Probing Ferroelectricity, Charge Density Wave Dynamics, and Magnetism with Submicron X-ray Diffraction Paul G. Evans Department of Materials Science and

$Page 1: Probing Ferroelectricity, Charge Density Wave Dynamics, and Magnetism with Submicron X-ray Diffraction Paul G. Evans Department of Materials Science and$
Probing Ferroelectricity, Charge Density Wave Dynamics, and

Magnetism with Submicron X-ray DiffractionPaul G. Evans

Department of Materials Science and Engineering

University of Wisconsin, Madison

[email protected]

APS SACMicrobeam ReviewJanuary 21, 2004

Outline• Brief Introduction to Microbeam Experiments at

Sector 7 of the APS• Overview: Physical Phenomena and Motivation• In Depth: Polarization Switching and Fatigue in

PZT Thin Films, Magnetic Domain Evolution in Antiferromagnetic Chromium

• Conclusions, Future Directions

MHATT-CAT’s mission:To develop a productive, open-access center for world-class research in x-ray science exploiting the unique characteristics of the APS, especially timing

and brilliance.

Unique Capabilities:• Ultrafast Laser Facility• Staff Expertise in Ultrafast Optics and X-ray Science, Diffraction, Scattering, and Spectroscopy• White Beam Diffraction with Submicron Focusing with G. Ice-style Mirrors• Small Angle Scattering/XPCS with Fast CCD Direct Detection

MHATT-CAT Sector 7

Center for Real-time X-ray

Studies www.mhatt.aps.anl.gov

• Relaxation dynamics of magnetic polymers using XPCS (collaboration with Ford Research, Dearborn, MI)

• Micro-fluorescence mapping of mineral intrusions (Dan Core, Geosciences Dept., Michigan)

• Development of Li-metal x-ray lenses (Nino Pereia, Ecopulse Inc., Fairfax, VA)

• Studies of core excitations in Kr microjets (Linda Young, ANL)

• Doped magnetic semiconductors (Frank Tsui, U. of North Carolina, Y. Chu, APS)

Microbeam General User

Research at Sector 7

DYNAMICS IN MAGNETORHEOLOGICAL ELASTOMERS

William F. Schlotter, Codrin Cionca, Sirinivas S. Paruchuri, Jevne B. Cunningham, Eric Dufresne, Steve B. Dierker, Dohn Arms and Roy ClarkeUniversity of Michigan, Ann Arbor John M. Ginder and Mark E. NicholsFord Motor Company, Research Laboratory, Dearborn, MI

CCD

Scintillation counter

electromagnet

sample

detectors

Coherent small angle scattering.

MR sample is rigidly mounted while direction of magnetic field is alternated

W. F. Schlotter, et al., Int. J. Mod. Phys. B 16 2426-32 (2002).

focused

unfocused

LITHIUM REFRACTIVE LENSES

Sawtooth Li Lens

Recent results: flux gain ~ 40spot size ~ 20 m

E. M. Dufresne, et al., Appl. Phys. Lett. 79, 4085 (2001).

N. R. Pereira, et al., Rev. Sci. Instrum. 75 37 (2004).

Sawtooth and Parabolic Lithium Lenses

X-ray Microdiffraction as a Tool for Physics and Materials Science

• 6 to 30 keV

• 1010 ph/s/0.01%BW

• Minimum spot (APS, sector 2)- 0.15 x 0.15 µm2

Contrast from: Diffraction, Composition, Ferroelectric Polarization, Magnetization.Problems with existing techniques: time resolution, electrodes, quantification.

Submicron Science with X-ray Diffraction

Strain in MEMS-inspired substrates for SiGe epitaxial growth. (Sector 2)

With B. Lai, Z. Cai (APS),P. Rugheimer, and M. Lagally (U. Wisc.)(P. G. Evans et al., submitted, Jan. 2004)

Charge density wave dynamics in NbSe3 (Sector 2)

With R. Thorne (Cornell U.)

-0.4

-0.2

0

0.2

0.4

-300 -200 -100 0 100 200 300

(220) reflection(004) reflectionsubstrate (220)

rock

ing

cu

rve

pe

ak

shift

(d

eg

.)

position (m)

Si3N

4Si

3N

4bridge

V=0 V=131.3 mV

Submicron Science

Spin density wave antiferromagnetism in Cr(Sector 2)

With E. Isaacs, B. Lai, Z. Cai (ANL)(P. G. Evans et al., Science 2002)

Polarization reversal and piezoelectric distortion in ferroelectric PZT thin films.(Sector 7)

With C.-B. Eom, (U. Wisc) and E. Dufresne (U. Mich/Sector 7)(D.-H. Do et al., submitted, Nov. 2003)

Outline• Brief Introduction to Microbeam Experiments at

Sector 7 of the APS• Overview: Physical Phenomena and Motivation• In Depth: Polarization Switching and Fatigue in

PZT Thin Films, Magnetic Domain Evolution in Antiferromagnetic Chromium

• Conclusions, Future Directions

Epitaxial PZT Thin Film Capacitors

2 Pr 95 C/cm2

angle relative to STO (002) (sec.)

inte

nsity

tetragonal PbZr1-xTixO3

x=0.5580 nm or 160 nm PZT thickness

Time Resolved Microdiffraction with Zone Plate Optics at Sector 7

Piezoelectric xyz stage

Electrical connection to sample

Avalanche photodiode detector (up to ~3 M cps).

