Characterization methods of epitaxial Sr2FeMoO6 thin films

Characterization methods of epitaxial Sr2FeMoO6 thin films

Journal of Crystal Growth 241 (2002) 448–454 Characterization methods of epitaxial Sr2FeMoO6 thin films M. Bessea, F. Paillouxa, A. Barthe! le! mya,*,...

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Journal of Crystal Growth 241 (2002) 448–454

Characterization methods of epitaxial Sr2FeMoO6 thin films M. Bessea, F. Paillouxa, A. Barthe! le! mya,*, K. Bouzehouanea, A. Ferta, J. Olivierb, O. Durandb, F. Wycziskb, R. Bisarob, J.-P. Contoura a

Unit!e mixte de Physique CNRS-Thales, Domaine de Corbeville, 91404 Orsay Cedex, France, and Universit!e Paris-Sud, 91405 Orsay Cedex, France b Thales Research and Technology, Domaine de Corbeville, 91404 Orsay Cedex, France Received 28 October 2001; accepted 12 March 2002 Communicated by M. Roth

Abstract We have investigated the microstructure and the magnetic properties of Sr2FeMoO6 thin films deposited on (0 0 1)SrTiO3 substrates by pulsed laser deposition. We have checked the influence of oxygen pressure on the roughness and the resistivity of the films. We found a narrow range of pressure (B5  106 Torr) leading to conductive films. Fixing the oxygen pressure at this value, we have performed a structural analysis for two characteristic samples grown at different temperatures. We have revealed the nucleation of iron-rich parasitic phases, the content of which depends on the growth temperature. The role of these Fe-rich inclusions on the electrical and magnetic properties of the SrFeMoO6 films is also discussed. r 2002 Elsevier Science B.V. All rights reserved. PACS: 81.15.F; 75.70.K Keywords: A1. Characterization; A3. Laser epitaxy; B2. Magnetic materials

1. Introduction The search of new compounds with a large spin polarization is actually in effervescence due to their potential applications in magnetoresistive devices and for the comprehension of the fundamental process involved in the magnetism of this class of materials. In this context a half-metallic material with a Curie temperature larger than room temperature is *Corresponding author. Tel.: +33-1693-39381; fax: +331693-30740. E-mail address: [email protected] (A. Barth!el!emy).

required. Manganites have been exhaustively studied, but their Curie temperatures are definitely too low. Recently, Kobayashi et al. predicted the half-metallic nature of double perovskites, Sr2BB0 O6, with BB0 =FeMo or FeRe [1]. Furthermore, the Curie temperatures of these compounds exceed room temperature by more than 100 K [2,3], which makes them good candidates for devices application. The large intergrain tunnel magnetoresistance (TMR) observed in polycrystalline Sr2FeMoO6 (SFMO) [4] and Sr2FeReO6 [5] reflects the half-metallic nature of these materials and is very promising. Unfortunately, large magnetic fields (of about few kG) are required to obtain this large intergrain TMR. A solution may

0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 2 ) 0 1 2 8 5 - X

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be offered by the possibility to grow magnetic tunnel junctions as already done with manganites [6]. The realization of such structures requires high-quality thin films of SFMO, i.e. films made of sufficiently large flat areas and exhibiting a magnetic moment close to the bulk value of 4.0mB /f.u. (300 emu/cm3), attempted in the present work. We have investigated the influence of the growth parameters on the macroscopic properties (magnetization and resistivity) and on the microstructure of Sr2FeMoO6 (SFMO) thin films deposited by pulsed laser deposition (PLD) on [0 0 1] SrTiO3 (STO) substrates.

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with 2 keV Ar+ ions during a couple of minutes. A rotating specimen holder was used to limit artefacts due to the ion etching. Maps were obtained considering the Mo-MNN (187 eV), OKLL (510 eV), Fe-LMM (705 eV) and Sr-LMM (1651 eV) peaks. The epitaxial relationship between the deposit and the substrate was also investigated by transmission electron microscopy (TEM). Cross-sections were obtained by mechanical thinning and ion milling (PIPS-Gatan) with 2.5 keV Ar+ ions during a few hours. The observations were performed with a TOPCON EM002B (Cs=0.4 mm) operated at 200 kV.

