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Effects of the substrate on properties of PTO thin film
Applied Surface Science 216 (2003) 318–322
Effects of the substrate on properties of PTO thin film K. Nishidaa,*, G. Matuokaa, M. Osadab, M. Kakihanac, T. Katodaa a
Department of Electronic and Photonic Systems Engineering, Kochi University of Technology, 185 Miyanokuchi, Tosayamada-cho, Kamigun, Kochi 782-8502, Japan b Precursory Research for Embryonic Science and Technology, Japan Science and Technology Corporation, 4259 Nagatsuta, Midori-ku, Yokohama-shi, Kanagawa 226-8503, Japan c Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama-shi, Kanagawa 226-8503, Japan
Abstract The lead titanate (PbTiO3: PTO) films grown by PE-CVD had (0 0 1) and (1 1 1) orientations on the substrates of Pt(1 0 0)/ MgO(1 0 0) and Pt(1 1 1)/MgO(1 1 1) while the PTO film had (1 0 0) orientation on the Pt(1 1 1)/MgO(1 0 0) substrate. The PTO thin film with (0 0 1) orientation had the best flatness. The PTO film on the Pt(1 0 0)/MgO(1 0 0) substrate having the crystallographic orientation corresponding to the polarization direction showed a ferroelectrical hystersis loop. # 2003 Elsevier Science B.V. All rights reserved. Keywords: PTO; Ferroelectrics; Substrate; Film orientation; P–E property
1. Introduction Lead titanate, PbTiO3 (PTO) is a well-known ferroelectric material with dielectric, piezoelectric and pyroelectric properties [1]. Currently, the use of PTO for various applications is the focus of much interest. PTO has the tetragonal perovskite structure at the room temperature [2] and a Curie temperature as high as 490 8C. In preparation of a PTO thin film, the following methods are used: chemical vapor phase deposition (CVD) methods [3–5], sputtering technique [6–8] and sol–gel method [9–11], etc. However, crystal growth of a PTO thin film has been still difficult because PTO consists of the three elements having different vapor *
Corresponding author. Tel.: þ81-887-53-1010; fax: þ81-887-57-2120. E-mail address: [email protected] (K. Nishida).
pressures. Control of a crystallographic orientation is very important to control various properties of PTO films. Although it is known that a crystallographic orientation of a PTO film is affected by a substrate, there are few reports of systematic study on effects of a substrate on a crystallographic orientation and electrical properties of a PTO thin film [12,13]. We will report effects of substrates on properties of PTO films in this paper.
2. Experimental PTO thin films were prepared by plasma enhanced chemical vapor phase deposition (PE-CVD) method. Fig. 1 schematically shows the PE-CVD system. PTO films were grown on three types of substrates: Pt(1 0 0)/MgO(1 0 0), Pt(1 1 1)/MgO(1 0 0) and
0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00450-1
K. Nishida et al. / Applied Surface Science 216 (2003) 318–322
319
Fig. 1. Schematic drawing of the PE-CVD chamber with in-situ Raman monitoring system.
Pt(1 1 1)/MgO(1 1 1). The Pt layer with a thickness of 100 nm was deposited by rf sputtering method. Pb(dpm)2 and titanium isopropoxide were used as a source material with high purity oxygen. A carrier gas of metalorganic sources was high purity nitrogen. Flow rates of the carrier gas Pb(dpm)2 and titanium isopropoxide were 50 and 5 sccm, respectively. A flow rate of high purity oxygen was 50 sccm. The growth conditions were as follows: substrate temperature, 546 8C; pressure, 10 Pa; and PF power, 170 W. The thickness of the samples was fixed 0.6 mm. The PTO films were characterized with X-ray diffraction (XRD), atomic force microscopy (AFM) and micro Raman spectroscopy. The growth system shown in Fig. 1 has an in-situ monitoring system based on laser Raman spectroscopy. The Curie temperature was estimated from change of the Raman spectra. A Raman equipment T64000 (JOBIN YVON) was used as the micro Raman spectroscopy and a laser Raman system SYSTEM1000 (RENISHAW) was used in the in-situ monitoring system. The excitation source was an Arþ laser with a wavelength of 514.5 nm for Raman measurement. A diameter of the laser probe was 1– 100 mm. A multi-channel detector was used for measurement with a high S/N ratio and in a short time. The electrical properties were characterized using the Sawyer-tower circuit after formation of an upper electrode of Au by evaporation method.
