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Multiferroic properties of polycrystalline Zn-substituted BiFeO3 thin films prepared by pulsed laser deposition
Current Applied Physics 11 (2011) S270eS273
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Multiferroic properties of polycrystalline Zn-substituted BiFeO3 thin films prepared by pulsed laser deposition Jung Min Park a, *, Fumiya Gotoda a, Seiji Nakashima b, Takeshi Kanashima a, Masanori Okuyama a a b
Graduated School of Engineering Science, Osaka University, 1-3 Machikaneyama-Cho, Toyonaka, Osaka 560-8531, Japan Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2201, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 June 2010 Received in revised form 28 January 2011 Accepted 7 March 2011 Available online 25 March 2011
BiFe1xZnxO3 (BFZO) thin films (x ¼ 0, 0.05, and 0.1) have been prepared on Pt/TiO2/SiO2/Si substrate by pulsed laser deposition and their multiferoic properties have been investigated. BFZO thin films shows polycrystalline perovskite single phase. The grains become small with increasing the substitution of Zn, and leakage current of BFZO thin film for x ¼ 0.05 was lower than that of BiFeO3 thin film. The dielectric constant of BFZO thin films for x ¼ 0, 0.05, and 0.1 are 107, 146, and 170 at room temperature, respectively. P-E hysteresis loops were obtained at room temperature and 80 K, and Pr slightly increases at low frequency from 500 Hz to 20 kHz. M-H hysteresis loops show weak ferromagnetic properties at 300 K and 80 K by substitution of Zn. Ó 2011 Elsevier B.V. All rights reserved.
Keywords: BFZO thin film Ferroelectirc properties Ferromagnetic properties
1. Introduction Multiferroic materials possess interesting properties such as ferroelectricity, ferromagnetism and ferroeleasticity simultaneously, and its magnetoelectric coupling is expected to be used for various functional devices [1]. Among of mutiferroic materials, BiFeO3 (BFO) has attracted much attention because ferroelectric and antiferromagnetic ordering temperatures are above room temperature (Curie temperature of w 1103 K and the Neel temperature of w 643 K) and large polarization in epitaxial and polycrystalline BFO thin film has been reported [2e4]. However, it is known that BFO thin films have large leakage current that make it difficult to obtain ferroelectric hysteresis loop at room temperature and weak magnetic properties. To solve these problems, element substitution at Bi-or Fe site in BFO thin film has been attempted. Some rare-earth ions such as La3þ, Nd3þ, and Gd3þ were substituted in Bi site and the substitution was reported to be effective as far as leakage current is concerned [5,6]. For Fe site, substitution of Ti4þ in BFO thin films have been reported in lower leakage current, but the hysteresis loop was unsaturated [7]. On the other hand, with the substitution of Mn4þ in BFO thin film, lower leakage current as well as saturated hysteresis loop have been reported [8]. However, these have been few reports about magnetic
* Corresponding author. E-mail address: [email protected] (J.M. Park). 1567-1739/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2011.03.032
properties and the ionic charge valence has not been confirmed, because Ti and Mn have mixed charge valence. In this study, we performed Zn substitution on BFO thin films prepared on Pt/TiO2/SiO2/Si substrate by pulsed laser deposition (PLD). The ionic charge valence of Zn2þ is stable and it is also expected that substitution of nonmagnetic ions like Zn in Fe site will exhibit ferromagnetism, and might lead to the removal of antiferromagnetically coupled spins [9,10]. Based on these facts, we have investigated electric, ferroelectric, and ferromagnetic properties in Zn-substituted BFO thin films. 2. Experiments BiFe1xZnxO3 thin films (x ¼ 0, 0.05, 0.1) on Pt (200 nm)/TiO2 (50 nm)/SiO2 (600 nm)/Si (625 mm) substrate were prepared using pulsed laser deposition (PLD) method. The fabrication of ceramic targets was started by mixing the appropriate amount of oxide powers of Bi2O3 (10% excess), Fe2O3 and ZnO. The mixed powder was then calcined, and then ground, then finally sintered at 800 Ce850 C after pressing. The films were deposited at 500 C and various oxygen pressures, using an ArF excimer laser (l ¼ 193 nm) with an energy of 130 mJ and a frequency of 5 Hz. Film thickness of 350 nm was obtained after a 30 min deposition. The crystalline structure of deposited thin films was identified by X-ray diffraction (XRD) analysis (Rigaku RINT 2000). The surface morphologies were observed by atomic force microscopy (AFM), (SIISPA400). In order to measure the electric, dielectric properties and the ferroelectric P-E
J.M. Park et al. / Current Applied Physics 11 (2011) S270eS273
Current Density (A/cm2)
a
Current Density (A/cm2)
b
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0
10 x=0 -1 10 x=0.05 -2 10 x=0.1 -3 10 -4 10 -5 10 -6 10 -7 10 -8 10 Temp. : RT -9 10 -150 -100 -50 0 50 100 150 Electric Field (kV/cm) 0
10 -1 10 -2 10 -3 10 -4 10 -5 10 -6 10 -7 10 -8 10 -9 10 -1000
x=0 x=0.05 x=0.1
Temp. : 80 K -500 0 500 Electric Field (kV/cm)
1000
Fig. 3. The leakage current of BiFe1xZnxO3 thin films for x ¼ 0, 0.05, and 0.1 measured at (a) RT and (b) 80 K.
