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

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

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 ???

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 )

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

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

Relative Dielectric Constant 3. Multiferroic BiMnO3 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)

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) 101906

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

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.245111 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)

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

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

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)

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

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: rainerxschmidt@googlemail.com r.schmidt@shef.ac.uk

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