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Structural and electrical properties of La0.5Ca0.5MnO3 films deposited on differently oriented SrTiO3 substrates
Materials Science and Engineering B 144 (2007) 123–126
Structural and electrical properties of La0.5Ca0.5MnO3 films deposited on differently oriented SrTiO3 substrates G.H. Aydogdu a,∗ , Y. Kuru b , H.-U. Habermeier a a
Max Planck Institute for Solid State Research, Heisenbergstr. 1, Stuttgart D-70569, Germany b Max Planck Institute for Metals Research, Heisenbergstr. 3, Stuttgart D-70569, Germany
Abstract La0.5 Ca0.5 MnO3 (LCMO) films of various thicknesses were epitaxially grown on (1 0 0) and (1 1 1) oriented SrTiO3 (STO) substrates by pulsed laser deposition (PLD) technique. It was observed that LCMO films on (1 0 0) STO behave as an insulator. However, the films on (1 1 1) STO show a metal to insulator transition, when their thicknesses exceed 145 nm. Jahn–Teller strain (εJ–T ) was seen to decrease with increasing film thickness for the films on (1 1 1) STO while the films on (1 0 0) STO have large compressive εJ–T values for all thicknesses. Moreover, rocking curves and asymmetries of the diffraction peaks for the films on (1 1 1) STO substrates may indicate the formation of a mosaic structure, which can act as a relaxation mechanism, with the increasing film thickness. These observations suggest that the electronic properties of the LCMO thin films can be modified by structural changes imposed by the substrate. © 2007 Elsevier B.V. All rights reserved. Keywords: La0.5 Ca0.5 MnO3 films; SrTiO3 substrates; Epitaxy of films; Structural distortion; Electrical and Magnetic measurements
1. Introduction Compared to ordinary metals or metallic heterostructures the magnetoresistance of doped rare earth manganite perovskites, Ln(1−x) Ax MnO3 (where Ln = rare earth, A = a divalent cation) is larger by orders of magnitude [1]. Hence, this effect is called colossal magnetoresistance (CMR). Doped rare earth manganites have recently been the focus of many researchers due to their potential use in microelectronics industry as read/write heads in computer discs and as sensors [2–6]. Depending on the doping level of the divalent Ca2+ causing a mixed value state of the Mn, there are several phases with different electronic and magnetic properties in the phase diagram of La(1−x) Cax MnO3 . The reason why the properties of manganites vary significantly with composition is that several mechanisms such as double exchange, charge, orbital, spin ordering and Jahn–Teller distortion compete with each other at similar energy scales. The possibility to control the formation of these electronic phases can provide an opportunity to use these phases as components of a device in different purposes [7]. It has recently been shown that it is possible to modify the properties of manganites by external perturbations without ∗
Corresponding author. E-mail addresses: [email protected] (G.H. Aydogdu), [email protected] (Y. Kuru), [email protected] (H.-U. Habermeier). 0921-5107/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2007.07.016
changing the chemical composition of the sample. One-way of achieving this is to employ strain as a tool for controlling the phase separation and formation phenomena [8–11]. In the case of epitaxial films, lattice parameter mismatch between the film and the substrate can be used in order to impose strain to the film [12–14]. In this study, it is demonstrated that differently oriented SrTiO3 (STO) substrates can be exploited as a tool to obtain La0.5 Ca0.5 MnO3 (LCMO) thin films with adjustable electronic and magnetic properties even if chemical compositions and thicknesses of the films remain unchanged. This composition has been chosen because according to the bulk phase diagram, it is at the boundary between a ferromagnetic metal and a charge-ordered antiferromagnetic insulator and small changes in the structure can cause a transition to either side. 2. Experimental Epitaxial LCMO films in various thicknesses were grown on (1 0 0) STO and (1 1 1) STO planar substrates by pulsed laser deposition technique (PLD) using a ceramic disc-shaped target at 1073 K in vacuum chamber with 40 Pa O2 pressure. After deposition, samples were annealed at ambient O2 pressure at a temperature of 1173 K for 30 min. The PLD system had an excimer laser with KrF gas mixture emitting UV radiation with a wavelength (λ) of 248 nm. Laser fluence and pulse frequency
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Fig. 1. Representative (a) 2θ-ω scan and (b) pole figure for films on (1 0 0) STO, and (c) 2θ-ω scan and (d) pole figure for films on (1 1 1) STO.