Fig. 1. (a) XRD patterns and (b) expanded scan around (010) of BiFe1xZnxO3 thin films for x ¼ 0, 0.05, and 0.1, respectively.
hysteresis loop, circular Pt electrodes of 200 mm diameters were formed on thin films by RF sputtering through a shadow mask. The electric and dielectric properties were measured using a semicoconductor parameter analyzer (Agilent 4155C). The ferroelectric and magnetic hysteresis loops were measured using the ferroelectric test system (Toyotechnica FCE-1) and the superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS-5S). A magnetic field of 0.5 to 0.5 T was applied perpendicularly to the film plane. 3. Results and discussion Fig. 1(a) shows the XRD (qe2q) diffraction patterns of BiFe1xZnxO3 (BFZO) thin films for x ¼ 0, 0.05 and 0.1, respectively.
The XRD patterns of thin films were indexed for rhombohedral structure and exhibit polycrystalline perovskite single phase without secondary phases. However, the impurity phase of Bi2O3 is observed in BFZO thin film for x ¼ 0.1. Compared to pure BFO thin film, no shift and change of reflections were observed for BFZO thin films for x ¼ 0.05 and 0.1. This indicates that the crystalline BFO structure has been little affected by the substitution of Zn. However, the peak widths of (010) in BFZO thin films became broader with increasing the fraction of Zn, which might have also lead to the decrease in grain size, as shown in Fig. 1(b). Fig. 2 (a)e(c) show the surface morphologies of BiFe1xZnxO3 thin films for x ¼ 0, 0.05, and 0.1 measured by atomic force microscopy (AFM), respectively. It was observed that the grains of BFZO thin films become small compared to those of pure BFO thin film, although large grain still remain in the small grains as shown in Fig. 1(b), and surface morphologies of BFZO thin films consist of fine and dense, compared to that of BFO thin film. Similar results were confirmed in reports about substitution of Mn, Sc, and Ti doped in BFO thin film [11e13]. Fig. 3(a) shows the current density-electric field (J-E) characteristics of BiFe1xZnxO3 thin films for x ¼ 0, 0.05, and 0.1, measured at room temperature. The leakage current density of BFZO thin film for x ¼ 0.05 shows lower values than that of a pure BFO thin film, while the leakage current density of BFZO thin film for x ¼ 0.1 is
Fig. 2. Surface morphologies of BiFe1xZnxO3 thin films for x ¼ (a) 0, (b) 0.05, and (c) 0.1.
J.M. Park et al. / Current Applied Physics 11 (2011) S270eS273
x=0 x=0.05 x=0.1
200
0.5 0.4
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0.3
100
0.2
50
0.1
0 3 10
4
5
10 10 Frequency (Hz)
)
Dielectric Constant ( r)
250
Loss Factor (tan
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Fig. 4. Dielectric constant (3r) and loss factor (tan d) of BiFe1xZnxO3 thin films for x ¼ 0, 0.05, and 0.1 measured at RT.
