Post on 03-Jan-2016
University of Wisconsin-MadisonUniversity of Wisconsin-Madison Department of Materials Science and EngineeringDepartment of Materials Science and Engineering
Opportunities for Coherent Scattering in Ferroelectrics and
MultiferroicsPaul G. Evans
Department of Materials Science and Engineering
University of Wisconsin, Madison
evans@engr.wisc.edu
University of Wisconsin-MadisonUniversity of Wisconsin-Madison Department of Materials Science and EngineeringDepartment of Materials Science and Engineering
Outline• Ferroelectrics and multiferroics
– Manipulate electrical polarization and magnetism using applied fields
– Time resolution is crucial– Knowing where the atoms are in steady state is
helpful, but there are many other opportunities.
• Our work: Dynamics in complex oxides (extreme conditions, short times, coupling of ferroelectricity with magnetism)
• Goals: What can be uniquely probed by coherent techniques?
University of Wisconsin-MadisonUniversity of Wisconsin-Madison Department of Materials Science and EngineeringDepartment of Materials Science and EngineeringUniversity of Wisconsin-MadisonUniversity of Wisconsin-Madison Department of Materials Science and EngineeringDepartment of Materials Science and Engineering
100s MV/cm or more: Bond breaking?
Several MV/cm to tens of MV/cm: High-field regimes of interatomic interactions.
Up to 1 MV/cm: Polarization domain dynamics controls electromechanical and switching properties.
Two motivations: 1. Electric field scales for ferroelectric phenomena
University of Wisconsin-MadisonUniversity of Wisconsin-Madison Department of Materials Science and EngineeringDepartment of Materials Science and Engineering
More Generally: Electrostatically Driven Materials
Ahn et al., Rev. Mod. Phys. 78, 1185 (2006)
University of Wisconsin-MadisonUniversity of Wisconsin-Madison Department of Materials Science and EngineeringDepartment of Materials Science and Engineering
Applicable to a Wide Range of Systems
Ahn et al., Rev. Mod. Phys. 78, 1185 (2006)
University of Wisconsin-MadisonUniversity of Wisconsin-Madison Department of Materials Science and EngineeringDepartment of Materials Science and Engineering
What Can be Learned?
• How fast can these transitions be? How homogeneous are they in space?
• What other structural transitions can be driven?• Fundamental physics of these phase transitions
have been previously available only by changing T (or doping, or H, etc.). Nothing fast!
• Short pulses go along with high E fields.
University of Wisconsin-MadisonUniversity of Wisconsin-Madison Department of Materials Science and EngineeringDepartment of Materials Science and Engineering
New Potential to Understand the High-Field Regime
Souza et al., Phys. Rev. Lett. 89, 117602 (2002).
University of Wisconsin-MadisonUniversity of Wisconsin-Madison Department of Materials Science and EngineeringDepartment of Materials Science and Engineering
Ti-O3 and Ba-O1 pairs move rigidly at high fields Phonon modes becomes stiffer and dielectric constant becomes smaller. Piezoelectric response should get weaker at high electric fieldsE > 16 MV/cm for BaTiO3 or E > 2.5 MV/cm for PbTiO3
Changes in Atomic Interactions at High Fields
N. Sai, K. M. Rabe, and D. Vanderbilt, Phys. Rev. B 66, 104108 (2002).
BaTiO3
University of Wisconsin-MadisonUniversity of Wisconsin-Madison Department of Materials Science and EngineeringDepartment of Materials Science and Engineering
Two motivations: 2. Timescales of Dynamic Phenomena
NSLS II: 15 ps
Other sources ~100 ps
FELs?
University of Wisconsin-MadisonUniversity of Wisconsin-Madison Department of Materials Science and EngineeringDepartment of Materials Science and Engineering
Epitaxial PZT Thin Film Capacitors
Tunability1) Composition: throughout tetragonal range, x >
0.5.2) Thickness: few unit cells to hundreds of nm.3) Device configuration.