were fixed to 1.6 J/cm2 and 5 Hz, respectively. The thicknesses of the films were adjusted by calculating the ablation period during the growth process. Before deposition, single crystal STO substrates were cleaned in ultrasonic baths of acetone and ethanol; then, it is mounted to a radiatively heated sapphire sample holder. The determination of the epitaxial relationship between the film and the substrate and phase analysis were carried out by Xray diffraction (XRD) using Cu K␣ radiation (λ = 0.154056 nm). Pole figures were measured in a Philips X’Pert MRD diffractometer equipped with an Eulerian cradle. The 2θ-ω (where 2θ is the angle between the incident and the diffracted X-ray beams and ω is the angle between the incident X-ray beam and the sample surface) scans between 10◦ and 120◦ at various inclination angles (ψ) were performed with a four-circle Bruker D8 Discover diffractometer. The temperature dependence of
magnetization was investigated by a Quantum Design MPMS superconducting quantum interference device (SQUID) magnetometer. The magnetic field (0.01 T) was oriented parallel to the film surface during field cooling. Resistivities of the films were measured within a temperature range between 5 K and 300 K by four-point probe method. Evaporated chromium–gold pads and silver epoxy were used to attach the gold wires to the specimen. 3. Results and discussion The 2θ-ω scans and {1 2 1} pole figures demonstrate that single phase and epitaxial LCMO thin films with orthorhombic crystal structure were successfully grown on both kinds of substrates (Fig. 1). For films on (1 0 0) STO, the (0 2 0) plane of the
Fig. 2. Magnetization vs. temperature graphs of films on (a) (1 1 1) STO and (b) (1 0 0) STO.
G.H. Aydogdu et al. / Materials Science and Engineering B 144 (2007) 123–126
125
Fig. 3. Resistivity vs. temperature graphs of 145 nm and 290 nm thick films on (a) (1 1 1) STO and (b) (1 0 0) STO.
LCMO films was identified to be parallel to the substrate surface. Films on (1 1 1) STO were (0 2 2) oriented. It should also be noted that the films deposited on (1 0 0) STO have four-fold symmetry; there are four peaks at ψ = 43.9◦ . For the pole figure of the LCMO films deposited on (1 1 1) STO substrates, three peaks are observed at ψ = 34.7◦ (three-fold symmetry). To determine the lattice parameters the peak positions in the 2θ-ω scans at various inclination angles have been used employing Nelson–Riley equation [15]. The mismatch parameters, which represent the relative change of the lattice parameters of the films with respect to the lattice parameter of the substrate, were determined by the formula (asubstrate − afilm )*100/asubstrate [16] for two in-plane directions. A distinct trend is not observed for mismatch values of the films on (1 0 0) STO. However, mismatch is seen to decrease monotonically from 2.4% to 1.7% with increasing film thickness for both of the in-plane directions for the films on (1 1 1) STO. It is important to note that these mismatch values are results of the combination of the epitaxial strain, the growth strain and possible relaxation mechanisms. Magnetization measurements reveal that LCMO films on (1 0 0) STO and (1 1 1) STO behave differently. For instance, saturation magnetization values of the films on (1 1 1) STO substrate are considerably larger than the magnetization values of the films on (1 0 0) STO substrate although compositions and thicknesses of the films on both kinds of the substrates are nominally identical (Fig. 2). Curie temperatures (Tc ) of the films on (1 1 1) STO are increasing with thickness and for the 290 nm thick film 230 K, lower than Tc of the bulk ceramic (270 K), has been measured. The variation of magnetization with temperature for LCMO films on (1 0 0) STO is almost linear in the entire temperature range and determination of Tc is difficult due to the absence of a sharp ferromagnetic transition. Moreover, it should be noted that magnetization per unit cell is proportional to the film thickness for the films on both kinds of substrates (Fig. 2). Variations in the resistivities of the LCMO films (for two different thicknesses) on both kinds of substrates with temperature are presented in Fig. 3. LCMO films on (1 0 0) STO substrates are insulators for all thicknesses; a metal to insulator transition is not observed. Resistivities of the films are steeply increasing when the temperature is gradually decreased. The films on (1 1 1) STO substrates show a metal to insulator transition with increasing temperature when the thickness exceeds 145 nm. Decrease of
