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Influence of epitaxial growth on phase competition in Pr0.5Sr0.5MnO3 films
Journal of Magnetism and Magnetic Materials 324 (2012) 1189–1192
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Influence of epitaxial growth on phase competition in Pr0.5Sr0.5MnO3 films Liping Chen a,b,n, Yuansha Chen b, Yubin Ma b, Guijun Lian b, Yan Zhang b, Guangcheng Xiong b a b
Department of Physics, Zhejiang Normal University, Jinhua 321004, PR China Department of Physics, Peking University, Beijing 100871, PR China
a r t i c l e i n f o
abstract
Article history: Received 1 February 2011 Received in revised form 1 September 2011 Available online 12 November 2011
A series of Pr0.5Sr0.5MnO3 (PSMO) films with various thickness were epitaxially grown on substrates of (0 0 1)-oriented (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 (LSAT), LaAlO3 (LAO) and SrTiO3 (STO), and (0 1 1)oriented STO using pulse laser deposition. Influence of epitaxial growth on phase competition was investigated. A ferromagnetic metal to antiferromagnetic insulator (FMM–AFI) transition upon cooling is present in both largely compressed situations deposited on LAO (0 0 1) and tensile cases deposited on STO (0 0 1) but absent in little strained films grown on LSAT (0 0 1), indicating that the antiferromagnetic insulating state is favored by strains. On the other hand, the 400 nm films deposited on (0 1 1)oriented STO as well as LAO substrates show FMM–AFI transition. These results reveal that both the orientation of epitaxial growth and substrate-induced strain affect the FMM–AFI transition. & 2011 Elsevier B.V. All rights reserved.
Keywords: Manganite Phase competition Pr0.5Sr0.5MnO3 Antiferromagnetic
1. Introduction Arising from the interactions among charge, orbital, spin and lattice, doped perovskite manganites exhibit rich phase diagrams, which may be favored in application [1]. As one of the most intriguing phenomena in the strongly correlated electron system, charge/orbital ordering usually appears in half doped manganites such as Pr0.5Ca0.5MnO3 [2,3], La0.5Ca0.5MnO3 [4], Pr0.5Sr0.5MnO3 (PSMO) [5–8] and Nd0.5Sr0.5MnO3 (NSMO) [6,9]. Among these materials PSMO and NSMO have been especially interested as they locate at phase boundary among paramagnetic-insulating (PMI), ferromagnetic-metallic (FMM) and antiferromagnetic-insulating (AFI) phases. The phase competition therein yields a ferromagnetic metal to antiferromagnetic insulator (FMM–AFI) transition at 140 K with a very sharp metal–insulator transition [5,6]. The stripe-type charge ordering in PSMO, which is specific to the manganites with a relatively wide one-electron bandwidth, has d(x2 y2) orbital state, A-type AFM spin ordering along the z direction, and is quite different from the CE-type one in NSMO [6,8,9]. As a result, PSMO shows some interesting properties. Application of small external perturbs, such as magnetic/electric field, strain or light, gives rise to drastic changes of resistivity and phase competition [5,10–12]. Sharp FMM–AFI transition is present in the films deposited on (0 1 1) substrates of LSAT and STO [8,13,14], but absent in the films grown on (0 0 1) substrates of LAO [10]. It seems that the properties of the films are determined by the orientation of the film growth. However, Prellier and Mercey
found that the deposition temperature also directly influences the substrate-induced strain and, therefore, the phase of the films on (0 0 1)LAO substrates [15]. These observations indicated that the phase transition of the films depend on several factors such as the orientation of the film growth, deposition temperature and strain. Due to the various deposition conditions, the results reported by different groups were not so comparable. Therefore, a more systematic study is needed.