3. Results and discussion Fig. 2 shows the XRD spectra from the PTO thin films on various substrates. The PTO thin films
Fig. 2. XRD spectra of the PTO thin films on substrates with various crystallographic orientations.
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Fig. 3. AFM images of the PTO thin films on substrates with various crystallographic orientations. The PTO films had orientations: (a) (1 0 0); (b) (0 0 1); and (c) (1 1 1).
grown on the Pt(1 0 0)/MgO(1 0 0) and the Pt(1 1 1)/ MgO(1 1 1) substrates were orientated to (0 0 1) and (1 1 1), respectively. PTO(0 0 1) and PTO(1 1 1) thin films were affected by the substrate and had orientations same as those of the substrate. On the other hand, the PTO film had grown on the Pt(1 1 1)/MgO(1 0 0) substrate to the (1 0 0) direction by itself. The crystallinity seems to be poor because the full-width of the half-maximum (FWHM) of XRD spectrum was larger than those of the other PTO films. The reason for orientating why the PTO(1 0 0) film self-oriented on Pt(1 1 1) and the Pt(1 1 1) self-oriented on MgO(1 0 0) has not been made clear.
Fig. 4. Raman spectra of the PTO thin films on substrates with various crystallographic orientations. Raman spectrum of powder PTO is shown as a reference.
The AFM images of the PTO films grown on various substrates are shown in Fig. 3. Triangular crystal grains were observed which corresponded to the orientations of the Pt layers in the case of Pt(1 1 1)/ MgO(1 1 1) while the surface of the PTO film formed on the Pt(1 0 0)/MgO(1 0 0) substrate was very flat. The PTO(0 0 1) and PTO(1 1 1) film of the root mean
Fig. 5. Raman shift vs. temperature of the B1 þ EðTOÞ mode obtained at the cooling process.
K. Nishida et al. / Applied Surface Science 216 (2003) 318–322
square (rms) of roughness were 6.87 and 69.04 nm, respectively (Fig. 3(b) and (c)). While the MgO(1 0 0) substrate had a very flat surface, MgO(1 1 1) substrate received much scratch damage though the surface polishing. Therefore, these different substrates for crystal growth caused such a difference in surface roughness of the PTO films. Fine crystal grains of PTO with a quadrangular shape were observed on the surface of a PTO film grown on Pt(1 1 1)/MgO(1 0 0). The rms of the last film was 97.31 nm which was the highest degree of roughness observed for the films on three types of the substrates (Fig. 3(a)). The results of characterization with XRD shown above and AFM means that growth mode of PTO on the Pt(1 1 1)/ MgO(1 1 1) substrate is different from that on other types of substrates. Stress in the PTO films was characterized based on extra shift of the phonon frequency measured using micro Raman spectroscopy. The Raman spectra were shown in Fig. 4. The Raman spectrum of powder PTO
321
is simultaneously shown as a reference. Each peak in the Raman spectra was assigned as shown in Fig. 4. The E(ITO) peak is called ‘‘soft mode’’. The appearance of this peak denotes that the material is ferroelectric [6,14–17]. Stresses accumulated in the PTO films with orientations (1 0 0), (0 0 1) and (1 1 1) were 1.96, 0.93 and 2.66 GPa, respectively, compressive. Larger stress accumulated in the PTO(1 1 1) films because the difference of lattice mismatch between the substrate and the PTO thin film was larger for the former than for the latter. Effects of substrate orientations on the Curie temperature of PTO films, Raman spectra were measured at various temperatures in the cooling process using the in-situ Raman spectroscopy. Relations between the Raman shift of the B1 þ EðTOÞ mode and temperature are plotted in Fig. 5. The B1 þ EðTOÞ mode shifted to a lower wave number with decrease in substrate temperature after growth and it took a minimum value. Then, it shifted to a higher wave number.
Fig. 6. P–E properties of the PTO thin films on substrates with various crystallographic orientations. The PTO films had orientations: (a) (1 0 0); (b) (0 0 1); and (c) (1 1 1).
