University of Wisconsin-Madison Department of Materials Science and Engineering Opportunities for...

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

[email protected]


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


More Generally: Electrostatically Driven Materials

Ahn et al., Rev. Mod. Phys. 78, 1185 (2006)


Applicable to a Wide Range of Systems

Ahn et al., Rev. Mod. Phys. 78, 1185 (2006)


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.


New Potential to Understand the High-Field Regime

Souza et al., Phys. Rev. Lett. 89, 117602 (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 > 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


Two motivations: 2. Timescales of Dynamic Phenomena

NSLS II: 15 ps

Other sources ~100 ps

FELs?


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.


Avalanche photodiode

monochromator

sample

Advanced Photon Source

Synchrotron X-ray Microscopy

Fresnel zone plate

e-


Synchronization

With B. Adams and S. Ross (APS).


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 (%)


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?


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


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


Ultrahigh piezoelectric strain 2.69%

Elastic piezoelectric strain of 2.69%.

inte

nsit

y

35 nm PZT, ~24.4 V pulse, 8 ns duration


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?


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.


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.


Where do we stand?

N. A. Spaldin and M. Fiebig, Science 309, 391 (2005).


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.


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.


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)


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


Nonresonant Magnetic X-ray Scattering

Non-resonant magnetic x-ray scattering, after DeBergevin and Brunel, (1980)


Antiferromagnetic Domains in Cr

Evans and Isaacs J. Phys. D (2006).Evans, et al. Science (2002).


Magnetism in BiFeO3

Fe

Fe

Fe

Fe

(Is this the spin polarization?)


{111} FamilyStructural

{½ ½ ½} FamilyMagnetic

Reflections withunmixed indices

Reflections with indices having mixed signs


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).


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.

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