2. Experimental details PSMO films were grown on (0 0 1)-oriented substrates of LSAT, LAO and STO, and (0 1 1)-oriented substrates of STO using pulse laser deposition. The pulsed KrF laser has a wavelength of 248 nm. The energy density was 300 J/cm2. The deposition temperature is 800 1C, and a pure O2 of 30 Pa was maintained throughout the deposition. After deposition, films were annealed and cooled to room temperature in 0.5 atm pressure of O2 to avoid possible oxygen deficiency. The film thickness ranges from 60 to 400 nm, as controlled by deposition time. Bulk PSMO crystal has a pseudocubic lattice with ap 0.384 nm [7,8,16]. Therefore, epitaxial PSMO films on LAO (a¼0.379 nm, lattice mismatch d ¼(ap asubstrate)/asubstrate 100% 1.3%) and STO substrates (a¼0.390 nm, d 1.5%) should be compressed and expanded in in-plane lattice, respectively, whereas the films on LSAT (a¼ 0.387 nm, d 0.8%) should be little strained.
3. Results and discussion n
Corresponding author at. Department of Physics, Zhejiang Normal University, Jinhua 321004, PR China. E-mail address: [email protected] (L. Chen). 0304-8853/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2011.11.006
The epitaxial relationship and crystalline quality of the films were assessed by four-circle X-ray diffraction (XRD). Fig. 1(a)
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L. Chen et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 1189–1192
displays the y 2y scan of the films grown on STO (0 0 1). Besides the diffraction of the substrate, there are only reflection peaks arising from (0 0 l) PSMO planes, showing a c-axis growth of the films. We investigated the in-plane orientation of the film by scanning an off-axis peak. Fig. 1(b) shows the j scans of (1 1 3) PSMO and (1 1 3) STO reflection for the films with a same beginning azimuth angle. The periodically spaced peaks of the (1 1 3) PSMO are separated by 901, and appear at the angles where the peaks of (1 1 3) STO reflection appear. This result implies a four-fold symmetry characteristic of the film and an in-plane epitaxial relationship of PSMO [1 0 0]//STO [1 0 0]. Similar XRD data were obtained for the films on (0 0 1)-oriented LAO and LSAT substrates. Consequently, all the films on (0 0 1) substrates are epitaxially grown and have a tetragonal structure. Fig. 1(c) shows the thickness dependence of out-of-plane lattice parameter for the films on (0 0 1) substrates. The out-of-plane lattice of films on LSAT is little
dependent on thickness. The value of the lattice (c¼0.382 nm) approaches that of the bulk PSMO crystal (ap 0.384 nm), which indicates that there are little strain and consequent strain relaxation due to the minor mismatch between the films and LSAT. An elongated/compressed out-of-plane lattice was seen in the 60 nm thick film on LAO/STO, suggesting compressive/tensile in-plane strain. Such a lattice parameter tends to approach to that of the bulk crystal with increasing thickness, implying a gradual relaxation of the strain. The out-of-plane strain ezz can be estimated by ezz ¼ (cfilm cbulk)/cbulk 100% where the cfilm and cbulk are out-ofplane parameters of PSMO film and crystal, respectively. As shown in Fig. 1(d), when the film thickness enhances from 60 to 400 nm, the ezz of PSMO films on LAO and STO relax from 2.4% to 0.2% and from 1.8% to 0.8%, respectively. Fig. 2 shows the XRD spectra of the films grown on STO (0 1 1). In the y 2y scan (Fig. 2(a)) besides the reflection of (0 l l) STO, the
Fig. 1. (a) y 2y scan of the films grown on STO (1 1 0). (b) j scans of the (1 1 3) PSMO reflection and (1 1 3) STO for the films on STO (1 1 0). (c) The thickness dependence of out-of-plane parameters cfilm for films on LAO (0 0 1) (’), STO (0 0 1) (K) and LSAT (0 0 1) (D). The arrows are guides for eyes. The line shows the value of pseudocubic lattice of bulk PSMO crystal (ap 0.384 nm).
Fig. 2. XRD spectra of the films on STO (0 1 1): (a) y 2y scan; (b) j scans of the (1 1 1) PSMO reflection and (1 1 1) STO for the films; (c) the (0 2 2) PSMO reflection of 60 nm and 400 nm films.
