Journal of Alloys and Compounds 369 (2004) 205–208

Local effects in strained manganite thin films N.M. Souza-Neto a,b,∗ , A.Y. Ramos a,c , H.C.N. Tolentino a , E. Favre-Nicolin d , L. Ranno d a

LNLS—Laboratório Nacional de Luz Sincrotron, Caixa Postal 6192, 13084-971 Campinas, São Paulo, Brazil b IFGW-UNICAMP, Campinas, São Paulo, Brazil c LMCP UMR7590 CNRS, Paris, France d Lab. Louis Néel, Grenoble, France

Abstract We report on polarized X-ray absorption spectroscopy (P-XAS) study of La0.7 Sr0.3 MnO3 (LSMO) films, epitaxially grown by laser ablation on tensile (SrTiO3 ) and compressive (LaAlO3 ) substrates. In-plane and out-of-plane bond information was obtained by setting the angle between electric field vector and film surface close to 0 and 90◦ , respectively. Measurements show significant modifications in the average MnO6 octahedron around manganese atoms in the film plane for tensile and compressive substrates. The modifications of the XANES spectra were correlated to a modification in the average Mn–O distance and a distortion of the MnO6 octahedra. Ab initio calculations using the full multiple scattering approach confirm the structural model of distorted octahedron. © 2003 Elsevier B.V. All rights reserved. Keywords: Magnetic films; Thin films; EXAFS; Synchrotron radiation

1. Introduction Materials showing the Colossal-Magnetoresistance effect have attracted considerable interest due to their potential for technological applications. High quality thin films of manganites can be grown using deposition techniques similar to the ones developed for high-TC superconductors and these films have received special attention due to their potential for construction of new generation magneto-electronic devices. Since La0.7 Sr0.3 MnO3 (LSMO) manganites shows a ferromagnetic transition around room temperature, and has a fully spin-polarized conduction band, this material is of special interest for magnetic tunnel junction construction, as low cost magnetic sensors. The study of these materials can establish an explicit connection between local structure and global properties. However, it is not well established how the substrate-induced lattice strain affects the local structure and the magnetism and transport properties [1,2]. Studies of thin films on slightly mismatched substrates have shown significant sensitivity of manganite properties to distortions. It is well known that the versatility of these properties in the bulk manganites is correlated to variations

of the local structure of Mn ions—variations of the local Jahn–Teller distortion within the MnO6 octahedral and/or in the Mn–O–Mn angle (octahedral tilt) [3]. For bulk rare-earth manganese oxides the local structure such as Mn–O–Mn angle and Mn–O bond length can be varied by changing the doping concentration or by applying hydrostatic pressure. In thin films, the substrate-induced crystallographic distortions are anisotropic, unlike the distortions induced by hydrostatic pressure or cation substitution. However, there is no consensus on how these strains are related to the local atomic organization around the manganese atoms, in spite of the importance of this link in understanding the magnetoresistance in perovskites [1]. At the local scale, unusual and complex effects may be expected, as, for example a static anisotropic distortion of the MnO6 octahedron, which could lead to an increase of the Jahn–Teller splitting of the eg levels tending to localize the charge carriers, contrary to the effect of hydrostatic pressure. On the other hand, it might be argued that the modifications of the magnetic properties are more related to modifications in the Mn–O–Mn angle. 2. Experiments and methods

∗ Corresponding author. Tel.: +55-19-3287-4520; fax: +55-19-3287-4632. E-mail address: [email protected] (N.M. Souza-Neto).

