@ece.neu.edu)_
- _[www.cm3ic.neu.edu](http://www.cm3ic.neu.edu/)_
- 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!

- 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

- 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 details

**Experimental details**
- MBE grown Fe (0.8nm)
- EXAFS measurements on X23B NSLS
- Sample size is 2 mm x 3 mm
- TEY, room temp, std. conditions
- Spot size ~250 m x 250 m
- FEFF modeling (FEFF3?)
**Goal**
- Determine nature of metallization
- Structure identification
- Structure stability: bcc vs fcc
- Bond distances, coordination

**PRB Brief Report 52(11) 7890 (1995)**

## Experimental details

**Experimental details**
- MBE grown Fe (0.8nm)
- EXAFS measurements on X23B NSLS
- Sample size is 2 mm x 3 mm
- TEY, room temp, std. conditions
- Spot size ~250 m x 250 m
- FEFF modeling (FEFF3?)
**Goal**
- Determine nature of metallization
- Structure identification
- Structure stability: bcc vs fcc
- Bond distances, coordination

**PRB Brief Report 52(11) 7890 (1995)**

## Results

**Results**
- FEFF modeling (FEFF 3.11 single scattering)
-  determines Fe-Fe and Fe-C bonds
- 2-11 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

**Challenges**
- Very thin films (6-8 Å, 4-5 ml)
- Small and very valuable substrates
- Synthetic diamond 2 mm x 3 mm substrates
- Beam spot size: 250 m x 250 m
- If 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 counts
- Smaller spots
- Beam stability
- Linearity in detector circuitry, challenging.

- PRB Brief Report 52(11) 7890 (1995)

## Phys. Rev. B43, 3, 15 Jan. 1991 (Brief Report)

**Phys. Rev. B43, 3, 15 Jan. 1991 (Brief Report)**

**Goals**
- Nature of strain in GeSi epitaxial films
**Experimental**
- Samples are 34 nm GeSi (31% Ge)
- CVD grown films
- In-plane vs out-of-plane bond distances
- and lattice parameters
- EXAFS using FY
- Sample spinning
- X23A2 beam does not walk
- Good linearity and reproducibility
- Typically JW many scans merged
**Results**
- Raw data with the first neighbor signal superimposed
- Bottom image is the fit to the FF data
- Very 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

**Appl. Phys. Lett., Vol. 73, No. 9, 31 August 1998**

**Goals and Samples**
- Investigate nature of local
- distortions in buried layers (InP caps)
- Important semiconductor materials
- Samples are in some cases 5-6 nm
- MOCVD processed

**Experimental**
- Fluorescent yield
- Spinning samples, wobble
- In-plane and glancing incidence
**Results**
- Raw data with best fit of NN bonds

## Findings

**Findings**
- In-As and Ga-As bonds are measured
- to a high accuracy
- Epitaxial films experience tetragonal
- distortions except for x=0.53 sample which
- had a perfect lattice match to the substrate
**Noteworthy**
- Large 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

**Appl. Phys. Lett. 68 (15), 8 April 1996**

**Experimental details**
- Ion beam sputtered Fe80B20 MetglassTM films
- 15 nm thick
- Annealed at different temperatures and different times
- EXAFS measurements on X23B NSLS
- Sample size is 5 mm x 5 mm
- TEY, room temp, std. conditions
- Spot size ~1 mm x 1 mm
- No FEFF modeling
- Linear modeling of standards (poor mans PCA)
**Goal**
- Determine phases present
- Determine atomic fraction of each
- Structural stability
- Onset of crystallization

## I wish we could have....

**Results**
- Mass of sampled data: ~1 nanogram
- DSC and DTA have 10-3-10-4 gram sensitivity
- Standards’ data are fit linearly with the
- following adjustable parameters:
- -atomic fraction of each phase
- -Debye-Waller coefficient of the amorphous phase
**Error bars**
- Determined by fitting the mean, +/- standard deviations
- Systematic variation of atom fractions
- until 2 doubles (this is larger)
- Counting statistics (10 scans of each sample)
- result in mean and standard deviations
**Findings**
- Onset of crystallizations is 200K below bulk samples
- Effective 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 

- 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**

**Experimental**
- Melt spun ribbon samples having
- excellent soft magnetic properties
- Amorphous as spun
- Crystallized by annealing at
- 550oC (opt. T) for various times
- EXAFS transmission, RT
**Goals**
- Does the 1% Cu make a difference and how?

