1. Low Temperature Impedance of Multiferroic BiMnO3 Thin Films
The University of Sheffield, Department of Engineering Materials Rainer Schmidt
Low Temperature Impedance of Multiferroic BiMnO3 Thin Films
Rainer Schmidt
Speaker: Antonio Feteira
The University of Sheffield
Engineering Materials
Electroceramics Research Group
14th November 2007
7th Pacific Rim Conference on
Glass and Ceramics Technology
S12-32-O

2. Low Temperature Impedance of Multiferroic BiMnO3 Thin Films
The University of Sheffield, Department of Engineering Materials Rainer Schmidt
Low Temperature Impedance of Multiferroic BiMnO3 Thin Films
1. Introduction
2. Impedance Spectroscopy
3. Multiferroic BiMnO3
4. Thin Film Epitaxy
5. Equivalent Circuit Analysis
6. Conclusions

3. Any Useful for Interface
1. Introduction Rainer Schmidt
Dielectric and Resistive Characterisations by Impedance Spectroscopy
Well Established:
Hardly ever Used:
Polycrystalline Bulk Material
Separation of Contributions from
Electrode - Sample Interface Layer
Grain Boundary Areas
Grain Interior Bulk Material
Thin Epitaxial Layers
Epitaxial Layer
Electrodes
Wafer Substrate
No Grain Boundaries
Any Useful for Interface
and Film Separation ???

4. Impedance Spectroscopy
2. Impedance Spectroscopy Rainer Schmidt
Impedance Spectroscopy
Application of an Alternating Voltage Signal to a Sample:
Measurement of the Alternating Current Response:
Time Dependent Definition of the Impedance:
U(w,t )=U0 cos(w t )
I(w,t ) = I0 cos(w t +d )
U(w,t ) U0 cos(w t )
Z(w,t )
I(w,t ) I0 cos(w t + d )
Time Independent Complex Impedance: Z*
(d )
(d )
(id )

5. U, I Ideal Capacitor Time Dependent Notation Phase Arrow Diagram
2. Impedance Spectroscopy Rainer Schmidt
Time Dependent Notation
Phase Arrow Diagram
U, I
U=U0 cos(w t)
IC=I0 cos(w t-p/2) C R
IRC=I0 cos(w t –d ) d
IR=I0 cos(w t) g
CPE
IC-CPE=I0 cos(w t -p/2+g)
ZC-CPE=1/(iw)n C ;
n ~ 1
Ideal Capacitor
ZR=R
ZC=1/iwC
IL=I0 cos(w t +p/2)
ZL=iwL
Time Independent Complex Impedance
g = (1-n) 90º

6. Complex Relationship Dielectric Constant – Capacitance Relationship
2. Impedance Spectroscopy Rainer Schmidt
Complex Relationship
Dielectric Constant – Capacitance Relationship
Contact Area A d
Contact Distance
Capacitance of the Measuring Cell in Vacuum

7. Relative Dielectric Constant
3. Multiferroic BiMnO Rainer Schmidt
Multiferroic Magneto-Electric BiMnO3 (BMO)
Ferroelectric below TCurie ~ 760 K
Kimura et al., Phys.Rev.B 67 (2003) R180401
Ferromagnetic below TCurie ~ 105K
Chiba et al., J.Solid State Chem. 132 (1997) p 139
Remnant Polarisation Pr at 200 K:
43 nC/cm2 (Ceramic)
Moreira dos Santos et al., Solid State Commun. 122 (2002) p 49
Remnant Polarisation Pr at 120 K:
4.1 nC/cm2 (Thin Film)
Spontaneous Magnetisation below TC:
~ 3.6 mB/ Mn (Ceramic)
Chiba et al., J.Solid State Chem. 132 (1997) p 139
~ 2.2 mB/ Mn (Thin Film)
Eerenstein et al., Science, 307 (2005) p 1203a
Magneto-Capacitance
in BMO Polycrystalline Bulk
Kimura et al., Phys.Rev.B 67 (2003) R180401
Relative Dielectric Constant
Weak Dielectric
Response at TC
Temperature (K)

8. Pulsed Laser Deposition of BiMnO3 Thin Films
4. Thin Film Epitaxy Rainer Schmidt
Pulsed Laser Deposition of BiMnO3 Thin Films
1. Vacuum Annealing of the
1%at. Nb doped STO Substrate
at 700°C for 1 h
2. Deposition at 0.1 Pa O2
Partial Pressure at 450°C
- Variable Deposition Time
- 1.5 J·cm-2 Laser Fluence
- 1 Hz Laser Pulse
Frequency
3. Post Deposition Annealing
at 60 kPa O2 Partial Pressure
at 500°C for 1 Hour
See also: W.
Eerenstein et al., Appl.Phys.Lett. 87 (2005)

