Power Point Presentation

Report
Analysis of Ferromagnetic-Multiferroic
interfaces in Epitaxial Multilayers of
LSMO and BFO
Student: Peter Knapp
Research Advisor: Professor Jeremiah Abiade
Overview
I. Bilayers were fabricated from ferromagnetic
(FM) LSMO (La0.7Sr0.3MnO3) and antiferromagnetic (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)
All Perovskites have the same basic
chemical formula: ABO3
(B)
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
characteristics
II. 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.
Thin Film
or
Multilayer
Thin Film or
Multilayer
• 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
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
LaAlO3
5 nm
LaAlO3
5 nm
BFO
inverse contrast
BFO
TEM Results – Contd.
Unknown
LaAlO3
300 nm
BFO
Glue
film (40cm)
100 nm
Clean Diffraction Pattern Indicates highly
crystalline film
Growth rate of BFO twice what was
expected
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 Work
X
S3
S2
S1
Rigaku-ATXG diffractometer
Slit Collimation
Geometry
S1 = 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
Crystallinity Scans
– Single Peak Single Crystal
– Multiple Peaks Polycrystalline
– No clear Peaks Amorphous
150nm_LSMO_150nm_BFO_LaAlO3
25
Counts Per Second
• Hold Omega at 0.5 degrees
• Scan 2Theta from 20o to 600
• If resulting graph has
20
15
10
5
0
20
25
30
35
40
45
50
55
60
50
55
60
2Theta (deg)
Amorphous
150nm_BFO_SrTiO3
350
300
250
200
150
100
50
0
100
Counts Per Second
Counts Per Second
150nm_BFO_LaAlO3
80
60
40
20
0
20
25
30
35
40
45
2Theta (deg)
Polycrystalline
50
55
60
20
25
30
35
40
45
2Theta (deg)
Nanocrystaline
Sample Scans
150nm_BFO_150nm_LSMO_SrTiO3
150nm_LSMO_150nm_BFO_LaAlO3
25
Counts Per Second
Counter Per Second
250
200
150
100
50
0
20
15
10
5
0
20
25
30
35
40
45
50
55
60
20
25
30
35
2Theta (deg)
45
50
55
60
50
55
60
2Theta (deg)
Approaching Single Crystal
Amorphous
150nm_BFO_LaAlO3
150nm_BFO_SrTiO3
350
300
250
200
150
100
50
0
100
Counts Per Second
Counts Per Second
40
80
60
40
20
0
20
25
30
35
40
45
2Theta (deg)
Polycrystalline
50
55
60
20
25
30
35
40
45
2Theta (deg)
Nanocrystaline
Crystallinity Scan Contd.
150nm_LSMO_LaAlO3
150nm_LSMO_SrTiO3
50
Counts Per Second
Counts Per Second
60
40
30
20
10
0
20
30
40
50
60
70
80
70
60
50
40
30
20
10
0
20
25
30
35
45
50
55
60
2Theta (deg)
Amorphous
Nanocrystalline or Amorphous
150nm_BFO_150nm_LSMO_LaAlO
3
150nm_BFO_150nm_LSMO_SrTiO3
250
Counter Per Second
Counts Per Second
2Theta (deg)
40
100
80
60
40
20
0
200
150
100
50
0
20
30
40
50
2Theta (deg)
Nanoctystalline or Amorphous
60
20
25
30
35
40
2Theta (deg)
Polycrystaline
45
50
55
60
Crystallinity Scans Contd.
150nm_LSMO_150nm_BFO_SrTiO3
25
30
20
25
Counts Per Second
Counts Per Second
150nm_LSMO_150nm_BFO_LaAlO3
15
10
5
0
15
10
5
0
20
25
30
35
40
45
50
55
60
20
25
30
35
40
45
50
55
2Theta (deg)
2Theta (deg)
Amorphous
Amorphous
200nm_BFO_150nm_LSMO_LaAlO3
200nm_BFO_150nm_LSMO_SrTiO3
35
30
25
20
15
10
5
0
60
120
Counts Per Second
Counts Per Second
20
100
80
60
40
20
0
20
25
30
35
40
2Theta (deg)
Amorphous
45
50
55
60
20
25
30
35
40
45
50
2Theta (deg)
Amorphous or Nanocrystalline
55
60
Crystallinity Scans Contd.
200nm_LSMO_150nm_BFO_LaAlO3
200nm_LSMO_150nm_BFO_SrTiO3
Counts Per Second
Counts Per Second
25
20
15
10
5
0
20
25
30
35
40
45
50
55
16
14
12
10
8
6
4
2
0
60
20
25
30
35
45
50
55
60
2Theta (deg)
Amorphous
Amorphous
250nm_BFO_150nm_LSMO_LaAlO3
250nm_BFO_150nm_LSMO_SrTiO3
16
14
12
10
8
6
4
2
0
20
Counts Per Second
Counts Per Second
2Theta (deg)
40
15
10
5
0
20
25
30
35
40
2Theta (deg)
Amorphous
45
50
55
60
20
25
30
35
40
2Theta (deg)
Amorphous
45
50
55
60
Crystallinity Scans Contd.
250nm_LSMO_150nm_BFO_SrTiO3
35
30
25
20
15
10
5
0
120
Counts Per Second
Counts Per Second
250nm_LSMO_150nm_BFO_LaAlO3
100
80
60
40
20
0
20
25
30
35
40
45
50
55
60
2Theta (deg)
20
25
30
35
40
45
50
2Theta (deg)
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
55
60
Nanocrystaline Samples
Film

(radians)
B(2)
(radians)
Crystallite
Width
(nm)
150nm_BFO_SrTiO
0.263
0.111
5
K
150nm_LSMO_SrTi
O3
0.256
0.0803
8
L cos 
150nm_BFO_150n
m_LSMO_LaAlO3
0.265
0.111
5
200nm_BFO_150n
m_LSMO_SrTiO3
0.259
0.0986
6
250nm_LSMO_150
nm_BFO_SrTiO3
0.254
0.116
5
Possible to determine
the size of crystallites
using the Scherrer Eqn.
B  2  
B(2) = Peak Width (radians)
λ = .1542 nm
L = Crystallite Width (nm)
 = d-spacing (radians)
K = Scherrer Constant (Assumed to be
1)
3
New 150 nm BFO Film on SrTiO3
150 nm BFO on SrTiO3 Newly
Prepared
60
50
Counts Per Second
• 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
40
30
20
10
0
20
30
40
50
60
70
80
2Theta (deg)
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 Crystal
S1 = 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
4.49*10-5
1.95*10-6
49.8
SrTiO3 Substrate INF
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 RSF
(CPS)
Atomic
Mass
Atomic
Mass
Conc. (%) 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
Peak
Position
BE (eV)
FWHM
(eV)
Raw Area RSF
(CPS)
Atomic
Mass
Atomic
Mass
Conc. (%) 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
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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