University of Wisconsin-MadisonUniversity of Wisconsin-Madison Department of Materials Science and EngineeringDepartment of Materials Science and Engineering
Avalanche photodiode
monochromator
sample
Advanced Photon Source
Synchrotron X-ray Microscopy
Fresnel zone plate
e-
University of Wisconsin-MadisonUniversity of Wisconsin-Madison Department of Materials Science and EngineeringDepartment of Materials Science and Engineering
Synchronization
With B. Adams and S. Ross (APS).
University of Wisconsin-MadisonUniversity of Wisconsin-Madison Department of Materials Science and EngineeringDepartment of Materials Science and Engineering
Knife edge scan
Microdiffraction and time-resolved x-ray scattering
Spatial Resolution
Piezoelectric lattice distortion in Pb(ZrxTi1-x)O3,
Grigoryev et al., Phys. Rev. Lett. 96, 187601 (2006)
Time Resolution
-400 -200 0 200 4000
200
400
600
800
1000
1200
1400
data fit
Co
un
ts
Knife edge position (nm)
~110 nm
Cr
fluo
resc
ence
inte
nsit
y
Cr knife edge position (nm)
2 (
deg.
)
time (ps)
strain (%)
University of Wisconsin-MadisonUniversity of Wisconsin-Madison Department of Materials Science and EngineeringDepartment of Materials Science and EngineeringUniversity of Wisconsin-MadisonUniversity of Wisconsin-Madison Department of Materials Science and EngineeringDepartment of Materials Science and Engineering
Local piezoelectric response
switching
Each point is a result of 21000 switching cycles
Switching is reproducible
Measure domain wall velocities
Structural signatures of polarization switching
What is the structure during switching?
University of Wisconsin-MadisonUniversity of Wisconsin-Madison Department of Materials Science and EngineeringDepartment of Materials Science and Engineering
0 20 40 60 804.24
4.25
4.26
4.27
4.28
4.29
4.30
0.00
0.25
0.50
0.75
1.00
latt
ice
cons
tant
c (
Å)
time (ns)
RC time constant ~ 12 ns
str
ain
(%)
E
2.19 MV/cm
Voltage pulseturned on
Voltage pulseturned off
O
Pb
Ti
a
c
E
Piezoelectricity in large electric fields
University of Wisconsin-MadisonUniversity of Wisconsin-Madison Department of Materials Science and EngineeringDepartment of Materials Science and Engineering
Three regimes:1) E < 1.8 MV/cm Linear piezoelectricity similar to low fields.
2) E ~ 2 MV/cm Meets predicted bond elongation induced by high tetragonality.
3) E > 2.5 MV/cm Indicates the system might be approaching the regime of strong repulsive interaction.
Piezoelectric strain at high electric fields
tetragonalityc/a = 1.1
3 = d33 E3,
d33 45 pm/V
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Ultrahigh piezoelectric strain 2.69%
Elastic piezoelectric strain of 2.69%.
inte
nsit
y
35 nm PZT, ~24.4 V pulse, 8 ns duration
University of Wisconsin-MadisonUniversity of Wisconsin-Madison Department of Materials Science and EngineeringDepartment of Materials Science and Engineering
Can we see polarization switching at the intrinsic coercive electric field?
Mechanical hysteresis with 50 ns pulses.
Ec (low frequency)
Ec (intrinsic, prediction)
This would involve using pulses so fast that the domains cannot respond. What is the structure during intrinsic switching?
University of Wisconsin-MadisonUniversity of Wisconsin-Madison Department of Materials Science and EngineeringDepartment of Materials Science and Engineering
Questions• Interfaces are important. What is the
structure of the entire device? How does it change in applied fields?
• So far we’ve discussed the film independently of its electrodes, and as a homogeneous structure. But this is clearly not the case.
University of Wisconsin-MadisonUniversity of Wisconsin-Madison Department of Materials Science and EngineeringDepartment of Materials Science and Engineering
Potential first step: Coherent probes for domain dynamics
Partially switched: large disorder
More completely switched: less disorder
Hypothetical domain configuration
Resulting coherent scattering pattern
Results give domain dynamics, structure, nucleation physics.
