- Email: [email protected]
Multiferroic properties and enhanced leakage current characteristics of polycrystalline Bi2MnFeO6 thin films grown on glass substrates
Journal Pre-proof
Multiferroic properties and enhanced leakage current characteristics of polycrystalline Bi2 MnFeO6 thin films grown on glass substrates Hyun Wook Shin , Yoonho Ahn , Jong Yeog Son PII: DOI: Reference:
S0040-6090(19)30682-0 https://doi.org/10.1016/j.tsf.2019.137655 TSF 137655
To appear in:
Thin Solid Films
Received date: Revised date: Accepted date:
16 October 2018 19 October 2019 21 October 2019
Please cite this article as: Hyun Wook Shin , Yoonho Ahn , Jong Yeog Son , Multiferroic properties and enhanced leakage current characteristics of polycrystalline Bi2 MnFeO6 thin films grown on glass substrates, Thin Solid Films (2019), doi: https://doi.org/10.1016/j.tsf.2019.137655
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Highlights ▶ Polycrystalline Bi2MnFeO6 thin films doped with Fe on BiMnO3 ▶ Leakage current characteristics and multiferroic properties ▶ High domain wall energies of Bi2MnFeO6 thin films
1
Multiferroic properties and enhanced leakage current characteristics of polycrystalline Bi2MnFeO6 thin films grown on glass substrates Hyun Wook Shin,a Yoonho Ahn,b and Jong Yeog Son a,* a
Department of Applied Physics, Kyung Hee University, Yongin 17104, Republic of Korea
b
School of Liberal Arts, Korea University of Technology and Education, Cheonan 31253,
Republic of Korea Abstract Although BiMnO3 (BMO) thin films have multiferroic properties, they also have drawbacks such as poor leakage current and ferroelectric properties. In this study, we fabricated polycrystalline Bi2MnFeO6 (BMFO) thin films and investigated their leakage current characteristics and multiferroic properties. On Pt/Ta/glass substrates, BMFO thin films were preferentially (111)-oriented as a polycrystalline thin film. The BMFO thin film showed good multiferroic properties: excellent ferroelectric properties with high remanent polarization of about 19.2 μC/cm2 and typical ferromagnetic properties with remanent magnetization of about 7×10-3 A/m. The BMFO thin film had better leakage current characteristics than the BMO thin film. The bandgap measurements confirmed that the BMFO thin film has a band gap of 2.46 eV, which is larger than that of the BMO thin film. This is considered to be a cause of the improved leakage current characteristics of the BMFO thin film compared to the BMO thin film. Keywords: Multiferroic properties; Enhanced leakage current; Polycrystalline thin films; Bismuth manganese iron oxide; Glass Author to whom correspondence should be addressed: *e-mail: [email protected]
2
1. Introduction Multiferroic thin films such as those of BiFeO3 (BFO) and BiMnO3 (BMO) have been widely researched as lead-free ferroelectric thin films due to their good ferroelectric properties compared to those of lead zirconate titanate (Pb(Ti,Zr)O3, PZT) [1-9]. Additional merits of multiferroic thin films include magnetism and magnetoelectric properties [9, 10]. Thus, multiferroic thin films can be used in various applications, including non-volatile randomaccess memory, actuators, sensors, high-sensitivity AC magnetic field sensors, electrically tunable microwave devices, and magnetoelectronic devices [1-10]. Most multiferroic thin films have high leakage currents, and the ferroelectric polarizations of the multiferroic thin films are smaller than those of PZT thin films [1-10]. Phase control using the thickness and doping processes with certain elements such as La and Mn makes it possible to enhance the physical properties of multiferroic thin films [11-14]. Recently, double perovskite structured (A2BB’O6) ferroelectric materials such as Sr2FeMoO6 and Bi2MnFeO6 (BMFO) have been investigated intensively because they can potentially overcome the disadvantages of leakage currents and poor ferroelectric properties [15, 16]. In the case of epitaxial Bi2FeCrO6 thin films, the bandgap can be adjusted from 1.6 to 2.5 eV by controlling the ordering of Fe and Cr via the growth rate of the film for applications in photovoltaic conversion. [17]. In addition, Ba2FeMnO6 thin films show multiferroic properties, simultaneously exhibiting ferroelectricity and ferromagnetism at room temperature, but showed relatively large leakage currents [18]. In this work, we investigated multiferroic properties of polycrystalline BMFO thin films grown on Pt/Ta/glass substrates by pulsed laser deposition (PLD). The polycrystalline BMFO thin film had multiferroic characteristics with good ferroelectricity (remanent polarization of about 19.2 μC/cm2) as well as a ferromagnetic hysteresis loop (remanent magnetization of 3
about 7×10-3 A/m). Additionally, we used piezoelectric force microscope (PFM) observations to obtain information on the domain wall energy from the ferroelectric domain according to the thickness of the BMFO film.
