- Email: [email protected]
Transport and magnetic properties of Pr1−xCaxMnO3 epitaxial films grown on LaAlO3 substrates
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 310 (2007) 2237–2238 www.elsevier.com/locate/jmmm
Transport and magnetic properties of Pr1xCaxMnO3 epitaxial films grown on LaAlO3 substrates A. Maniwaa, K. Okanoa, I. Ohkuboa, H. Kumigashiraa,, M. Oshimaa, M. Lippmaab, M. Kawasakic, H. Koinumad a
Department of Applied Chemistry, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan b Institute for Solid State Physics, The University of Tokyo, Kashiwa 277-8581, Japan c Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan d National Institute for Materials Science, Tsukuba 305-0047, Japan Available online 17 November 2006
Abstract We have measured physical properties of Pr1xCaxMnO3 (PCMO) epitaxial thin films with different hole concentrations (x ¼ 0.2, 0.3, 0.4, and 0.5) grown on LaAlO3 (1 0 0) substrates by laser molecular beam epitaxy technique. The temperature dependence of the resistivity shows insulating behavior in all temperature regions and the resistivity itself monotonously decreases as x increases. This insulating nature of PCMO films is similar to that of bulk PCMO crystals. However, we did not find any indication of the resistivity anomaly associated with the onset of charge ordering irrespective of x. These results suggest that the compressive strain strongly suppresses charge-ordered states in PCMO. r 2006 Elsevier B.V. All rights reserved. PACS: 71.30.+h; 73.61.r; 75.70.i Keywords: Pr1xCaxMnO3; Laser MBE; Epitaxial strain; Charge order; Thin film
Hole-doped perovskite manganese oxides exhibit various ground states that can be tuned by the control of bandwidth and/or filling. Among these manganites, Pr1xCaxMnO3 (PCMO, 0.3pxp0.5) is a charge-ordered antiferromagnetic insulator at low temperature (lower than TCO230 K for 0.3pxp0.5) and shows stimulus-induced metal–insulator transition [1]. Recently, PCMO has attracted considerable attention because of their colossal electric-pulse-induced resistance change effect in a film and potential applications to non-volatile random-access-memory devices [2–5]. However, little is known on the transport and magnetic properties of PCMO epitaxial films themselves, though the physical properties of manganese oxides can be easily affected by epitaxial strain [6]. In this study, we have investigated the effects of epitaxial strain by measuring the physical properties of PCMO epitaxial thin
Corresponding author. Tel.: +81 3 5841 7192; fax: +81 3 5841 8744.
E-mail address: [email protected] (H. Kumigashira). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.817
films with different hole concentrations (x ¼ 0.2, 0.3, 0.4, and 0.5) grown on LaAlO3 (1 0 0) substrates. The PCMO epitaxial films with different hole concentrations (x ¼ 0.2, 0.3, 0.4, and 0.5) were grown on LaAlO3 (1 0 0) substrates by laser molecular beam epitaxy technique. A Nd: YAG laser was used for ablation in its frequency-tripled mode (l ¼ 355 nm) at a repetition rate of 2 Hz. PCMO films with a thickness of about 400 A˚ were deposited on the LaAlO3 substrates at a substrate temperature of 500 1C at an oxygen pressure of 0.1 mTorr. The intensity of the specular spot in the reflection highenergy electron diffraction (RHEED) pattern was monitored during the deposition to determine the surface morphology and the film growth rate. The epitaxial growth of PCMO thin films on LaAlO3 substrates was confirmed by the observation of clear oscillations due to the layer-bylayer growth mode. The PCMO thin films were subsequently annealed at 400 1C for 45 min. In the atmospheric pressure of oxygen to remove oxygen vacancies. The surface morphology of the measured films was analyzed
ARTICLE IN PRESS A. Maniwa et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 2237–2238
2238
Table 1 Physical properties of fabricated PCMO thin films grown on LaAlO3 substrates d|| (A˚)
d? (A˚)
Ground state
0.2 0.3 0.4 0.5
3.79 3.79 3.79 3.79
3.98 3.93 3.91 3.85
PI PI PI PI
The out-of-plane (c-axis) and the in-plane (a-axis) lattice constants (d|| and d?, respectively) are determined by the 4cXRD measurements.
