Impedance Spectroscopy on Nano-sized Multiferroic Thin Films University of Cambridge, Department of Materials Science and Metallurgy Rainer Schmidt Impedance Spectroscopy on Nano-sized Multiferroic Thin Films Rainer Schmidt Department of Materials Science Device Materials Group Wilma Eerenstein Department of Earth Sciences Centre for Ferroics Finlay Morrison James Scott 26th July 2006 ICYS-ICMR Summer School on Nanomaterials Poster II-26

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. Introduction 2. Impedance Spectroscopy 3. Multiferroic BiFeO3 4. Thin Film Epitaxy 5. Equivalent Circuit Analysis 6. Conclusions

Any Useful for Interface 1. Introduction Rainer Schmidt Dielectric and Resistive Characterisations by Impedance Spectroscopy Well Established: Never 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 Waver Substrate No Grain Boundaries Any Useful for Interface and Film Separation ???

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 )

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

Multiferroic Magneto-Electric BiFeO3 3. 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 RT: 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 RT: 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) Pseudo-Tetragonal Phase (Thin Film Epitaxy)

Pulsed Laser Deposition of BiFeO3 Thin Films 4. Thin Film Epitaxy 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 7 Pa O2 Partial Pressure, 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 4. Thin Film Epitaxy Rainer Schmidt Sample Geometry Copper Wires to Impedance Analyzer Agilent 4294A Spring Loaded Stainless-Steel Probes Pt - Electrodes Film Thickness d: 50/100/200 nm BiFeO3 Layer Nb-Doped STO Substrate r ~ 5 mΩ·cm AC Signal Current Path

Data Collected at 150ºC / 50 nm Film 5. Equivalent Circuit Analysis Rainer Schmidt Equivalent Circuit [C1] [C2] Irvine et al., Advanced Materials, Vol. 2 (3), (1990) p132 Data Collected at 150ºC / 50 nm Film 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

Film Thickness Dependence of C1 and C2 5. Equivalent Circuit Analysis Rainer Schmidt Film Thickness Dependence of C1 and C2 Data Normalisation d A Interface : A Film : A / d A / d Capacitance Conversion: Hsu, Mansfeld, Corrosion 57, (2001), p747

Temperature Dependence of Resistance 5. Equivalent Circuit Analysis Rainer Schmidt Temperature Dependence of Resistance

Temperature Dependence of the Dielectric Constant e’2 5. Equivalent Circuit Analysis 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

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 two Contributions Suggests That Single Frequency Measurements are Unreliable Contributions from Measuring Leads and Electrodes can be Separated and Quantified Poster II-26

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

Temperature Dependence of R0 and L1 Equivalent Circuit Analysis Rainer Schmidt Temperature Dependence of R0 and L1

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’

- 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 10-12 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

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