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The minimum value of a temperature can be considered to correspond with a change in crystallographic structure. Therefore, the minimum temperature can be said to be the Curie point. The Curie points of the (1 0 0), (0 0 1) and (1 1 1) orientated PTO thin films were 470, 440 and 460 8C, respectively. Stress accumulated in the PTO(1 1 1) thin film was larger than that in PTO(0 0 1) thin film. The difference in the stress is the reason why the Curie point of the PTO(1 1 1) thin film was higher than that of the PTO(0 0 1) thin film. However, it is impossible to discuss the difference in the Curie point of the PTO(1 0 0) thin film only from a point of stress because the PTO films are polycrystalline. The P–E measurement was carried out in order to discern a clear relationship between crystallographic orientation of a film and its electrical properties. P–E curves obtained from the PTO films are shown in Fig. 6. The PTO film having (0 0 1) orientation grown on the Pt(1 0 0)/MgO(1 0 0) substrate showed the ferroelectric hystersis. The residual polarization Pr was 18 mC/cm2 and the anti-electric field Ec was 50 kV/cm. Other PTO films grown on Pt(1 1 1)/ MgO(1 1 1) or Pt(1 1 1)/MgO(1 0 0) substrates did not shown such hysteresis. The reason of these results is that the polarization axis of PTO is (0 0 1).
4. Summary In this paper, effects of the substrates with various types of crystallographic and electrical properties of PTO films were reported. The PTO films grown by PE-CVD had (0 0 1) and (1 1 1) orientation on the substrates of Pt(1 0 0)/MgO(1 0 0) and Pt(1 1 1)/ MgO(1 1 1), respectively. However, a PTO thin films with (1 0 0) orientation was obtained on the Pt(1 1 1)/ MgO(1 0 0) substrate. Flatness of the surface depended
on the crystallographic orientation and the PTO films with (0 0 1) orientation having the best flatness. P–E characteristics depended also on the orientation of the surface. Especially, the PTO(0 0 1) on the Pt(1 0 0)/ MgO(1 0 0) substrate that had an orientation corresponding to the polarization direction showed a ferroelectrical hysteria loop. The results shown in this paper indicate that it is possible to control P–E of the PTO films characteristics by controlling the crystallographic orientation of the substrate. References [1] J.S. Wrigh, L.F. Francis, J. Mater. Res. 8 (1993) 1712. [2] G. Burns, B.A. Scott, Phys. Rev. B 7 (1973) 3088. [3] B.S. Kwak, E.P. Boyd, A. Erbil, Appl. Phys. Lett. 53 (18) (1988) 1702. [4] T. Li, Y. Zhu, S.B. Desu, C.H. Peng, M. Nagata, Appl. Phys. Lett. 68 (1996) 616. [5] G.R. Bai, H.L.M. Chang, C.M. Foster, Z. Shen, D.J. Lam, J. Mater. Res. 9 (1994) 156. [6] L. Taguchi, A. Pignolet, L. Wand, M. Proctor, F. Levy, P.E. Schmid, J. Appl. Phys. 73 (1993) 394. [7] K. Iijima, Y. Tomita, R. Takayama, I. Ueda, J Appl. Phys. 60 (1986) 361. [8] T. Ogawa, A. Senda, T. Kasamani, Jpn. J. Appl. Phys., Part 1 30 (1991) 2145. [9] M. Maeda, H. Ishida, K.K.K. Soe, I. Suzuki, Jpn. J. Appl. Phys. 32 (1993) 4136. [10] S.S. Dana, F.K. Etzod, J. Clabes, J. Appl. Phys. 69 (1991) 4398. [11] C. Chen, D.F. Ryder Jr., J. Am. Ceram. Soc. 72 (1989) 1495. [12] S.W. Chung, S.O. Chung, K. No, W.J. Lee, Thin Solid Films 295 (1997) 299. [13] B.M. Yen, H. Chen, J. Appl. Phys. 85 (1999) 853. [14] W.H. Ma, M.S. Zhang, Z. Tin, J. Korean Phys. Soc. 32 (1998) S1137. [15] M.D. Fontana, A. Ridah, G.E. Kugel, C. Carabatos Nedelec, J. Phys. C: Solid State Phys. 21 (1988) 5853. [16] G. Burns, B.A. Scott, Phys. Rev. B 7 (1973) 3088. [17] M. Maglione, R. Bohmer, A. Loidl, U.T. Hochli, Phys. Rev. B 40 (1989) 11441.
