Determination of Magnetic Directions in Multiferroic BiFeO3 Thin Films

1
Determination of Magnetic Directions in
Multiferroic BiFeO3 Thin Films 
Mikel Barry Holcomb Department of Materials
Science and Engineering Department of Physics,
University of California, Berkeley Lawrence
Berkeley National Laboratory
Supported by LBL-LDRD SRC-NRI
2
Introduction to Multiferroics

• Multifunctional ? Material possess multiple order
  parameters
• Multiferroic ? Materials possess two or more of
  the following
• (Anti-)Ferromagnetism, (Anti-)Ferroelectricity,
  (Anti-)Ferroelasticity

• Coupling between order parameters

During and after applied field
Before applied field
field
3
BiFeO3 only room temp multiferroic
Ederer and Spaldin, PRB 71(2005)
TC 1103K
TN 643K
Polarization points in one of 8 possible lt111gt
directions. Magnetic plane is perpendicular to
the polarization direction.
4
Probing ME coupling
AFM X-ray dichroism PEEM
Ferroelectricity Piezoelectric force microscopy

• Parameters
• Film orientation
• Variants
• Strain

5
Absorption Studies of AFMs
Element Specific
A
B
Intensity map
LaFeO3
B/A
Before
6 mm
X-rays
XMLD
Ratio image Magnetic contrast
After
A. Scholl, Science 287 1014 (2000)
6
Electrical Control of AFM in BFO
PFM
OP PFM
Matching domains Same temperature dependence of
both as grown and poled areas
AFM is controlled by FE!
E
E
T. Zhao, M. Barry et al., Nat Mat 5, 823 (2006)
PEEM
7
Expected signal for XMCD
xrays
RCP
Ip (cos2 ?PH cos2 ?PV) q (cos2 ?MH cos2
?MV)
Top view
E
1
65
4
Intensity for Easy Plane
2
Intensity for Easy Axis
3
1,3
1,3
2
2
4
4
Sample Rotation
Linear
Linear
8
Split into darker medium grey in RCP
Linear
RCP
65
90 rotation
65
E
-25
-25
There may be a preferred magnetic axis in BFO
thin films!
9
Determining Which Axis with Grazing Studies
plane
P
lt112gt
a

• First observation of an easy axis in BFO
• Choice of axis is controllable by strain

lt110gt
Expected 110 Expected Plane Expected 112 Thin STO
Thick STO Thick DSO
10
Summary
Dichroism is rich for exploration in probing AFM
properties in magnetoelectric materials
Multiple order parameters can be separated by
taking advantage of temperature and
crystallography
Epitaxial BFO (001) has an easy magnetic axis,
instead of the easy plane predicted for bulk
11
Many Applications
Physics of AFM (Optical Studies)
FM
AFM/FE
Exchange Bias FM/AFM
MagnetoelectricCoupling AFM/FE
12
Many Applications
Elke Arenholz Cinthia Piamonteze  Joe Orienstein
Group Andrew Doran Andreas Scholl
Physics of AFM (Optical Studies)
S.Y. Yang Chan-Ho Yang
Aranta Fraile Lane Martin Eddie Chu Martin Gajek
Jan Seidel Ramesh
FM
AFM/FE
Exchange Bias FM/AFM
MagnetoelectricCoupling AFM/FE
Tong Zhao Kilho Lee Florin Zavaliche
13
Thick BiFeO3
At least 3 colors No unique axis for each
polarization direction Whole plane? Why
different? Epitaxial strain? Migloiri, Millis,
JAP, 83 (3), 1588
Another Color?
Grey
White
Black
5 ?m
XMLD
15 Micky Barry Holcomb
14
Temperature Dependent PEEM
T. Zhao, M. Barry et al., Nat Mat 5, 823 (2006)
We get roughly equal contributions from both
magnetic and ferroelectric prop. Can decouple the
contributions with temperature dependence Angle
dep. also allows separation and can determine
magnetic behavior.
Ferroelectric Contribution
8 Micky Barry Holcomb
15
Angle Dependence of AFM
Alders, PRB, 57 11 623
Pure AFM XMLD I (3 cos2 ?M -1)ltM2gt
background At given temperature Ip cos2 ?P
q cos2 ?M background, ?p??q?1 ?p?percentage
polarization contribution p and q can be positive
or negative cos2xsin2x1
L
X-rays
E
10 Micky Barry Holcomb
16
Expected signal for XMLD
1,3
e-
p
s
xrays
1
2,4
p
4
2
3
Ip cos2 ?P q cos2 ?M
11 Micky Barry Holcomb
17
Expected magnetic behavior
P
AFM domain
Ferroelectric domain
P
Look similar in XMLD images
Might there be a preferred axis?
Easy Magnetic Axis
9 Micky Barry Holcomb
18
FM-AFM coupling/switching with BFO
20x20 ?
In plane PFM
10x10 ?
10 ?
OOP PFM
18 Micky Barry Holcomb
19
Thank you!