9. Sample Geometry Equivalent Circuit 50 nm
4. Thin Film Epitaxy Rainer Schmidt
Equivalent
Circuit
Sample Geometry
50 nm

10. Complex Impedance Data of Epitaxial BiMnO3
5. Equivalent Circuit Analysis Rainer Schmidt
Complex Impedance Data of Epitaxial BiMnO3
Modulus M ''
Discrimination
Between
Interface and
Intrinsic Film
Contributions
can be Achieved
by Analysing the
Capacitance C1
and C2 in
Dependence on
Film Thickness
Schmidt et al.,
Phys.Rev.B 75 (2007) p
Frequency Range: 40 Hz – 2.5 MHz ; Voltage Signal: 50 mV
= Data = Model; measured at 75 K
Specific impedances z', -z'' in Ω·cm normalised by g = A (contact area) / d (2 x film thickness)

11. Temperature Dependence of Resistance
5. Equivalent Circuit Analysis Rainer Schmidt
Temperature Dependence of Resistance
R1 (Interface)
Non-insulating
R2 (Film) R2
125 K-155 K

12. Temperature Dependence of Capacitance
5. Equivalent Circuit Analysis Rainer Schmidt
Temperature Dependence of Capacitance
Massive Dielect.
Response in
Thin Films
C1 (Interface)
C2 (Film)

13. Weak Dielectric Response at TC
5. Equivalent Circuit Analysis Rainer Schmidt
Magneto-Capacitance in BiMnO3 Polycrystalline Bulk
Kimura et al., Phys.Rev.B 67 (2003) R180401
Relative Dielectric Constant
Weak Dielectric
Response at TC
Temperature (K)

14. Conclusions Impedance Spectroscopy is a Useful Tool for Dielectric
6. Conclusions Rainer Schmidt
Conclusions
Impedance Spectroscopy is a Useful Tool for Dielectric
Characterization of Multiferroic Thin Films
Multiferroic BiMnO3 Thin Film Impedance Spectra
Display an Interface and a Film Contribution
The BiMnO3 Thin Film Resistance is Non-insulating below TC
Massive Response of the Dielectric Constant at TC was
Detected Indicating Magneto-Electric Coupling or a Phase Transition

15. Acknowledgments rainerxschmidt@googlemail.com r.schmidt@shef.ac.uk
7. Acknowledgments Rainer Schmidt
Acknowledgments
Wilma Eerenstein (Film Growth)
Paul Midgley (Supervision)
University of Cambridge
Department of Materials Science
Please Send Your Questions to:

16. Equivalent Circuit wmax = 2p fmax - Ferroelectrics - Dielectrics
Impedance on the Complex Plane Rainer Schmidt
Equivalent Circuit
- Ferroelectrics
- Dielectrics
- Thermistors R
Real – and Imaginary Parts of the Impedance
Nyquist Plot
106
105
104
103
102
101
100
10-1
10-2
106
105
104
103
102
101
100
10-1
10-2
1.0E5
7.5e4
5.0E4
2.5E4
wmax
= 2p fmax Z’
-Z’’
-Z’’
R = 100 kΩ
C’ = 10 nF
E E E E5
Frequency in Hz Z’

17. - Z’’ Z’ Brick Layer Model Narrow, Homogeneous
Data Analysis: The Brick-Layer Model Rainer Schmidt
Brick Layer Model
Narrow, Homogeneous
Distribution of Grain Sizes
Identical Grain Boundary,
Electrode and Bulk
Properties and
Homogeneous Shapes
Bulk
Cb ~ 1 x F
Grain Boundary
Cgb ~ 1 x 10-9 F
Electrodes
Cel ~ 1 x 10-6 F Rb Cb
Rgb
Cgb
Rel
Cel
Equivalent Circuit
- Z’’
Nyquist Plot
Negative Imaginary Part of Impedance –Z’’ vs Real Part Z’
frequency Z’ Rb
Rb+Rgb
Rb+Rgb+Rel

18. Identification of the Origin of Relaxation Times
Data Analysis: The Brick Layer Model Rainer Schmidt
Identification of the Origin of Relaxation Times
Normalised Capacitance in F/cm
Origin of the RC Element
10-12
10-11
10-11 – 10-8
10-10 – 10-9
10-9 – 10-7
10-7 – 10-5
10-4
bulk
minor second phase
grain boundary
bulk ferroelectric
surface layer
sample – electrode interface
electrochemical reaction
Irvine et al., Advanced Materials, 2 (3) (1990) 132
The Values are Valid Only for Samples of the Size 1 cm x 1cm x 1cm
For Bulk Contributions Data can be Normalized Using the
Geometrical Factor g: d
For Electrode Contributions Data can be Normalized Using the Contact Size A