analysis of ferromagnetic-multiferroic interfaces in epitaxial multilayers of lsmo and bfo student:...
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Analysis of Ferromagnetic-Multiferroic interfaces in Epitaxial Multilayers of LSMO and BFO
Student: Peter KnappResearch Advisor: Professor Jeremiah Abiade
OverviewI. Bilayers were fabricated from ferromagnetic
(FM) LSMO (La0.7Sr0.3MnO3) and anti-ferromagnetic (AFM) BFO (BiFeO3) via Pulsed Laser Deposition (PLD)
II. Layers were analyzed using TEM (Transmission Electron Microscopy), XRD (X-ray Diffraction), and XPS (X-ray Photoelectron Spectroscopy) in order to confirm composition and observe structural detiails
Motivation For Project• Need to control the structure of oxide thin films and
multilayers
• Understand effects of structure & layering on magnetic interaction
• Preliminary work for future experiments on properties of ferromagnetic/ferroelectric systems
Introduction to Multiferroic Bilayers• Materials where electric polarization
influences ferromagnetic polarization, allowing manipulation of electric/magnetic order1
• Contemporary research focuses on bilayers of FM and AFM materials
• These structures demonstrate exchange bias (EB), exchange enhancement (EE), and exchange coupling (EC)
Particular Interest in LSMO and BFO• On their own LSMO and
BFO possess useful characteristics
• Combined they clearly exhibit exchange interactions that characterize multiferroic systems
• Additional advantages include common perovskite structure and a close lattice parameter
(A) (B)
All Perovskites have the same basic chemical formula: ABO3
Interfacial Effects• Researchers know little about how interfacial
effects impact magnetic effects• It is known that there is lattice mismatch and
diffusion between LSMO and BFO layers.• It is necessary to understand how these
phenomena can effect film properties
Lattice Mismatch
Controlling Structure
• These experiments will focus on achieving structural control during deposition
• Substrate will be varied between LaAlO3 or SrTiO3
• The thickness of the layers will be varied
• Layer order will be varied
Potential Applications of Work
• Could help demonstrate novel uses for materials like LMSO and BFO in memory devices and sensors, for instance Hard Drives and SQUIDs (superconducting quantum interference devices)
• Development of novel heterostructures for unusual uses i.e. LMSO as electrode for ferroelectric films
• Tailor structures to realize multicomponent multiferroic systems (e.g. electrical control of magnetism)
Experimental Procedures
I. PLD for synthesis of the Bilayers.II. TEM to observe local
characteristicsII. XRD to observe interlayer
interaction and structural characteristics
III. XPS to confirm composition
Pulsed Laser Deposition
1. Physical Vapor Deposition Technique
2. High Powered (Excimer Laser) focused on target (material to be deposited) in vacuum
3. Material is vaporized into plasma plume which extends from target
4. Proceeds to land on substrate forming a thin film
5. Highly Advantageous
Transmission Electron Microscopy
• Beam of Electrons fired through specimen
• Electrons interact with material in film
• Image created on photographic film or a CCD camera
II. X-Ray Reflectivity
• Measurement: Specular reflection as a function of angle of incidence.
• Result: electron density profile along substrate normal
• Thickness and average electron density of the film.
• Thickness and electron density can be used to infer roughness and structural defects like diffusion and lattice mismatch
• X-ray techniques can also be used to analyze strain in the films
Thin Film or Multilayer
Thin Film or
Multilayer
III. X-ray Photoelectron Spectroscopy
• XPS = X-Ray Photoelectron Spectroscopy
• Kinetic Energy and Intensity of electrons emitted from material irradiated with X-Rays is measured
• Yields elemental composition, empirical formula, chemical state, and electronic state XPS Mechanism
PLD Results: Films Deposited
• Target Substrate Distance=4.5 cm
• Deposition Temp=6500 Celsius
• O2 Background=0.02 Torr
• Pulse Frequency=5 Hz• Laser Fluence =1.5 Jcm-2
• Wavelength=248 nm• Used KrF Excimer Laser
Thickness LSM0 (nm)
Pulses for LSMO
Deposition
Thickness BFO (nm)
Pulses for BFO
Deposition
Order of layers on substrate (bottom/top)
0 0 150 10,580 BFO
150 10,580 150 10,580 BFO/LSMO
200 14,100 150 10,580 BFO/LSMO
250 17,630 150 10,580 BFO/LSMO
150 10,580 0 0 LSMO
150 10,580 150 10,580 LSMO/BFO
150 10,580 200 14,100 LSMO/BFO
150 10,580 250 17,630 LSMO/BFO
Films deposited on both LaAlO3 and SrTiO3
TEM Results – 150nm_BFO_LaAlO3
5 nm5 nm 5 nm5 nm inverse contrast
LaAlO3
BFO
LaAlO3
BFO
TEM Results – Contd.