University of Wisconsin-MadisonUniversity of Wisconsin-Madison Department of Materials Science and EngineeringDepartment of Materials Science and Engineering
Where do we stand?
N. A. Spaldin and M. Fiebig, Science 309, 391 (2005).
University of Wisconsin-MadisonUniversity of Wisconsin-Madison Department of Materials Science and EngineeringDepartment of Materials Science and Engineering
Nanomagnetism and Spintronics
Krivorotov et al., Science 307, 228 (2005).
Coherent Magnetic Oscillations due to Spin Transfer Torque
Perspective by Covington, Science 307, 215 (2005).
Dynamics are relatively slow now, but only beginning to be explored.
University of Wisconsin-MadisonUniversity of Wisconsin-Madison Department of Materials Science and EngineeringDepartment of Materials Science and Engineering
Spintronics with Complex Materials
H. Bea et al., Appl. Phys. Lett. 89, 242114 (2006).
• Exchange Bias in Complex Oxide Systems
BiFeO3 is ferroelectric, so now expect dynamics in the structure and the magnetism – and coupling between them.
University of Wisconsin-MadisonUniversity of Wisconsin-Madison Department of Materials Science and EngineeringDepartment of Materials Science and Engineering
e.g. Wang et al. Science 299, 1719 (2003).
Pt electrode (150 nm)
BiFeO3 (400 nm)
SrTiO3 (001) substrate
BiFeO3 Thin Films
SrRuO3 (15 nm)
University of Wisconsin-MadisonUniversity of Wisconsin-Madison Department of Materials Science and EngineeringDepartment of Materials Science and Engineering
Polarization Switching in BiFeO3
poled with +10 VIntegrated intensity: 1
5 m
poled with -10 VIntegrated intensity: 0.76
5 m
poled with +10 VIntegrated intensity: 1.1
5 m
University of Wisconsin-MadisonUniversity of Wisconsin-Madison Department of Materials Science and EngineeringDepartment of Materials Science and Engineering
Nonresonant Magnetic X-ray Scattering
Non-resonant magnetic x-ray scattering, after DeBergevin and Brunel, (1980)
University of Wisconsin-MadisonUniversity of Wisconsin-Madison Department of Materials Science and EngineeringDepartment of Materials Science and Engineering
Antiferromagnetic Domains in Cr
Evans and Isaacs J. Phys. D (2006).Evans, et al. Science (2002).
University of Wisconsin-MadisonUniversity of Wisconsin-Madison Department of Materials Science and EngineeringDepartment of Materials Science and Engineering
Magnetism in BiFeO3
Fe
Fe
Fe
Fe
(Is this the spin polarization?)
University of Wisconsin-MadisonUniversity of Wisconsin-Madison Department of Materials Science and EngineeringDepartment of Materials Science and Engineering
{111} FamilyStructural
{½ ½ ½} FamilyMagnetic
Reflections withunmixed indices
Reflections with indices having mixed signs
University of Wisconsin-MadisonUniversity of Wisconsin-Madison Department of Materials Science and EngineeringDepartment of Materials Science and Engineering
Dynamics of the “Other” Multiferroic Relationships
How does antiferromagnetism respond to applied electric fields?
1. Rearrangement of spin polarization domains?2. Canted (slightly) ferromagnetic spin arrangement?
Can we reach a tetragonal phase? What happens to domains?
Canted structure?Ederer and Spaldin, Phys. Rev. B (2005).
University of Wisconsin-MadisonUniversity of Wisconsin-Madison Department of Materials Science and EngineeringDepartment of Materials Science and Engineering
Conclusion• Ferroelectrics in extreme electric fields: non-linear piezoelectricity.
Can we exploit high strains (2.7% so far) and high bandwidths (few GHz so far)?
Other phenomena in extreme electric fields?What is the structure under high electric fields?
• Multiferroics: piezoelectricity, switching.Dynamics of relationship between magnetism and polarization?What is the magnetic structure?Can we probe magnetism coherently?
This work was supported by DOE through the BES X-ray and Neutron Scattering Program and by NSF through the Ceramics program of the Division of Materials Research.