generation and analysis of magnetic flux pinning...
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Generation and Analysis of Magnetic Flux Pinning Sites in YBa2Cu3O7-δ
Thin Films
Haiyan WangSuperconductivity Technology Center
Materials Science & Technology DivisionLos Alamos National Laboratory
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OutlineI. High Temperature Superconductor Thin Films• Pulsed laser deposition and domain matching
epitaxy• Normal architecture of coated conductors;• Generation of magnetic flux pinning sites in YBCO
thin films.a. Nanoparticles generated on the buffer surface; b. Y2O3 nanoparticles doped in YBCO;c. BaZrO3 nanoparticles doped in YBCO.
II. Opportunities and Future Work
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Pulsed Laser Deposition
Heatedsubstrate
Plume of evaporant
Target
KrF excimer laser (248nm,pulse duration 25ns, 10Hz,deposition rate~2nm/s, a typical pulse~6J/cm2,
target-substrate distance 4-5cm);
Advantages: simple configuration, Reproduction of the target stoichiometry, low contamination,
In-situ control of the thin film properties.
Figure from Physical vapor Deposition of Thin Films, John E. Mahan (Wiley-interscience Publication)
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Domain matching epitaxy
Small lattice mismatchLattice matching epitaxy
Large lattice mismatchDomain matching epitaxy
af ≈ as maf ≈ nas
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Epitaxial TiN film on Si(100)Domain Matching Epitaxy
sf
sf
namanama
f+
−×= 2
Large lattice mismatch
4aTiN ≈ 3aSi
f=24.6%
sf
sf
aaaa
f+
−×= 2
Domain matching epitaxy
TiN af=0.4256nm, Si as=0.5430nm
f’=4.4%
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Domain epitaxy growth of TiN films on Si(100) TS=700oC
])110[2
( ab =Two set of extra half planes generated in TiN film form one 90o dislocation.
Wang et al. J. Mater. Res., 16, 2733, (2001).
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Normal architecture of YBa2Cu3O7-δ coated conductors
Ni alloy tape
IBAD MgObufferYBCO
Polished polycrystalline substrate
Ion Beam Assisted Deposition (IBAD) produces single-crystal-like MgO template.
PLD produce epitaxial buffer layer and YBCO film.
Tape is moving slowly .
Target rotation
For the growth of meter-long YBCO wires, the substrate is slowly moving during deposition.
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TEM and HRTEM show the microstructure of a typical coated conductor.
Ni Alloy
IBAD MgOBuffer
YBCO
Wang et al. J. Mater. Res., 19, 1869 (2004).
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sf
sf
namanama
f+
−×= 2
Large lattice mismatch
14aSrTiO3 ~ 13aMgO
f=7.38%
sf
sf
aaaa
f+
−×= 2
Domain matching epitaxy
STO af=3.905A, MgO as=4.21A
f’=0.02%~0%
Domain matching epitaxy between STO and MgO
Wang et al. J. Mater. Res., 19, 1869 (2004).
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Domain match epitaxy between STO and MgO: 14 to13 match
STO
MgO
14aSTOmatches13aMgO~0.02%
Fast Fourier Filtered Image
Wang et al. J. Mater. Res., 19, 1869 (2004).
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10 62J cE A /cm
Generators&
Transformers
Cables
10 4
10 5
10 3
2 64
77 K
0
Magnetic Energy Storage
Fault Current limiters
B / Tesla || to c
Motors
Current status,<1m
What in-field performance is needed for the future applications of YBCO coated conductors?
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Flux pinning in type II superconductors
Ideal type II superconductors (no extended defects)Force →dissipative flux movement →carrying transport current with losses.
2
0
1
oan
nB
=
Φ=
Ba 0
0Φ
= Flux line arrangement containing high density dislocations in Pb-6.3 at.% In sample, B=35mT, T=1.2K.
Most type II superconductors have extended defects (dislocations, grain boundaries and precipitates) →pinning centers for flux lines
Lorentz force between flux and current. FL=Jtr x Φ0 . Need Fp>FL
nmaTB
cmGe
h
201
1007.22
270
=⇒=
⋅×≈≈Φ −
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Flux pinning measurements→ Field dependence and angular dependence
HAligned vortex
DefectJc
angle
Extended defects•Line defects and columnar defects•Planar defects (stacking faults and twin boundaries)
Jc ∝ length of vortex that is pinned.
Larger angle →shorter pinned segments →lower Jc
Maximum pinning
Always H⊥J (maximum Lorentz force)
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Flux pinning → directional pinning by correlated defects → angular dependence
H
H
Aligned vortex
Staircase vortex
Defect
Defect
Jc
angle
Extended defects•Line defects and columnar defects•Planar defects (stacking faults and twin boundaries)
Jc ∝ length of vortex that is pinned.