10 Resistivity (Ω cm)
x
1
x = 0.2 x = 0.3 x = 0.4
0.1
x = 0.5
50
100
150 200 250 Temperature (K)
300
350
Fig. 2. Temperature dependence of resistivity for PCMO (x ¼ 0.2, 0.3, 0.4, and 0.5) thin films on LaAlO3.
Fig. 1. A typical AFM image (scanned area; 1000 600 nm2) of a 400-A˚thick Pr0.5Ca0.5MnO3 epitaxial film grown on the LaAlO3 substrate.
by atomic force microscopy (AFM) in air. The crystal structure was characterized by four-circle X-ray diffraction (4cXRD) measurements, and the epitaxial growth of the films on the substrates was confirmed. The lattice parameters determined by the 4cXRD are summarized in Table 1. The electrical resistivity was measured by a fourprobe method. The magnetization was measured by a super-conducting quantum-interference device (SQUID) magnetometer. Fig. 1 shows a typical AFM image of the fabricated PCMO thin films. Atomically flat step-and-terrace structures, which reflect the surface structures of LaAlO3 substrates, are clearly observed. Such structures are also observed in all the films grown with different compositions. Fig. 2 shows the temperature dependence of resistivity for the PCMO thin films with different x. The resistivity shows the insulating behavior in all temperature regions and the resistivity itself monotonously decreases as x increases. The transport activation energies are estimated to be 113, 130, 115, 103, and 90.5 meV for PCMO films with x ¼ 0.2, 0.3, 0.4, and 0.5, respectively. These values are comparable to the formation energy of small polarons [7]. The insulating behavior of PCMO films is similar to that of bulk PCMO crystals [1]. However, we did not find any indication of the resistivity anomaly associated with the onset of charge ordering irrespective of x. In order to investigate the existence of a possible ferromagnetic phase, we have measured the magnetization of these films (not shown), but we did not find the ferromagnetic states. In the
analogy of bulk phase diagram [1], it is reasonable to conclude that the PCMO film is paramagnetic insulator in the region of 0.2oxo0.5, although it is difficult to eliminate the possibility of antiferromagnetic ground states owing to the small volume of the films. The suppression of charge-ordered states in the PCMO films is in a sharp contrast with their stabilization in the PCMO films grown on SrTiO3 (1 0 0) substrates, where the superlattice spots due to the charge ordering are clearly observed in electron diffraction measurements [8]. The possible reason to reconcile two contradictory experimental results is the difference of epitaxial strain, namely compressive (PCMO/ LaAlO3) or tensile (PCMO/SrTiO3) strain. The tensile strain may make the charge-ordered states stable, while the compressive strain makes them unstable [7] owing to the lack of structural flexibility [9]. These results suggest that the compressive strain strongly suppresses the chargeordered states in PCMO. This work was supported by a Grant-in-Aid for Scientific Research (S17101004) from JSPS.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Y. Tomioka, et al., Phys. Rev. B 53 (1996) R1689. S.Q. Liu, et al., Appl. Phys. Lett. 76 (2000) 2749. T. Oka, et al., Phys. Rev. Lett. 95 (2005) 266403. A. Baikalov, et al., Appl. Phys. Lett. 83 (2003) 957. A. Sawa, et al., Appl. Phys. Lett. 85 (2004) 4073. Y. Konishi, et al., J. Phys. Soc. Japan 68 (1999) 3790. T.T.M. Plastra, et al., Phys. Rev. B 56 (1997) 51046. Z.Q. Yang, et al., Appl. Phys. Lett. 88 (2006) 072507. M. Nakamura, et al., Appl. Phys. Lett. 86 (2005) 182504.