increased. The leakage current densities were 1.4 102, 2.6 103, and 2.7 102 A/cm2 for x ¼ 0, 0.05, and 0.1 at an electric field of 142 kV/cm, respectively. The leakage current densities of BiFe1xZnxO3 thin films for x ¼ 0, 0.05, and 0.1 measured at 80 K become lower than those of RT, as shown in Fig. 3(b). Although the leakage current density of a pure BFO thin film at maximum electric field of 860 kV/cm is 3.7 104 A/cm2 which is lower than that of RT, it was still larger than the BFZO thin film of 3.5 105 A/cm2 for x ¼ 0.05. These results indicate that the leakage current of Zn-substituted BFO thin film for x ¼ 0.05 exhibits significantly lower values than that of BFO thin film at both temperature of RT and 80 K, and P-E hysteresis loop can also be expected at room temperature. Sakamoto et al. described that substitution of Mn2þ in BFO thin film exhibits low leakage current and doped Mn2þ might play an important role as acceptor for improving the electrical properties in BFO thin film [14]. On the other hand, Hu et al. described that formation of defect complexes between the
a
acceptors (Zn2þFe3þ) and (VO2) in BFO lead to low leakage current because applied electric field requires to overcome the electrostatic attraction force between the (Zn2þFe3þ) and (VO2) [13]. Based on the above reports, reduction of leakage current by substitution of Znþ2 can be established. Fig. 4 shows the frequency dependence of the dielectric constant (3r) and loss factor (tan d) of BiFe1xZnxO3 thin films for x ¼ 0, 0.05, and 0.1 measured at RT, throughout the frequency range of 103e106 Hz. The 3r and tan d of both BFO and BFZO thin films show a slightly decreasing tendency with increasing the frequency from 103 to 106 Hz. At low frequency, increased 3r and tan d might be due to charged defects [15]. The 3r and tan d of BFZO thin films for x ¼ 0, 0.05, and 0.1 were 107, 146, 170, and 0.04, 0.03 and 0.05 at 1 MHz, respectively. This result indicates that the Zn-substituted BFO thin films have higher relative dielectric constants than those of pure BFO thin film which is also confirmed by previous reports [5,15,16]. The P-E hysteresis loops of BiFe1xZnxO3 thin films for x ¼ 0, 0.05, and 0.1 were measured using a 20 kHz triangular waveform at room temperature and 80 K, as shown in Fig. 5 (a) and (b). P-E hysteresis loops were obtained in BFO and BFZO thin films at room temperature as shown in Fig. 5 (a). However, BFZO thin film for
b
Fig. 5. P-E hysteresis loops of BiFe1xZnxO3 thin films for x ¼ 0, 0.05, and 0.1 measured at (a) RT and (b) 80 K.
Fig. 6. Frequency dependence of P-E hysteresis loops with sweeping frequency changed from 500 Hz to 20 kHz for x ¼ 0, 0.05, and 0.1, respectively.
J.M. Park et al. / Current Applied Physics 11 (2011) S270eS273
a 10
3
Magnetization (emu/cm )
0 -5
Temp. : 300 K 2.5 -2.5 0 Magnetic Field (kOe)
Temp. : 80 K
Magnetization (emu/cm )
3
5 0 -5
-10 -5
-2.5 0 2.5 Magnetic Field (kOe)
0 -5
-10 -5
Temp. : 300 K
2.5 -2.5 0 Magnetic Field (kOe)
5
x=0.05
0
Fig. 7(a)e(b) show magnetic hysteresis (M-H) loops of BiFe1xZnxO3 thin films for x ¼ 0, 0.05, and 0.1 at 300 and 80 K for the maximum magnetic field of 5 kOe, respectively. The pure BFO thin film shows antiferromagnetic behavior, while BFZO thin films show weak ferromagnetic behavior with increasing the substitution of Zn at 300 and 80 K. The remanent magnetization (Mr) measured at 80 K were 0.15, 0.6 and 0.8 emu/cm3, for x ¼ 0, 0.05, and 0.1, respectively. The enhanced magnetic properties have been explained by Tanaka et. al. and J. Wei et. al. [9,17].. They described that antiferromagnetically coupled spins might be perturbed by substitution of nonmagnetical ions such as Zr4þ and Zn2þ, and when trivalent Fe ions are replaced by Zr4þ and Zn2þ ions, ferromagnetic-like coupling rather than an antiferromagnetic behavior might be shown. Therefore, we suggest that substitution of Zn2þ in BFO thin film might affect magnetic properties, with possibility of breaking the antimagnetic coupling in BFO thin film.
-5
4. Conclusion
Temp. : 80 K 2.55 0 -2.5 Magnetic Field (kOe)
10
x=0.1
3
x=0.1 5
5
5
-10 -5
5
10
0 2.5 -2.5 Magnetic Field (kOe)
-5
10
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-5 -10
x=0.05
3
0
5
10
Magnetization (emu/cm )
x=0
5
Magnetization (emu/cm )
3
Magnetization (emu/cm )
x=0 5
-10 -5
80 K
b 10
RT
S273
5 0 -5
-10 -5
Temp. : 80 K
0 2.5 -2.5 Magnetic Field (kOe)
5
Fig. 7. M-H hysteresis loops of BiFe1xZnxO3 thin films for x ¼ 0, 0.05, and 0.1 measured at (a) RT and (b) 80 K.