Effects of the substrate on properties of PTO thin film K. Nishidaa,*, G. Matuokaa, M. Osadab, M. Kakihanac, T. Katodaa a
Department of Electronic and Photonic Systems Engineering, Kochi University of Technology, 185 Miyanokuchi, Tosayamada-cho, Kamigun, Kochi 782-8502, Japan b Precursory Research for Embryonic Science and Technology, Japan Science and Technology Corporation, 4259 Nagatsuta, Midori-ku, Yokohama-shi, Kanagawa 226-8503, Japan c Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama-shi, Kanagawa 226-8503, Japan
Abstract The lead titanate (PbTiO3: PTO) films grown by PE-CVD had (0 0 1) and (1 1 1) orientations on the substrates of Pt(1 0 0)/ MgO(1 0 0) and Pt(1 1 1)/MgO(1 1 1) while the PTO film had (1 0 0) orientation on the Pt(1 1 1)/MgO(1 0 0) substrate. The PTO thin film with (0 0 1) orientation had the best flatness. The PTO film on the Pt(1 0 0)/MgO(1 0 0) substrate having the crystallographic orientation corresponding to the polarization direction showed a ferroelectrical hystersis loop. # 2003 Elsevier Science B.V. All rights reserved. Keywords: PTO; Ferroelectrics; Substrate; Film orientation; P–E property
1. Introduction Lead titanate, PbTiO3 (PTO) is a well-known ferroelectric material with dielectric, piezoelectric and pyroelectric properties [1]. Currently, the use of PTO for various applications is the focus of much interest. PTO has the tetragonal perovskite structure at the room temperature [2] and a Curie temperature as high as 490 8C. In preparation of a PTO thin film, the following methods are used: chemical vapor phase deposition (CVD) methods [3–5], sputtering technique [6–8] and sol–gel method [9–11], etc. However, crystal growth of a PTO thin film has been still difficult because PTO consists of the three elements having different vapor *
Corresponding author. Tel.: þ81-887-53-1010; fax: þ81-887-57-2120. E-mail address: [email protected] (K. Nishida).
pressures. Control of a crystallographic orientation is very important to control various properties of PTO films. Although it is known that a crystallographic orientation of a PTO film is affected by a substrate, there are few reports of systematic study on effects of a substrate on a crystallographic orientation and electrical properties of a PTO thin film [12,13]. We will report effects of substrates on properties of PTO films in this paper.
2. Experimental PTO thin films were prepared by plasma enhanced chemical vapor phase deposition (PE-CVD) method. Fig. 1 schematically shows the PE-CVD system. PTO films were grown on three types of substrates: Pt(1 0 0)/MgO(1 0 0), Pt(1 1 1)/MgO(1 0 0) and
0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00450-1
K. Nishida et al. / Applied Surface Science 216 (2003) 318–322
319
Fig. 1. Schematic drawing of the PE-CVD chamber with in-situ Raman monitoring system.
Pt(1 1 1)/MgO(1 1 1). The Pt layer with a thickness of 100 nm was deposited by rf sputtering method. Pb(dpm)2 and titanium isopropoxide were used as a source material with high purity oxygen. A carrier gas of metalorganic sources was high purity nitrogen. Flow rates of the carrier gas Pb(dpm)2 and titanium isopropoxide were 50 and 5 sccm, respectively. A flow rate of high purity oxygen was 50 sccm. The growth conditions were as follows: substrate temperature, 546 8C; pressure, 10 Pa; and PF power, 170 W. The thickness of the samples was fixed 0.6 mm. The PTO films were characterized with X-ray diffraction (XRD), atomic force microscopy (AFM) and micro Raman spectroscopy. The growth system shown in Fig. 1 has an in-situ monitoring system based on laser Raman spectroscopy. The Curie temperature was estimated from change of the Raman spectra. A Raman equipment T64000 (JOBIN YVON) was used as the micro Raman spectroscopy and a laser Raman system SYSTEM1000 (RENISHAW) was used in the in-situ monitoring system. The excitation source was an Arþ laser with a wavelength of 514.5 nm for Raman measurement. A diameter of the laser probe was 1– 100 mm. A multi-channel detector was used for measurement with a high S/N ratio and in a short time. The electrical properties were characterized using the Sawyer-tower circuit after formation of an upper electrode of Au by evaporation method.
