1. R Schmidt1, W Eerenstein1, F D Morrison2, P A Midgley1
Impedance Spectroscopy on Multiferroic BiFeO3 Epitaxial Films
R Schmidt1, W Eerenstein1, F D Morrison2, P A Midgley1
1 University of Cambridge, Department of Materials Science & Metallurgy, Pembroke Street, Cambridge CB2 3QZ, United Kingdom
2 University of Cambridge, Department of Earth Sciences, Downing Street, Cambridge CB2 3EQ, United Kingdom
Introduction
In recent years the interest in multiferroic materials exhibiting a magnetoelectric coupling effect has been revived.1 BiFeO3 (BFO) has attracted considerable interest, because it is the only known material that is both magnetically ordered and ferroelectric (multiferroic) at room temperature (RT).2 Single crystal BFO has a distorted perovskite structure,3 and is an antiferromagnet with a Neel temperature of TN = 645 K.4 Fe3+ cations are mutually coupled antiferromagnetically, the magnetic moments are slightly canted and form a spiral spin structure resulting in a zero net magnetic moment.5 Contrarily, in epitaxial BFO thin films grown on SrTiO3 (STO) substrates the latent magnetisation can be released resulting in a magnetic moment of ~ 0.05 B/Fe.6 This may be associated to the absence of spiral structures due to epitaxial constraint 7 and/or the fact that the film thickness is comparable to the spiral wavelength.
Ferroelectricity in the perovskite unit cell arises from a displacement of Fe3+ cations from the centro-symmetric positions in the FeO6 octahedra and the resulting dipole moment. In single crystals, the Curie temperature TC is reported as 1100 K.8 In granular and epitaxial BFO thin films a spontaneous RT polarisation of mC·cm-2 has been measured.9,10
Magnetoelectric coupling between ferroelectricity and antiferromagnetism in BFO films has been claimed,10 implying that an applied magnetic field may induce changes in the ferroelectric polarisation and vice versa. BFO may function as a model magnetoelectric multiferroic for studies at RT and above.
1.N.A. Spaldin and M. Fiebig, The Renaissance of Magnetoelectric Multiferroics, Science 309 (15 July) (2005) 391.
2.J.R. Teague, et al., Dielectric Hysteresis in Single Crystal BiFeO3, Solid State Communications 8 (1970) 1073.
3.C. Michel, et al., The Atomic Structure of BiFeO3, Solid State Communications 7 (1969) 701.
4.G.A. Smolenskii and Yudin, Sov.Phys.JETP 16 (1963) 622.// .S.V. Kiselev, et al., Sov.Phys.Dokl. 7 (1963) 742.
5.I. Sosnovska, et al., Spiral Magnetic-Ordering in Bismuth Ferrite, J.Phys.C Solid State Phys. 15 (1982) 4835.
6.W. Eerenstein, et al., Comment on"Epitaxial BiFeO3 Multiferroic Thin Film Heterostructures", Science 307 (25 February) (2005) 1203a.
7.M. Bai, et al.,, Applied Physics Letters 86 (3) (2005)
8.I.G. Ismailzade,, Phys.Status Solidi B 46 (1971) K39.
9.K.Y. Yun, M. Noda and M. Okuyama, Prominent ferroelectricity of BiFeO3 thin films prepared by pulsed-laser deposition, Applied Physics Letters 83 (19) (2003) 3981.
10.J. Wang, et al., Epitaxial BiFeO3 Multiferroic Thin Film Heterostructures, Science 299 (14 March) (2003) 1719.
BiFeO3 Thin Films
1. Vacuum Annealing of the
STO Substrate at 700°C
for 1 h
2. Deposition at 7 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
XRD Analysis
(002) and (013) Crystal Reflections
STO-BFO Lattice Mismatch: 1.4%
(002) Rocking Curve FWHM: 0.05º
Uniform Epitaxial Strain up to
100 nm Film Thickness
Strain Relaxation for d > 100 nm
is associated with a gradual
increase of the (002) FWHM
up to 0.1º
Spring Loaded
Stainless-Steel
Probes
BiFeO3 Layer
AC Signal Current Path
Pt - Electrodes
Copper Wires
to Impedance
Analyzer
Film Thickness d:
50/100/200 nm
Nb-Doped STO waver:
r ~ 5 mΩ·cm
Pulsed Laser Deposition
Rhombohedral Phase (Bulk Single Crystal) Pseudo-Tetragonal Phase (Epitaxial)
a = Å
c/a = 1.027
Impedance Spectroscopy of Epitaxial BiFeO3
[C1]
[C2]
Equivalent Circuit Model:
= Data = Model measured at 150ºC
Specific impedances z’, -z’’ in Ω·cm normalised
by g = A (contact area) / d (film thickness)
Agilent 4294A Precision Impedance Analyzer
Applied Frequency Range : 40 Hz – 2.5 MHz
Voltage Signal Amplitude : 100 mV
Constant-Phase-
Element (CPE)
Impedance:
C.H. Hsu, F. Mansfeld, Corrosion, 57 (9) (2001) 747
wmax’’ = Frequency at Semicircle Maximum, i.e. Maximum in -Z’’
n ≤1 describes the “non-ideality” of a capacitance (ideal: n =1)
Conversion for
R-CPE circuits:
Temperature and Film Thickness Dependencies of the Equivalent Circuit Components C1, C2, R1,R2
C1 is Approximately Constant With Varying Film Thickness by Normalising C1 by the Contact Area A
R1-CPE1 is an Electrode-Sample Interface Contribution
C2 is Approx. Constant by Normalisation with g (=A/d)
R2-CPE2 is the Film Contribution
Temperature in Celsius
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
R1 and R2 are both plotted using normalisation by g, which is meaningless for the interface resistance, and R1 is therefore regarded as a relative resistance and not resistivity.
Conclusions
Ac impedance spectroscopy applied over a wide frequency and temperature range has to be carried out to comprehensively characterise the dielectric and resistive properties of epitaxial multiferroic BFO thin films. The impedance spectra emphasized the composite character of the sample response, and numerical equivalent circuit fitting was required in order to obtain reliable values for the intrinsic dielectric constant and resistivity of the film. Single frequency measurements can be affected or dominated by the interface response and may be inappropriate to extract information. The impedance of epitaxial BFO films was dominated by the interface contribution indicating an interface barrier effect, which results in an artificially large value of the resistivity.
Thermally activated charge transport in BFO films showed an activation energy of ~ 0.72 eV in the range of a transport mechanism dominated by oxygen vacancies typical for ferroelectrics. The dielectric constant of the film was found to be ~ 285 ± 75. The lack of extended defects such as grain boundaries and the superior crystallinity in epitaxial films may facilitate the production of highly polarised ferroelectric devices.