the resistivity below the transition temperature (TMI ) is sharper for the thinner films. Moreover, TMI is positively correlated with the film thickness. Difference between the electrical and magnetic properties of LCMO thin films on (1 0 0) STO and (1 1 1) STO may stem from two reasons. First is the anisotropy of the resistivity and the magnetization in the LCMO structure because orientation of the unit cell varies according to the measurement direction. Second is the structural modifications in the lattice of film due to conditions (e.g. epitaxial orientation and strain) dictated by the substrate. In this study, the latter point is highlighted and investigated in detail. The strain tensor can be separated into two parts. Hydrostatic strain represents the volume change while the second part (deviatoric component) is related to the structural distortions and does not affect the volume. Jahn–Teller strain (εJ–T ) is proposed to evaluate this distortion and it is proportional to √ electron lattice coupling. εJ–T = 1/6 (2εzz − εxx − εyy ), where εzz is out-of-plane and εxx and εyy are in-plane strain components [17]) for films of various thicknesses on both (1 0 0) STO and (1 1 1) STO are plotted in Fig. 4. Although the data is scattered, it can be seen that, εJ–T slightly decreases with increasing thickness for the films on (1 1 1) STO, which can be related
Fig. 4. Jahn–Teller strain vs. thickness graphs for films on (1 1 1) and (1 0 0) STO.
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4. Conclusions Single phase, epitaxial LCMO thin films are successfully deposited on (1 0 0) and (1 1 1) planar STO substrates by PLD method. The films grown on these substrates demonstrate different electrical and magnetic properties. This difference and the observation of metallic behavior for the films on (1 1 1) STO when film thickness exceeds 145 nm can be explained by decrease of the distortion (evaluated by εJ–T ) for thicker films on (1 1 1) STO. Formation of a mosaic structure parallel to the relaxation of εJ–T is identified from the rocking curves. Acknowledgements Fig. 5. Rocking curves of films on (1 1 1) STO.
The authors thank Mr. G. Maier and Dr. U. Welzel of Central Scientific Facility X-ray diffraction and department of Prof. Dr. Ir. E.J. Mittemeijer in Max Planck Institute for Metals Research for assistance with the pole figure measurements. G.H.A. also gratefully thanks Mrs. E. Bruecher for the magnetization measurements. This work was supported by NMP4-CT-2005-517039 controlling mesoscopic phase separation (COMEPHS) project. References
Fig. 6. (0 4 4) diffraction peaks of films on (1 1 1) STO.
to the observed metal to insulator transition in resistivity measurements of thicker films. However, LCMO films on (1 0 0) STO have large compressive εJ–T values, and furthermore, εJ–T becomes even more compressive when thickness is increased. This large distortion can explain why films on (1 0 0) STO are insulators for all thicknesses [18]. In Fig. 5, rocking curves of the LCMO films on (1 1 1) STO are presented. Full width at half of the maximum intensity of the rocking curve increases and two shoulders on two sides of the main peak develop with increasing film thickness. Mosaic spread increases and orientation of the (0 4 4) planes deviates more from their initial orientation, parallel to the substrate. In addition (0 4 4), diffraction peak becomes more asymmetric (Fig. 6). A strain gradient through the film thickness and asymmetry in the diffraction peaks can be caused by the above mentioned mosaic structure, which may act as a relaxation mechanism in the surface region. Formation of this mosaic structure may decrease the distortion (represented by εJ–T ) that LCMO lattice experience and in turn lead to enhancement metallic behavior.