2. Experimental procedure 3. Results and discussion A frequency tripled (l ¼ 355 nm) Nd:YAG laser was focused on a stoichiometric target of SFMO. The pulse rate was set to 5 Hz and the substrate– target distance to about 55 mm. In situ monitoring of the growth was performed by reflection highenergy electron diffraction (RHEED) using a differentially pumped electron gun (EK-35 Staib Instrumente Gmbh). RHEED patterns were recorded at 25 keV with a grazing incidence of about 21. In order to get an insight into the lattice parameter of the films, in the growth direction, X-ray diffraction (XRD) experiments were carried out. Spectra were acquired using a y22y diffractometer in the Bragg–Brentano geometry, with the Cu-Ka radiation and a back curved graphite monochromator. As these films aimed to be integrated in multilayer devices, we have inspected the surface morphology and roughness of the thin layers by atomic force microscopy (AFM). Images were recorded using a Dimension 3100 AFM instrument from Digital Instruments which was used in its tapping mode configuration, in air at room temperature. The chemical homogeneity of the films was examined using Auger electron spectroscopy (AES) mapping using a Phi 680 Field Emission Scanning Auger Nanoprobe operated at 20 keV, 10 nA (probe size B20 nm). The carboneous contamination layer was removed by ion etching

3.1. In situ monitoring by RHEED We have used RHEED experiments in order to verify the flatness of the STO substrate surface after the cleaning procedure and also to continuously check the evolution of the growing surface during the deposition. Results obtained for a sample grown at 8301C under 5  106 Torr of oxygen are summarized in Fig. 1. The pattern recorded at the early stages of growth exhibits spots characteristic of a three-dimensional (3D) growth. The lattice parameter measured on this pattern (0.32 nm) does not correspond to the lattice constant of SFMO, indicating the nucleation of clusters of a parasitic phase. The curvature of the spots also indicates a slight misalignment between these clusters. As the deposition proceeds, we observe the evolution of the RHEED pattern. For a deposition time exceeding 5 min, bright lines indicative of the formation of flat terraces are observed. The lattice parameter evaluated from this last pattern roughly corresponds to the SFMO lattice constant (0.788 nm). 3.2. Effect of oxygen pressure Three samples have been grown at the same deposition temperature (9301C) under different oxygen pressures. AFM pictures of these samples (Fig. 2) obviously show three kinds of topology.

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Fig. 1. RHEED patterns recorded during the growing of the SFMO film (deposition rate 0.04 nm/pulse).

Fig. 2. AFM pictures (1 mm  1 mm) of SFMO thin films grown at 9301C under various oxygen pressure: (a) 5  106 Torr, (b) 1  105 Torr, (c) 6  105 Torr.

These structural differences are correlated with the physical properties. Indeed, resistivity measurements give evidence to progressive change from a metallic behaviour (r ¼ 0:4 mO cm) at 5  106 Torr to an insulating one (r > 1 MO cm) at 6  105 Torr. Magnetization also varies with the oxygen pressure: the sample grown at 5  106 Torr exhibits a magnetization of about 90710 emu/cm3; assuming a single-phase thin film, this value corresponds to a magnetic moment of 1.2mB /f.u. The samples grown under higher oxygen pressure are almost nonmagnetic. It appears that the optimum oxygen pressure to grow magnetic thin films exhibiting a metallic behaviour should be lower than 5  106 Torr, which is in good agreement with values already reported in the literature [7]. 3.3. Effect of temperature The optimum oxygen pressure being well established, we have mainly focused this study on two samples: one grown at 8651C under