2. Experimental Procedure BMFO thin films were grown on Pt/Ta/fused silica glass substrates via PLD. For PLD ablation, BMFO bulk targets were prepared by a conventional solid-state reaction method. Commercially available, high purity powders of Bi2O3 (99.99% pure), Mn2O3 (99.99%), and Fe2O3 (99.99%) were mixed in molar ratios. For calcination, the mixtures were well ground, pelletized, and fired in air at 400 °C for 10 hours. After each intermediate grinding process, the mixture was subjected to continuous sintering at 700 °C for 12 hours and finally at 900 °C for 5 hours. A KrF excimer laser with a wavelength of 248 nm and an energy density of 0.5 J/cm2 was focused onto the targets. The laser pulse frequency was set at 2 Hz, and the PLD growth rate was about 0.12 nm per pulse, which required a deposition time of about 417 seconds to grow a 100-nm-thick thin film. The distance between the target and substrate was maintained at ~4 cm. Once the base pressure reached ~6.7×10-5 Pa, the substrate temperature was set to 820 °C with an oxygen partial pressure of 13.3 Pa. After deposition, all the thin films were cooled slowly to room temperature at a rate of 100 °C/h in oxygen ambient at 4×104 Pa. The structural analysis of the thin films was carried out by X-ray diffraction (XRD) (CuKα1 radiation with a wavelength of 1.542). The surface morphology, root-mean-square (RMS) roughness, and ferroelectric domain structure of the thin films were observed by atomic force microscope (AFM) and PFM measurements. To fabricate circular top electrodes with a radius of 100 μm, 100-nm-thick Pt was deposited on the BMFO thin film by radio frequency magnetron sputtering through a dot-patterned shadow mask. Subsequently, all the 4
samples were annealed at 400 °C for five minutes prior to obtaining the ferroelectric hysteresis loops, which were measured using an RT66A (Radiant Technologies, Inc.) test system. The magnetic hysteresis loops were obtained using a superconducting quantum interference device (SQUID) magnetometer (Quantum Design, Inc.).
3. Results and Discussion Figure 1 (a) shows a schematic drawing of a 100-nm-thick BMFO thin film grown on a Pt/Ta/glass substrate. The Ta buffer layer was used to improve the crystallinity of the Pt bottom electrode for deposition of the BMFO thin film. First, we assessed the crystal structure of the BMFO thin film grown on the Pt/Ta/glass substrate. Figure 1 (b) shows the θ2θ scans of the BMFO, BMO, and BFO thin films on the Pt/Ta/glass substrates. The BMFO and BFO thin films exhibited only one XRD peak: the (111) peak related to the (111) orientation growth of the Pt bottom electrode. However, in the BMO thin film, (100) and (200) peaks were observed in addition to the (111) peak. This indicates that the BMFO and BFO films were preferentially (111) oriented, unlike the BMO film. For the BMFO thin film, a rocking curve measurement was also employed to evaluate the degree of preferred orientation for the (111) peak. It was estimated that the full width at half maximum (FWHM) of the (111) peak is approximately 1.2°, indicating the poor crystallinity of the BMFO thin film along the out-of-plane direction. It is well known that BMO thin films have significantly poorer leakage current characteristics than conventional ferroelectric thin films such as PZT and SrBi 2Ta2O9 thin films. To assess the enhancement in leakage current of the double perovskite, we measured the leakage current behavior of the polycrystalline BMFO, BMO, and BFO thin films. To clarify the conduction mechanism, a plot of current density (J) versus applied electric field 5
(E) characteristics of Pt/BMFO/Pt, Pt/BMO/Pt, and Pt/BFO/Pt capacitors is shown in Figure 2 (a). The Pt/BMFO/Pt capacitor exhibited lower leakage currents than the Pt/BMO/Pt capacitor and higher leakage currents than the Pt/BFO/Pt capacitor. At weak electric fields less than 250 kV/cm, the polycrystalline BMFO thin film had an Ohmic conduction behavior value of n=1.0. At an electric field of approximately 250 kV/cm, the Ohmic conduction behavior transitions to space-charge-limited conduction (SCLC). To investigate the ferroelectric properties, we observed the hysteresis loops of the polycrystalline BMFO, BMO, and BFO thin films at a measurement frequency of 1 kHz (Figure 2 (b)). To measure the hysteresis loops, we prepared Pt/BMFO/Pt, Pt/BMO/Pt, and Pt/BFO/Pt capacitors. The BMFO thin film exhibited good ferroelectricity, with a high remanent polarization of 19.2 μC/cm2 (2Pr ~38.4 μC/cm2) and coercive electric field of approximately 160 kV/cm. This remanent polarization value of the BMFO thin film is smaller than that of the BFO thin film and higher than that of the BMO thin film. In particular, the BMFO thin film showed polarization degradation at electric fields over 220 kV/cm. This polarization degradation is influenced by the leakage current, as expected from the leakage current behavior of the polycrystalline BMFO thin film. To determine the multiferroic characteristics of the polycrystalline BMFO thin film, it is necessary to measure ferroelectricity and magnetism simultaneously. Thus, we successively examined the magnetic properties of the polycrystalline BMFO thin film. Figure 2(c) shows the magnetic hysteresis loop of the BMFO thin film at room temperature. As expected, the polycrystalline BMFO thin film exhibited a ferromagnetic hysteresis loop with a remanent magnetization of approximately 7×10-3 A/m and a coercive magnetic field of approximately 3.14x104 A/m. It was confirmed that the polycrystalline BMFO thin film has room