• Publications
• T. Zhao, A. Scholl, F. Zavaliche, K. Lee, M.
  Barry, A. Doran, M.P. Cruz, Y.H. Chu, C. Ederer,
  N.A. Spaldin, R.R. Das, D.M. Kim, S. H. Baek,
  C.B. Eom, R. Ramesh, Electrical control of
  antiferromagnetic domains inmultiferroic BiFeO3
  films at room temperature, Nature Materials 5
  823 (Oct 2006).
• T. Zhao, A. Scholl, F. Zavaliche, H. Zheng, M.
  Barry, A. Doran, K. Lee, M.P. Cruz, R. Ramesh,
  Nanoscale x-ray magnetic circular dichroism
  probing of electric-field-induced magnetic
  switching in multiferroic nanostructures, APL
  90, 123104 (2007).
• R. Ramesh, N. Spaldin, M. Barry ... Thin film
  multiferroics Progress and prospects
• M. Barry, First observation of Easy Magnetic
  Axes in BiFeO3 Thin Films, Nature Physics, near
  submission
• L.W. Martin, Y.H. Chu, M. Barry, S.J. Han, Q.
  Zhan, P.L. Yang, K. Lee, A. Scholl, S.X. Wang,
  Z.Q. Qiu, R. Ramesh, Science, near submission
• K. Lee, M. Barry, Magnetic Anisotropy and
  Exchange Coupling in Multiferroic BiFeO3 Thin
  Films, PRL, near submission

19 Micky Barry Holcomb
20
Magnetoelectric Multiferroics Equations
Magnetoelectric Effect ? Describes the coupling
between electric and magnetic fields in
matter Contributions to the ME effect are
obtained from a materials free
energy Differentiations lead to both
polarization and to magnetization
Fiebig, J. Phys. D Appl Phys. 38 (2005) R1123-R152
1 Micky Barry Holcomb
21
Imaging Ferroelectric domains in BiFeO3
Piezoelectric Force Microscopy
White Black Brown
5 Micky Barry Holcomb
22
Electrical Control
Bi Fe O
8 possible polarization variants 3 types (180,
71 and 109) of ferroelectric polarization
switching
BiFeO3
SrRuO3
SrTiO3
F. Kubel, H. Schmid, Acta Cryst. B46, 698
(1990)
T. Zhao, M. Barry et al., Nat Mat 5, 823 (2006)
1 Micky Barry Holcomb
23
Magnetoelectric Multiferroics Materials

• Cr2O3
• Prototypical ME, first discovered
• P and M are field induced
• aZZ 4.13 ps m-1 _at_ E 106 V
• Corresponds to the magnetization observed after
  reversal of only 5 of every 106 spins in the AFM
  lattice!!
• Other ME Materials
• Ti2O3
• GaFeO3
• Boracites and Phosphates
• PbFe0.5Nb0.5O3
• Garnet films
• 80 total materials have ME effects

• Top Performers
• LiCoPO4 (aYX 30.6 ps m-1)
• Y3Fe5O12 films (a 30 ps m-1)
• TbPO4 (aAA 36.7 ps m-1)
• Principal weakness of ME Effect ? Perturbative
  nature of the microscopic mechanisms driving ME
  behavior
• and ME response is limited by
• aij2 lt ?iie ?jjm
• Where ?iie and ?jjm are the magnetic
  susceptibilities

Back
24
Devices

• Permanent Magnets
• Magnetic recording media or domain stabilizers
  in recording heads based on anisotropic MR
• GMR systems--reduction of the saturation fields
  to observe GMR

25
Total Electron Yield

• X-rays are absorbed by exciting a core electron
• Recombination of the core hole leads to emission
  of an Auger electron
• As they leave the sample, the primary Auger
  electrons inelastically scatter creating
  secondary electrons which dominate the total
  electron yield intensity.
• The sampling depth is typically a few nanometers.

1 Micky Barry Holcomb
26
Why does XMCD tell us about magnetism?
Photon ? Photoelectron ? Spin ? d holes
Transfer of angular momentum
Absorption Intensity ? angle between E and FM ?
number of d holes with a certain spin
3d
2p3/2
2p1/2
Back
27
Two Magnetic Axes
-
110
110
28
The Linear Magnetoelectric Effect

• Polarization and magnetization of a medium

• Covariant relativistic formulation

with

• Relativistic equivalence of electric and magnetic
  fields requires "magneto-electric"
  cross-correlation ( a) in matter

Back
29
Future Work
Focus 1 Thickness Dependence
Focus 2 Orientation Dependence
Physics of AFM (Optical Studies)
1 Micky Barry Holcomb
30
Piezoforce Microscopy of FE Domain Structure
(001)
(111)
(110)
P1-
1.5µm
?
1.5µm
IP
IP
IP
OP
001
1-10
(110) plane
(111) plane
P -
P2 -
P1 -
1 Micky Barry Holcomb
31
Future Work
Focus 1 Thickness Dependence
Focus 2 Orientation Dependence
Physics of AFM (Optical Studies)
MagnetoelectricCoupling AFM/FE
1 Micky Barry Holcomb
32
Electrical Control of AFM in BFO
OP PFM
Matching domains Same temperature dependence of
both as grown and poled areas
AFM is controlled by FE!
E
E
T. Zhao, M. Barry et al., Nat Mat 5, 823 (2006)
1 Micky Barry Holcomb