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Epitaxial A-Site Ordered Perovskite Manganite SmBaMn2O6 Film on SrTiO3(001): Fabrication, Structure, and Physical Property Tomohiko Nakajima,*,† Tetsuo Tsuchiya,† Kais Daoudi,† Masaki Ichihara,‡ Yutaka Ueda,‡ and Toshiya Kumagai† National Institute of AdVanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan, and Materials Design and Characterization Laboratory, Institute for Solid State Physics, UniVersity of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan ReceiVed April 25, 2007. ReVised Manuscript ReceiVed August 29, 2007
An A-site ordered perovskite manganite SmBaMn2O6-δ film was fabricated by means of an excimer laser-assisted metal-organic deposition (ELAMOD) and epitaxially grown on a SrTiO3(001) substrate. The SmBaMn2O6-δ film was crystallized in [010]- and [001]-oriented domains that were confirmed by X-ray diffraction and cross-section transmittance electron microscopy (TEM). The A-site ordered structure formed at 500 °C under KrF laser irradiation at a laser fluence of 140 mJ/cm2 for 60 min in an Ar flow, whereas disordered A-site cations formed under laser irradiation at fluences less than 120 mJ/cm2 and/or in an oxygen atmosphere. A SmBaMn2O6 film was obtained by oxygen annealing of the as-prepared SmBaMn2O6-δ film at 500 °C for 3 h. The effects of structural strain due to the substrate on the physical properties of the SmBaMn2O6 film are discussed in terms of the results of structural analysis and electrical resistivity data. The electrical resistivity for the SmBaMn2O6 film on SrTiO3(001) show insulating behavior without a first-order charge/orbital order (CO) transition caused by constraint of lattice change associated with the CO because of the undeformable substrate lattice. The ELAMOD process is expected to serve as a new key technique for the fabrication of A-site ordered perovskite manganite thin films.
Introduction During the past decade, many materials, such as granular films,1 tunneling junctions,2 and transition metal oxides,3-5 have been found to exhibit the magnetoresistance (MR) effect. In particular, research on MR oxides has focused on the perovskite manganites, R1-xAxMnO3 (R ) rare earth elements and A ) Ca, Sr, and Ba), because they exhibit the so-called colossal magnetoresistance (CMR) effect along with the various fascinating physical phenomena such as the generation of a ferromagnetic metal (FM) by the doubleexchange interaction, and an insulator-metal (IM) transition accompanied by a charge/orbital order (CO).6 Moreover, the light-induced IM transition has also been realized in the CMR manganites by Takubo et al.7 Because of these properties, the perovskite manganites show promise for applications such as electronic switching devices that use a magnetically or optically induced phase change. However, CMR manganites * Corresponding author. E-mail:
[email protected]. † National Institute of Advanced Industrial Science and Technology. ‡ University of Tokyo.
(1) Coey, J. M. D.; Berkowitz, A. E.; Balcells, Li.; Putris, F. F.; Barry, A. Phys. ReV. Lett. 1998, 80, 3815. (2) Miyazaki, T.; Tezuka, N. J. Magn. Magn. Mater. 1995, 139, L231. (3) Penney, T.; Schafer, M. W.; Torrance, J. B. Phys. ReV. B 1972, 5, 3669. (4) Urushibara, A.; Moritomo, Y.; Arima, T.; Asamitsu, A.; Kido, G.; Tokura, Y. Phys. ReV. B 1995, 51, 14103. (5) Maignan, A.; Martin, C.; Pelloquin, D.; Nguyen, N.; Raveau, B. J. Solid State Chem. 1999, 142, 247. (6) See reviews, Rao, C. N. R.; Raveau, B. Colossal Magnetoresistance, Charge Ordering and Related Properties of Manganese Oxides; World Scientific: Singapore, 1998. (7) Takubo, N.; Ogimoto, Y.; Nakamura, M.; Tamaru, H.; Izumi, M.; Miyano, K. Phys. ReV. Lett. 2005, 95, 017404.