x ¼ 0.1 shows unsaturated hysteresis loop. The remanent polarization (Pr) and coercive field (Ec) were 103, 78, and 89 mC/cm2 and 250, 185, and 209 kV/cm, for x ¼ 0, 0.05, and 0.1 at maximum field of 415 kV/cm, respectively. In order to suppress the leakage current and to measure saturated polarization, we performed P-E hysteresis measurement at 80 K. The saturated P-E hysteresis loops are obtained for both BFO and BFZO thin films, as shown in Fig. 5(b). The remanent polarization (Pr) and coercive field (Ec) were 116, 112, and 94 mC/cm2 and 300, 284, and 295 kV/cm, for x ¼ 0, 0.05, 0.1 at maximum field of 845 kV/cm, respectively. Remanent polarization (Pr) values at 80 K are larger compared to Pr values at room temperature, and remanent polarization (Pr) and coercive field (Ec) of BFZO thin films were decreased at RT and 80 K, compared to those of pure BFO thin film. These results might be due to decrease of the grain size (and hence, more defects formed at the grain boundaries could have deteriorated the ferroelectric properties) [6]. Fig. 6(a)e(c) show frequency dependence of P-E hysteresis loops measured at 80 K, for x ¼ 0, 0.05, and 0.1, respectively. Pr (500 Hz) e Pr (20 kHz) values were 14, 3, and 17 mC/cm2 for x ¼ 0, 0.05, and 0.1 at sweeping frequencies of 500 Hze20 kHz, respectively. We were able to point out that for the BFO and BFZO thin films at x ¼ 0.1, the increase of Pr values is found with decreasing the frequency from 500 to 20 kHz, while BFZO thin film for x ¼ 0.05 shows weak dependence on frequency.
We have investigated the characteristics of polycrystalline Zn-substituted BiFeO3 thin films prepared by pulsed laser deposition. From XRD patterns, we found that the crystalline BFO structure has not been affected by substitution of Zn2þ. The grain sizes of BFZO thin films decrease, and leakage currents decrease for 5%-Zn doped BFO thin film, compared to those of pure BFO thin film. The P-E hysteresis loops of BFZO thin films were obtained at RT and 80 K. For BFZO thin film of x ¼ 0.05, weak dependence was exhibited at frequencies from 500 to 20 kHz, compared to BFZO thin film of x ¼ 0, and 0.1. The M-H hysteresis loops of BFZO thin films were obtained at 300 and 80 K. The BFZO thin films show weak ferromagnetic behavior, while pure BFO thin film exhibits antiferromagnetic behavior. Thus, it was confirmed that Zn-substituted BFO thin films for x ¼ 0.05 show good ferroelectric properties due to reducing the leakage current and weak ferromagnetic behavior. References [1] W. Eerenstein, N.D. Mathur, J.F. Scott, Nature 442 (2006) 759. [2] G.A. Smolenskii, I. Chupis, Sov. Phys. Usp. 26 (1982) 475. [3] J. Wang, J. Neaton, H. Zheng, V. Nagarajan, S. Ogale, B. Liu, D. Vihland, V. Vaithyanathan, D. Schlom, U. Waghmare, N. Spaldin, K. Rabe, M. Wuttig, R. Ramesh, Science 299 (2003) 1719. [4] K.Y. Yun, D. Ricinshi, T. Kanashima, M. Okuyama, Appl. Phys. Lett. 89 (2006) 192902. [5] H. Uchida, R. Ueno, H. Nakaki, H. Funakubo, S. Koda, Jpn. J. Appl. Phys. 44 (2005) 561. [6] G.D. Hu, X. Cheng, W.B. Wu, C.H. Yang, Appl. Phys. Lett. 91 (2007) 232909. [7] Y. Wang, C.W. Nan, Appl. Phys. Lett. 89 (2006) 052903. [8] S.K. Singh, H. Ishiwara, K. Maruyama, Appl. Phys. Lett. 88 (2006) 262908. [9] J. Takaobushi, H. Tanaka, T. Kawai, S. Ueda, E. Ikenaga, J.J. Kim, M. Kobata, E. Ikenaga, M. Yabashi, K. Kobayashi, Y. Nishino, D. Miwa, K. Tamasaku, T. Ishikawa, Appl. Phys. Lett. 89 (2006) 242507. [10] D. Venkateshvaran, M. Althammer, A. Nielsen, S. Geprags, M. Rao, T. Goennenwein, M. Opel, R. Gross, Phys. Rev. B. 79 (2009) 134405. [11] C.F. Chung, J.P. Lin, J.M. Wu, Appl. Phys. Lett. 88 (2006) 242909. [12] S.R. Shannigrahi, A. Huang, N. Chadrasekhar, D. Tripathy, A.O. Adeyeye, Appl. Phys. Lett. 90 (2007) 022901. [13] G.D. Hu, S.H. Fan, C.H. Yang, W.B. Wu, Appl. Phys. Lett. 92 (2008) 192905. [14] W. Sakamoto, A. Iwata, T. Yogo, J. Appl. Phys. 104 (2008) 104106. [15] G.L. Yuan, S.W. Or, J. Appl. Phys. 100 (2008) 024109. [16] Y.H. Lee, J.M. Wu, C.H. Lai, Appl. Phys. Lett. 88 (2006) 042903. [17] J. Wei, R. Haumont, R. Jarrier, P. Berhtet, B. Dkhill, Appl. Phys. Lett. 96 (2010) 102509.