Fig. 3. r–T curves for the films on (0 0 1)-oriented LSAT (a), LAO (b) and STO (c). The respective inset is the M–T curve of 60 nm film.
L. Chen et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 1189–1192
well defined (0 1 1) and (0 2 2) PSMO reflections are identified, implying PSMO [0 1 1] perpendicular to the substrate surface. Fig. 2(b) presents the j scans of (1 1 1) PSMO and (1 1 1) STO with a same beginning azimuth angle. The simultaneous reflection peaks of PSMO and STO were both separated by 1801. This result indicates a two-fold symmetry characteristic of the film and an in-plane epitaxial relationship of PSMO [1 0 0]//STO [1 0 0]. Since the in-plane lattice mismatch between lattice parameters for the (0 1 1) substrate and film d is 1.4%, the in-plane lattice suffers tensile strain. Calculated from the XRD (shown in Fig. 2(c)), when the thickness increases from 60 to 400 nm, the out-of-plane lattice of the film increases from 0.269 nm to 0.271 nm, and the consequent out-of-plane strain e relaxes from 0.9% to 0%. Using the standard four-probe technique, we measured the temperature dependence of resistivity for the films. By a superconducting quantum interference device magnetometer, we performed the magnetic measurements. To clarify possible in-plane anisotropy, we measured the r–T along [1 0 0] and [1 1 0] direction for films grown on (0 0 1) substrates. As a result, the r–T along [1 0 0] direction is consistent with the r–T along [1 1 0] direction, indicating isotropic properties within plane. For films on the STO (0 1 1), the r–T along [0 –1 1] direction in the plane agrees with the r–T along [1 0 0] direction. Therefore, we only exhibit the r–T and M–T along [1 0 0] direction for films grown on (0 0 1) substrates and along [0 1 1] direction for the films on STO (0 1 1). Fig. 3 shows the r–T curves of films grown on (0 0 1)-oriented, LAO (a), STO (b) LSAT (c). The inset shows the field-cooled M–T
Fig. 4. r–T curves for the films on (0 1 1)-oriented STO; the top inset is the M–T curve of 400 nm thick film; the bottom inset is the r–T curves for the 400 nm thick films on (0 1 1) LAO substrates.
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curve for the 60 nm film, respectively. The films on LSAT exhibit a PMI–FMM transition at the temperature of 277 K, which is little influenced by the film thickness. Besides the PMI–FMM transition at high temperature, the 60 nm thick films on LAO also have a FMM–AFI transition at low temperature, which is quite different from the films grown on LSAT. The Curie temperature and Neel temperature are TC ¼235 K and TN ¼163 K, respectively. With increasing thickness, the PMI–FMM transition shifts to higher as the TC of the films on LAO rises to 253 K of 120 nm thick films, and then to 4 290 K of 400 nm thick films. On the contrary, the FMM– AFI transition moves towards to lower temperature. The films on STO present a similar relationship between the phase transitions and thickness. The 60 nm thick films get into FMM state at TC ¼263 K, and turn to AFI phase at TN ¼151 K. When the thickness increases, the PMI–FMM transition temperature TC decreases to 274 K of 120 nm thick films and then to 4290 K for the 400 nm thick films; the FMM–AFI transition moves to lower temperature as the relevant metal–insulator transition temperature is lowered from 210 K of 60 nm thick films to 170 K of 120 nm thick film