film (40cm) 100 nm100 nm
300 nm
Clean Diffraction Pattern Indicates highly crystalline film
Growth rate of BFO twice what was expected
LaAlO3
BFO
Glue
Unknown
TEM - Results
1. PLD Allowed for deposition of films that are highly crystalline
2. At the interface there is a slight rotation (30o to 40o) between the crystalline plane of the substrate and film
3. Growth Rate of BFO is twice that of LSMO
XRD Preliminary WorkSlit Collimation GeometryS1 = 0.5 mm (h) 2 mm (v) S2 = 0.1 mm (h) 2 mm (v)S3 and X Replaced with Soller Slit to lock out reflection from excess crystal planes/substrate Sample : 5mmX5mmX0.5mm substrates
Rigaku-ATXG diffractometer
X
S3
S2
S1
Crystallinity Scans• Hold Omega at 0.5 degrees• Scan 2Theta from 20o to 600
• If resulting graph has – Single Peak Single Crystal– Multiple Peaks Polycrystalline– No clear Peaks Amorphous
Polycrystalline Nanocrystaline
20 25 30 35 40 45 50 55 600
50100150200250300350
150nm_BFO_LaAlO3
2Theta (deg)
Coun
ts P
er S
econ
d
20 25 30 35 40 45 50 55 600
20
40
60
80
100
150nm_BFO_SrTiO3
2Theta (deg)
Coun
ts P
er S
econ
d
20 25 30 35 40 45 50 55 600
5
10
15
20
25
150nm_LSMO_150nm_BFO_LaAlO3
2Theta (deg)
Coun
ts P
er S
econ
d
Amorphous
Sample Scans
Polycrystalline Nanocrystaline
20 25 30 35 40 45 50 55 600
50100150200250300350
150nm_BFO_LaAlO3
2Theta (deg)
Coun
ts P
er S
econ
d
20 25 30 35 40 45 50 55 600
20
40
60
80
100
150nm_BFO_SrTiO3
2Theta (deg)
Coun
ts P
er S
econ
d
20 25 30 35 40 45 50 55 600
5
10
15
20
25
150nm_LSMO_150nm_BFO_LaAlO3
2Theta (deg)
Coun
ts P
er S
econ
d
Amorphous
20 25 30 35 40 45 50 55 600
50
100
150
200
250
150nm_BFO_150nm_LSMO_SrTiO3
2Theta (deg)
Coun
ter P
er S
econ
d
Approaching Single Crystal
Crystallinity Scan Contd.
20 30 40 50 60 70 800
10
20
30
40
50
60
150nm_LSMO_LaAlO3
2Theta (deg)
Coun
ts P
er S
econ
d
20 25 30 35 40 45 50 55 600
10203040506070
150nm_LSMO_SrTiO3
2Theta (deg)
Coun
ts P
er S
econ
d
Amorphous Nanocrystalline or Amorphous
20 25 30 35 40 45 50 55 600
20
40
60
80
100
150nm_BFO_150nm_LSMO_LaAlO3
2Theta (deg)
Coun
ts P
er S
econ
d
20 25 30 35 40 45 50 55 600
50
100
150
200
250
150nm_BFO_150nm_LSMO_SrTiO3
2Theta (deg)
Coun
ter P
er S
econ
d
Nanoctystalline or Amorphous Polycrystaline
Crystallinity Scans Contd.
20 25 30 35 40 45 50 55 600
5
10
15
20
25
150nm_LSMO_150nm_BFO_LaAlO3
2Theta (deg)
Coun
ts P
er S
econ
d
20 25 30 35 40 45 50 55 600
5
10
15
20
25
30
150nm_LSMO_150nm_BFO_SrTiO3
2Theta (deg)
Coun
ts P
er S
econ
d
20 25 30 35 40 45 50 55 6005
101520253035
200nm_BFO_150nm_LSMO_LaAlO3
2Theta (deg)
Coun
ts P
er S
econ
d
20 25 30 35 40 45 50 55 600
20
40
60
80
100
120
200nm_BFO_150nm_LSMO_SrTiO3
2Theta (deg)
Coun
ts P
er S
econ
d
Amorphous Amorphous
Amorphous Amorphous or Nanocrystalline
Crystallinity Scans Contd.
20 25 30 35 40 45 50 55 600
5
10
15
20
25
200nm_LSMO_150nm_BFO_LaAlO3
2Theta (deg)
Coun
ts P
er S
econ
d
20 25 30 35 40 45 50 55 6002468
10121416
200nm_LSMO_150nm_BFO_SrTiO3
2Theta (deg)
Coun
ts P
er S
econ
d
20 25 30 35 40 45 50 55 6002468
10121416
250nm_BFO_150nm_LSMO_LaAlO3
2Theta (deg)
Coun
ts P
er S
econ
d
20 25 30 35 40 45 50 55 6002468
1012141618
250nm_BFO_150nm_LSMO_SrTiO3
2Theta (deg)
Coun
ts P
er S
econ
d
Amorphous Amorphous
Amorphous Amorphous
Crystallinity Scans Contd.