2  106 Torr (sample I) and the other grown at 9301C under 5  106 Torr (sample II) in order to check the effect of the growth temperature. Both samples are magnetic and exhibit a metallic behaviour (with a resistivity of about 60 mO cm at 4 K). The XRD spectrum of sample I (Fig. 3a) shows diffraction peaks corresponding to the SFMO layer, close to the substrate peaks. The out-ofplane lattice parameter ‘‘c’’ of the SFMO layer determined from this spectrum is about 0.797570.0005 nm. The HRTEM picture (Fig. 3b) reveals the pseudo-morphic growth of the SFMO film on the substrate. Lattice parameters evaluated from this image are a ¼ 0:78870:002 nm (along the interface) and c ¼ 0:79570:002 nm (perpendicular to the interface). Assuming that the unit-cell is tetragonal, its volume is 0.495 nm3. This quite large value as compared to the bulk (0.490 nm3), and may be attributed to a small concentration (o1%) of oxygen vacancies [8] or to an offstoichiometric compound with excess of Mo (the volume of Sr2Mo2O6 is 0.502 nm3 [9]). Because of

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Fig. 3. (a) XRD spectrum (y22y) of sample I (8651C) showing (0 0 l) diffraction peaks of SFMO and extra-peaks. (b) HRTEM picture of a SFMO part of the same sample showing the cube/ cube epitaxy between the film and the substrate.

the compressive stress induced by the lattice mismatch between SFMO and STO, the epitaxial strain could play an important role in the expansion of the c-axis with respect to the bulk value (0.7900 nm) [10]. These values are in good

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agreement with values previously reported in the literature [11]. Nevertheless, the lattice parameter should not be used as a criterion to estimate the quality of the deposit. Indeed, slight O, Fe or Mo off-stoichiometry would lead to lattice variations of the same order of magnitude as epitaxy-induced strains. Unfortunately, extra-peaks are also present in the XRD spectrum of sample I (Fig. 3a). In the same way, the magnetization versus temperature curve exhibits a shape which suggest the presence of two magnetic materials (Fig. 7c). Indeed, this curve seems to be a combination of two curves having different Curie points, one about 250 K and the other above 350 K. This measurement together with the extra-peaks observed in XRD are related to a peculiar morphology of the film surface, as shown on the AFM image (Fig. 4a). Three main features clearly appear in this picture. Sample I is basically composed of wide (hundreds squarenanometers) flat terraces. These terraces are separated by deep valleys oriented along the / 1 1 0S crystallographic directions of the substrate. Some outgrowths rising up from the film surface are also visible. Typical TEM cross-section pictures of this sample show atomically flat areas (Fig. 4b) corresponding to the terraces observed by AFM. The V shape (Fig. 4b) of the valleys suggest a lower surface energy for the (1 1 1) planes of the SFMO pseudo-cubic unit cell. The outgrowths seen in the AFM picture have also been observed during the TEM experiments. One kind of outgrowth having a square shape and a flat top is shown in Fig. 5a; this kind of grains nucleate on the substrate during the early stage of deposition, and then grow along their [0 0 1] direction. As they

Fig. 4. (a) AFM picture of the film surface. Flat terraces are separated by valleys oriented along the /1 1 0S crystallographic directions. (b) Conventional bright-field TEM picture of the flat terrace and V-shaped valley.

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Fig. 5. (a) HRTEM image of a flat-square outgrowth imbeded in the SFMO layer and (b) corresponding diffractogram. This kind of iron oxide outgrowth epitaxially grow on the substrate along their [0 0 1] direction.

Fig. 6. (a) SEM image of sample I and (b) corresponding AES map. Iron oxides appear in black, pure iron in red, oxygen in light blue.

grow faster than the SFMO part of the film, they lead to a severe roughness of the deposit. The corresponding lattice parameter measured (B0.834 nm) on the corresponding digital diffractogram is close to the lattice constant of iron oxides, such as Fe2O3 (0.835 nm) or Fe3O4 (0.839 nm). A thorough investigation of the XRD spectrum (fig. 3a) reveals that the extra-peak observed at 2y ¼ 43:151 corresponds to this kind of parasitic phase. EDX experiments performed in the TEM corroborates the presence of iron in these grains and also indicates a lack of strontium and molybdenum. We have used AES mapping to check the homogeneity of this film over wide areas. The map shown in Fig. 6b obviously reveals three types of regions: (1) the red dots are indicative of the presence of pure metallic iron located at the bottom of the valleys or in big outgrowths, (2) the black dots (combination of red and blue components) unambiguously confirm that the flat square outgrowths are composed of iron oxide, but its exact stoichiometry remains unknown, and (3) the