and then to o20 K of 400 nm thick films. Comparison between these (0 0 1) films suggested that the phase diagram in the films is drastically dependent on strain. Unlike the absence of the FMM–AFI transition in the little strained films on LSAT, both the strained films on LAO and STO substrates show a lower TC and a higher TN than the bulk crystal (TC ¼270 K of the crystal TN ¼140 K), showing the strain effect on the phase transitions. The FMM–AFI transition is favored by the strain no matter of compressive or tensile strain. Fig. 4 shows the r–T curves of films grown on STO (0 1 1). The top inset plots the M–T of the 400 m thick films. Both the 60 nm and 400 nm films present a FMM–AFI transition at 130 K. It seems that the orientation of the film growth is another way to control the FMM–AFI transition besides strain. To confirm this point, we deposited the films on (0 1 1) LAO as well. As displayed in the bottom inset of Fig. 4, the r–T curve of the mostly strain-relaxed 400 nm thick films implies that a FMM–AFI transition occurs at temperature of 140 K. These observations demonstrate that the (0 1 1) epitaxial growth of the films brings a FMM–AFI transition. Accompanying the FMM–AFI transition at 140 K, a structural transition described in the inset of Fig. 5(a) occurs [7,8,16]. The lattice changes from tetragonal in FMM state to monoclinic in AFI state. The structure of the (0 0 1)-oriented film on LSAT was described in Fig. 5(a). The in-plane lattice of the film clamps to substrate. Due to tetragonal substrate, tetragonal symmetry is imposed in the film. Also, the lattice deformation such as compression or extension of the in-plane lattice is severely restricted. Only the out-of-plane lattice constant can vary while the in-plane lattice constants are clamped to the substrates. In such case, the FMM–AFI transition is difficult to take place. Fig. 5(b) exhibits the schematic structure of the PSMO films grown on STO (0 1 1), as reported previously. The in-plane cfilm(or afilm) is along STO [1 0 0], bfilm and afilm(or cfilm) are 451 with respect to the surface. In this situation, the desired structural
Fig. 5. Schematic lattice structure of (0 0 1) epitaxial PSMO films (a) and (0 1 1) epitaxial PSMO films (b); the arrows denote the lattice changes in FMM–AFI transition for bulk PSMO crystal.
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transition is easy to be realized. On the other hand, the appearance of the FMM–AFI transition in the thinner films on LAO (0 0 1) and STO (0 0 1) substrates may be ascribed to large strain. The strain induces J–T distortion might provide a freedom for the occurrence of FMM–AFI transition [17]. As a result of strain relaxation with increasing thickness, the FMM–AFI is moved to lower temperature and finally disappears. In the films on LSAT (0 0 1), the lattice mismatch is small, and consequently they do not show FMM–AFI transition.
4. Conclusions In conclusion, we systematically investigated the influence of epitaxial growth on phase competition between FMM and AFI sates for PSMO films on (0 0 1)-oriented LSAT, LAO and STO and (0 1 1)-oriented STO substrates. It was found that both the orientation of epitaxial growth and substrate-induced strain affect the FMM–AFI transition significantly. We believe our investigation will be beneficial for the physics research and device application.