6
temperature
multiferroic
behavior,
simultaneously
exhibiting
ferroelectricity
and
ferromagnetism. The surface morphologies and ferroelectric domain structures of the polycrystalline BMFO thin films were simultaneously observed by AFM and PFM. On the surface of the polycrystalline BMFO thin film, there are triangular grains with an average size of approximately 100 nm (Figure 3(a)). It is suggested that these triangular grains reflect the crystallinity of the preferentially (111)-oriented polycrystalline BMFO thin film. The RSM roughness of the polycrystalline BMFO thin films was estimated to be 8.7 nm. To obtain the PFM signal, an AC voltage of about 15 kHz was applied to a Pt-coated tip with a radius of about 15 nm, and PFM images were observed. Figure 3 (b) shows a PFM image of the polycrystalline BMFO thin film. In general, ferroelectric thin films have a stripe or mosaic domain structure [19]. The polycrystalline BMFO thin film has a mosaic-like ferroelectric domain structure corresponding to the AFM image shown in Figure 3(a). To understand ferroelectric domain structures of the polycrystalline BMFO thin films, Landau, Lifshitz, and Kittel’s (LLK) scaling law gives information for the domain wall energies of ferroelectric thin films [19, 20]. The relationship between domain width and thickness is well defined by the LLK law, and the domain width w is proportional to dγ, where d is the film thickness, and γ is the scaling exponent with a value close to 0.5. In the LLK law, the scaling exponent γ is proportional to the domain wall energy. The domain widths of the polycrystalline BMFO thin films were obtained as functions of thickness (Figure 4 (c)). The polycrystalline BMFO thin films have a scaling exponent of about 0.57, compared to 0.49 for the PbTiO3 thin films, indicating that the BMFO thin films have higher domain wall energy than the PbTiO3 thin films [19, 20]. The high domain wall energies of the
7
BMFO thin films are probably due to the magnetoelectric couplings of the polycrystalline BMFO thin films [19]. In general, dielectric materials are band insulators; the larger the bandgap, the better the insulation characteristics, and the better the leakage current characteristics [21]. We further investigated the band gap energies of the BMFO thin films via optical absorption experiments because physical properties such as leakage current characteristics are greatly influenced by the band gap energies of ferroelectric thin films. Figure 4 shows Tauc plots of the BMFO, BMO, and BFO thin films. The band gap energies of the BMFO, BMO, and BFO thin films were estimated to be about 1.75, 2.42, and 2.74 eV, respectively. These band gap energies are similar to those reported by previous works [22, 23]. The BMO thin film has the smallest band gap, which is consistent with the high leakage current of the BMO thin film discussed above. Based on comparison of the band gaps, the BMFO thin film has a larger bandgap than the BMO thin film and showed improved leakage current and ferroelectric characteristics.
4. Conclusions We fabricated polycrystalline BMFO thin films on Pt/Ta/fused silica glass substrates via PLD. The BMFO thin film had a preferentially (111)-oriented polycrystalline structure. The BMFO thin film exhibited a transition from Ohmic conduction to SCLC near an electric field of approximately 220 kV/cm. The Pt/BMFO/Pt capacitor had high remnant polarization of 19.2 μC/cm2 (2Pr ~38.4 μC/cm2), with a coercive electric field of approximately 160 kV/cm. The polycrystalline BMFO thin films have room temperature multiferroicity, comprising both ferroelectricity and ferromagnetism. The LLK scaling law revealed that the polycrystalline BMFO thin films have a scaling exponent of about 0.57, compared to 0.49 for the PbTiO3
8
thin films, indicating that the BMFO thin films have higher domain wall energy than the PbTiO3 thin films.
Declaration of Competing Interest
The authors whose names are listed immediately below certify that they have NO affiliations with or involvement in any organization or entity with any financial interest (such as honoraria; educational grants; participation in speakers' bureaus; membership, employment, consultancies, stock ownership, or other equity interest; and expert testimony or patentlicensing arrangements), or non-financial interest (such as personal or professional relationships, affiliations, knowledge or beliefs) in the subject matter or materials discussed in this manuscript.
Acknowledgment This work was supported by a National Research Foundation of Korea grant under contract no. 2015R1A2A2A05027951.
9