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Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Multiferroic properties of polycrystalline Zn-substituted BiFeO3 thin films prepared by pulsed laser deposition Jung Min Park a, *, Fumiya Gotoda a, Seiji Nakashima b, Takeshi Kanashima a, Masanori Okuyama a a b
Graduated School of Engineering Science, Osaka University, 1-3 Machikaneyama-Cho, Toyonaka, Osaka 560-8531, Japan Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2201, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 June 2010 Received in revised form 28 January 2011 Accepted 7 March 2011 Available online 25 March 2011
BiFe1xZnxO3 (BFZO) thin films (x ¼ 0, 0.05, and 0.1) have been prepared on Pt/TiO2/SiO2/Si substrate by pulsed laser deposition and their multiferoic properties have been investigated. BFZO thin films shows polycrystalline perovskite single phase. The grains become small with increasing the substitution of Zn, and leakage current of BFZO thin film for x ¼ 0.05 was lower than that of BiFeO3 thin film. The dielectric constant of BFZO thin films for x ¼ 0, 0.05, and 0.1 are 107, 146, and 170 at room temperature, respectively. P-E hysteresis loops were obtained at room temperature and 80 K, and Pr slightly increases at low frequency from 500 Hz to 20 kHz. M-H hysteresis loops show weak ferromagnetic properties at 300 K and 80 K by substitution of Zn. Ó 2011 Elsevier B.V. All rights reserved.
Keywords: BFZO thin film Ferroelectirc properties Ferromagnetic properties
1. Introduction Multiferroic materials possess interesting properties such as ferroelectricity, ferromagnetism and ferroeleasticity simultaneously, and its magnetoelectric coupling is expected to be used for various functional devices [1]. Among of mutiferroic materials, BiFeO3 (BFO) has attracted much attention because ferroelectric and antiferromagnetic ordering temperatures are above room temperature (Curie temperature of w 1103 K and the Neel temperature of w 643 K) and large polarization in epitaxial and polycrystalline BFO thin film has been reported [2e4]. However, it is known that BFO thin films have large leakage current that make it difficult to obtain ferroelectric hysteresis loop at room temperature and weak magnetic properties. To solve these problems, element substitution at Bi-or Fe site in BFO thin film has been attempted. Some rare-earth ions such as La3þ, Nd3þ, and Gd3þ were substituted in Bi site and the substitution was reported to be effective as far as leakage current is concerned [5,6]. For Fe site, substitution of Ti4þ in BFO thin films have been reported in lower leakage current, but the hysteresis loop was unsaturated [7]. On the other hand, with the substitution of Mn4þ in BFO thin film, lower leakage current as well as saturated hysteresis loop have been reported [8]. However, these have been few reports about magnetic
* Corresponding author. E-mail address: [email protected] (J.M. Park). 1567-1739/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2011.03.032
properties and the ionic charge valence has not been confirmed, because Ti and Mn have mixed charge valence. In this study, we performed Zn substitution on BFO thin films prepared on Pt/TiO2/SiO2/Si substrate by pulsed laser deposition (PLD). The ionic charge valence of Zn2þ is stable and it is also expected that substitution of nonmagnetic ions like Zn in Fe site will exhibit ferromagnetism, and might lead to the removal of antiferromagnetically coupled spins [9,10]. Based on these facts, we have investigated electric, ferroelectric, and ferromagnetic properties in Zn-substituted BFO thin films. 2. Experiments BiFe1xZnxO3 thin films (x ¼ 0, 0.05, 0.1) on Pt (200 nm)/TiO2 (50 nm)/SiO2 (600 nm)/Si (625 mm) substrate were prepared using pulsed laser deposition (PLD) method. The fabrication of ceramic targets was started by mixing the appropriate amount of oxide powers of Bi2O3 (10% excess), Fe2O3 and ZnO. The mixed powder was then calcined, and then ground, then finally sintered at 800 Ce850 C after pressing. The films were deposited at 500 C and various oxygen pressures, using an ArF excimer laser (l ¼ 193 nm) with an energy of 130 mJ and a frequency of 5 Hz. Film thickness of 350 nm was obtained after a 30 min deposition. The crystalline structure of deposited thin films was identified by X-ray diffraction (XRD) analysis (Rigaku RINT 2000). The surface morphologies were observed by atomic force microscopy (AFM), (SIISPA400). In order to measure the electric, dielectric properties and the ferroelectric P-E
J.M. Park et al. / Current Applied Physics 11 (2011) S270eS273
Current Density (A/cm2)
a
Current Density (A/cm2)
b
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0
10 x=0 -1 10 x=0.05 -2 10 x=0.1 -3 10 -4 10 -5 10 -6 10 -7 10 -8 10 Temp. : RT -9 10 -150 -100 -50 0 50 100 150 Electric Field (kV/cm) 0
10 -1 10 -2 10 -3 10 -4 10 -5 10 -6 10 -7 10 -8 10 -9 10 -1000
x=0 x=0.05 x=0.1
Temp. : 80 K -500 0 500 Electric Field (kV/cm)
1000
Fig. 3. The leakage current of BiFe1xZnxO3 thin films for x ¼ 0, 0.05, and 0.1 measured at (a) RT and (b) 80 K.