such as La1-xCaxMnO38 and (Nd,Sm)0.5Sr0.5MnO39 have a disadvantage for the application: namely, the CMR is observed far below room temperature. Therefore, the development of new materials that exhibit CMR near room temperature is desirable. A-site (R and A-sites in the perovskite structure) ordered manganites, RBaMn2O6 (R ) Y and rare earth elements) were recently synthesized, and their structural and physical properties have been reported.10-16 RBaMn2O6 has a layertype ordering of R3+ and Ba2+ ions along the c-axis, and the MnO2 square sublattice is sandwiched between two types of rock-salt layers, RO and BaO. In RBaMn2O6, the FM is dominant in large R compounds (R ) La, Pr, and Nd) with a relatively small mismatch between RO and BaO lattices; whereas in small R materials (R ) Sm-Ho, Y) with a large mismatch, the CE-type CO insulator is stabilized at remarkable high temperatures (far above 300 K).10-13 The high CO (8) Martin, C.; Maignan, A.; Hervieu, M.; Raveau, B. Phys. ReV. B 1999, 60, 12191. (9) Kuwahara, H.; Moritomo, Y.; Tomioka, Y.; Asamitsu, A.; Kasai, M.; Kumai, R.; Tokura, Y. Phys. ReV. B 1997, 56, 9386. (10) Nakajima, T.; Kageyama, H.; Ueda, Y. J. Phys. Chem. Solids 2002, 63, 913. (11) Nakajima, T.; Kageyama, H.; Yoshizawa, H.; Ueda, Y. J. Phys. Soc. Jpn. 2002, 71, 2843. (12) Nakajima, T.; Kageyama, H.; Ohoyama, K.; Yoshizawa, H.; Ueda, Y. J. Solid State Chem. 2004, 177, 987. (13) Nakajima, T.; Yoshizawa, H.; Ueda, Y. J. Phys. Soc. Jpn. 2004, 73, 2283. (14) Millange, F.; Caignaert, V.; Domenge´s, B.; Raveau, B.; Suard, E. Chem. Mater. 1998, 10, 1974. (15) Trukhanov, S. V.; Troyanchuk, I. O.; Hervieu, M.; Szymczak, H.; Barner, K. Phys. ReV. B 2002, 66, 184424. (16) Akahoshi, D.; Uchida, M.; Tomioka, Y.; Arima, T.; Matsui, Y.; Tokura, Y. Phys. ReV. Lett. 2003, 90, 177203.
10.1021/cm071118i CCC: $37.00 © 2007 American Chemical Society Published on Web 10/09/2007
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temperature (TCO) is considered to originate mainly in the layer-type ordering of R3+ and Ba2+, which results in the absence of electrostatic potential disorder in the structure. Very recently, we investigated CMR at room temperature by introducing disorder in A-site ordered perovskite manganites; Sm1-xLax+yBa1-yMn2O6 shows a quite large MR effect of about 1000% at room temperature under 9 T.17 However, the thin films of A-site ordered perovskite manganites have not been prepared so far by conventional film-forming methods such as pulsed laser deposition (PLD) and metal-organic deposition (MOD) processes, whereas the fabrication of films is essential for the practical applications. There could be some difficulties in making the A-site ordered structures by means of these methods, although the fabrication of A-site disordered manganites thin films has already been realized by these methods.18-20 Therefore, we have investigated the fabrication of A-site ordered manganite thin films using a new process, an excimer laser-assisted metal-organic deposition (ELAMOD) process. When the ELAMOD process is used to fabricate oxide films by excimer laser irradiation, a metal-organic solution is spin-coated onto a substrate, and the film is then crystallized by direct irradiation with an excimer laser after preheating.21-23 We have already prepared thin films of the manganite La1-xSrxMnO3 using this process.21 In this work, we achieved epitaxial growth of an A-site ordered manganite SmBaMn2O6 thin film on a single-crystal SrTiO3(001) (STO(001)) substrate by means of the ELAMOD process. In this paper, we report on the fabrication method, and the structure and the physical properties of the SmBaMn2O6 film. In addition, to clarify the key aspects of the preparation of A-site ordered structure in this process, we compared the fabrication conditions and structural properties of the SmBaMn2O6 film to those of an A-site disordered Sm0.5Ba0.5MnO3 film that was also prepared on STO(001) by means of ELAMOD. Experimental Section The starting solutions for the preparation of SmBaMn2O6 and Sm0.5Ba0.5MnO3 were prepared by mixing 2-ethylhexanoate solutions of the constituent metals diluted with toluene to obtain the required concentration and viscosity for spin-coating. The Sm:Ba: Mn molar ratio in the coating solution was 1.0:1.0:2.0, respectively. This solution was spin-coated onto the STO(001) substrate at 4000 rpm for 10 s. The coated films were dried at 100 °C in air to remove the solvent, and then heated to 500 °C in air for 10 min to decompose the organic components of the film. After the spin coating and preheating, the films were irradiated with a KrF laser (17) Nakajima, T.; Ueda, Y. J. Appl. Phys. 2005, 98, 046108. (18) Prellier, W.; Haghiri-Gosnet, A. M.; Mercey, B.; Lecoeur, Ph.; Hervieu, M.; Simon, Ch.; Raveau, B. Appl. Phys. Lett. 2000, 77, 1023. (19) Konishi, Y.; Fang, Z.; Izumi, M.; Manako, T.; Kasai, M.; Kuwahara, H.; Kawasaki, M.; Terakura, K.; Tokura, Y. J. Phys. Soc. Jpn. 1999, 68, 3790. (20) Manabe, T.; Yamaguchi, I.; Kondo, W.; Mizuta, S.; Kumagai, T. J. Mater. Res. 1997, 12, 541. (21) Tsuchiya, T.; Watanabe, A.; Kumagai, T.; Mizuta, S. Appl. Surf. Sci. 2005, 248, 118. (22) Tsuchiya, T.; Yamaguchi, I.; Manabe, T.; Kumagai, T.; Mizuta, S. Mater. Sci. Eng., B 2004, 109, 131. (23) Tsuchiya, T.; Yoshitake, T.; Shimakawa, Y.; Yamaguchi, I.; Manabe, T.; Kumagai, T.; Kubo, Y.; Mizuta, S. J. Photochem. Photobiol., A 2004, 166, 123.