Journal of Magnetism and Magnetic Materials 310 (2007) 2237–2238 www.elsevier.com/locate/jmmm
Transport and magnetic properties of Pr1xCaxMnO3 epitaxial films grown on LaAlO3 substrates A. Maniwaa, K. Okanoa, I. Ohkuboa, H. Kumigashiraa,, M. Oshimaa, M. Lippmaab, M. Kawasakic, H. Koinumad a
Department of Applied Chemistry, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan b Institute for Solid State Physics, The University of Tokyo, Kashiwa 277-8581, Japan c Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan d National Institute for Materials Science, Tsukuba 305-0047, Japan Available online 17 November 2006
Abstract We have measured physical properties of Pr1xCaxMnO3 (PCMO) epitaxial thin films with different hole concentrations (x ¼ 0.2, 0.3, 0.4, and 0.5) grown on LaAlO3 (1 0 0) substrates by laser molecular beam epitaxy technique. The temperature dependence of the resistivity shows insulating behavior in all temperature regions and the resistivity itself monotonously decreases as x increases. This insulating nature of PCMO films is similar to that of bulk PCMO crystals. However, we did not find any indication of the resistivity anomaly associated with the onset of charge ordering irrespective of x. These results suggest that the compressive strain strongly suppresses charge-ordered states in PCMO. r 2006 Elsevier B.V. All rights reserved. PACS: 71.30.+h; 73.61.r; 75.70.i Keywords: Pr1xCaxMnO3; Laser MBE; Epitaxial strain; Charge order; Thin film
Hole-doped perovskite manganese oxides exhibit various ground states that can be tuned by the control of bandwidth and/or filling. Among these manganites, Pr1xCaxMnO3 (PCMO, 0.3pxp0.5) is a charge-ordered antiferromagnetic insulator at low temperature (lower than TCO230 K for 0.3pxp0.5) and shows stimulus-induced metal–insulator transition [1]. Recently, PCMO has attracted considerable attention because of their colossal electric-pulse-induced resistance change effect in a film and potential applications to non-volatile random-access-memory devices [2–5]. However, little is known on the transport and magnetic properties of PCMO epitaxial films themselves, though the physical properties of manganese oxides can be easily affected by epitaxial strain [6]. In this study, we have investigated the effects of epitaxial strain by measuring the physical properties of PCMO epitaxial thin
Corresponding author. Tel.: +81 3 5841 7192; fax: +81 3 5841 8744.
E-mail address: [email protected] (H. Kumigashira). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.817
films with different hole concentrations (x ¼ 0.2, 0.3, 0.4, and 0.5) grown on LaAlO3 (1 0 0) substrates. The PCMO epitaxial films with different hole concentrations (x ¼ 0.2, 0.3, 0.4, and 0.5) were grown on LaAlO3 (1 0 0) substrates by laser molecular beam epitaxy technique. A Nd: YAG laser was used for ablation in its frequency-tripled mode (l ¼ 355 nm) at a repetition rate of 2 Hz. PCMO films with a thickness of about 400 A˚ were deposited on the LaAlO3 substrates at a substrate temperature of 500 1C at an oxygen pressure of 0.1 mTorr. The intensity of the specular spot in the reflection highenergy electron diffraction (RHEED) pattern was monitored during the deposition to determine the surface morphology and the film growth rate. The epitaxial growth of PCMO thin films on LaAlO3 substrates was confirmed by the observation of clear oscillations due to the layer-bylayer growth mode. The PCMO thin films were subsequently annealed at 400 1C for 45 min. In the atmospheric pressure of oxygen to remove oxygen vacancies. The surface morphology of the measured films was analyzed
ARTICLE IN PRESS A. Maniwa et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 2237–2238
2238
Table 1 Physical properties of fabricated PCMO thin films grown on LaAlO3 substrates d|| (A˚)
d? (A˚)
Ground state
0.2 0.3 0.4 0.5
3.79 3.79 3.79 3.79
3.98 3.93 3.91 3.85
PI PI PI PI
The out-of-plane (c-axis) and the in-plane (a-axis) lattice constants (d|| and d?, respectively) are determined by the 4cXRD measurements.