20 25 30 35 40 45 50 55 6005
101520253035
250nm_LSMO_150nm_BFO_LaAlO3
2Theta (deg)
Coun
ts P
er S
econ
d
20 25 30 35 40 45 50 55 600
20
40
60
80
100
120
250nm_LSMO_150nm_BFO_SrTiO3
2Theta (deg)
Coun
ts P
er S
econ
d
Amorphous Amorphous or Nanocrystalline
Results• Majority of Films are amorphous
• Several Films appear to be Polycrystalline or Nanocrystalline
• New BFO film created with alternate deposition parameters
Nanocrystaline Samples
Possible to determine the size of crystallites using the Scherrer Eqn.
cos
2L
KB
B(2) = Peak Width (radians)λ = .1542 nmL = Crystallite Width (nm) = d-spacing (radians)K = Scherrer Constant (Assumed to be 1)
Film (radians)
B(2)(radians)
Crystallite Width(nm)
150nm_BFO_SrTiO3
0.263 0.111 5
150nm_LSMO_SrTiO3
0.256 0.0803 8
150nm_BFO_150nm_LSMO_LaAlO3
0.265 0.111 5
200nm_BFO_150nm_LSMO_SrTiO3
0.259 0.0986 6
250nm_LSMO_150nm_BFO_SrTiO3
0.254 0.116 5
New 150 nm BFO Film on SrTiO3• Used standard Laser
Fluence and Pulse Frequency
• Modified Annealing Process
• Deposition at 670o C at .02 Torr
• Cool to 390o C, anneal for 1 hour
• Cool to room temperature at 5o C/min
20 30 40 50 60 70 800
10
20
30
40
50
60
150 nm BFO on SrTiO3 Newly Prepared
2Theta (deg)
Coun
ts P
er S
econ
d
Data indicates Amorphous film. XPS analysis used to confirm
composition allowing us to draw a more accurate conclusion.
Crystallinity Scans - Results
• Majority of films are amorphous with some polycrystalline and nanocrystalline samples
• Likely due to diffusion of oxygen during annealing
• Indicated deposition process still requires optimization
X-Ray Reflectivity 150nm_BFO_150nm_LSMO_SrTiO3
GE111 Compressor CrystalS1 = 0.5 mm (h) 2 mm (v)
S2 = 0.1 mm (h) 2 mm (v)
S3 = 0.2 mm (h) 5 mm (v)
X = 0.2 mm (h)Flux: ~ 2.1*106 photons/s Sample : 5mmX5mmX0.5mm substrates
Layer Thickness (Å) SLD (Real) SLD (Imaginary) Roughness (Å)
Air INF 0 0 0
Residue 84.3 4.27*10-6 3.32*10-8 30.1
BFO 1450 6.57*10-5 7.90*10-6 77.5
LSMO 1550 5.09*10-5 1.59*10-5 51.5
SrTiO3 Substrate INF 4.49*10-5 1.95*10-6 49.8
Conclusion - XRR
• Thickness and SLD data seems reasonable but contrasts with data on growth rate from TEM
• Unfitted drop results from having a high roughness film and low X-ray intensity during scanning
• Top residue Layer is Likely a Combination of organics and silver particles from adhesive
XPS Analysis
Peak Position BE (eV)
FWHM (eV)
Raw Area (CPS)
RSF Atomic Mass
Atomic Conc. (%)
Mass Conc. (%)
Bi 4f 157 2.881 1481870 9.140 208.98 24 74
Fe 2p 708 5.099 245382.5 2.957 55.846 13 11
O 1s 527 3.380 331482.5 0.780 15.99 63 15
Peak Position BE (eV)
FWHM (eV)
Raw Area (CPS)
RSF Atomic Mass
Atomic Conc. (%)
Mass Conc. (%)
Bi 4f 156 2.767 1302850 9.140 208.98 21 68
Fe 2p 708 4.572 378805.0 2.957 55.846 19 17
O 1s 527 3.162 318230.0 0.780 15.99 60 15
XPS Results for original 150nm_BFO_SrTiO3: Proper Stoichiometry Observed
XPS Results for New 150nm_BFO_SrTiO3: Proper Stoichiometry not Observed
XPS - Results
• Stoichiometry of films very similar to target material
• Currently no explanation for iron deficiency in the new BFO film
Summary/Conclusion
• While the constructed films were not epitaxial many were highly crystalline
• The Stoichiometry of films examined by XPS was consistent with the target material
• XRR indicated the films have a large roughness
• The deposition process for LSMO and BFO still requires optimization.
Acknowledgements
The financial support from the National Science Foundation, EEC-NSF Grant # 1062943 is gratefully acknowledged. I would like to thank Professors Jursich and Takoudis for organizing the REU Program. I would like to thank the LORE lab in general and Professor Jeremiah Abiade specifically for providing me with the opportunity to work in their lab.
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