References [1] Y.H. Chu, L.W. Martin, M.B. Holcomb, M. Gajek, S.J. Han, Q. He, N. Balke, C.H. Yang, D. Lee, W. Hu, Q. Zhan, P.L. Yang, A. Fraile-Rodríguez, A. Scholl, S.X. Wang, R. Ramesh, Electric-field control of local ferromagnetism using a magnetoelectric multiferroic, Nature Materials, 7 (2008) 478-482. [2] M. Dawber, K.M. Rabe, J.F. Scott, Physics of thin-film ferroelectric oxides, Reviews of Modern Physics, 77 (2005) 1083-1130. [3] W. Eerenstein, N.D. Mathur, J.F. Scott, Multiferroic and magnetoelectric materials, Nature, 442 (2006) 759-765. [4] V. Nagarajan, A. Roytburd, A. Stanishevsky, S. Prasertchoung, T. Zhao, L. Chen, J. Melngailis, O. Auciello, R. Ramesh, Dynamics of ferroelastic domains in ferroelectric thin films, Nature Materials, 2 (2003) 43-47. [5] C.W. Nan, M.I. Bichurin, S. Dong, D. Viehland, G. Srinivasan, Multiferroic magnetoelectric composites: Historical perspective, status, and future directions, Journal of Applied Physics, 103 (2008) 031101. [6] W. Prellier, M.P. Singh, P. Murugavel, The single-phase multiferroic oxides: From bulk to thin film, Journal of Physics Condensed Matter, 17 (2005) R803-R832. [7] D.G. Schlom, L.Q. Chen, C.B. Eom, K.M. Rabe, S.K. Streiffer, J.M. Triscone, Strain tuning of ferroelectric thin films, Annual Review of Materials Research, 2007, pp. 589-626. [8] N. Setter, D. Damjanovic, L. Eng, G. Fox, S. Gevorgian, S. Hong, A. Kingon, H. Kohlstedt, N.Y. Park, G.B. Stephenson, I. Stolitchnov, A.K. Taganstev, D.V. Taylor, T. Yamada, S. Streiffer, Ferroelectric thin films: Review of materials, properties, and applications, Journal of Applied Physics, 100 (2006) 051606. [9] J. Wang, J.B. Neaton, H. Zheng, V. Nagarajan, S.B. Ogale, B. Liu, D. Viehland, V. Vaithyanathan, D.G. Schlom, U.V. Waghmare, N.A. Spaldin, K.M. Rabe, M. Wuttig, R. 10
Ramesh, Epitaxial BiFeO3 multiferroic thin film heterostructures, Science, 299 (2003) 17191722. [10] H. Zheng, J. Wang, S.E. Lofland, Z. Ma, L. Mohaddes-Ardabili, T. Zhao, L. SalamancaRiba, S.R. Shinde, S.B. Ogale, F. Bai, D. Viehland, Y. Jia, D.G. Schlom, M. Wuttig, A. Roytburd, R. Ramesh, Multiferroic BaTiO3-CoFe2O4 Nanostructures, Science, 303 (2004) 661-663. [11] R.J. Zeches, M.D. Rossell, J.X. Zhang, A.J. Hatt, Q. He, C.H. Yang, A. Kumar, C.H. Wang, A. Melville, C. Adamo, G. Sheng, Y.H. Chu, J.F. Ihlefeld, R. Erni, C. Ederer, V. Gopalan, L.Q. Chen, D.G. Schldin, N.A. Spaldin, L.W. Martin, R. Ramesh, A strain-driven morphotropic phase boundary in bifeO3, Science, 326 (2009) 977-980. [12] J. Seidel, P. Maksymovych, Y. Batra, A. Katan, S.Y. Yang, Q. He, A.P. Baddorf, S.V. Kalinin, C.H. Yang, J.C. Yang, Y.H. Chu, E.K.H. Salje, H. Wormeester, M. Salmeron, R. Ramesh, Domain wall conductivity in La-doped BiFeO3, Physical Review Letters, 105 (2010) 197603. [13] Y. Ahn, J. Seo, J.Y. Son, J. Jang, Enhanced multiferroic properties in epitaxial Yb-doped BiFeO3 thin films, Electronic Materials Letters, 11 (2015) 609-613. [14] W.-H. Kim, J. Yeog Son, The effects of La substitution on ferroelectric domain structure and multiferroic properties of epitaxially grown BiFeO3 thin films, Applied Physics Letters, 103 (2013) 132907. [15] W.A. Adeagbo, M. Hoffmann, A. Ernst, W. Hergert, M. Saloaro, P. Paturi, K. Kokko, Tuning the probability of defect formation via substrate strains in Sr2FeMoO6 films, Physical Review Materials, 2 (2018) 083604. [16] K. Koumpouras, I. Galankis, Magnetic Configurations in Cubic Bi2MnFeO6 Alloys from First-Principles, Journal of Spintronics and Magnetic Nanomaterials, 1 (2012) 57-62.
11
[17] A. Quattropani, D. Stoeffler, T. Fix, G. Schmerber, M. Lenertz, G. Versini, J.L. Rehspringer, A. Slaoui, A. Dinia, S. Colis, Band-Gap Tuning in Ferroelectric Bi2FeCrO6 Double Perovskite Thin Films, The Journal of Physical Chemistry C, 122 (2018) 1070-1077. [18] S. Ravi, C. Senthilkumar, Room temperature multiferroicity in a new Ba2FeMnO6 double perovskite material, Ceramics International, 43 (2017) 14441-14445. [19] G. Catalan, H. Béa, S. Fusil, M. Bibes, P. Paruch, A. Barthélémy, J.F. Scott, Fractal dimension and size scaling of domains in thin films of multiferroic BiFeO3, Physical Review Letters, 100 (2008). [20] J.Y. Son, S. Song, J.H. Lee, H.M. Jang, Anomalous domain periodicity observed in ferroelectric PbTiO 3 nanodots having 180° stripe domains, Scientific Reports, 6 (2016). [21] K.R. Olasupo, M.K. Hatalis, Leakage current mechanism in sub-micron polysilicon thinfilm transistors, IEEE Transactions on Electron Devices, 43 (1996) 1218-1223. [22] J.H. Lee, X. Ke, R. Misra, J.F. Ihlefeld, X.S. Xu, Z.G. Mei, T. Heeg, M. Roeckerath, J. Schubert, Z.K. Liu, J.L. Musfeldt, P. Schiffer, D.G. Schlom, Adsorption-controlled growth of BiMnO3 films by molecular-beam epitaxy, Applied Physics Letters, 96 (2010) 262905. [23] X.S. Xu, J.F. Ihlefeld, J.H. Lee, O.K. Ezekoye, E. Vlahos, R. Ramesh, V. Gopalan, X.Q. Pan, D.G. Schlom, J.L. Musfeldt, Tunable band gap in Bi(Fe1−xMnx)O3 films, Applied Physics Letters, 96 (2010) 192901.
12
Figure captions
Figure 1. (a) Schematic drawing of a BMFO thin film grown on a Pt/Ta/glass substrate. (b) XRD patterns of the BMFO, BMO, and BFO thin films grown on Pt/Ta/glass substrates.
Figure 2. (a) Current density curves (J-E) as a function of applied electric field (E) for the BMFO, BMO, and BFO thin films. (b) Ferroelectric hysteresis loops of the BMFO, BMO, and BFO thin films. (c) Ferromagnetic hysteresis loop of the BMFO thin film at room temperature. 13
Figure 3. (a) AFM image of the BMFO thin film. There are triangular grains on the surface of the thin film. (b) PFM image of the BMFO thin film. (c) Domain sizes of the BMFO thin films as a function of thickness. The scaling exponent (γ) of the PbTiO3 thin film (0.49) is smaller than that of the BMFO thin film (0.59).