3. Results and discussion Fig. 2 shows the XRD spectra from the PTO thin films on various substrates. The PTO thin films
Fig. 2. XRD spectra of the PTO thin films on substrates with various crystallographic orientations.
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K. Nishida et al. / Applied Surface Science 216 (2003) 318–322
Fig. 3. AFM images of the PTO thin films on substrates with various crystallographic orientations. The PTO films had orientations: (a) (1 0 0); (b) (0 0 1); and (c) (1 1 1).
grown on the Pt(1 0 0)/MgO(1 0 0) and the Pt(1 1 1)/ MgO(1 1 1) substrates were orientated to (0 0 1) and (1 1 1), respectively. PTO(0 0 1) and PTO(1 1 1) thin films were affected by the substrate and had orientations same as those of the substrate. On the other hand, the PTO film had grown on the Pt(1 1 1)/MgO(1 0 0) substrate to the (1 0 0) direction by itself. The crystallinity seems to be poor because the full-width of the half-maximum (FWHM) of XRD spectrum was larger than those of the other PTO films. The reason for orientating why the PTO(1 0 0) film self-oriented on Pt(1 1 1) and the Pt(1 1 1) self-oriented on MgO(1 0 0) has not been made clear.
Fig. 4. Raman spectra of the PTO thin films on substrates with various crystallographic orientations. Raman spectrum of powder PTO is shown as a reference.
The AFM images of the PTO films grown on various substrates are shown in Fig. 3. Triangular crystal grains were observed which corresponded to the orientations of the Pt layers in the case of Pt(1 1 1)/ MgO(1 1 1) while the surface of the PTO film formed on the Pt(1 0 0)/MgO(1 0 0) substrate was very flat. The PTO(0 0 1) and PTO(1 1 1) film of the root mean
Fig. 5. Raman shift vs. temperature of the B1 þ EðTOÞ mode obtained at the cooling process.
K. Nishida et al. / Applied Surface Science 216 (2003) 318–322
square (rms) of roughness were 6.87 and 69.04 nm, respectively (Fig. 3(b) and (c)). While the MgO(1 0 0) substrate had a very flat surface, MgO(1 1 1) substrate received much scratch damage though the surface polishing. Therefore, these different substrates for crystal growth caused such a difference in surface roughness of the PTO films. Fine crystal grains of PTO with a quadrangular shape were observed on the surface of a PTO film grown on Pt(1 1 1)/MgO(1 0 0). The rms of the last film was 97.31 nm which was the highest degree of roughness observed for the films on three types of the substrates (Fig. 3(a)). The results of characterization with XRD shown above and AFM means that growth mode of PTO on the Pt(1 1 1)/ MgO(1 1 1) substrate is different from that on other types of substrates. Stress in the PTO films was characterized based on extra shift of the phonon frequency measured using micro Raman spectroscopy. The Raman spectra were shown in Fig. 4. The Raman spectrum of powder PTO
321
is simultaneously shown as a reference. Each peak in the Raman spectra was assigned as shown in Fig. 4. The E(ITO) peak is called ‘‘soft mode’’. The appearance of this peak denotes that the material is ferroelectric [6,14–17]. Stresses accumulated in the PTO films with orientations (1 0 0), (0 0 1) and (1 1 1) were 1.96, 0.93 and 2.66 GPa, respectively, compressive. Larger stress accumulated in the PTO(1 1 1) films because the difference of lattice mismatch between the substrate and the PTO thin film was larger for the former than for the latter. Effects of substrate orientations on the Curie temperature of PTO films, Raman spectra were measured at various temperatures in the cooling process using the in-situ Raman spectroscopy. Relations between the Raman shift of the B1 þ EðTOÞ mode and temperature are plotted in Fig. 5. The B1 þ EðTOÞ mode shifted to a lower wave number with decrease in substrate temperature after growth and it took a minimum value. Then, it shifted to a higher wave number.
Fig. 6. P–E properties of the PTO thin films on substrates with various crystallographic orientations. The PTO films had orientations: (a) (1 0 0); (b) (0 0 1); and (c) (1 1 1).