[1] J. Volger, Physica 20 (1954) 49. [2] R. vonHelmolt, J. Wecker, B. Holzapfel, L. Schultz, K. Samwer, Phys. Rev. Lett. 71 (1993) 2331. [3] K. Chahara, T. Ohno, M. Kasai, Y. Kozono, Appl. Phys. Lett. 63 (1993) 1990. [4] S. Jin, T.H. Tiefel, M. McCormack, R.A. Fastnacht, R. Ramesh, L.H. Chen, Science 264 (1994) 413. [5] A. Urushibara, Y. Moritomo, T. Arima, A. Asamitsu, G. Kido, Y. Tokura, Phys. Rev. B 51 (1995) 14103. [6] C.N.R. Rao, A.K. Cheetham, R. Mahesh, Chem. Mater. 8 (1996) 2421. [7] H.-U. Habermeier, Physica B 321 (2002) 9. [8] O.I. Lebedev, G. Van Tendeloo, S. Amelinckx, F. Razavi, H.-U. Habermeier, Phil. Mag. A 81 (2001) 797. [9] J.-P. Locquet, J. Perret, J. Fompeyrine, E. Machler, J.W. Seo, G. Van Tendeloo, Nature 394 (1998) 453. [10] F.S. Razavi, G. Gross, H.-U. Habermeier, O. Lebedev, S. Amelinckx, G. Van Tendeloo, A. Vigliante, Appl. Phys. Lett. 76 (2000) 155. [11] E. Dagotto, Nanoscale Phase Separation and Colossal Magnetoresistance, vol. 136, Springer-Verlag, Berlin, 2003, p. 9. [12] S.-Y. Bae, S.X. Wang, Appl. Phys. Lett. 69 (1996) 121. [13] V.S. Achutharaman, P.A. Kraus, V.A. Vasko, C.A. Nordman, A.M. Goldman, Appl. Phys. Lett. 67 (1995) 1019. [14] K.M. Krishnan, A.R. Modak, C.A. Lucas, R. Michel, H.B. Cherry, J. Appl. Phys. 79 (1996) 5169. [15] B.D. Cullity, S.R. Stock, Elements of X-ray Diffraction, 3rd ed., Prentice Hall, New Jersey, 2001, p. 363. [16] M. Ohring, Materials Science of Thin Films Deposition and Structure, 2nd ed., Academic Press, New Jersey, 2002, p. 417. [17] A.J. Millis, T. Darling, A. Migliori, J. Appl. Phys. 83 (1998) 1588. [18] C.N.R. Rao, B. Raveau (Eds.), Colossal Magnetoresistance, Charge Ordering and Related Properties of Manganese Oxides, World Scientific, Singapore, 1998, p. 10.
Structural and electrical properties of La0.5Ca0.5MnO3 films deposited on differently oriented SrTiO3 substrates G.H. Aydogdu a,∗ , Y. Kuru b , H.-U. Habermeier a a
Max Planck Institute for Solid State Research, Heisenbergstr. 1, Stuttgart D-70569, Germany b Max Planck Institute for Metals Research, Heisenbergstr. 3, Stuttgart D-70569, Germany
Abstract La0.5 Ca0.5 MnO3 (LCMO) films of various thicknesses were epitaxially grown on (1 0 0) and (1 1 1) oriented SrTiO3 (STO) substrates by pulsed laser deposition (PLD) technique. It was observed that LCMO films on (1 0 0) STO behave as an insulator. However, the films on (1 1 1) STO show a metal to insulator transition, when their thicknesses exceed 145 nm. Jahn–Teller strain (εJ–T ) was seen to decrease with increasing film thickness for the films on (1 1 1) STO while the films on (1 0 0) STO have large compressive εJ–T values for all thicknesses. Moreover, rocking curves and asymmetries of the diffraction peaks for the films on (1 1 1) STO substrates may indicate the formation of a mosaic structure, which can act as a relaxation mechanism, with the increasing film thickness. These observations suggest that the electronic properties of the LCMO thin films can be modified by structural changes imposed by the substrate. © 2007 Elsevier B.V. All rights reserved. Keywords: La0.5 Ca0.5 MnO3 films; SrTiO3 substrates; Epitaxy of films; Structural distortion; Electrical and Magnetic measurements
1. Introduction Compared to ordinary metals or metallic heterostructures the magnetoresistance of doped rare earth manganite perovskites, Ln(1−x) Ax MnO3 (where Ln = rare earth, A = a divalent cation) is larger by orders of magnitude [1]. Hence, this effect is called colossal magnetoresistance (CMR). Doped rare earth manganites have recently been the focus of many researchers due to their potential use in microelectronics industry as read/write heads in computer discs and as sensors [2–6]. Depending on the doping level of the divalent Ca2+ causing a mixed value state of the Mn, there are several phases with different electronic and magnetic properties in the phase diagram of La(1−x) Cax MnO3 . The reason why the properties of manganites vary significantly with composition is that several mechanisms such as double exchange, charge, orbital, spin ordering and Jahn–Teller distortion compete with each other at similar energy scales. The possibility to control the formation of these electronic phases can provide an opportunity to use these phases as components of a device in different purposes [7]. It has recently been shown that it is possible to modify the properties of manganites by external perturbations without ∗
