Opportunities for Coherent Scattering in Ferroelectrics and Multiferroics

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Opportunities for Coherent Scattering in
Ferroelectrics and Multiferroics

• Paul G. Evans
• Department of Materials Science and Engineering
• University of Wisconsin, Madison
• evans_at_engr.wisc.edu

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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?

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Two motivations 1. Electric field scales for
ferroelectric phenomena
Several MV/cm to tens of MV/cm High-field
regimes of interatomic interactions.
100s MV/cm or more Bond breaking?
Up to 1 MV/cm Polarization domain dynamics
controls electromechanical and switching
properties.
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More Generally Electrostatically Driven Materials
Ahn et al., Rev. Mod. Phys. 78, 1185 (2006)
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Applicable to a Wide Range of Systems
Ahn et al., Rev. Mod. Phys. 78, 1185 (2006)
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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.

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New Potential to Understand the High-Field Regime
Souza et al., Phys. Rev. Lett. 89, 117602 (2002).
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Changes in Atomic Interactions at High Fields
BaTiO3
N. Sai, K. M. Rabe, and D. Vanderbilt, Phys. Rev.
B 66, 104108 (2002).
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 gt 16 MV/cm for BaTiO3 or E gt
2.5 MV/cm for PbTiO3
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Two motivations 2. Timescales of Dynamic
Phenomena
Other sources 100 ps
NSLS II 15 ps
FELs?
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Epitaxial PZT Thin Film Capacitors

• Tunability
• Composition throughout tetragonal range, x gt
  0.5.
• Thickness few unit cells to hundreds of nm.
• Device configuration.

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Synchrotron X-ray Microscopy
Fresnel zone plate
e-
sample
monochromator
Avalanche photodiode
Advanced Photon Source
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Synchronization
With B. Adams and S. Ross (APS).
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Microdiffraction and time-resolved x-ray
scattering
Spatial Resolution
Time Resolution
Knife edge scan
Piezoelectric lattice distortion in
Pb(ZrxTi1-x)O3, Grigoryev et al., Phys. Rev.
Lett. 96, 187601 (2006)
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Structural signatures of polarization switching
Local piezoelectric response
Each point is a result of 21000 switching cycles
Switching is reproducible Measure domain wall
velocities
What is the structure during switching?
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Piezoelectricity in large electric fields
Voltage pulse turned on
Voltage pulse turned off
E
2.19 MV/cm
E
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Piezoelectric strain at high electric fields
Three regimes 1) E lt 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 gt 2.5 MV/cm Indicates
the system might be approaching the regime of
strong repulsive interaction.
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Ultrahigh piezoelectric strain 2.69
35 nm PZT, 24.4 V pulse, 8 ns duration
Elastic piezoelectric strain of 2.69.
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Can we see polarization switching at the
intrinsic coercive electric field?
Ec (low frequency)
Ec (intrinsic, prediction)
Mechanical hysteresis with 50 ns pulses.
This would involve using pulses so fast that the
domains cannot respond. What is
the structure during intrinsic switching?
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Questions

• Interfaces are important. What is the structure
  of the entire device? How does it change in
  applied fields?
• So far weve discussed the film independently of
  its electrodes, and as a homogeneous structure.
  But this is clearly not the case.

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Potential first step Coherent probes for domain
dynamics
Hypothetical domain configuration
Resulting coherent scattering pattern
Partially switched large disorder
More completely switched less disorder
Results give domain dynamics, structure,
nucleation physics.
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Where do we stand?
N. A. Spaldin and M. Fiebig, Science 309, 391
(2005).
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Nanomagnetism and Spintronics
Coherent Magnetic Oscillations due to Spin
Transfer Torque
Perspective by Covington, Science 307, 215 (2005).
Krivorotov et al., Science 307, 228 (2005).
Dynamics are relatively slow now, but only
beginning to be explored.
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Spintronics with Complex Materials

• Exchange Bias in Complex Oxide Systems

H. Bea et al., Appl. Phys. Lett. 89, 242114
(2006).
BiFeO3 is ferroelectric, so now expect dynamics
in the structure and the magnetism and coupling
between them.
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BiFeO3 Thin Films
Pt electrode (150 nm)
BiFeO3 (400 nm)
SrRuO3 (15 nm)
e.g. Wang et al. Science 299, 1719 (2003).
SrTiO3 (001) substrate
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Polarization Switching in BiFeO3
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Nonresonant Magnetic X-ray Scattering
Non-resonant magnetic x-ray scattering, after
DeBergevin and Brunel, (1980)
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Antiferromagnetic Domains in Cr
Evans and Isaacs J. Phys. D (2006). Evans, et
al. Science (2002).
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Magnetism in BiFeO3
Fe
Fe
Fe
Fe
(Is this the spin polarization?)
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Reflections with unmixed indices
Reflections with indices having mixed signs
111 Family Structural
½ ½ ½ Family Magnetic
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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?

Canted structure? Ederer and Spaldin, Phys. Rev.
B (2005).
Can we reach a tetragonal phase? What happens to
domains?
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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.