14
Figure 4. Tauc plots of the BMFO, BMO, and BFO thin films.
15
Multiferroic properties and enhanced leakage current characteristics of polycrystalline Bi2 MnFeO6 thin films grown on glass substrates Hyun Wook Shin , Yoonho Ahn , Jong Yeog Son PII: DOI: Reference:
S0040-6090(19)30682-0 https://doi.org/10.1016/j.tsf.2019.137655 TSF 137655
To appear in:
Thin Solid Films
Received date: Revised date: Accepted date:
16 October 2018 19 October 2019 21 October 2019
Please cite this article as: Hyun Wook Shin , Yoonho Ahn , Jong Yeog Son , Multiferroic properties and enhanced leakage current characteristics of polycrystalline Bi2 MnFeO6 thin films grown on glass substrates, Thin Solid Films (2019), doi: https://doi.org/10.1016/j.tsf.2019.137655
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Highlights ▶ Polycrystalline Bi2MnFeO6 thin films doped with Fe on BiMnO3 ▶ Leakage current characteristics and multiferroic properties ▶ High domain wall energies of Bi2MnFeO6 thin films
1
Multiferroic properties and enhanced leakage current characteristics of polycrystalline Bi2MnFeO6 thin films grown on glass substrates Hyun Wook Shin,a Yoonho Ahn,b and Jong Yeog Son a,* a
Department of Applied Physics, Kyung Hee University, Yongin 17104, Republic of Korea
b
School of Liberal Arts, Korea University of Technology and Education, Cheonan 31253,
Republic of Korea Abstract Although BiMnO3 (BMO) thin films have multiferroic properties, they also have drawbacks such as poor leakage current and ferroelectric properties. In this study, we fabricated polycrystalline Bi2MnFeO6 (BMFO) thin films and investigated their leakage current characteristics and multiferroic properties. On Pt/Ta/glass substrates, BMFO thin films were preferentially (111)-oriented as a polycrystalline thin film. The BMFO thin film showed good multiferroic properties: excellent ferroelectric properties with high remanent polarization of about 19.2 μC/cm2 and typical ferromagnetic properties with remanent magnetization of about 7×10-3 A/m. The BMFO thin film had better leakage current characteristics than the BMO thin film. The bandgap measurements confirmed that the BMFO thin film has a band gap of 2.46 eV, which is larger than that of the BMO thin film. This is considered to be a cause of the improved leakage current characteristics of the BMFO thin film compared to the BMO thin film. Keywords: Multiferroic properties; Enhanced leakage current; Polycrystalline thin films; Bismuth manganese iron oxide; Glass Author to whom correspondence should be addressed: *e-mail: [email protected]
2
1. Introduction Multiferroic thin films such as those of BiFeO3 (BFO) and BiMnO3 (BMO) have been widely researched as lead-free ferroelectric thin films due to their good ferroelectric properties compared to those of lead zirconate titanate (Pb(Ti,Zr)O3, PZT) [1-9]. Additional merits of multiferroic thin films include magnetism and magnetoelectric properties [9, 10]. Thus, multiferroic thin films can be used in various applications, including non-volatile randomaccess memory, actuators, sensors, high-sensitivity AC magnetic field sensors, electrically tunable microwave devices, and magnetoelectronic devices [1-10]. Most multiferroic thin films have high leakage currents, and the ferroelectric polarizations of the multiferroic thin films are smaller than those of PZT thin films [1-10]. Phase control using the thickness and doping processes with certain elements such as La and Mn makes it possible to enhance the physical properties of multiferroic thin films [11-14]. Recently, double perovskite structured (A2BB’O6) ferroelectric materials such as Sr2FeMoO6 and Bi2MnFeO6 (BMFO) have been investigated intensively because they can potentially overcome the disadvantages of leakage currents and poor ferroelectric properties [15, 16]. In the case of epitaxial Bi2FeCrO6 thin films, the bandgap can be adjusted from 1.6 to 2.5 eV by controlling the ordering of Fe and Cr via the growth rate of the film for applications in photovoltaic conversion. [17]. In addition, Ba2FeMnO6 thin films show multiferroic properties, simultaneously exhibiting ferroelectricity and ferromagnetism at room temperature, but showed relatively large leakage currents [18]. In this work, we investigated multiferroic properties of polycrystalline BMFO thin films grown on Pt/Ta/glass substrates by pulsed laser deposition (PLD). The polycrystalline BMFO thin film had multiferroic characteristics with good ferroelectricity (remanent polarization of about 19.2 μC/cm2) as well as a ferromagnetic hysteresis loop (remanent magnetization of 3
about 7×10-3 A/m). Additionally, we used piezoelectric force microscope (PFM) observations to obtain information on the domain wall energy from the ferroelectric domain according to the thickness of the BMFO film.