Corresponding author. E-mail addresses: [email protected] (G.H. Aydogdu), [email protected] (Y. Kuru), [email protected] (H.-U. Habermeier). 0921-5107/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2007.07.016
changing the chemical composition of the sample. One-way of achieving this is to employ strain as a tool for controlling the phase separation and formation phenomena [8–11]. In the case of epitaxial films, lattice parameter mismatch between the film and the substrate can be used in order to impose strain to the film [12–14]. In this study, it is demonstrated that differently oriented SrTiO3 (STO) substrates can be exploited as a tool to obtain La0.5 Ca0.5 MnO3 (LCMO) thin films with adjustable electronic and magnetic properties even if chemical compositions and thicknesses of the films remain unchanged. This composition has been chosen because according to the bulk phase diagram, it is at the boundary between a ferromagnetic metal and a charge-ordered antiferromagnetic insulator and small changes in the structure can cause a transition to either side. 2. Experimental Epitaxial LCMO films in various thicknesses were grown on (1 0 0) STO and (1 1 1) STO planar substrates by pulsed laser deposition technique (PLD) using a ceramic disc-shaped target at 1073 K in vacuum chamber with 40 Pa O2 pressure. After deposition, samples were annealed at ambient O2 pressure at a temperature of 1173 K for 30 min. The PLD system had an excimer laser with KrF gas mixture emitting UV radiation with a wavelength (λ) of 248 nm. Laser fluence and pulse frequency
124
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Fig. 1. Representative (a) 2θ-ω scan and (b) pole figure for films on (1 0 0) STO, and (c) 2θ-ω scan and (d) pole figure for films on (1 1 1) STO.
were fixed to 1.6 J/cm2 and 5 Hz, respectively. The thicknesses of the films were adjusted by calculating the ablation period during the growth process. Before deposition, single crystal STO substrates were cleaned in ultrasonic baths of acetone and ethanol; then, it is mounted to a radiatively heated sapphire sample holder. The determination of the epitaxial relationship between the film and the substrate and phase analysis were carried out by Xray diffraction (XRD) using Cu K␣ radiation (λ = 0.154056 nm). Pole figures were measured in a Philips X’Pert MRD diffractometer equipped with an Eulerian cradle. The 2θ-ω (where 2θ is the angle between the incident and the diffracted X-ray beams and ω is the angle between the incident X-ray beam and the sample surface) scans between 10◦ and 120◦ at various inclination angles (ψ) were performed with a four-circle Bruker D8 Discover diffractometer. The temperature dependence of
magnetization was investigated by a Quantum Design MPMS superconducting quantum interference device (SQUID) magnetometer. The magnetic field (0.01 T) was oriented parallel to the film surface during field cooling. Resistivities of the films were measured within a temperature range between 5 K and 300 K by four-point probe method. Evaporated chromium–gold pads and silver epoxy were used to attach the gold wires to the specimen. 3. Results and discussion The 2θ-ω scans and {1 2 1} pole figures demonstrate that single phase and epitaxial LCMO thin films with orthorhombic crystal structure were successfully grown on both kinds of substrates (Fig. 1). For films on (1 0 0) STO, the (0 2 0) plane of the
Fig. 2. Magnetization vs. temperature graphs of films on (a) (1 1 1) STO and (b) (1 0 0) STO.
G.H. Aydogdu et al. / Materials Science and Engineering B 144 (2007) 123–126
125
Fig. 3. Resistivity vs. temperature graphs of 145 nm and 290 nm thick films on (a) (1 1 1) STO and (b) (1 0 0) STO.