10 Resistivity (Ω cm)
x
1
x = 0.2 x = 0.3 x = 0.4
0.1
x = 0.5
50
100
150 200 250 Temperature (K)
300
350
Fig. 2. Temperature dependence of resistivity for PCMO (x ¼ 0.2, 0.3, 0.4, and 0.5) thin films on LaAlO3.
Fig. 1. A typical AFM image (scanned area; 1000 600 nm2) of a 400-A˚thick Pr0.5Ca0.5MnO3 epitaxial film grown on the LaAlO3 substrate.
by atomic force microscopy (AFM) in air. The crystal structure was characterized by four-circle X-ray diffraction (4cXRD) measurements, and the epitaxial growth of the films on the substrates was confirmed. The lattice parameters determined by the 4cXRD are summarized in Table 1. The electrical resistivity was measured by a fourprobe method. The magnetization was measured by a super-conducting quantum-interference device (SQUID) magnetometer. Fig. 1 shows a typical AFM image of the fabricated PCMO thin films. Atomically flat step-and-terrace structures, which reflect the surface structures of LaAlO3 substrates, are clearly observed. Such structures are also observed in all the films grown with different compositions. Fig. 2 shows the temperature dependence of resistivity for the PCMO thin films with different x. The resistivity shows the insulating behavior in all temperature regions and the resistivity itself monotonously decreases as x increases. The transport activation energies are estimated to be 113, 130, 115, 103, and 90.5 meV for PCMO films with x ¼ 0.2, 0.3, 0.4, and 0.5, respectively. These values are comparable to the formation energy of small polarons [7]. The insulating behavior of PCMO films is similar to that of bulk PCMO crystals [1]. However, we did not find any indication of the resistivity anomaly associated with the onset of charge ordering irrespective of x. In order to investigate the existence of a possible ferromagnetic phase, we have measured the magnetization of these films (not shown), but we did not find the ferromagnetic states. In the
analogy of bulk phase diagram [1], it is reasonable to conclude that the PCMO film is paramagnetic insulator in the region of 0.2oxo0.5, although it is difficult to eliminate the possibility of antiferromagnetic ground states owing to the small volume of the films. The suppression of charge-ordered states in the PCMO films is in a sharp contrast with their stabilization in the PCMO films grown on SrTiO3 (1 0 0) substrates, where the superlattice spots due to the charge ordering are clearly observed in electron diffraction measurements [8]. The possible reason to reconcile two contradictory experimental results is the difference of epitaxial strain, namely compressive (PCMO/ LaAlO3) or tensile (PCMO/SrTiO3) strain. The tensile strain may make the charge-ordered states stable, while the compressive strain makes them unstable [7] owing to the lack of structural flexibility [9]. These results suggest that the compressive strain strongly suppresses the chargeordered states in PCMO. This work was supported by a Grant-in-Aid for Scientific Research (S17101004) from JSPS.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Y. Tomioka, et al., Phys. Rev. B 53 (1996) R1689. S.Q. Liu, et al., Appl. Phys. Lett. 76 (2000) 2749. T. Oka, et al., Phys. Rev. Lett. 95 (2005) 266403. A. Baikalov, et al., Appl. Phys. Lett. 83 (2003) 957. A. Sawa, et al., Appl. Phys. Lett. 85 (2004) 4073. Y. Konishi, et al., J. Phys. Soc. Japan 68 (1999) 3790. T.T.M. Plastra, et al., Phys. Rev. B 56 (1997) 51046. Z.Q. Yang, et al., Appl. Phys. Lett. 88 (2006) 072507. M. Nakamura, et al., Appl. Phys. Lett. 86 (2005) 182504.