Nakajima et al. (Lambda Physik, Compex110) at a fluence of 140 mJ/cm2 and a repetition rate of 10 Hz at 500 °C for 60 min under an Ar flow. The obtained film, SmBaMn2O6-δ (0.0 < δ e 1.0), which contained high oxygen vacancy in the SmO layer, was annealed in an O2 flow at 500 °C for 3 h: this process led to full oxidation of the film, that is, to SmBaMn2O6. When the film was irradiated with the KrF laser in air and/or at a fluence below 120 mJ/cm2, the A-site disordered phase was formed. Sm0.5Ba0.5MnO3 films were prepared by KrF laser irradiation at a fluence of 80 mJ/cm2 and a repetition rate of 10 Hz at 500 °C for 60 min in air. The as-prepared Sm0.5Ba0.5MnO3-δ film contained low oxygen vacancy, and the fully oxidized Sm0.5Ba0.5MnO3 film was fabricated by oxygen annealing at 800 °C for 6 h. The crystallinity and epitaxy of the obtained films were examined by X-ray diffraction (MAC Science, MXP3A) θ -2θ scans and pole-figure analysis using the Schulz reflection method. The lattice parameters of epitaxial films were determined by X-ray diffraction reciprocal space mapping. The microscopic structures at room temperature were probed by means of cross-section transmittance electron microscopy (TEM) and electron diffraction (ED) analysis using a JEM-2010 (JEOL) instrument operating at 200 kV. The thickness of the obtained films was estimated to be about 250 Å by TEM observation. The resistivity curves of the films as a function of temperature were measured by the DC four-probe method.
Results Figure 1a shows the X-ray diffraction pattern of the A-site ordered SmBaMn2O6-δ film on the STO(001) substrate. Although the SmBaMn2O6-δ film was well-crystallized on the substrate, the observed diffraction peaks of the film could not be indexed to only the tetragonal SmBaMn2O6-δ (δ ) 0.0 or 1.0) or the orthorhombic SmBaMn2O6-δ (δ ) 0.5) unit cell. The film appears not to be a single-crystal film, and careful analysis of the data led us to conclude that two kinds of mutually perpendicular crystallographic domains associated with [010]- and [001]-oriented phases of SmBaMn2O6-δ (0.0 < δ e 1.0) exist. It is generally difficult to determine exact oxygen content of thin films; however, we speculate that these domains are assigned to SmBaMn2O6-δ, which have different δ by the obtained structural data and a comparison with crystal structures of bulk samples: one domain has 0.5 e δ e 1.0, and the other has 0.0 < δ < 0.5. The broad 001/2p reflection at around 2θ ) 12° (Figure 1a, inset) indicates that we successfully fabricated a film in which the Sm3+ and Ba2+ cations alternate along the c-axis originating from [001]-oriented domains, where subscript p represents the simple perovskite cell. In-plane alignments of the [001]- and [010]-oriented domains in the SmBaMn2O6-δ film were confirmed by X-ray diffraction pole-figure measurements. Figure 1b-d shows the pole figures for the (220)p, (202)p, and (220)p reflections originating from SmBaMn2O6-δ(001), SmBaMn2O6-δ(010), and STO(001), respectively. The four sharp peaks were observed for all patterns, and the peaks were located at the same Φ angles. These results indicate that the (001) and (010) domains of the SmBaMn2O6-δ film grew on the STO(001) substrate and that the epitaxial relationships were [100]SmBaMn2O6-δ(001)//[100]STO(001) and [100]SmBaMn2O6-δ(010)//[100]STO(001). The X-ray diffraction pattern of the A-site disordered Sm0.5Ba0.5MnO3-δ film on the STO(001) substrate is shown
Epitaxial A-Site Ordered SmBaMn2O6 Film on SrTiO3(001)
Figure 1. (a) X-ray diffraction pattern of the SmBaMn2O6-δ film on the STO(001) substrate at room temperature, indexed to the reduced unit cell with ap × bp × cp, where subscript p represents the simple perovskite cell. The inset shows the enlarged diffraction pattern at around 2θ ) 12°. X-ray diffraction Φ scans of (b) (220)p peaks for SmBaMn2O6-δ(001) domains, (c) (202)p peaks for SmBaMn2O6-δ(010) domains, and (d) (220) peaks for the STO(001) substrate.