Fig. 1. (a) XRD patterns and (b) expanded scan around (010) of BiFe1xZnxO3 thin films for x ¼ 0, 0.05, and 0.1, respectively.
hysteresis loop, circular Pt electrodes of 200 mm diameters were formed on thin films by RF sputtering through a shadow mask. The electric and dielectric properties were measured using a semicoconductor parameter analyzer (Agilent 4155C). The ferroelectric and magnetic hysteresis loops were measured using the ferroelectric test system (Toyotechnica FCE-1) and the superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS-5S). A magnetic field of 0.5 to 0.5 T was applied perpendicularly to the film plane. 3. Results and discussion Fig. 1(a) shows the XRD (qe2q) diffraction patterns of BiFe1xZnxO3 (BFZO) thin films for x ¼ 0, 0.05 and 0.1, respectively.
The XRD patterns of thin films were indexed for rhombohedral structure and exhibit polycrystalline perovskite single phase without secondary phases. However, the impurity phase of Bi2O3 is observed in BFZO thin film for x ¼ 0.1. Compared to pure BFO thin film, no shift and change of reflections were observed for BFZO thin films for x ¼ 0.05 and 0.1. This indicates that the crystalline BFO structure has been little affected by the substitution of Zn. However, the peak widths of (010) in BFZO thin films became broader with increasing the fraction of Zn, which might have also lead to the decrease in grain size, as shown in Fig. 1(b). Fig. 2 (a)e(c) show the surface morphologies of BiFe1xZnxO3 thin films for x ¼ 0, 0.05, and 0.1 measured by atomic force microscopy (AFM), respectively. It was observed that the grains of BFZO thin films become small compared to those of pure BFO thin film, although large grain still remain in the small grains as shown in Fig. 1(b), and surface morphologies of BFZO thin films consist of fine and dense, compared to that of BFO thin film. Similar results were confirmed in reports about substitution of Mn, Sc, and Ti doped in BFO thin film [11e13]. Fig. 3(a) shows the current density-electric field (J-E) characteristics of BiFe1xZnxO3 thin films for x ¼ 0, 0.05, and 0.1, measured at room temperature. The leakage current density of BFZO thin film for x ¼ 0.05 shows lower values than that of a pure BFO thin film, while the leakage current density of BFZO thin film for x ¼ 0.1 is
Fig. 2. Surface morphologies of BiFe1xZnxO3 thin films for x ¼ (a) 0, (b) 0.05, and (c) 0.1.
J.M. Park et al. / Current Applied Physics 11 (2011) S270eS273
x=0 x=0.05 x=0.1
200
0.5 0.4
150
0.3
100
0.2
50
0.1
0 3 10
4
5
10 10 Frequency (Hz)
)
Dielectric Constant ( r)
250
Loss Factor (tan
S272
0.0 6 10
Fig. 4. Dielectric constant (3r) and loss factor (tan d) of BiFe1xZnxO3 thin films for x ¼ 0, 0.05, and 0.1 measured at RT.