## 0% Nb

**EXAFS details**
- Ribbons: 25 mm thick, 2 mm wide, 20 mm long
- Problems with sample uniformity
- Mapping the transmitted beam we find regions of uniform
- thickness that are about 300 mm x 1000 mm
- Fe 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**

**Findings**
- The presence of Nb limits the nucleation
- and growth of the Cu atoms
- Without Nb, the Cu is fully crystallized after
- 2 minutes at temperature
- The 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 heating
- Sometimes samples are what they are....

- 3% Nb

- The role of 3% Nb

## Goals

**IEEE Trans. Magn. 31, 6, Nov. 1995**

**Goals**
- The goal is to gather information on both the
- element specific cation distribution and valency
- This is true for the manganites as well
- Magnetic and electronic properties

**Experimental details**
- Samples are ferrite films prepared by SSP or PLD
- Size is typically 3-5 mm on a side
- Thickness is typically 100-200 nm
- EXAFS: FY or TEY

## Results

**Results**
- As shown the peaks from 2-4 Å experience systematic changes
- FEFF 6: Multiple scattering fits without FEFFIT
- First determination of cation site distribution in ferrites
- (one step better than ME)
- Findings allow interpretation of M vs y
- trends with increasing Zn
- These 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**

**Notes:**
Here we present the FT of Fe in the Fe3O4 standard, the y axis of this plot corresponds with FT ampltidue and provides coordination and atomic disorder information and the x-axis is in angstroms and provides information of bond lengths and lattice distortions
These data are useful in identifying certain features that are helpful in understanding ferrite systems
The most important part of this data is the peaks over 2-4 angstroms (next slide) 

## Other ferrite systems studied by NRL and NEU researchers since 2000

**PLD Cu-ferrite**
- Aria Yang, et al., Appl. Phys. Lett., **86**,  252510 (2005).

**ATLAD Cu-ferrite**
- Aria Yang, et al., J. Appl. Phys., accepted, 2008.

**PLD and ATLAD Mn-ferrite**
- Aria 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-ferrite**
- S.A. Oliver, V.G. Harris, H. Hamdeh and J.C. Ho, Appl. Phys. Lett., **76** (19), 2761 (2000).

**NP Mn-ferrite (polyol)**
- _[C. N. ](http://0-scitation.aip.org.ilsprod.lib.neu.edu/vsearch/servlet/VerityServlet?KEY=ALL&possible1=Chinnasamy%2C+C.+N.&possible1zone=author&maxdisp=25&smode=strresults&aqs=true)__[Chinnasamy](http://0-scitation.aip.org.ilsprod.lib.neu.edu/vsearch/servlet/VerityServlet?KEY=ALL&possible1=Chinnasamy%2C+C.+N.&possible1zone=author&maxdisp=25&smode=strresults&aqs=true)_, 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-ferrite**
- K. 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-ferrite**
- G. 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-ferrite**
- C.N. Chinnasamy, et al., J. Appl. Phys., **101**, 09M517 (2007).

## Nanoparticle ferrites: Opportunities and Challenges

- Nano-particles are formed in the general sequence of nucleation,growth; Ostwald
- ripening; Aggregation/Agglomeration/Sintering/Coalescence (La Mer’s Law)

**Experimental**

- Technologies are varied, from biosensor, cancer remediation therapies, MRI contrast agents, laser printer toner, radar absorbing materials (stealth)

## Magnetism and Microstructure studies

-    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) **

-  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.

## EXAFS Constraints

- 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:

**EXAFS Constraints**

- S. Calvin, et. al., Phys. Scr., T 115, 744 (2005)

## Table. Results determined from the EXAFS best fits

| 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

**Results**
- Neel temperature reflects the
- strength of the exchange
- Neel 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:**
- 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 (SCalvin@slc.edu);
- Everett E. Carpenter, Virginia Commonwealth University Department of Chemistry

## Argonaut SurveyorTM Chemical Processor

**A **Monochromator
**B** Incident x-ray detector
**C **Flow cell
**D **Lytle detector

**E** Argonaut processor
**F **Waste collection
**G **Control computer
**H **Beamline computer

_**Configuration of system**_
_**tested at NSLS**_

- Configuration figure adapted from S. Calvin et al., *Rev. Sci. Instrum.* **76**, 016103 (2005).