LCMO films was identified to be parallel to the substrate surface. Films on (1 1 1) STO were (0 2 2) oriented. It should also be noted that the films deposited on (1 0 0) STO have four-fold symmetry; there are four peaks at ψ = 43.9◦ . For the pole figure of the LCMO films deposited on (1 1 1) STO substrates, three peaks are observed at ψ = 34.7◦ (three-fold symmetry). To determine the lattice parameters the peak positions in the 2θ-ω scans at various inclination angles have been used employing Nelson–Riley equation [15]. The mismatch parameters, which represent the relative change of the lattice parameters of the films with respect to the lattice parameter of the substrate, were determined by the formula (asubstrate − afilm )*100/asubstrate [16] for two in-plane directions. A distinct trend is not observed for mismatch values of the films on (1 0 0) STO. However, mismatch is seen to decrease monotonically from 2.4% to 1.7% with increasing film thickness for both of the in-plane directions for the films on (1 1 1) STO. It is important to note that these mismatch values are results of the combination of the epitaxial strain, the growth strain and possible relaxation mechanisms. Magnetization measurements reveal that LCMO films on (1 0 0) STO and (1 1 1) STO behave differently. For instance, saturation magnetization values of the films on (1 1 1) STO substrate are considerably larger than the magnetization values of the films on (1 0 0) STO substrate although compositions and thicknesses of the films on both kinds of the substrates are nominally identical (Fig. 2). Curie temperatures (Tc ) of the films on (1 1 1) STO are increasing with thickness and for the 290 nm thick film 230 K, lower than Tc of the bulk ceramic (270 K), has been measured. The variation of magnetization with temperature for LCMO films on (1 0 0) STO is almost linear in the entire temperature range and determination of Tc is difficult due to the absence of a sharp ferromagnetic transition. Moreover, it should be noted that magnetization per unit cell is proportional to the film thickness for the films on both kinds of substrates (Fig. 2). Variations in the resistivities of the LCMO films (for two different thicknesses) on both kinds of substrates with temperature are presented in Fig. 3. LCMO films on (1 0 0) STO substrates are insulators for all thicknesses; a metal to insulator transition is not observed. Resistivities of the films are steeply increasing when the temperature is gradually decreased. The films on (1 1 1) STO substrates show a metal to insulator transition with increasing temperature when the thickness exceeds 145 nm. Decrease of
the resistivity below the transition temperature (TMI ) is sharper for the thinner films. Moreover, TMI is positively correlated with the film thickness. Difference between the electrical and magnetic properties of LCMO thin films on (1 0 0) STO and (1 1 1) STO may stem from two reasons. First is the anisotropy of the resistivity and the magnetization in the LCMO structure because orientation of the unit cell varies according to the measurement direction. Second is the structural modifications in the lattice of film due to conditions (e.g. epitaxial orientation and strain) dictated by the substrate. In this study, the latter point is highlighted and investigated in detail. The strain tensor can be separated into two parts. Hydrostatic strain represents the volume change while the second part (deviatoric component) is related to the structural distortions and does not affect the volume. Jahn–Teller strain (εJ–T ) is proposed to evaluate this distortion and it is proportional to √ electron lattice coupling. εJ–T = 1/6 (2εzz − εxx − εyy ), where εzz is out-of-plane and εxx and εyy are in-plane strain components [17]) for films of various thicknesses on both (1 0 0) STO and (1 1 1) STO are plotted in Fig. 4. Although the data is scattered, it can be seen that, εJ–T slightly decreases with increasing thickness for the films on (1 1 1) STO, which can be related
Fig. 4. Jahn–Teller strain vs. thickness graphs for films on (1 1 1) and (1 0 0) STO.
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4. Conclusions Single phase, epitaxial LCMO thin films are successfully deposited on (1 0 0) and (1 1 1) planar STO substrates by PLD method. The films grown on these substrates demonstrate different electrical and magnetic properties. This difference and the observation of metallic behavior for the films on (1 1 1) STO when film thickness exceeds 145 nm can be explained by decrease of the distortion (evaluated by εJ–T ) for thicker films on (1 1 1) STO. Formation of a mosaic structure parallel to the relaxation of εJ–T is identified from the rocking curves. Acknowledgements Fig. 5. Rocking curves of films on (1 1 1) STO.