in Figure 2a. The film is a single phase of Sm0.5Ba0.5MnO3-δ with a simple cubic symmetry, and is [001]-oriented; only the 00lp reflections were observed. The 001p peak was observed at 23.22°. There is no trace of 001/2p reflection due to A-site ordering at around 2θ ) 12° (Figure 2a, inset). The four sharp peaks for the (220)p reflections in the Φ scan of the Sm0.5Ba0.5MnO3-δ film and STO(001) (Figure 2b,c) confirm the complete in-plane texture of the film. For the results described below, we evaluated the A-site ordered/ disordered structures on the basis of these structural analyses. What are the appropriate conditions for obtaining A-site ordering by means of ELAMOD process? We studied the X-ray diffraction profiles of obtained films that were prepared under various fabrication conditions. Figure 3a shows the X-ray diffraction patterns of the films prepared at laser fluences of 120, 130, and 140 mJ/cm2. At 120 mJ/cm2, the film crystallized in the A-site disordered cubic phase. When the laser fluence was increased to 130 mJ/cm2, the A-site ordered orthorhombic SmBaMn2O6-δ phase started to form, and at 140 mJ/cm2, SmBaMn2O6-δ was well-crystallized. Production of the A-site ordered phase also strongly depended on the atmosphere under which the laser irradiation was carried out. At 140 mJ/cm2, the A-site disordered phase was stabilized in air, whereas the A-site ordered phase was formed in an Ar gas flow (Figure 3b). Figure 3c exhibits
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Figure 2. (a) X-ray diffraction pattern of the Sm0.5Ba0.5MnO3-δ film on the STO(001) substrate at room temperature, indexed to the cubic unit cell with ap × ap × ap, where subscript p represent the simple perovskite cell. The inset shows the enlarged diffraction pattern at around 2θ ) 12°. X-ray diffraction Φ scans of (b) (220)p peaks for the Sm0.5Ba0.5MnO3-δ(001) film, and (c) (220) peaks for the STO(001) substrate.
the dependence of the X-ray diffraction patterns on fabrication temperature. A-site ordered phases crystallized at more than 500 °C under Ar at a laser fluence of 140 mJ/cm2, whereas the A-site disordered phase was observed at temperatures less than 400 °C. At 600 °C, the film surface became rough because of laser ablation, although the A-site ordered phase was formed. Unfortunately, the A-site ordered SmBaMn2O6-δ on STO(001) exhibited the twinning structure described above regardless of the fabrication conditions. We subjected the SmBaMn2O6-δ and Sm0.5Ba0.5MnO3-δ films to oxygen annealing to eliminate oxygen vacancies. Figure 4 shows the X-ray diffraction patterns around the 001p reflection of the STO(001) substrate for the A-site ordered, and disordered films before and after oxygen annealing. The X-ray diffraction pattern of SmBaMn2O6-δ at around 2θ ) 23° was drastically changed by oxygen annealing at 500 °C, whereas the 001/2p reflection was not varied. The observed large change of the lattice parameters due to the oxygen annealing agrees with the results for the bulk sample.25,26 In addition, the oxygen vacancy δ was easily changed to 0.0 even in the single crystal of A-site ordered RBaMn2O6-δ at 500 °C in oxygen flow as reported in previous literature;24,27 therefore, the oxidation from SmBaMn2O6-δ to SmBaMn2O6 in the thin film was also considered to be successful. By (24) Nakajima, T. Ph.D. dissertation, University of Tokyo, Tokyo, Japan, 2006; Chapter 6. (25) Caignaert, V.; Millange, F.; Domenge´s, B.; Raveau, B.; Suard, E. Chem. Mater. 1999, 11, 930. (26) Karppinen, M.; Okamoto, H.; Fjellvåg, H.; Motohashi, T.; Yamauchi, H. J. Solid State Chem. 2004, 177, 2122.
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Figure 4. X-ray diffraction patterns at around 2θ ) 23° before and after oxygen annealing of (a) SmBaMn2O6-δ and (b) Sm0.5Ba0.5MnO3-δ films. The inset shows the peak fitting for the diffraction pattern of the SmBaMn2O6 at around 2θ ) 23°. The open circles and thick line indicate the measured points and simulation curve.
Figure 3. X-ray diffraction patterns of SmBaMn2O6-δ films prepared for various (a) KrF laser fluences, (b) atmospheres and (c) temperatures, where subscripts O and D represent the A-site ordered and disordered films, respectively.
means of peak fitting using Voigt function, the diffraction peak at around 2θ ) 23° of the SmBaMn2O6 was estimated as two multiple peaks: the reflections were indexed to the 010p (2θ ) 23.22°, d010 ) 3.831 Å) and the 001p (2θ ) 23.35°, d001 ) 3.807 Å) originated from the SmBaMn2O6(010) and SmBaMn2O6(001) domains, respectively (inset of Figure 4a). The structural parameters of the bulk crystal are ap(a/x2) ) 3.917 Å and cp(c/2) ) 3.813 Å in the reduced tetragonal unit cell ap × ap × cp,24 and then the difference of the lattice parameters from that of the STO substrate (a ) 3.905 Å) causes a lattice misfit. The lattice misfit between the (001) plane of the SmBaMn2O6 and the STO(001) is small, whereas the (010) plane of the SmBaMn2O6 has large misfit with that of the STO(001), indicating the emergence of an orthorhombic distortion of the SmBaMn2O6(010) domain. In contrast, oxygen annealing slightly shifted the diffraction peaks of Sm0.5Ba0.5MnO3-δ to lower angles (∆2θ ) 0.02°), which indicates full oxidation (δ f 0.0). The cross-section TEM image of the SmBaMn2O6 film on STO(001) shows that SmBaMn2O6 grew epitaxially on the STO(001) substrate (Figure 5a), and the film thickness was estimated to be 250 Å. The 7.5 Å modulation structures (27) Akahoshi, D.; Okimoto, Y.; Kubota, M.; Kumai, R.; Arima, T.; Tomioka, Y.; Tokura, Y. Phys. ReV. B 2004, 70, 064418.
