Impedance Spectroscopy on Multiferroic BiFeO3 Epitaxial Thin Films University of Cambridge, Department of Materials Science and Metallurgy Rainer Schmidt Impedance Spectroscopy on Multiferroic BiFeO3 Epitaxial Thin Films Rainer Schmidt Department of Materials Science Device Materials Group Wilma Eerenstein Department of Earth Sciences Centre for Ferroics Finlay Morrison James Scott Electroceramics X Toledo 17th June 2006

Impedance Spectroscopy on Multiferroic BiFeO3 Epitaxial Thin Films University of Cambridge, Department of Materials Science and Metallurgy Rainer Schmidt Impedance Spectroscopy on Multiferroic BiFeO3 Epitaxial Thin Films 1. Multiferroic BiFeO3 2. Experimental 3. Impedance Spectroscopy 4. Data Analysis: The Brick Layer Model 5. Data Analysis: Equivalent Circuit Fitting 6. Conclusions

Multiferroic Magneto-Electric BiFeO3 1. Multiferroic BiFeO3 Rainer Schmidt Multiferroic Magneto-Electric BiFeO3 Ferroelectric below TCurie ~ 1125K Ismailzade, Phys.Status Solidi B, 46 (1971) K39 Antiferromagnetic below TNeel ~ 645K Smolenskii, Yudin, Sov.Phys.JETP, 16 (1963) 622 Spontaneous Polarization at Room Temperature: 3.5 – 8.9 m C/cm2 (Single Crystal/Ceramic) Teague et al., Solid State Commun., 8 (1970) 1073 Wang et al., Appl.Phys.Lett., 84 (2004) 1731 35 - 55 m C/cm2 (Granular /Epitaxial Thin Film) Wang et al., Science, 299 (2003) 1719 Yun et al., Appl.Phys.Lett., 83 (2003) 3981 Spontaneous Magnetization at Room Temperature: 0.5 - ~ 0.05 mB/ Unit Cell (Thin Film) Wang et al., Science, 299 (2003) 1719 // Eerenstein et al., Science, 307 (2005) 1203a Rhombohedral Phase (Bulk Single Crystal) Tetragonal Phase (Thin Film Epitaxy)

Pulsed Laser Deposition of BiFeO3 Thin Films 2. Experimental Rainer Schmidt Pulsed Laser Deposition of BiFeO3 Thin Films 1. Vacuum Annealing of the STO Substrate at 700°C for 1 h 2. Deposition at 15 Pa O2 Partial Pressure at 670°C - Variable Deposition Time - 1.6 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

Nb-Doped STO Substrate 2. Experimental Rainer Schmidt Sample Geometry Copper Wires to Impedance Analyzer Agilent 4294A Spring Loaded Stainless-Steel Probes Pt - Electrodes BiFeO3 Layer 50/100/200 nm Nb-Doped STO Substrate AC Signal Current Path

Impedance Spectroscopy 3. 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 ) Time Independent Complex Impedance:

Complex Relationship Dielectric Constant – Capacitance Relationship 3. Impedance Spectroscopy Rainer Schmidt Complex Relationship Dielectric Constant – Capacitance Relationship Contact Area A d Contact Distance Capacitance of the Measuring Cell in Vacuum

4. Data Analysis: The Brick-Layer Model Rainer Schmidt Composite Character of the Impedance Response of a Polycrystalline Sample Bulk Cb ~ 1 x 10-12 F Grain Boundary Cgb ~ 1 x 10-9 F Electrode Interface 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

Identification of the Origin of Relaxation Times 4. 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 – 10-8 10-10 – 10-9 10-7 – 10-5 Bulk Grain Boundary Bulk Ferroelectric Sample – Electrode Interface 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

Data collected at 150ºC / 50 nm film 5. Data Analysis: Equivalent Circuit Fitting Rainer Schmidt Equivalent Circuit [C1] [C2] 101 102 103 104 105 106 107 Frequency in Hz Z’ 101 102 103 104 105 106 107 108 107 106 105 104 103 10-10 10-11 10-12 10-13 C’ R1 R2 R0 C1 C2 Data collected at 150ºC / 50 nm film

Thickness Dependence of C1 and C2 5. Data Analysis: Equivalent Circuit Fitting Rainer Schmidt Thickness Dependence of C1 and C2 Capacitance Conversion: Hsu, Mansfeld, Corrosion 57 (2001) 747

Temperature Dependence of Resistance 5. Data Analysis: Equivalent Circuit Fitting Rainer Schmidt Temperature Dependence of Resistance

Temperature Dependence of the Dielectric Constant e 2 5. Data Analysis: Equivalent Circuit Fitting Rainer Schmidt Temperature Dependence of the Dielectric Constant e 2 Dielectric Constant of BiFeO3 Epitaxial Layer: ~ 285 ± 75 Dielectric Constant for Polycrystalline Films : ~ 110 V R Palkar et al., Appl.Phys.Lett. 80(9) 1628, 2002

Temperature Dependence of R0 and L1 5. Data Analysis: Equivalent Circuit Fitting Rainer Schmidt Temperature Dependence of R0 and L1

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 BiFeO3 Thin Film Impedance Spectra Display an Interface and a Film Contribution Strong Overlap of the two Contributions Suggests That Single Frequency Measurements are Unreliable Contributions from Measuring Leads and Wires can be Separated and Quantified

Acknowledgments Paul Midgley University of Cambridge 7. Acknowledgments Rainer Schmidt Acknowledgments Paul Midgley University of Cambridge Department of Materials Science

U, I Time Dependent Notation Arrow Diagram Impedance on the Complex Plane Rainer Schmidt Time Dependent Notation 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 ZR=R ZC=1/iwC IL=I0 cos(w t +p/2) ZL=iwL Time Independent Complex Impedance

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 101 102 103 104 105 106 107 108 0 2.5E4 5.0E4 7.5E4 1.0E5 Frequency in Hz Z’