The authors thank Mr. G. Maier and Dr. U. Welzel of Central Scientific Facility X-ray diffraction and department of Prof. Dr. Ir. E.J. Mittemeijer in Max Planck Institute for Metals Research for assistance with the pole figure measurements. G.H.A. also gratefully thanks Mrs. E. Bruecher for the magnetization measurements. This work was supported by NMP4-CT-2005-517039 controlling mesoscopic phase separation (COMEPHS) project. References
Fig. 6. (0 4 4) diffraction peaks of films on (1 1 1) STO.
to the observed metal to insulator transition in resistivity measurements of thicker films. However, LCMO films on (1 0 0) STO have large compressive εJ–T values, and furthermore, εJ–T becomes even more compressive when thickness is increased. This large distortion can explain why films on (1 0 0) STO are insulators for all thicknesses [18]. In Fig. 5, rocking curves of the LCMO films on (1 1 1) STO are presented. Full width at half of the maximum intensity of the rocking curve increases and two shoulders on two sides of the main peak develop with increasing film thickness. Mosaic spread increases and orientation of the (0 4 4) planes deviates more from their initial orientation, parallel to the substrate. In addition (0 4 4), diffraction peak becomes more asymmetric (Fig. 6). A strain gradient through the film thickness and asymmetry in the diffraction peaks can be caused by the above mentioned mosaic structure, which may act as a relaxation mechanism in the surface region. Formation of this mosaic structure may decrease the distortion (represented by εJ–T ) that LCMO lattice experience and in turn lead to enhancement metallic behavior.
[1] J. Volger, Physica 20 (1954) 49. [2] R. vonHelmolt, J. Wecker, B. Holzapfel, L. Schultz, K. Samwer, Phys. Rev. Lett. 71 (1993) 2331. [3] K. Chahara, T. Ohno, M. Kasai, Y. Kozono, Appl. Phys. Lett. 63 (1993) 1990. [4] S. Jin, T.H. Tiefel, M. McCormack, R.A. Fastnacht, R. Ramesh, L.H. Chen, Science 264 (1994) 413. [5] A. Urushibara, Y. Moritomo, T. Arima, A. Asamitsu, G. Kido, Y. Tokura, Phys. Rev. B 51 (1995) 14103. [6] C.N.R. Rao, A.K. Cheetham, R. Mahesh, Chem. Mater. 8 (1996) 2421. [7] H.-U. Habermeier, Physica B 321 (2002) 9. [8] O.I. Lebedev, G. Van Tendeloo, S. Amelinckx, F. Razavi, H.-U. Habermeier, Phil. Mag. A 81 (2001) 797. [9] J.-P. Locquet, J. Perret, J. Fompeyrine, E. Machler, J.W. Seo, G. Van Tendeloo, Nature 394 (1998) 453. [10] F.S. Razavi, G. Gross, H.-U. Habermeier, O. Lebedev, S. Amelinckx, G. Van Tendeloo, A. Vigliante, Appl. Phys. Lett. 76 (2000) 155. [11] E. Dagotto, Nanoscale Phase Separation and Colossal Magnetoresistance, vol. 136, Springer-Verlag, Berlin, 2003, p. 9. [12] S.-Y. Bae, S.X. Wang, Appl. Phys. Lett. 69 (1996) 121. [13] V.S. Achutharaman, P.A. Kraus, V.A. Vasko, C.A. Nordman, A.M. Goldman, Appl. Phys. Lett. 67 (1995) 1019. [14] K.M. Krishnan, A.R. Modak, C.A. Lucas, R. Michel, H.B. Cherry, J. Appl. Phys. 79 (1996) 5169. [15] B.D. Cullity, S.R. Stock, Elements of X-ray Diffraction, 3rd ed., Prentice Hall, New Jersey, 2001, p. 363. [16] M. Ohring, Materials Science of Thin Films Deposition and Structure, 2nd ed., Academic Press, New Jersey, 2002, p. 417. [17] A.J. Millis, T. Darling, A. Migliori, J. Appl. Phys. 83 (1998) 1588. [18] C.N.R. Rao, B. Raveau (Eds.), Colossal Magnetoresistance, Charge Ordering and Related Properties of Manganese Oxides, World Scientific, Singapore, 1998, p. 10.