Figure 5. (a) Cross-section TEM image of the SmBaMn2O6 film on STO(001). (b) Enlarged view of the lattice image of the SmBaMn2O6 film. The simulated image using the A-site ordered SmBaMn2O6 model calculated for a crystal thickness close to 100 Å and a defocus value ∆f ) -600 Å is shown in the inset. (c) ED pattern of the SmBaMn2O6 film taken along the zone axis [100]p.
along the c-axes corresponding to the doubling of the cp parameter were clearly observed in the film and there is a domain boundary between [001]- and [010]-oriented SmBaMn2O6.
Epitaxial A-Site Ordered SmBaMn2O6 Film on SrTiO3(001)
Figure 6. (a,b) Cross-section TEM image of the Sm0.5Ba0.5MnO3 film on STO(001). (c) Enlarged view of the lattice image of the Sm0.5Ba0.5MnO3 film. The simulated image using the A-site disordered Sm0.5Ba0.5MnO3 model calculated for a crystal thickness close to 100 Å and a defocus value ∆f ) -600 Å is shown in the inset. (d) ED pattern of the Sm0.5Ba0.5MnO3 film taken along the zone axis [100]p.
The simulated lattice pattern calculated by using the structural parameters of bulk A-site ordered SmBaMn2O624 agrees with the observed high-resolution lattice image, as shown in Figure 5b, and the ED pattern of the film at room temperature shows super-lattice reflections with a twofold periodicity along [001] (Figure 5c). These results demonstrate the A-site layer ordering of the Sm and Ba cations and are consistent with the analysis of X-ray diffraction experiments described above. No sign of A-site ordering was observed in the crosssection TEM image obtained along the [100] and [010] zone axes for the Sm0.5Ba0.5MnO3 film (Figure 6a-c). Like the SmBaMn2O6 film, the Sm0.5Ba0.5MnO3 film grew epitaxially on the STO(001) substrate; however, the superlattice spots originating from the A-site ordering were not observed in the ED pattern (Figure 6d). Thus, these results clearly indicate that the A-site ordered and disordered structures were successfully controlled by the fabrication conditions used in the ELAMOD process. Contour mapping in the reciprocal space around the 303p/ 330p peaks of the SmBaMn2O6(010)/ SmBaMn2O6(001) domains and the STO(001) substrate is plotted in Figure 7. From the Q| and the Q⊥ values of the SmBaMn2O6(010) domain, we calculated the in- and out-of-plane lattice parameters to be d| ) 3.902 Å and d⊥ ) 3.839 Å, respectively. The out-plane parameter d⊥ (bp for SmBaMn2O6(010)) is 1.99% longer than ap ) 3.917 Å for the bulk material, and the in-plane parameter d| (ap and cp for SmBaMn2O6(010)) is 3.902 Å, which is 0.38% shorter and 2.33% longer than ap ) 3.917 Å and cp ) 3.813 Å for the bulk material, respectively. The in- and out-of-plane parameters for the SmBaMn2O6(001) domain were also estimated to be d| ) 3.904 Å and d⊥ ) 3.816 Å, respectively. The out-of-plane parameter d⊥ (cp for SmBaMn2O6(001)) is 0.08% longer than cp for the bulk material, and the in-plane
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Figure 7. Contour mapping in the reciprocal space around the 303p/330p peaks of the SmBaMn2O6(010)/SmBaMn2O6(001) domains and the STO(001) substrate. The vertical (Q⊥) and horizontal (Q|) axes represent the [001] and [100] directions, respectively.
Figure 8. Temperature dependence of electrical resistivity for the SmBaMn2O6 and Sm0.5Ba0.5MnO3 films on STO(001) under 0 and 7 T. The inset shows the resistivity behaviors in the form of ln(F/T) vs 1000/T. The solid lines represent the curve fittings.