Larger angle →shorter pinned segments →lower Jc
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Flux pinning → directional pinning by correlated defects → angular dependence
H
H
H
Aligned vortex
Staircase vortex
Unpinned vortex
Defect
Defect
Defect
Jc
angle
Extended defects•Line defects and columnar defects•Planar defects (stacking faults and twin boundaries)
Jc ∝ length of vortex that is pinned.
Larger angle →shorter pinned segments →lower Jc
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Possible practical ways to increase pinning
More line defects– Miscut substrates– Lower growth temperatures (smaller island sizes)– Heteroepitaxial nanoparticles
Point defects (and associated strain)– cation or anion vacancies– RE-Ba cross substitution
Volume defects (and associated strain)– Second phase particles– Interlayers of non-superconducting material– Substrate surface roughening
Lorentz force between flux and current. FL=Jtr x Φ0 . Need Fp>FL
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Three simple and effective approaches to enhance pinning demonstrated
Method 1. Introduction of buffer roughness-epitaxial STO nanoparticles grown on the YBCO deposition surface
-more low angle grain boundaries // c;
Method 2. Introduction of heteroepitaxial second phases Y-rich YBCO films—self-assembled Y2O3 nanoparticles;
Method 3. Introduction of heteroepitaxial second phases Nanoparticles (BaZrO3) grown in the YBCO film.
-Increase c-axis dislocation density.Materials selection for second phase:a) Can grow heteroepitaxially with YBCOb) Lattice mismatch producing strain leading to misfit dislocationsc) High melting temperature phase, yielding slow growth kinetics andhence small particlesd) Chemical compatibility with YBCO
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Method 1-Introduction of buffer roughnessSTO nanoparticles can be generated at lower temperature.
670oC, STO
760oC, STO
820oC, STO
SEM
Wang et al. J. Mater. Res., 19, 1869 (2004).
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STO nanoparticles (outgrowth) at interface generated low angle boundaries in YBCO.
Wang et al. J. Mater. Res., 19, 1869 (2004).
STO at 820oCSmooth interface
STO at 700oCNanoparticleat interface
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STO at 820oCSmooth interface
STO at 700oCLattice distortions
Wang et al. J. Mater. Res., 19, 1869 (2004).
STO nanoparticles (outgrowth) at interface generated local lattice distortions in YBCO.
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STO nanoparticles enhance in-field performance at high field.
0 1 2 3 4 5 6 7
0.01
0.1
1
TSTO=700oCSTO nanoparticles
Jc [M
A cm
-2]
H [T]
TSTO=820oC
smooth surface
H//c T=75.5K
0.01 0.1 1
0.01
0.1
1
α=0.44
J c/Jcsf
µ0H[T]
α=0.55
Wang et al. to be published in App. Phys. Lett.
α value is temperature independent, architecture dependent and repetitive.Smaller α value, better field dependence.
α−∝ HJc
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The angular dependence of Jc shows a larger c-axis peak.
Wang et al. to be published in App. Phys. Lett.
The angular effect isconsistent with the presenceof low angle boundaries //c(introduced by tilted a-bplanes induced by buffersurface roughness)
0 1 2 3 4 5 6 7
0.01
0.1
1
TSTO=700oCSTO nanoparticles
Jc [M
A c
m-2
]
H [T]
TSTO=820oC
smooth surface
H//c T=75.5K
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Method 2. Introduction of heteroepitaxial second phasesSelf-assembled epitaxial Y2O3 nanoparticles in YBCO
Wang et al. submitted to Appl. Phys. Lett.
Y2O3
YBCOCeramic target of Y-rich YBCO fabricated and ablated
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Y2O3 nanoparticles enhance in-field performance at low field.
0 1 2 3 4 5 6 71E-3
0.01
0.1
1
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.40.1
1
YBCO YBCO with10mol% Y2O3
J c (MA
/cm
2 )
B (T)
J c (MA
/cm
2 )
B (T)
Wang et al. submitted to Appl. Phys. Lett.
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Method 3. Introduction of heteroepitaxial second phasesEpitaxial BaZrO3 nano-particles in YBCO films
Nature Materials, 3, 439-443, (2004)
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HRTEM and FFT shows the presence of BaZrO3 nano-particles.
Nature Materials, 3, 439-443, (2004)
20 40 60 80 1000
20
40
60
80
100
120
140
160
Par
ticle
Num
ber
Particle Size (nm)
1µmx1µm area
39
165
77
124 21 1 1 1 1
Fast Fourier Transform indicates FCC,a ~4.27Å
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Comparison of YBCO+BaZrO3 and YBCO shows a 1.5~5× increase in Jc.