Acknowledgements The authors acknowledge Jianfeng Wang for X-ray diffraction measurement.
References [1] C. Martin, A. Maignan, M. Hervieu, B. Raveau, Physical Review B 60 (1999) 12191. [2] V.N. Smolyaninova, A. Biswas, C. Hill, B.G. Kim, S.W. Cheong, R.L. Greene, Journal of Magnetism and Magnetic Materials 267 (2003) 300–306. [3] L. Pi, E. Fan, Y.Z. Zhang, J.X. Ma, S. Tan, Y.H. Zhang, Journal of Magnetism and Magnetic Materials 314 (2007) 87–92. [4] C.H. Chen, S.W. Cheong, Physical Review Letters 76 (1996) 4042. [5] Y. Tomioka, A. Asamitsu, Y. Moritomo, H. Kuwahara, Y. Tokura, Physical Review Letters 74 (1995) 5108. [6] H. Kawano, R. Kajimoto, H. Yoshizawa, Y. Tomioka, H. Kuwahara, Y. Tokura, Physical Review Letters 78 (1997) 4253–4256. [7] A. Llobet, J.L. Garcia-Munoz, C. Frontera, C. Ritter, Physical Review B 60 (1999) R9889–R9892. [8] R. Kajimoto, H. Yoshizawa, Y. Tomioka, Y. Tokura, Physical Review B 66 (2002) 180402 R. [9] H. Kawano-Furukawa, R. Kajimoto, H. Yoshizawa, Y. Tomioka, H. Kuwahara, Y. Tokura, Physical Review B 67 (2003). [10] B. Mercey, M. Hervieu, W. Prellier, J. Wolfman, C. Simon, B. Raveau, Applied Physics Letters 78 (2001) 3857–3859. [11] Y. Wakabayashi, D. Bizen, Y. Kubo, H. Nakao, Y. Murakami, M. Nakamura, Y. Ogimoto, K. Miyano, H. Sawa, Journal of the Physical Society of Japan 77 (2008). [12] M. Nakamura, Y. Ogimoto, H. Tamaru, N. Izumi, K. Miyano, Applied Physics Letters 86 (2005) 182504. [13] Y. Ogimoto, N. Takubo, M. Nakamura, H. Tamaru, M. Izumi, K. Miyano, Applied Physics Letters 86 (2005) 112513. [14] Y. Ogimoto, M. Nakamura, N. Takubo, H. Tamaru, M. Izumi, K. Miyano, Physical Review B 71 (2005) 060403 R. [15] W. Prellier, B. Mercey, Journal of Physics D—Applied Physics 38 (2005) 172–174. [16] K. Knizek, Z. Jirak, E. Pollert, F. Zounova, S. Vratislav, Journal of Solid State Chemistry 100 (1992) 292–300. [17] J. Wang, F.X. Hu, R.W. Li, J.R. Sun, B.G. Shen, Applied Physics Letters 96 (2010).
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Influence of epitaxial growth on phase competition in Pr0.5Sr0.5MnO3 films Liping Chen a,b,n, Yuansha Chen b, Yubin Ma b, Guijun Lian b, Yan Zhang b, Guangcheng Xiong b a b
Department of Physics, Zhejiang Normal University, Jinhua 321004, PR China Department of Physics, Peking University, Beijing 100871, PR China
a r t i c l e i n f o
abstract
Article history: Received 1 February 2011 Received in revised form 1 September 2011 Available online 12 November 2011
A series of Pr0.5Sr0.5MnO3 (PSMO) films with various thickness were epitaxially grown on substrates of (0 0 1)-oriented (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 (LSAT), LaAlO3 (LAO) and SrTiO3 (STO), and (0 1 1)oriented STO using pulse laser deposition. Influence of epitaxial growth on phase competition was investigated. A ferromagnetic metal to antiferromagnetic insulator (FMM–AFI) transition upon cooling is present in both largely compressed situations deposited on LAO (0 0 1) and tensile cases deposited on STO (0 0 1) but absent in little strained films grown on LSAT (0 0 1), indicating that the antiferromagnetic insulating state is favored by strains. On the other hand, the 400 nm films deposited on (0 1 1)oriented STO as well as LAO substrates show FMM–AFI transition. These results reveal that both the orientation of epitaxial growth and substrate-induced strain affect the FMM–AFI transition. & 2011 Elsevier B.V. All rights reserved.