Incident beam

Use avalanche photodiode with multichannel scaler to time-resolve the diffraction signal during voltage pulses.

Spatial Resolution: Knife EdgeFluorescence Scan

Aside: Spatial Resolution at Sector 7

0.6 m FWHM

Limitations at Sector 7: Vibrations, Be Windows (?)

Diamond Demonstration Version Diamond Demonstration Version Diamond Demonstration Version







(002) x-ray reflection

x-ray reflection

)200(

Intensities are different because PZT lacks inversion symmetry!

Probing Ferroelectric Polarization Switching with X-ray Microdiffraction

X-ray photon energy energy 10 keV.

Maps of the intensity of PZT (002) reflection vs. the position of the beam on the sample. Intensity following –10 V pulse is 25% higher than

Imaging PZT Polarization Switching

0.8V 1.2V 1.6V

2.0V 2.8V

-10V

20 m

1. Apply negative voltage pulse to produce uniform polarization.

Switching with E Ec

2. Apply a positive pulse near Ec.

Device switches in well defined areas.

500 s triangle pulses

-10 -5 0 5 10-20

0

20

40

60

80

100

120

140

Applied Voltage (V)

Δ Pr

SwitchedPolarization

Quantitative Relationship of Polarization to Switched Area

Switched Area Measured from X-ray Microdiffraction Maps is Proportional to the Total Switched Polarization.

Time Resolved Diffraction During Switching- 10 V

+ 10 V

20 m

time (ms)0 1

0 20 40 60 80 1000

10

20

30

40

50

20 40 60 80 100

1750

2000

2250

2500

2750

3000

inte

nsit

ytime (ms)

0 1

time (ms)0 1

20 40 60 80 100

2000

2500

3000

inte

nsit

y

0 20 40 60 80 1000

10

20

30

40

50 +10V

0V

-10V

Applied

Voltage

time (ms)0 1

two

thet

a (d

eg.)

34.52

35.02

Switching of 200 m diameter device limited by RC time constant.

+10V

0V

-10V

Applied

Voltage

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.timems

34.62

34.72

34.82

34.92

35.02

owt

atehtged.

Positive pulses

Negative pulses

Quantitative Measurement of Piezoelectric Distortion

Quantitative measurement: Requires No Standards.

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.timems

34.62

34.72

34.82

34.92

35.02

owt

atehtged.

+10V

0V

-10V

Applied

Voltage

Both polarizations haved33=60 pm V-1.

257-5

An Electromechanical Puzzle

We think: Difference between top and bottom contacts.

During switching experiments the piezoelectric coefficients for positive and negative polarizations are different.

-10 -5 0 5 10-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

Str

ain

(%)

Voltage(V)

257-5 257-7

0 20 40 60 80 1000

10

20

30

40

50

time (ms)

0 1

two

thet

a (d

eg.)

34.52

35.02

Polarization Fatigue

Polarization Fatigue: 160 nm, ± 10 V pulses

Higher Electric Field Pulses Partially Restore the Switchable

Polarization

Polarization Fatigue at Higher Electric Fields

Polarization Fatigue at Higher Electric Fields

Structural Signature of Fatigue at High Electric Fields

Compare Structures at Points Outside the Device With Areas Fatigued at High Electric Fields

Theta-Two Theta Scans at Two Locations

80 nm thickness,105 cycles, ± 10 V

Conclusions: Ferroelectrics

1. Fatigue in PZT devices with Pt electrodes pins the polarization in the state that would normally follow a positive voltage pulse to the bottom electrode.

2. Fatigue at higher electric fields in PZT thin films is accompanied by the gradual spread of a structurally distinct area.

Fatigue

1. X-ray microdiffraction provides a new avenue to studying polarization switching in ferroelectric devices.

2. Quantitative agreement of x-ray observations with electrical measurements of switching.

3. Potential to take advantage of the flexibility of x-ray scattering techniques: resonant scattering, time resolved diffraction…

Imaging

Spin density wave domains in chromium

• Domains are responsible for macroscopically observable magnetic, mechanical, electrical phenomena.

• Previous domain imaging experiments are at the 1 mm scale.

Cr is a spin density wave (SDW) antiferromagnet.

Antiferromagnetic Domains: •Modulation direction Q any <001>

Three possible Q domains.•Spin polarizations S, also <001>

Two S domains in transverse phase. Just one S (||Q) in longitudinal phase.