increased. The leakage current densities were 1.4 102, 2.6 103, and 2.7 102 A/cm2 for x ¼ 0, 0.05, and 0.1 at an electric field of 142 kV/cm, respectively. The leakage current densities of BiFe1xZnxO3 thin films for x ¼ 0, 0.05, and 0.1 measured at 80 K become lower than those of RT, as shown in Fig. 3(b). Although the leakage current density of a pure BFO thin film at maximum electric field of 860 kV/cm is 3.7 104 A/cm2 which is lower than that of RT, it was still larger than the BFZO thin film of 3.5 105 A/cm2 for x ¼ 0.05. These results indicate that the leakage current of Zn-substituted BFO thin film for x ¼ 0.05 exhibits significantly lower values than that of BFO thin film at both temperature of RT and 80 K, and P-E hysteresis loop can also be expected at room temperature. Sakamoto et al. described that substitution of Mn2þ in BFO thin film exhibits low leakage current and doped Mn2þ might play an important role as acceptor for improving the electrical properties in BFO thin film [14]. On the other hand, Hu et al. described that formation of defect complexes between the
a
acceptors (Zn2þFe3þ) and (VO2) in BFO lead to low leakage current because applied electric field requires to overcome the electrostatic attraction force between the (Zn2þFe3þ) and (VO2) [13]. Based on the above reports, reduction of leakage current by substitution of Znþ2 can be established. Fig. 4 shows the frequency dependence of the dielectric constant (3r) and loss factor (tan d) of BiFe1xZnxO3 thin films for x ¼ 0, 0.05, and 0.1 measured at RT, throughout the frequency range of 103e106 Hz. The 3r and tan d of both BFO and BFZO thin films show a slightly decreasing tendency with increasing the frequency from 103 to 106 Hz. At low frequency, increased 3r and tan d might be due to charged defects [15]. The 3r and tan d of BFZO thin films for x ¼ 0, 0.05, and 0.1 were 107, 146, 170, and 0.04, 0.03 and 0.05 at 1 MHz, respectively. This result indicates that the Zn-substituted BFO thin films have higher relative dielectric constants than those of pure BFO thin film which is also confirmed by previous reports [5,15,16]. The P-E hysteresis loops of BiFe1xZnxO3 thin films for x ¼ 0, 0.05, and 0.1 were measured using a 20 kHz triangular waveform at room temperature and 80 K, as shown in Fig. 5 (a) and (b). P-E hysteresis loops were obtained in BFO and BFZO thin films at room temperature as shown in Fig. 5 (a). However, BFZO thin film for
b
Fig. 5. P-E hysteresis loops of BiFe1xZnxO3 thin films for x ¼ 0, 0.05, and 0.1 measured at (a) RT and (b) 80 K.
Fig. 6. Frequency dependence of P-E hysteresis loops with sweeping frequency changed from 500 Hz to 20 kHz for x ¼ 0, 0.05, and 0.1, respectively.
J.M. Park et al. / Current Applied Physics 11 (2011) S270eS273
a 10
3
Magnetization (emu/cm )
0 -5
Temp. : 300 K 2.5 -2.5 0 Magnetic Field (kOe)
Temp. : 80 K
Magnetization (emu/cm )
3
5 0 -5
-10 -5
-2.5 0 2.5 Magnetic Field (kOe)
0 -5
-10 -5
Temp. : 300 K
2.5 -2.5 0 Magnetic Field (kOe)
5
x=0.05
0
Fig. 7(a)e(b) show magnetic hysteresis (M-H) loops of BiFe1xZnxO3 thin films for x ¼ 0, 0.05, and 0.1 at 300 and 80 K for the maximum magnetic field of 5 kOe, respectively. The pure BFO thin film shows antiferromagnetic behavior, while BFZO thin films show weak ferromagnetic behavior with increasing the substitution of Zn at 300 and 80 K. The remanent magnetization (Mr) measured at 80 K were 0.15, 0.6 and 0.8 emu/cm3, for x ¼ 0, 0.05, and 0.1, respectively. The enhanced magnetic properties have been explained by Tanaka et. al. and J. Wei et. al. [9,17].. They described that antiferromagnetically coupled spins might be perturbed by substitution of nonmagnetical ions such as Zr4þ and Zn2þ, and when trivalent Fe ions are replaced by Zr4þ and Zn2þ ions, ferromagnetic-like coupling rather than an antiferromagnetic behavior might be shown. Therefore, we suggest that substitution of Zn2þ in BFO thin film might affect magnetic properties, with possibility of breaking the antimagnetic coupling in BFO thin film.
-5
4. Conclusion
Temp. : 80 K 2.55 0 -2.5 Magnetic Field (kOe)
10
x=0.1
3
x=0.1 5
5
5
-10 -5
5
10
0 2.5 -2.5 Magnetic Field (kOe)
-5
10
3
Magnetization (emu/cm )
-5 -10
x=0.05
3
0
5
10
Magnetization (emu/cm )
x=0
5
Magnetization (emu/cm )
3
Magnetization (emu/cm )
x=0 5
-10 -5
80 K
b 10
RT
S273
5 0 -5
-10 -5
Temp. : 80 K
0 2.5 -2.5 Magnetic Field (kOe)
5
Fig. 7. M-H hysteresis loops of BiFe1xZnxO3 thin films for x ¼ 0, 0.05, and 0.1 measured at (a) RT and (b) 80 K.