Keywords: Manganite Phase competition Pr0.5Sr0.5MnO3 Antiferromagnetic
1. Introduction Arising from the interactions among charge, orbital, spin and lattice, doped perovskite manganites exhibit rich phase diagrams, which may be favored in application [1]. As one of the most intriguing phenomena in the strongly correlated electron system, charge/orbital ordering usually appears in half doped manganites such as Pr0.5Ca0.5MnO3 [2,3], La0.5Ca0.5MnO3 [4], Pr0.5Sr0.5MnO3 (PSMO) [5–8] and Nd0.5Sr0.5MnO3 (NSMO) [6,9]. Among these materials PSMO and NSMO have been especially interested as they locate at phase boundary among paramagnetic-insulating (PMI), ferromagnetic-metallic (FMM) and antiferromagnetic-insulating (AFI) phases. The phase competition therein yields a ferromagnetic metal to antiferromagnetic insulator (FMM–AFI) transition at 140 K with a very sharp metal–insulator transition [5,6]. The stripe-type charge ordering in PSMO, which is specific to the manganites with a relatively wide one-electron bandwidth, has d(x2 y2) orbital state, A-type AFM spin ordering along the z direction, and is quite different from the CE-type one in NSMO [6,8,9]. As a result, PSMO shows some interesting properties. Application of small external perturbs, such as magnetic/electric field, strain or light, gives rise to drastic changes of resistivity and phase competition [5,10–12]. Sharp FMM–AFI transition is present in the films deposited on (0 1 1) substrates of LSAT and STO [8,13,14], but absent in the films grown on (0 0 1) substrates of LAO [10]. It seems that the properties of the films are determined by the orientation of the film growth. However, Prellier and Mercey
found that the deposition temperature also directly influences the substrate-induced strain and, therefore, the phase of the films on (0 0 1)LAO substrates [15]. These observations indicated that the phase transition of the films depend on several factors such as the orientation of the film growth, deposition temperature and strain. Due to the various deposition conditions, the results reported by different groups were not so comparable. Therefore, a more systematic study is needed.
2. Experimental details PSMO films were grown on (0 0 1)-oriented substrates of LSAT, LAO and STO, and (0 1 1)-oriented substrates of STO using pulse laser deposition. The pulsed KrF laser has a wavelength of 248 nm. The energy density was 300 J/cm2. The deposition temperature is 800 1C, and a pure O2 of 30 Pa was maintained throughout the deposition. After deposition, films were annealed and cooled to room temperature in 0.5 atm pressure of O2 to avoid possible oxygen deficiency. The film thickness ranges from 60 to 400 nm, as controlled by deposition time. Bulk PSMO crystal has a pseudocubic lattice with ap 0.384 nm [7,8,16]. Therefore, epitaxial PSMO films on LAO (a¼0.379 nm, lattice mismatch d ¼(ap asubstrate)/asubstrate 100% 1.3%) and STO substrates (a¼0.390 nm, d 1.5%) should be compressed and expanded in in-plane lattice, respectively, whereas the films on LSAT (a¼ 0.387 nm, d 0.8%) should be little strained.
3. Results and discussion n
Corresponding author at. Department of Physics, Zhejiang Normal University, Jinhua 321004, PR China. E-mail address: [email protected] (L. Chen). 0304-8853/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2011.11.006
The epitaxial relationship and crystalline quality of the films were assessed by four-circle X-ray diffraction (XRD). Fig. 1(a)
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L. Chen et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 1189–1192
displays the y 2y scan of the films grown on STO (0 0 1). Besides the diffraction of the substrate, there are only reflection peaks arising from (0 0 l) PSMO planes, showing a c-axis growth of the films. We investigated the in-plane orientation of the film by scanning an off-axis peak. Fig. 1(b) shows the j scans of (1 1 3) PSMO and (1 1 3) STO reflection for the films with a same beginning azimuth angle. The periodically spaced peaks of the (1 1 3) PSMO are separated by 901, and appear at the angles where the peaks of (1 1 3) STO reflection appear. This result implies a four-fold symmetry characteristic of the film and an in-plane epitaxial relationship of PSMO [1 0 0]//STO [1 0 0]. Similar XRD data were obtained for the films on (0 0 1)-oriented LAO and LSAT substrates. Consequently, all the films on (0 0 1) substrates are epitaxially grown and have a tetragonal structure. Fig. 1(c) shows the thickness dependence of out-of-plane lattice parameter for the films on (0 0 1) substrates. The out-of-plane lattice of films on LSAT is little
dependent on thickness. The value of the lattice (c¼0.382 nm) approaches that of the bulk PSMO crystal (ap 0.384 nm), which indicates that there are little strain and consequent strain relaxation due to the minor mismatch between the films and LSAT. An elongated/compressed out-of-plane lattice was seen in the 60 nm thick film on LAO/STO, suggesting compressive/tensile in-plane strain. Such a lattice parameter tends to approach to that of the bulk crystal with increasing thickness, implying a gradual relaxation of the strain. The out-of-plane strain ezz can be estimated by ezz ¼ (cfilm cbulk)/cbulk 100% where the cfilm and cbulk are out-ofplane parameters of PSMO film and crystal, respectively. As shown in Fig. 1(d), when the film thickness enhances from 60 to 400 nm, the ezz of PSMO films on LAO and STO relax from 2.4% to 0.2% and from 1.8% to 0.8%, respectively. Fig. 2 shows the XRD spectra of the films grown on STO (0 1 1). In the y 2y scan (Fig. 2(a)) besides the reflection of (0 l l) STO, the
Fig. 1. (a) y 2y scan of the films grown on STO (1 1 0). (b) j scans of the (1 1 3) PSMO reflection and (1 1 3) STO for the films on STO (1 1 0). (c) The thickness dependence of out-of-plane parameters cfilm for films on LAO (0 0 1) (’), STO (0 0 1) (K) and LSAT (0 0 1) (D). The arrows are guides for eyes. The line shows the value of pseudocubic lattice of bulk PSMO crystal (ap 0.384 nm).