large blue areas correspond to the flat terraces composed of strontium, molybdenum, iron and oxygen. Even though the exact stoichiometry is not determined, these flat regions can correspond to the material we aim to obtain. The low Curie temperature of 250 K, and the apparent magnetization of 145715 emu/cm3 extracted from the magnetization measurements taking into account the whole volume, could be related to a slight cationic off-stoichiometry of the SFMO double perovskite. From AFM pictures, AES maps and TEM images, we roughly estimate that the volume proportion of parasitic phases is of 10–15%. By increasing the temperature we have improved the quality of the films (sample II). As shown in Fig. 7a and b, the intensity of the extrapeaks corresponding to parasitic phases decreases drastically and becomes of the order of experimental error as the growth temperature increases. This result is in good agreement with the magnetization versus temperature measurement shown in Fig. 7c. Indeed, the contribution of the

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Fig. 7. (a,b) Comparison of XRD spectra of sample I (red) and sample II (black). Extra-peaks are no more observed at high deposition temperature. (c) Magnetization versus temperature plot illustrating the decrease of parasitic phase proportion in sample II (black) compared to sample I (red).

Fig. 8. Comparison of surface morphology and chemical composition of sample I (a) and sample II (b).

parasitic phases to the total magnetization decreases to about 10% when compared to the 40% in sample I. Owing to its high spatial resolution, Auger mapping (Fig. 8) allows to detect the existence of minuscule amounts (few percents) of iron and iron oxide impurities distributed between

the terraces even for high-temperature deposition. This decrease of the volume of parasitic phases is related to a decrease of the apparent magnetization (Mapparent ) from 145710 to 90710 emu/cm3. This last result points out the fact that the parasitic phases may modify the apparent

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magnetization and lead to absurd values of the magnetic moment. As the parasitic phases synthesized during the PLD process are metallic iron and iron oxides, they are certainly responsible for the various components seen in our magnetization versus temperature curves. Making the reasonable assumption that the global magnetization measured in our samples is the sum of at least two components: one due to the SFMO part of the films and the other coming from the iron-rich parasitic phases, we can evaluate the magnetization of SFMO. From the structural analysis, we can estimate a volumic proportion (x) of parasitic phases of 10–15% and about 1% or 2% in samples I and II, respectively. Assuming that only two components are involved in our curves (SFMO and Fe), the corresponding value of the SFMO magnetization is given by MSFMO ¼ ðMapparent  xMFe Þ=ð1  xÞ with MFe ¼ 1700 emu/cm3. A straightforward calculation shows that for an apparent magnetization of 145 emu/cm3 (sample I), the content of iron greater than 5% leads to MSFMO o63 emu/cm3. Also, for Mapparent ¼ 90 emu/cm3 (sample II) and 1% of Fe, we obtain MSFMO o90 emu/cm3.

4. Conclusion In this study, we present the first comprehensive structural characterization of ‘‘Sr2FeMoO6’’ thin films grown by PLD using the optimum conditions published earlier [7,9,12]. We have evidenced the presence of Fe-rich parasitic phases nucleating during the early stage of the growth. This leads to a rough surface with L-shape valleys and outgrowths, as already reported by other groups [12]. Another consequence is an overvaluation of the effective magnetization of the SFMO grains (for example, in the case of 5% of iron grains, with an apparent magnetization equal to that of the bulk SFMO, a straightforward calculation shows that the effective value of the magnetization is reduced to about 225 emu/cm3). We point out that it is of

prime importance to carry out a detailed structural characterization to conclude about the nature of such deposits. Taking into account the lack of structural characterization reported until now in the literature [9–12] and the conclusions of the present study, we assume that optimized SFMO thin films cannot be achieved using a single-step growth, but rather requires a complex process to avoid iron-rich parasitic phase nucleation.

Acknowledgements The authors are indebted to Dr P. Decorse, Professors A. Revcolevschi and P. Berthet for providing the Sr2FeMoO6 target, we are also grateful to M. Magis for technical support. Financial support by the CEE project AMORE (G5RD-2000-0138).

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