parameter d| (ap for SmBaMn2O6(001)) is 3.904 Å, which is 0.33% shorter than ap for the bulk material. The in-plane parameters d| of both domains almost equal to the lattice parameter of STO (a ) 3.905 Å). These transformations of the lattice parameters indicate that the SmBaMn2O6(010)/ (001) domains are distorted by the tensile strain caused by the misfit of the lattice parameters of the film and the substrate, and the MnO6 octahedra of the film are considered to be elongated and compressed along the apical and equatorial oxygen directions, respectively, compared with the octahedral of the bulk material. Figure 8 shows the temperature dependence of electrical resistivity (F) for the SmBaMn2O6 and Sm0.5Ba0.5MnO3 films under 0 and 7 T. Both films were insulating, and the MR effect was not observed even at 7 T. The F-T curves of the SmBaMn2O6 and Sm0.5Ba0.5MnO3 films at high-temperature (300-400 K), are well-fitted to the equation for the small polaron hopping model: F ) FaT exp(EA/kBT), where Fa is a constant and EA is the thermal activation energy.28 The EA
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values for SmBaMn2O6 and Sm0.5Ba0.5MnO3 were calculated to be 173.3(8) and 213.8(9) meV, respectively. Discussion Fabrication Conditions for A-Site Ordered SmBaMn2O6-δ. In previous work, we showed that films can be crystallized and epitaxially grown on single-crystal substrates by means of the ELAMOD process; for example, La1-xSrxMnO3 can be grown on LaAlO3.21 In this study, we observed the epitaxial growth of Sm0.5Ba0.5MnO3-δ on STO(001) prepared by KrF laser irradiation at 80 mJ/cm2 in air. Both of these A-site disordered manganite films were fabricated by laser irradiation at relatively mild laser fluence in air. However, the A-site ordered structure could not be obtained under these fabrication conditions. Structural analysis of the prepared films indicated that the A-site ordered structure in perovskite manganite was formed only under special irradiation conditions, that is, the A-site cations of Sm3+ and Ba2+ were ordered when the film was irradiated with a KrF laser at fluences higher than 130 mJ/cm2 and at 500 °C in an Ar atmosphere. These fabrication conditions are similar to those used to prepare bulk RBaMn2O6-δ; polycrystalline RBaMn2O6-δ can be prepared by solid-state reaction at 1300 °C under Ar or N2 or in a vacuum.13-16,26 The reductive atmosphere is quite important for A-site ordering in RBaMn2O6-δ. The layertype ordering of A-site cations is realized by the general preference of the small R3+ ions for adopting 8-fold coordination in the reductive atmosphere; the lack of oxygen ions around the R3+ ions leads to the A-site layer ordering, and then the oxygen-deficient RBaMn2O6-δ is formed. In contrast, the A-site ordered structure is easily converted to the disordered structure in a slight oxidizing atmosphere at high temperature.13,24 Hence, we consider the reductive atmosphere to be essential for generating A-site cation ordering in the ELAMOD process as well. To order the A-site cations by means of ELAMOD process, strong laser fluence (>130 mJ/cm2) and an Ar flow were necessary. The intense laser irradiation seems to provide not only crystallization energy but also further reducing power for A-site ordering. The oxygen-deficient SmBaMn2O6-δ film exhibited epitaxial growth on the STO(001) substrate; however, the film had a twinning structure, that is, [010]- and [001]-oriented domains with different oxygen content. We would like to discuss how to estimate the oxygen contents of the SmBaMn2O6-δ film and the origin of domain coexistence. Figure 9 shows the cell parameters of bulk SmBaMn2O6-δ (δ ) 0.0, 0.5, and 1.0) and the degree of misfit between each crystallographic plane and the STO(001) lattice.24,29 If the thin film was epitaxially grown on the substrate, the inplane parameter of the film could approach to the cell parameter of the substrate, and the out-of-plane parameter of the film was then changed to keep the original cell volume. The diffraction peak for the SmBaMn2O6-δ film at 2θ ) (28) Holstein, T. Ann. Phys. 1959, 8, 325. (29) Troyanchuk, I. O.; Trukhanov, S. V.; Szymczak, G. Crystallogr. Rep. 2002, 47, 658.
Figure 9. (a) Oxygen content δ dependence of the lattice parameters and the cell volumes for the bulk SmBaMn2O6-δ crystals (0.0 e δ e 1.0).24,29 (b) Area misfit between each crystallographic plane for the SmBaMn2O6-δ and the STO(001) substrate.
22.38° (Figure 1a), which was calculated as d1 ) 3.969 Å corresponding to the out-of-plane parameter of the film was considered to be assigned to [010]-oriented SmBaMn2O6-δ1 (0.5 e δ1 e 1.0), because the d1 value would be associated with a shortening bp parameter of bulk SmBaMn2O5.5 because of the large negative misfit of SmBaMn2O5.5(010) plane, or near bp parameter of bulk SmBaMn2O5.0 because of the little misfit of SmBaMn2O5.0(010) plane. Therefore, the δ1 for SmBaMn2O6-δ1 corresponding to this domain could be in the range of 0.5 e δ1 e 1.0. On the basis of these speculatio ns from the lattice misfit values, the lower limit of δ1 ) 0.5 cannot be strictly determined; however, this estimation is almost appropriate combined with the other facts such as the relationship of cell volumes for each domain to be described. The reflection for the SmBaMn2O6-δ film at 2θ ) 23.42° (Figure 1a), which was calculated as d2 ) 3.795 Å, corresponding to the out-of-plane parameter of the film, was considered to be assigned to [001]-oriented SmBaMn2O6-δ2 (0.0 < δ2 < 0.5). The δ2 for SmBaMn2O6-δ2 seems to be not just 0.5 and 0.0 as following two reasons: (1) the positive misfit was large for the short d2 value when the δ2 was 0.5, and (2) no oxygen deficiency (δ2 ) 0.0) was quite disadvantage for the A-site ordering. Hence, the d2 value could be in the range of 0.0 < δ2 < 0.5. The shorter d2 than the d1 (δ2 < δ1) indicates that the [001]-oriented domain has smaller cell volume than that of the [010] domain. This behavior that the cell volume increases with the increasing δ is consistent with the cell volume change for bulk samples (Figure 9a). The misfit between the SmBaMn2O6-δ and the STO(001) is considered to be originated from two origins in this
Epitaxial A-Site Ordered SmBaMn2O6 Film on SrTiO3(001)
Figure 10. Schematic illustration for (a) the lattice misfit, and (b) the specific plane misfit at the interface between the (001) and (010) planes for SmBaMn2O6-δ and the STO(001) plane.