x ¼ 0.1 shows unsaturated hysteresis loop. The remanent polarization (Pr) and coercive field (Ec) were 103, 78, and 89 mC/cm2 and 250, 185, and 209 kV/cm, for x ¼ 0, 0.05, and 0.1 at maximum field of 415 kV/cm, respectively. In order to suppress the leakage current and to measure saturated polarization, we performed P-E hysteresis measurement at 80 K. The saturated P-E hysteresis loops are obtained for both BFO and BFZO thin films, as shown in Fig. 5(b). The remanent polarization (Pr) and coercive field (Ec) were 116, 112, and 94 mC/cm2 and 300, 284, and 295 kV/cm, for x ¼ 0, 0.05, 0.1 at maximum field of 845 kV/cm, respectively. Remanent polarization (Pr) values at 80 K are larger compared to Pr values at room temperature, and remanent polarization (Pr) and coercive field (Ec) of BFZO thin films were decreased at RT and 80 K, compared to those of pure BFO thin film. These results might be due to decrease of the grain size (and hence, more defects formed at the grain boundaries could have deteriorated the ferroelectric properties) [6]. Fig. 6(a)e(c) show frequency dependence of P-E hysteresis loops measured at 80 K, for x ¼ 0, 0.05, and 0.1, respectively. Pr (500 Hz) e Pr (20 kHz) values were 14, 3, and 17 mC/cm2 for x ¼ 0, 0.05, and 0.1 at sweeping frequencies of 500 Hze20 kHz, respectively. We were able to point out that for the BFO and BFZO thin films at x ¼ 0.1, the increase of Pr values is found with decreasing the frequency from 500 to 20 kHz, while BFZO thin film for x ¼ 0.05 shows weak dependence on frequency.
We have investigated the characteristics of polycrystalline Zn-substituted BiFeO3 thin films prepared by pulsed laser deposition. From XRD patterns, we found that the crystalline BFO structure has not been affected by substitution of Zn2þ. The grain sizes of BFZO thin films decrease, and leakage currents decrease for 5%-Zn doped BFO thin film, compared to those of pure BFO thin film. The P-E hysteresis loops of BFZO thin films were obtained at RT and 80 K. For BFZO thin film of x ¼ 0.05, weak dependence was exhibited at frequencies from 500 to 20 kHz, compared to BFZO thin film of x ¼ 0, and 0.1. The M-H hysteresis loops of BFZO thin films were obtained at 300 and 80 K. The BFZO thin films show weak ferromagnetic behavior, while pure BFO thin film exhibits antiferromagnetic behavior. Thus, it was confirmed that Zn-substituted BFO thin films for x ¼ 0.05 show good ferroelectric properties due to reducing the leakage current and weak ferromagnetic behavior. References [1] W. Eerenstein, N.D. Mathur, J.F. Scott, Nature 442 (2006) 759. [2] G.A. Smolenskii, I. Chupis, Sov. Phys. Usp. 26 (1982) 475. [3] J. Wang, J. Neaton, H. Zheng, V. Nagarajan, S. Ogale, B. Liu, D. Vihland, V. Vaithyanathan, D. Schlom, U. Waghmare, N. Spaldin, K. Rabe, M. Wuttig, R. Ramesh, Science 299 (2003) 1719. [4] K.Y. Yun, D. Ricinshi, T. Kanashima, M. Okuyama, Appl. Phys. Lett. 89 (2006) 192902. [5] H. Uchida, R. Ueno, H. Nakaki, H. Funakubo, S. Koda, Jpn. J. Appl. Phys. 44 (2005) 561. [6] G.D. Hu, X. Cheng, W.B. Wu, C.H. Yang, Appl. Phys. Lett. 91 (2007) 232909. [7] Y. Wang, C.W. Nan, Appl. Phys. Lett. 89 (2006) 052903. [8] S.K. Singh, H. Ishiwara, K. Maruyama, Appl. Phys. Lett. 88 (2006) 262908. [9] J. Takaobushi, H. Tanaka, T. Kawai, S. Ueda, E. Ikenaga, J.J. Kim, M. Kobata, E. Ikenaga, M. Yabashi, K. Kobayashi, Y. Nishino, D. Miwa, K. Tamasaku, T. Ishikawa, Appl. Phys. Lett. 89 (2006) 242507. [10] D. Venkateshvaran, M. Althammer, A. Nielsen, S. Geprags, M. Rao, T. Goennenwein, M. Opel, R. Gross, Phys. Rev. B. 79 (2009) 134405. [11] C.F. Chung, J.P. Lin, J.M. Wu, Appl. Phys. Lett. 88 (2006) 242909. [12] S.R. Shannigrahi, A. Huang, N. Chadrasekhar, D. Tripathy, A.O. Adeyeye, Appl. Phys. Lett. 90 (2007) 022901. [13] G.D. Hu, S.H. Fan, C.H. Yang, W.B. Wu, Appl. Phys. Lett. 92 (2008) 192905. [14] W. Sakamoto, A. Iwata, T. Yogo, J. Appl. Phys. 104 (2008) 104106. [15] G.L. Yuan, S.W. Or, J. Appl. Phys. 100 (2008) 024109. [16] Y.H. Lee, J.M. Wu, C.H. Lai, Appl. Phys. Lett. 88 (2006) 042903. [17] J. Wei, R. Haumont, R. Jarrier, P. Berhtet, B. Dkhill, Appl. Phys. Lett. 96 (2010) 102509.