2. Experimental Procedure BMFO thin films were grown on Pt/Ta/fused silica glass substrates via PLD. For PLD ablation, BMFO bulk targets were prepared by a conventional solid-state reaction method. Commercially available, high purity powders of Bi2O3 (99.99% pure), Mn2O3 (99.99%), and Fe2O3 (99.99%) were mixed in molar ratios. For calcination, the mixtures were well ground, pelletized, and fired in air at 400 °C for 10 hours. After each intermediate grinding process, the mixture was subjected to continuous sintering at 700 °C for 12 hours and finally at 900 °C for 5 hours. A KrF excimer laser with a wavelength of 248 nm and an energy density of 0.5 J/cm2 was focused onto the targets. The laser pulse frequency was set at 2 Hz, and the PLD growth rate was about 0.12 nm per pulse, which required a deposition time of about 417 seconds to grow a 100-nm-thick thin film. The distance between the target and substrate was maintained at ~4 cm. Once the base pressure reached ~6.7×10-5 Pa, the substrate temperature was set to 820 °C with an oxygen partial pressure of 13.3 Pa. After deposition, all the thin films were cooled slowly to room temperature at a rate of 100 °C/h in oxygen ambient at 4×104 Pa. The structural analysis of the thin films was carried out by X-ray diffraction (XRD) (CuKα1 radiation with a wavelength of 1.542). The surface morphology, root-mean-square (RMS) roughness, and ferroelectric domain structure of the thin films were observed by atomic force microscope (AFM) and PFM measurements. To fabricate circular top electrodes with a radius of 100 μm, 100-nm-thick Pt was deposited on the BMFO thin film by radio frequency magnetron sputtering through a dot-patterned shadow mask. Subsequently, all the 4
samples were annealed at 400 °C for five minutes prior to obtaining the ferroelectric hysteresis loops, which were measured using an RT66A (Radiant Technologies, Inc.) test system. The magnetic hysteresis loops were obtained using a superconducting quantum interference device (SQUID) magnetometer (Quantum Design, Inc.).
3. Results and Discussion Figure 1 (a) shows a schematic drawing of a 100-nm-thick BMFO thin film grown on a Pt/Ta/glass substrate. The Ta buffer layer was used to improve the crystallinity of the Pt bottom electrode for deposition of the BMFO thin film. First, we assessed the crystal structure of the BMFO thin film grown on the Pt/Ta/glass substrate. Figure 1 (b) shows the θ2θ scans of the BMFO, BMO, and BFO thin films on the Pt/Ta/glass substrates. The BMFO and BFO thin films exhibited only one XRD peak: the (111) peak related to the (111) orientation growth of the Pt bottom electrode. However, in the BMO thin film, (100) and (200) peaks were observed in addition to the (111) peak. This indicates that the BMFO and BFO films were preferentially (111) oriented, unlike the BMO film. For the BMFO thin film, a rocking curve measurement was also employed to evaluate the degree of preferred orientation for the (111) peak. It was estimated that the full width at half maximum (FWHM) of the (111) peak is approximately 1.2°, indicating the poor crystallinity of the BMFO thin film along the out-of-plane direction. It is well known that BMO thin films have significantly poorer leakage current characteristics than conventional ferroelectric thin films such as PZT and SrBi 2Ta2O9 thin films. To assess the enhancement in leakage current of the double perovskite, we measured the leakage current behavior of the polycrystalline BMFO, BMO, and BFO thin films. To clarify the conduction mechanism, a plot of current density (J) versus applied electric field 5
(E) characteristics of Pt/BMFO/Pt, Pt/BMO/Pt, and Pt/BFO/Pt capacitors is shown in Figure 2 (a). The Pt/BMFO/Pt capacitor exhibited lower leakage currents than the Pt/BMO/Pt capacitor and higher leakage currents than the Pt/BFO/Pt capacitor. At weak electric fields less than 250 kV/cm, the polycrystalline BMFO thin film had an Ohmic conduction behavior value of n=1.0. At an electric field of approximately 250 kV/cm, the Ohmic conduction behavior transitions to space-charge-limited conduction (SCLC). To investigate the ferroelectric properties, we observed the hysteresis loops of the polycrystalline BMFO, BMO, and BFO thin films at a measurement frequency of 1 kHz (Figure 2 (b)). To measure the hysteresis loops, we prepared Pt/BMFO/Pt, Pt/BMO/Pt, and Pt/BFO/Pt capacitors. The BMFO thin film exhibited good ferroelectricity, with a high remanent polarization of 19.2 μC/cm2 (2Pr ~38.4 μC/cm2) and coercive electric field of approximately 160 kV/cm. This remanent polarization value of the BMFO thin film is smaller than that of the BFO thin film and higher than that of the BMO thin film. In particular, the BMFO thin film showed polarization degradation at electric fields over 220 kV/cm. This polarization degradation is influenced by the leakage current, as expected from the leakage current behavior of the polycrystalline BMFO thin film. To determine the multiferroic characteristics of the polycrystalline BMFO thin film, it is necessary to measure ferroelectricity and magnetism simultaneously. Thus, we successively examined the magnetic properties of the polycrystalline BMFO thin film. Figure 2(c) shows the magnetic hysteresis loop of the BMFO thin film at room temperature. As expected, the polycrystalline BMFO thin film exhibited a ferromagnetic hysteresis loop with a remanent magnetization of approximately 7×10-3 A/m and a coercive magnetic field of approximately 3.14x104 A/m. It was confirmed that the polycrystalline BMFO thin film has room