system: (1) the lattice parameter misfit (Figure 10a) and (2) a specific plane misfit (Figure 10b). Under an Ar gas flow at high temperature, the A-site order was realized in bulk samples and the SmBaMn2O6-δ was stabilized in 0.5 e δ e 1.0.24 Therefore, the A-site ordered SmBaMn2O6-δ film could have also similar oxygen content with the bulk compounds at the moment of the crystallization under the heating process by the excimer laser irradiation. If the δ was close to 1.0, the lattice parameter misfit of the SmBaMn2O5.0(010) was smaller than that of the SmBaMn2O5.0(001). On the other hand, the area misfit for the SmBaMn2O5.5(001) domain was little compared to the SmBaMn2O5.5(010), when the δ was near from 0.5. However, the SmBaMn2O5.5(001) domain was strongly anisotropically distorted for the cubic STO lattice. Therefore, we expected that the SmBaMn2O6-δ(010) domain had an advantage for the epitaxial growth on the STO(001). Nevertheless, the single domain growth was practically not realized. The reason seems to be that the SmBaMn2O6-δ(010) also has another misfit with the STO(001) lattice because of the A-site ordering of SmBaMn2O6-δ. The A-site ordered structure produces an unconventional distortion as shown in Figure 10b: the MnO2 plane is 10-50%, depending on the oxygen deficiency, shifted to the SmO layer because of the
Chem. Mater., Vol. 19, No. 22, 2007 5361
large size mismatch of Sm3+(1.24 Å)/Ba2+(1.61 Å) cations;24-26 consequently, the large misfit between the SmBaMn2O6-δ (MnO2 layer) and the STO (TiO2 layer) occurrs. Hence, the stability of these two kinds of domains could countervail, and as a consequence, the SmBaMn2O6-δ(010)/SmBaMn2O6-δ(001) domain coexistence was considered to emerge. The as-prepared [001]-oriented domain seems to absorb slight oxygen in order to relax the lattice parameter misfit as mentioned above after the crystallization, and then the SmBaMn2O6-δ1(010) and SmBaMn2O6-δ2(001) domains could have different oxygen contents δ1 and δ2. To realize the single domain epitaxial growth for the A-site ordered SmBaMn2O6-δ, we have to diminish the lattice misfit between the film and substrate until it is as low as possible. However, the control of the specific plane misfit mentioned above was difficult. Therefore, the lattice parameter misfit should be lowered by the other substrate, which has a smaller in-plane parameter than the STO(001), and then the SmBaMn2O5.0(001) domain is expected to be strongly stabilized on the substrate. Structural and Physical Properties of the SmBaMn2O6 Film on STO(001). The bulk SmBaMn2O6 crystal shows the IM transition at 375 K associated with the CO, and the crystal has a unit cell 2x2ap × x2bp × 4cp corresponding to the piling up of CE-type CO layers with four-fold periodicity along the c-axis at room temperature.12,30,31 In contrast, the SmBaMn2O6 film grown on STO(001) never exhibited the IM transition, and the four-fold periodicity along the c-axis seen in the bulk material was not confirmed. This result indicates that the SmBaMn2O6 film has different or no long-range CO states at room temperature. Ogimoto et al. have recently reported that the first-order CO transition does not appear in the perovskite manganite Pr0.5Ca0.5MnO3 film epitaxially grown on (001) substrates of STO and [(LaAlO3)0.3-(SrAl0.5Ta0.5O3)0.7] (LSAT), and the IM transition was observed only in the film on (110) substrates.32 Ogimoto et al. concluded that the lattice deformation accompanied by the d(3x2 - r2)/(3y2 - r2) orbital ordering in the CE-type CO state is allowed on the (110) substrates; however, the (001) substrate on which the d(3x2 - r2)/(3y2 - r2) orbital plane lies prevents the firstorder lattice change to the CE-type CO phase. Therefore, the IM transition was considered not to occur in the manganite films on (001) substrates. This reasoning is also thought to explain the fact that the abrupt resistivity change that accompanied the CO transition was not observed for the A-site ordered SmBaMn2O6 thin film on STO(001). The orbital ordered plane corresponding to the CO transition for the [010]-oriented SmBaMn2O6 domains is perpendicular to the substrate surface; therefore, the constraint of lattice deformation for the [010]-oriented domain is considered to be weaker than that for the [001]-oriented domain. However, the first-order CO transition of [010]-oriented domains also seems to be suppressed by the surrounding rigid [001](30) Kageyama, H.; Nakajima, T.; Ichihara, M.; Ueda, Y.; Yoshizawa, H.; Ohoyama, K. J. Phys. Soc. Jpn. 2003, 72, 241. (31) Uchida, M.; Akahoshi, D.; Kumai, R.; Tomioka, Y.; Arima, T.; Tokura, Y.; Matsui, Y. J. Phys. Soc. Jpn. 2002, 71, 2605. (32) Ogimoto, Y.; Nakamura, M.; Takubo, N.; Tamaru, H.; Izumi, M.; Miyano, K. Thin Solid Films 2005, 486, 104.