Nature Materials, 3, 439-443, (2004)
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Angular dependence shows a huge c-aixspeak indicating extra c-axis pinning.
Nature Materials, 3, 439-443, (2004)
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Summary
• By pulsed laser deposition, high quality YBCO thin films can be grown on Ni substrates using domain matching epitaxy.
• We have discovered three methods to introduce nanoparticles (c-axis defects in YBCO).
• Nanoparticles significantly improve the in-field performance of YBCO thin films.
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Opportunities and future workSuperconductors
Explore fundamental physics of nanoparticle enhanced pinning mechanism in YBCO thin films (modeling and simulation)Precisely control the size and distribution of nanoparticles in YBCO thin films.
Nitride materials for electronic and optical devicesElectrical properties of nanocrystalline TiN;Cu diffusion mechanism in epitaxial and polycrystalline TaN;Electrical property and Cu diffusion in epitaxial TiN/TaNsuperlattice.GaN LED devices
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Nanocrystalline TiN films grown on Si(100)Ts=25oC
AverageGrain size size=8nm
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Nanocrystalline TiN films grown on Si(100)Ts=200oC
Average grain size ~ 20nm, with high angle grain boundaries.
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Nanocrystalline TiN Films Electrical Properties
0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14
40
60
80
100
120
140
160 25oC
200oC400oC500oC
600oCRes
istiv
ity(x
10-6
Ohm
cm
)
1/d(nm-1)
ratio of atoms on the grainboundary to that in the bulk;
grain boundary width;
grain size.
Resistivity increasing is mainly controlled by grain boundaryscattering.
dδ3=Ω
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Epitaxial and polycrystalline TaN
Single crystal cubic TaNwith TiN buffer layer
Hexagonal Polycrystalline TaN
Latticematching
Domain matching
Si a=5.43ATiN a=4.24ATaN a=4.33A
Cubic: metastable
phase;Hexagonal:
Stable phase.
Wang et al. Appl. Phys. Lett., 80, 2323 (2002).
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Single crystal cubic TaN on Si(100) and Si(111) using TiN buffer layer--TEM & STEM
Low Mag TEM and STEM HRTEM
Wang et al. Appl. Phys. Lett., 80, 2323 (2002).
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Cu diffusion into single crystal cubic TaNand polycrystalline TaN after 700oC, 30min
Single crystal TaN Polycrystalline TaN
Wang et al. Appl. Phys. Lett., 81, 1453 (2002).
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Cu diffusion characteristics in single crystal cubic TaN
1.00 1.02 1.04 1.06 1.08 1.10 1.12 1.14 1.16 1.18 1.20-41
-40
-39
-38
-37
-36
-35
-34
-33574 oC604636670707
Cu diffusion coefficient in single crystalline TaN
Activation energy Q0=3.27±0.1eV
600oC
650oC
700oC
lnD
(cm
2 /s)
1000/T(K)
12]/)1.027.3(exp[)5.9160( −±−±= scmTkeVD B
eVQTaNgb 6.127.35.0 ≈×=
eVQTiNgb 29.0=
TaN has a denser interstitial structure (more closely packed than TiN)
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TaN/TiN Superlattice as advanced Cu diffusion barrier
Low mag and SAD
Wang et al. Appl. Phys. Lett., 83, 3072 (2003)
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STEM of TaN/TiN Superlattice
Z-contrast intensity profile
STEM (Z-contrast)
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Electrical Property of TiN/TaN SL
0 50 100 150 200 250 300200
220
240
260
280
300
320
340
360
ρ (µΩ
-cm
)
T(K)0 50 100 150 200 250 300
70
80
90
100
110
120
130
Res
istiv
ity (µ
Ω-c
m)
T(K)
•TaN/TiN SL RT resistivity is ~130 µΩ-cm with a small temperature coefficient of resistivity of 0.002 K-1.•RT resistivity of TaN/TiN SL is lower than TaN itself.•semiconductor (TaN only) to metallic behavior.
TaN/TiN SLTaN only
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Growth defects in GaNsuperlattice LED devices
Threading dislocations terminate near the pits.
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Colossal magnetoresistance materials (CMR)
The influence of defects (misfit dislocations) on the performance of Colossal Magnetoresistance (CMR) materials. (Nd0.67Sr0.33MnO3 on LaAlO3)
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Acknowledgements
Drs. Quanxi Jia, Stephen Foltyn(Los Alamos National Laboratory)
Profs. Jagdish Narayan,Carl Koch, Mike Rigsbee, Hans Conrad.(North Carolina State University)