6
temperature
multiferroic
behavior,
simultaneously
exhibiting
ferroelectricity
and
ferromagnetism. The surface morphologies and ferroelectric domain structures of the polycrystalline BMFO thin films were simultaneously observed by AFM and PFM. On the surface of the polycrystalline BMFO thin film, there are triangular grains with an average size of approximately 100 nm (Figure 3(a)). It is suggested that these triangular grains reflect the crystallinity of the preferentially (111)-oriented polycrystalline BMFO thin film. The RSM roughness of the polycrystalline BMFO thin films was estimated to be 8.7 nm. To obtain the PFM signal, an AC voltage of about 15 kHz was applied to a Pt-coated tip with a radius of about 15 nm, and PFM images were observed. Figure 3 (b) shows a PFM image of the polycrystalline BMFO thin film. In general, ferroelectric thin films have a stripe or mosaic domain structure [19]. The polycrystalline BMFO thin film has a mosaic-like ferroelectric domain structure corresponding to the AFM image shown in Figure 3(a). To understand ferroelectric domain structures of the polycrystalline BMFO thin films, Landau, Lifshitz, and Kittel’s (LLK) scaling law gives information for the domain wall energies of ferroelectric thin films [19, 20]. The relationship between domain width and thickness is well defined by the LLK law, and the domain width w is proportional to dγ, where d is the film thickness, and γ is the scaling exponent with a value close to 0.5. In the LLK law, the scaling exponent γ is proportional to the domain wall energy. The domain widths of the polycrystalline BMFO thin films were obtained as functions of thickness (Figure 4 (c)). The polycrystalline BMFO thin films have a scaling exponent of about 0.57, compared to 0.49 for the PbTiO3 thin films, indicating that the BMFO thin films have higher domain wall energy than the PbTiO3 thin films [19, 20]. The high domain wall energies of the
7
BMFO thin films are probably due to the magnetoelectric couplings of the polycrystalline BMFO thin films [19]. In general, dielectric materials are band insulators; the larger the bandgap, the better the insulation characteristics, and the better the leakage current characteristics [21]. We further investigated the band gap energies of the BMFO thin films via optical absorption experiments because physical properties such as leakage current characteristics are greatly influenced by the band gap energies of ferroelectric thin films. Figure 4 shows Tauc plots of the BMFO, BMO, and BFO thin films. The band gap energies of the BMFO, BMO, and BFO thin films were estimated to be about 1.75, 2.42, and 2.74 eV, respectively. These band gap energies are similar to those reported by previous works [22, 23]. The BMO thin film has the smallest band gap, which is consistent with the high leakage current of the BMO thin film discussed above. Based on comparison of the band gaps, the BMFO thin film has a larger bandgap than the BMO thin film and showed improved leakage current and ferroelectric characteristics.
4. Conclusions We fabricated polycrystalline BMFO thin films on Pt/Ta/fused silica glass substrates via PLD. The BMFO thin film had a preferentially (111)-oriented polycrystalline structure. The BMFO thin film exhibited a transition from Ohmic conduction to SCLC near an electric field of approximately 220 kV/cm. The Pt/BMFO/Pt capacitor had high remnant polarization of 19.2 μC/cm2 (2Pr ~38.4 μC/cm2), with a coercive electric field of approximately 160 kV/cm. The polycrystalline BMFO thin films have room temperature multiferroicity, comprising both ferroelectricity and ferromagnetism. The LLK scaling law revealed that the polycrystalline BMFO thin films have a scaling exponent of about 0.57, compared to 0.49 for the PbTiO3
8
thin films, indicating that the BMFO thin films have higher domain wall energy than the PbTiO3 thin films.
Declaration of Competing Interest
The authors whose names are listed immediately below certify that they have NO affiliations with or involvement in any organization or entity with any financial interest (such as honoraria; educational grants; participation in speakers' bureaus; membership, employment, consultancies, stock ownership, or other equity interest; and expert testimony or patentlicensing arrangements), or non-financial interest (such as personal or professional relationships, affiliations, knowledge or beliefs) in the subject matter or materials discussed in this manuscript.
Acknowledgment This work was supported by a National Research Foundation of Korea grant under contract no. 2015R1A2A2A05027951.
9