Cr unit cell

S || [100]

Q || [001]

•SDW leads to strain wave and charge density wave (CDW).

•Spins are transverse T=123 to 311 K, longitudinal for T<123 K.

Fermi surface nesting in Cr(q) susceptibility, response of hypothetical non-magnetic

system to magnetic perturbation with wavevector q

Difficult to calculate (q) directly, but common feature is

K KqK

qEE

1)(

where EK+q and EK are pairs of filled and empty states differing in wavevector by q.

QSDW

Band structure of Cr: Fermi surfaces nest with Q=(0,0,1-) , incommensurate with lattice.

Force Radiation

-eE electric dipole “Thomson scattering”

-eEmagnetic quadrupole

electric dipole)( H

H magnetic dipole

After F. de Bergevin and M. Brunel, Acta Cryst. A 37 314 (1981).

EE

EH

H E

H H

-e

-e

-e

-e

(Non-Resonant) Magnetic X-ray Scattering: Classical Picture

Cr in reciprocal space

CDWnear (0 0 2)

SDWnear (0 0 1)

L

K

Magnetic scattering appears near forbidden lattice reflections.

Also: Strain wave (CDW) reflections near allowed lattice reflections.

Form images using either type of reflection.

H

incident beam k

diffracted beam k’

ST

SL

ST

Non-resonant magnetic x-ray diffraction from Cr

Most important term of cross section scales as:

2)ˆˆ( 'kkS

Polar plot of cross section as a function of spin direction for a Q || (001) domain in our geometry.

k’-k || Cr (0 0 1-)

APS Station 2ID-D

All three Q domains are present

Room temperature laboratory diffractometer scans with large mm-scale beam.

h scan near (2 0 0)

k scan near (0 2 0)

l scan near (0 0 2)0.4

0.6

0.8

1

1.2

0.04 0.06 0.08 0.1

inte

nsity

(a.u

.)

(2+,0,0)

(0,2+,0)

(0,0,2+)

Domain populationsH : K : L4.65 : 1.35 : 1 Visit one CDW

reflection from each family.

B C D W 1 0 0 m

0

4

8

0 . 9 5 0 . 9 5 3 0 . 9 5 6L

C

1

2

SD

W

inte

nsity

co

unts

s-1

A S D W 1 0 0 m

2 1

*

SDW Domains at 130 KSDW magnetic reflection

CDWcharged reflection

incident beam h=5.8 keV

incident beam h=11.6 keV

Spin-flip transitionTransverse SDW phase Longitudinal SDW phase

TSF=123 Kin bulk Cr

Image SDW reflection as a function of temperature.

Magnetic reflection disappears!

10 m 2

4 3

1

10 m 10 m 10 m

10 m 10 m 10 m

140 K 130 K 125 K 120 K

110 K 119 K 115 K 1

4

Q || (001) and SQ

Q (001) or S||Q

Repeat with charged CDW reflectionSDW Magnetic reflection CDW Charged reflection

T=130 K T=130 K

T=110 K T=110 K

Spin flip transition begins at Q domain edges

Nominally first order transition is broadened by several degrees, even at micron scale.

8.6

8.7

0

5

10

110 120 130 140

T (K)

2

3

4

1

mea

n in

tens

ity

(cou

nts

s-1

)

mag

net

iza

tion

(em

u/cm

3 )

H=2 kG

H=0

T (K)

10 m 2

4 3

1

10 m 10 m 10 m

10 m 10 m 10 m

140 K 130 K 125 K 120 K

110 K 119 K 115 K 1

4

Q || (001) and SQ

Q (001) or S||Q

Learning more about domain walls

CDWnear (0 0 2)

SDWnear (0 0 1)

L

K

H

red Q || [100] green Q || [010] blue Q || [001]

1) So far we’ve looked at Q || [001]. What happens in neighboring Q domains?

2) Two spin polarizations within transverse phase.

transverse phase

S || [010]

transverse phase

S || [001]

longitudinal phase

S || [100]

Magnetic cross sections in a Q || [100] domain

incident beam k

diffracted beam k’

Cross section with S along [100] or [010] than along [001].


Magnetic imaging of three domains

Conclusions: Chromium• Self organized or artificial domains at small scales are

key to macroscopic properties. Imaging is important.• Spin flip transition in Cr begins at domain walls upon

cooling.• Future work in Cr:

– Control of domain walls

– Separation of bulk and interface effects

Conclusion

• Today: Cr antiferromagnetism and PZT ferroelectricity

• Future directions:– Coupling of strain between layers in multilayer

films– Direct measurements of ferroelectric

polarization domain wall velocity– Size effects in ferroelectric materials

Documents

Probing Ferroelectricity, Charge Density Wave Dynamics, and Magnetism with Submicron X-ray Diffraction Paul G. Evans Department of Materials Science and