Fig. 2. XRD spectra of the films on STO (0 1 1): (a) y 2y scan; (b) j scans of the (1 1 1) PSMO reflection and (1 1 1) STO for the films; (c) the (0 2 2) PSMO reflection of 60 nm and 400 nm films.
Fig. 3. r–T curves for the films on (0 0 1)-oriented LSAT (a), LAO (b) and STO (c). The respective inset is the M–T curve of 60 nm film.
L. Chen et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 1189–1192
well defined (0 1 1) and (0 2 2) PSMO reflections are identified, implying PSMO [0 1 1] perpendicular to the substrate surface. Fig. 2(b) presents the j scans of (1 1 1) PSMO and (1 1 1) STO with a same beginning azimuth angle. The simultaneous reflection peaks of PSMO and STO were both separated by 1801. This result indicates a two-fold symmetry characteristic of the film and an in-plane epitaxial relationship of PSMO [1 0 0]//STO [1 0 0]. Since the in-plane lattice mismatch between lattice parameters for the (0 1 1) substrate and film d is 1.4%, the in-plane lattice suffers tensile strain. Calculated from the XRD (shown in Fig. 2(c)), when the thickness increases from 60 to 400 nm, the out-of-plane lattice of the film increases from 0.269 nm to 0.271 nm, and the consequent out-of-plane strain e relaxes from 0.9% to 0%. Using the standard four-probe technique, we measured the temperature dependence of resistivity for the films. By a superconducting quantum interference device magnetometer, we performed the magnetic measurements. To clarify possible in-plane anisotropy, we measured the r–T along [1 0 0] and [1 1 0] direction for films grown on (0 0 1) substrates. As a result, the r–T along [1 0 0] direction is consistent with the r–T along [1 1 0] direction, indicating isotropic properties within plane. For films on the STO (0 1 1), the r–T along [0 –1 1] direction in the plane agrees with the r–T along [1 0 0] direction. Therefore, we only exhibit the r–T and M–T along [1 0 0] direction for films grown on (0 0 1) substrates and along [0 1 1] direction for the films on STO (0 1 1). Fig. 3 shows the r–T curves of films grown on (0 0 1)-oriented, LAO (a), STO (b) LSAT (c). The inset shows the field-cooled M–T
Fig. 4. r–T curves for the films on (0 1 1)-oriented STO; the top inset is the M–T curve of 400 nm thick film; the bottom inset is the r–T curves for the 400 nm thick films on (0 1 1) LAO substrates.