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K. Nishida et al. / Applied Surface Science 216 (2003) 318–322
The minimum value of a temperature can be considered to correspond with a change in crystallographic structure. Therefore, the minimum temperature can be said to be the Curie point. The Curie points of the (1 0 0), (0 0 1) and (1 1 1) orientated PTO thin films were 470, 440 and 460 8C, respectively. Stress accumulated in the PTO(1 1 1) thin film was larger than that in PTO(0 0 1) thin film. The difference in the stress is the reason why the Curie point of the PTO(1 1 1) thin film was higher than that of the PTO(0 0 1) thin film. However, it is impossible to discuss the difference in the Curie point of the PTO(1 0 0) thin film only from a point of stress because the PTO films are polycrystalline. The P–E measurement was carried out in order to discern a clear relationship between crystallographic orientation of a film and its electrical properties. P–E curves obtained from the PTO films are shown in Fig. 6. The PTO film having (0 0 1) orientation grown on the Pt(1 0 0)/MgO(1 0 0) substrate showed the ferroelectric hystersis. The residual polarization Pr was 18 mC/cm2 and the anti-electric field Ec was 50 kV/cm. Other PTO films grown on Pt(1 1 1)/ MgO(1 1 1) or Pt(1 1 1)/MgO(1 0 0) substrates did not shown such hysteresis. The reason of these results is that the polarization axis of PTO is (0 0 1).
4. Summary In this paper, effects of the substrates with various types of crystallographic and electrical properties of PTO films were reported. The PTO films grown by PE-CVD had (0 0 1) and (1 1 1) orientation on the substrates of Pt(1 0 0)/MgO(1 0 0) and Pt(1 1 1)/ MgO(1 1 1), respectively. However, a PTO thin films with (1 0 0) orientation was obtained on the Pt(1 1 1)/ MgO(1 0 0) substrate. Flatness of the surface depended
on the crystallographic orientation and the PTO films with (0 0 1) orientation having the best flatness. P–E characteristics depended also on the orientation of the surface. Especially, the PTO(0 0 1) on the Pt(1 0 0)/ MgO(1 0 0) substrate that had an orientation corresponding to the polarization direction showed a ferroelectrical hysteria loop. The results shown in this paper indicate that it is possible to control P–E of the PTO films characteristics by controlling the crystallographic orientation of the substrate. References [1] J.S. Wrigh, L.F. Francis, J. Mater. Res. 8 (1993) 1712. [2] G. Burns, B.A. Scott, Phys. Rev. B 7 (1973) 3088. [3] B.S. Kwak, E.P. Boyd, A. Erbil, Appl. Phys. Lett. 53 (18) (1988) 1702. [4] T. Li, Y. Zhu, S.B. Desu, C.H. Peng, M. Nagata, Appl. Phys. Lett. 68 (1996) 616. [5] G.R. Bai, H.L.M. Chang, C.M. Foster, Z. Shen, D.J. Lam, J. Mater. Res. 9 (1994) 156. [6] L. Taguchi, A. Pignolet, L. Wand, M. Proctor, F. Levy, P.E. Schmid, J. Appl. Phys. 73 (1993) 394. [7] K. Iijima, Y. Tomita, R. Takayama, I. Ueda, J Appl. Phys. 60 (1986) 361. [8] T. Ogawa, A. Senda, T. Kasamani, Jpn. J. Appl. Phys., Part 1 30 (1991) 2145. [9] M. Maeda, H. Ishida, K.K.K. Soe, I. Suzuki, Jpn. J. Appl. Phys. 32 (1993) 4136. [10] S.S. Dana, F.K. Etzod, J. Clabes, J. Appl. Phys. 69 (1991) 4398. [11] C. Chen, D.F. Ryder Jr., J. Am. Ceram. Soc. 72 (1989) 1495. [12] S.W. Chung, S.O. Chung, K. No, W.J. Lee, Thin Solid Films 295 (1997) 299. [13] B.M. Yen, H. Chen, J. Appl. Phys. 85 (1999) 853. [14] W.H. Ma, M.S. Zhang, Z. Tin, J. Korean Phys. Soc. 32 (1998) S1137. [15] M.D. Fontana, A. Ridah, G.E. Kugel, C. Carabatos Nedelec, J. Phys. C: Solid State Phys. 21 (1988) 5853. [16] G. Burns, B.A. Scott, Phys. Rev. B 7 (1973) 3088. [17] M. Maglione, R. Bohmer, A. Loidl, U.T. Hochli, Phys. Rev. B 40 (1989) 11441.