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oriented region. Hence, epitaxial growth on substrates with the other orientation, such as STO(110) and LSAT(110), is thought to be necessary for the first-order CO transition in the SmBaMn2O6 film as well. We compared the EA values of electrical resistivity for SmBaMn2O6 and Sm0.5Ba0.5MnO3 films to those of bulk polycrystalline samples. The EA of polycrystalline Sm0.5Ba0.5MnO3 was in the 160-230 meV range, depending on the degree of A-site disorder;24 and the values were similar to the value for the Sm0.5Ba0.5MnO3 film (213.8(9) meV). This large EA value was caused by the charge incoherent localization; a strong local lattice distortion derived from the large size variance of the Sm and Ba ions produced a high-energy barrier for eg electron hopping. In contrast, the EA value for the SmBaMn2O6 film (173.3(8) meV) was quite large compared to that of a polycrystalline sample (86.9(3) meV24). For the bulk crystal, we found that the A-site ordered structure is an advantage for electron hopping because of the absence of structural randomness due to disordering of the Sm and Ba cations.24 Therefore, the high EA value for the SmBaMn2O6 film and the high resistivity at temperatures above 300 K indicate the presence of an energy barrier for eg electron hopping, a barrier not caused by the structural disorder. For a Pr0.5Ca0.5MnO3 film, high resistivity above the TCO is explained as follows: the in- and out-of-plane lattice parameters are elongated and compressed, respectively, by the misfit with the lattice constant of the substrate. This distortion indicates apically compressed MnO6 octahedra along the c-axis and tensile strain in the ab plane (d(3x2 r2)/(3y2 - r2) orbital ordered plane) and this deformation favors the high-resistivity CO state.32 However, the [010]and [001]-oriented domains in SmBaMn2O6 film have different and opposing strains; that is, the MnO6 octahedron of the SmBaMn2O6 film was elongated in the [001] direction, and its shape approached that of a regular octahedron. This type of strain should be advantageous for eg electron hopping, but in reality, the resistivity of the SmBaMn2O6 film is large at high temperature around the TCO of bulk SmBaMn2O6. Therefore, there may be some other explanation for the high resistivity in the SmBaMn2O6 film at high temperature. To explain this high resistivity, we must carry out microscopic studies of, for example, the coherent length of the CO phase and its temperature dependence by means of ED and TEM techniques. These studies are in progress. In this study, the resistivity of the SmBaMn2O6 film showed only insulating behavior without an MR effect;
Nakajima et al.
however, we expect that our successful fabrication of a thin film of A-site ordered perovskite manganite will lead to the fabrication of CMR thin film materials that function at room temperature by means of control of the film constituents and film forming on other substrates, such as STO(110) and LSAT(110). Conclusion By means of an ELAMOD process, we successfully prepared an A-site ordered perovskite manganite SmBaMn2O6-δ film and achieved epitaxial growth of the film on an STO(001) substrate. The SmBaMn2O6-δ film was crystallized in [010]- and [001]-oriented domains that were confirmed by X-ray diffraction and cross-section TEM measurements. The A-site ordered structure was well-formed at 500 °C by means of KrF laser irradiation at a fluence of 140 mJ/cm2 for 60 min in an Ar flow, whereas the A-site cations were disordered when the laser irradiation was carried out at laser fluences lower than 120 mJ/cm2 and/or in an oxygen atmosphere. The fully oxidized SmBaMn2O6 film was obtained by oxygen annealing of the as-prepared SmBaMn2O6-δ film at 500 °C for 3 h. Thus, we achieved the fabrication of an A-site ordered perovskite SmBaMn2O6 thin film at low temperature (500 °C). This result suggests that the ELAMOD process has great potential not only as a technique for the fabrication of A-site ordered manganite films but also for industrial use for switching devices, if properties such as the magnetic- or light-induced IM transitions at room temperature can be developed in this system. Because the films can be effectively fabricated on substrates at low temperature, the microfabrication of the materials will be also realized by the scanning and partly masked laser light. The SmBaMn2O6 film on STO(001) showed insulating behavior without the first-order CO transition, because of the constraint of the lattice change associated with the CO due to the undeformable substrate lattice. However, the large MR effect observed at room temperature for bulk RBaMn2O6 is also expected to be observed for A-site ordered manganite thin films if the constituents of the films and the orientation of substrates can be optimized. Acknowledgment. This work was partly supported by a Grant-in-Aid for Scientific Research (18850028) from the Ministry of Education, Culture, Sports, Science, and Technology. CM071118I