## Custom flow cell

_**Custom flow cell**_

- Flow cell figures adapted from S. Calvin et al., *Rev. Sci. Instrum.* **76**, 016103 (2005).

## Time-resolved mode

_**Time-resolved mode**_

- Example of time-resolved EXAFS data at the iron edge collected at X23B using the SurveyorTM 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.

## Time-resolved mode

_**Time-resolved mode**_

- Example of time-resolved EXAFS data at the iron edge collected at X23B using the SurveyorTM 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)

## Time-resolved mode

_**Time-resolved mode**_

- Example of time-resolved EXAFS data at the iron edge collected at X23B using the SurveyorTM 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:**
- 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 (SCalvin@slc.edu);
- 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) |

**Notes:**
The valence information presented in the last slide is determined by energy shift, cation distribution information.
We calcalued the net charge (for Mn and Fe ions in MnFe2O4) to double check our calculation accuracy, since we did not use the net balance to constrain the results we got previously.
And it turned out not bad which approves our study.

## Inversion parameters

-  4Ms decreased as Inversion parameter (percentage of Mn ions at B sites) increased, which is similar with Cu-ferrite system

**Notes:**
As shown in picture, 4pMs decreased when inversion parameter increased, which is similar to Cu-ferrite system.

## X-ray absorption spectra

- U4B, NSLS, BNL

-  Sample processed above Pox>5 mTorr more oxidized than samples processed less than 2 mTorr.
-  Mn3+: 4B
-  Mn2+: 5B
-  Fe3+: 5B
-  over oxidized Mn ions at B site could reduce magnetization
-  Could Mn and Fe have different valence at different sites?

**Notes:**
By looking at absorption spectra, we can tell the valence by edge shift.
For those films grown at 1 and 2mTorr, IIIB peak sits at the similar position, which is at left of those grown above 3mTorr (5, 10, 20, 50mTorr). 
So, probably, those grown above 5mTorr is more oxidized, which might be the reason introducing the reduction of magnetization.

## 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

- f’(E) + if’’(E)

**Notes:**
By tracking Bragg peaks, DAFS can give us the information for a specific atom at specific sites.
By doing DAFS information, we can get the valence and structure information for Mn/Fe at A, and B sites respectively.
f”(E) is simply proportional to the product of the x-ray absorption cross-section and the x-ray energy
f’(E) and f”(E) are interralted by their mutual Kramers-Kronig dispersion integrals

## 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)

**Notes:**
Only because of spinel structure, by tracking <222> reflection, scattering intensity only gives the information of those on B sites.
Similarly, <422> reflections tells information on A sites only.

## Table. Oxidation charge of Mn and Fe at specific site for both ATLAD MnFe2O4 films

|  | 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

**Notes:**
A sites valence are calculated by coordination charge method, while B valence are calculated directly, so that they are normalized.
Those valence information is determined by energy shift. 

## Table. First oxygen neighbor bond distance of Mn and Fe at specific site for both ATLAD MnFe2O4 films

|  | 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 |

**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.**

**Notes:**
Fe@B-O, Fe@A-O distance similar for both 1mT and 50mTorr samples.
Only Mn@A-O expands. Others all shrink.
In 1mTorr sample, Mn@B sites shrinks, while Mn@A expands, both at same size.
While in 50mTorr sample, Mn@A, and B all shrinks at same size.

## These parameters, and nothing short, are needed as input for first principles and MF theories

|  | 1mT  | 50mT | Bulk |
| --- | --- | --- | --- |
| SEM-EDX measured stoichiometry, Mn: Fe ratio | 1:2 | 7:13 | 1:2
 |
| 4Ms (G)
(20oC) | 4547 | 2251 | 50001 |
| 4Ms (G)
(4K) | 7212 | 4321 | 70001 |
| Predicted 4Ms (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)**

- This is underway

## 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.....?