References [1] Y.H. Chu, L.W. Martin, M.B. Holcomb, M. Gajek, S.J. Han, Q. He, N. Balke, C.H. Yang, D. Lee, W. Hu, Q. Zhan, P.L. Yang, A. Fraile-Rodríguez, A. Scholl, S.X. Wang, R. Ramesh, Electric-field control of local ferromagnetism using a magnetoelectric multiferroic, Nature Materials, 7 (2008) 478-482. [2] M. Dawber, K.M. Rabe, J.F. Scott, Physics of thin-film ferroelectric oxides, Reviews of Modern Physics, 77 (2005) 1083-1130. [3] W. Eerenstein, N.D. Mathur, J.F. Scott, Multiferroic and magnetoelectric materials, Nature, 442 (2006) 759-765. [4] V. Nagarajan, A. Roytburd, A. Stanishevsky, S. Prasertchoung, T. Zhao, L. Chen, J. Melngailis, O. Auciello, R. Ramesh, Dynamics of ferroelastic domains in ferroelectric thin films, Nature Materials, 2 (2003) 43-47. [5] C.W. Nan, M.I. Bichurin, S. Dong, D. Viehland, G. Srinivasan, Multiferroic magnetoelectric composites: Historical perspective, status, and future directions, Journal of Applied Physics, 103 (2008) 031101. [6] W. Prellier, M.P. Singh, P. Murugavel, The single-phase multiferroic oxides: From bulk to thin film, Journal of Physics Condensed Matter, 17 (2005) R803-R832. [7] D.G. Schlom, L.Q. Chen, C.B. Eom, K.M. Rabe, S.K. Streiffer, J.M. Triscone, Strain tuning of ferroelectric thin films, Annual Review of Materials Research, 2007, pp. 589-626. [8] N. Setter, D. Damjanovic, L. Eng, G. Fox, S. Gevorgian, S. Hong, A. Kingon, H. Kohlstedt, N.Y. Park, G.B. Stephenson, I. Stolitchnov, A.K. Taganstev, D.V. Taylor, T. Yamada, S. Streiffer, Ferroelectric thin films: Review of materials, properties, and applications, Journal of Applied Physics, 100 (2006) 051606. [9] J. Wang, J.B. Neaton, H. Zheng, V. Nagarajan, S.B. Ogale, B. Liu, D. Viehland, V. Vaithyanathan, D.G. Schlom, U.V. Waghmare, N.A. Spaldin, K.M. Rabe, M. Wuttig, R. 10
Ramesh, Epitaxial BiFeO3 multiferroic thin film heterostructures, Science, 299 (2003) 17191722. [10] H. Zheng, J. Wang, S.E. Lofland, Z. Ma, L. Mohaddes-Ardabili, T. Zhao, L. SalamancaRiba, S.R. Shinde, S.B. Ogale, F. Bai, D. Viehland, Y. Jia, D.G. Schlom, M. Wuttig, A. Roytburd, R. Ramesh, Multiferroic BaTiO3-CoFe2O4 Nanostructures, Science, 303 (2004) 661-663. [11] R.J. Zeches, M.D. Rossell, J.X. Zhang, A.J. Hatt, Q. He, C.H. Yang, A. Kumar, C.H. Wang, A. Melville, C. Adamo, G. Sheng, Y.H. Chu, J.F. Ihlefeld, R. Erni, C. Ederer, V. Gopalan, L.Q. Chen, D.G. Schldin, N.A. Spaldin, L.W. Martin, R. Ramesh, A strain-driven morphotropic phase boundary in bifeO3, Science, 326 (2009) 977-980. [12] J. Seidel, P. Maksymovych, Y. Batra, A. Katan, S.Y. Yang, Q. He, A.P. Baddorf, S.V. Kalinin, C.H. Yang, J.C. Yang, Y.H. Chu, E.K.H. Salje, H. Wormeester, M. Salmeron, R. Ramesh, Domain wall conductivity in La-doped BiFeO3, Physical Review Letters, 105 (2010) 197603. [13] Y. Ahn, J. Seo, J.Y. Son, J. Jang, Enhanced multiferroic properties in epitaxial Yb-doped BiFeO3 thin films, Electronic Materials Letters, 11 (2015) 609-613. [14] W.-H. Kim, J. Yeog Son, The effects of La substitution on ferroelectric domain structure and multiferroic properties of epitaxially grown BiFeO3 thin films, Applied Physics Letters, 103 (2013) 132907. [15] W.A. Adeagbo, M. Hoffmann, A. Ernst, W. Hergert, M. Saloaro, P. Paturi, K. Kokko, Tuning the probability of defect formation via substrate strains in Sr2FeMoO6 films, Physical Review Materials, 2 (2018) 083604. [16] K. Koumpouras, I. Galankis, Magnetic Configurations in Cubic Bi2MnFeO6 Alloys from First-Principles, Journal of Spintronics and Magnetic Nanomaterials, 1 (2012) 57-62.
11
[17] A. Quattropani, D. Stoeffler, T. Fix, G. Schmerber, M. Lenertz, G. Versini, J.L. Rehspringer, A. Slaoui, A. Dinia, S. Colis, Band-Gap Tuning in Ferroelectric Bi2FeCrO6 Double Perovskite Thin Films, The Journal of Physical Chemistry C, 122 (2018) 1070-1077. [18] S. Ravi, C. Senthilkumar, Room temperature multiferroicity in a new Ba2FeMnO6 double perovskite material, Ceramics International, 43 (2017) 14441-14445. [19] G. Catalan, H. Béa, S. Fusil, M. Bibes, P. Paruch, A. Barthélémy, J.F. Scott, Fractal dimension and size scaling of domains in thin films of multiferroic BiFeO3, Physical Review Letters, 100 (2008). [20] J.Y. Son, S. Song, J.H. Lee, H.M. Jang, Anomalous domain periodicity observed in ferroelectric PbTiO 3 nanodots having 180° stripe domains, Scientific Reports, 6 (2016). [21] K.R. Olasupo, M.K. Hatalis, Leakage current mechanism in sub-micron polysilicon thinfilm transistors, IEEE Transactions on Electron Devices, 43 (1996) 1218-1223. [22] J.H. Lee, X. Ke, R. Misra, J.F. Ihlefeld, X.S. Xu, Z.G. Mei, T. Heeg, M. Roeckerath, J. Schubert, Z.K. Liu, J.L. Musfeldt, P. Schiffer, D.G. Schlom, Adsorption-controlled growth of BiMnO3 films by molecular-beam epitaxy, Applied Physics Letters, 96 (2010) 262905. [23] X.S. Xu, J.F. Ihlefeld, J.H. Lee, O.K. Ezekoye, E. Vlahos, R. Ramesh, V. Gopalan, X.Q. Pan, D.G. Schlom, J.L. Musfeldt, Tunable band gap in Bi(Fe1−xMnx)O3 films, Applied Physics Letters, 96 (2010) 192901.
12
Figure captions
Figure 1. (a) Schematic drawing of a BMFO thin film grown on a Pt/Ta/glass substrate. (b) XRD patterns of the BMFO, BMO, and BFO thin films grown on Pt/Ta/glass substrates.
Figure 2. (a) Current density curves (J-E) as a function of applied electric field (E) for the BMFO, BMO, and BFO thin films. (b) Ferroelectric hysteresis loops of the BMFO, BMO, and BFO thin films. (c) Ferromagnetic hysteresis loop of the BMFO thin film at room temperature. 13
Figure 3. (a) AFM image of the BMFO thin film. There are triangular grains on the surface of the thin film. (b) PFM image of the BMFO thin film. (c) Domain sizes of the BMFO thin films as a function of thickness. The scaling exponent (γ) of the PbTiO3 thin film (0.49) is smaller than that of the BMFO thin film (0.59).
14
Figure 4. Tauc plots of the BMFO, BMO, and BFO thin films.
15