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curve for the 60 nm film, respectively. The films on LSAT exhibit a PMI–FMM transition at the temperature of 277 K, which is little influenced by the film thickness. Besides the PMI–FMM transition at high temperature, the 60 nm thick films on LAO also have a FMM–AFI transition at low temperature, which is quite different from the films grown on LSAT. The Curie temperature and Neel temperature are TC ¼235 K and TN ¼163 K, respectively. With increasing thickness, the PMI–FMM transition shifts to higher as the TC of the films on LAO rises to 253 K of 120 nm thick films, and then to 4 290 K of 400 nm thick films. On the contrary, the FMM– AFI transition moves towards to lower temperature. The films on STO present a similar relationship between the phase transitions and thickness. The 60 nm thick films get into FMM state at TC ¼263 K, and turn to AFI phase at TN ¼151 K. When the thickness increases, the PMI–FMM transition temperature TC decreases to 274 K of 120 nm thick films and then to 4290 K for the 400 nm thick films; the FMM–AFI transition moves to lower temperature as the relevant metal–insulator transition temperature is lowered from 210 K of 60 nm thick films to 170 K of 120 nm thick film and then to o20 K of 400 nm thick films. Comparison between these (0 0 1) films suggested that the phase diagram in the films is drastically dependent on strain. Unlike the absence of the FMM–AFI transition in the little strained films on LSAT, both the strained films on LAO and STO substrates show a lower TC and a higher TN than the bulk crystal (TC ¼270 K of the crystal TN ¼140 K), showing the strain effect on the phase transitions. The FMM–AFI transition is favored by the strain no matter of compressive or tensile strain. Fig. 4 shows the r–T curves of films grown on STO (0 1 1). The top inset plots the M–T of the 400 m thick films. Both the 60 nm and 400 nm films present a FMM–AFI transition at 130 K. It seems that the orientation of the film growth is another way to control the FMM–AFI transition besides strain. To confirm this point, we deposited the films on (0 1 1) LAO as well. As displayed in the bottom inset of Fig. 4, the r–T curve of the mostly strain-relaxed 400 nm thick films implies that a FMM–AFI transition occurs at temperature of 140 K. These observations demonstrate that the (0 1 1) epitaxial growth of the films brings a FMM–AFI transition. Accompanying the FMM–AFI transition at 140 K, a structural transition described in the inset of Fig. 5(a) occurs [7,8,16]. The lattice changes from tetragonal in FMM state to monoclinic in AFI state. The structure of the (0 0 1)-oriented film on LSAT was described in Fig. 5(a). The in-plane lattice of the film clamps to substrate. Due to tetragonal substrate, tetragonal symmetry is imposed in the film. Also, the lattice deformation such as compression or extension of the in-plane lattice is severely restricted. Only the out-of-plane lattice constant can vary while the in-plane lattice constants are clamped to the substrates. In such case, the FMM–AFI transition is difficult to take place. Fig. 5(b) exhibits the schematic structure of the PSMO films grown on STO (0 1 1), as reported previously. The in-plane cfilm(or afilm) is along STO [1 0 0], bfilm and afilm(or cfilm) are 451 with respect to the surface. In this situation, the desired structural
Fig. 5. Schematic lattice structure of (0 0 1) epitaxial PSMO films (a) and (0 1 1) epitaxial PSMO films (b); the arrows denote the lattice changes in FMM–AFI transition for bulk PSMO crystal.
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transition is easy to be realized. On the other hand, the appearance of the FMM–AFI transition in the thinner films on LAO (0 0 1) and STO (0 0 1) substrates may be ascribed to large strain. The strain induces J–T distortion might provide a freedom for the occurrence of FMM–AFI transition [17]. As a result of strain relaxation with increasing thickness, the FMM–AFI is moved to lower temperature and finally disappears. In the films on LSAT (0 0 1), the lattice mismatch is small, and consequently they do not show FMM–AFI transition.
4. Conclusions In conclusion, we systematically investigated the influence of epitaxial growth on phase competition between FMM and AFI sates for PSMO films on (0 0 1)-oriented LSAT, LAO and STO and (0 1 1)-oriented STO substrates. It was found that both the orientation of epitaxial growth and substrate-induced strain affect the FMM–AFI transition significantly. We believe our investigation will be beneficial for the physics research and device application.
Acknowledgements The authors acknowledge Jianfeng Wang for X-ray diffraction measurement.
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