0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2003.09.085

In this paper we report the local scale structural characterization around the manganese ions in LSMO thin films epitaxially grown by pulsed laser deposition under tensile

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Table 1 Films characteristics [4,6] εxx and εzz describe the strain conditions and are defined as εxx = εyy = (afilm − aref )/aref and εzz = (cfilm − cref )/cref Substrate

Thickness (nm)

εxx

εzz

SrTiO3 MgO LaAlO3

60 60 45

Tension (∼0.9%) Relaxed (0%) Compression (∼2.0%)

Compression (∼0.8%) Relaxed (0%) Tension (∼2.3%)

(SrTiO3 (STO) [1 1 0]) and compressive (LaAlO3 (LAO) [1 1 0]) substrate. LSMO has a perovskite structure with a = 3.87 Å [5]. STO and LAO have cubic and pseudo-cubic structures with, a = 3.905 and 3.793 Å, respectively. The low lattice mismatch between LSMO and STO and LAO allows a pseudomorphic growth for film thickness below 100 and 25 nm, respectively [4,6]. MgO substrate (cubic, with a = 4.21 Å) was used to obtain 3D texturized fully relaxed film. On the MgO the high lattice mismatch is (9%) and the growth is non-pseudomorphic. Films characteristics correlated to substrate induced strain are presented on Table 1. Further description of the growth conditions and structural characterization by X-ray diffraction can be found elsewhere, together with a characterization of their transport and magnetization properties [4,6,7]. The X-ray absorption experiments at the Mn K and L edges were performed at the Brazilian synchrotron light laboratory (Laboratório Nacional de Luz S´ıncrotron, LNLS) in Campinas, Brazil [8]. The data at the Mn K (6539 eV) were collected at the D04B-XAS1 beam line using a Si(1 1 1) monochromator. An ion chamber monitored the incident beam and the data were collected in the fluorescence mode using a Ge 15-elements solid state detector from Camberra.

The XANES (X-ray Absorption Near Edge Structure) spectra were collected in the range 6440–6700 eV with energy steps of 0.3 eV. The energy calibration was checked after each spectrum using a Mn metal foil. The XANES were normalized over an interval of 200–250 eV above the edge. In these conditions the edge structure in all spectra can be compared in position and intensity. All procedures allow us to certify effects greater than 0.2 eV energy shifts for XANES region. The Mn L3 and L2 edges data (639 and 650 eV, respectively) were collected by total electron yield at the D08A-SGM beamline. The experimental resolution for these energies was around 0.4 eV. The linear polarization of the synchrotron light has been used to gain selective information in the plane and out-of-plane of the film. This selection was achieved by setting the angle between electric field vector of the incident photon beam and the film surface to values close to 0 and 90◦ , respectively [9].

3. Results and discussion In Fig. 1 we show the Mn absorption K edge of the tensile and compressive strained films for the direction in-plane and out of the film plane. The shift in the edge jump and the modification on the shape of main line are related to modifications in the coordination shell [10–13]. The negative energy shift (−0.4 eV) in the spectra for LSMO/STO films are correlated to a increase of average Mn–O bond distance in the plane of film, as compared to the average distance out of the plane of the film. In the film grown on compressive substrate, the energy shift (+0.8 eV)

Fig. 1. Near edge X-ray absorption spectra at the Mn K edge for the tensile (SrTiO3 ) and compressive (LaAlO3 ) LSMO film in-plane (solid line) and out-of-plane (points line) measurements. Inset: zoom.

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Fig. 2. Near edge X-ray absorption spectra at the Mn K edge for the tensile (SrTiO3 ), compressive (LaAlO3 ) and relaxed (MgO) LSMO films in the out-of-plane measurements.

indicate the decrease of average Mn–O bond distance in the plane of film, as compared to the average distance out of the plane of the film. As expected the effects in the tensile and compressive substrate are in opposite directions. It is

noteworthy that the amplitude of the energy shift between in-plane and out-of-plane spectra roughly proportional to the amplitude of the relative strain εxx − εzz . This result indicates that the modification of the average size of the

Fig. 3. Near edge X-ray absorption calculated spectra at the Mn K edge for different models for the LSMO structure. The upper spectra correspond to the structure of LSMO with a simple lattice parameter modification for each substrate case. The middle spectra are the same simulations but with a distorted octahedron propose, equivalent to effects observed by EXAFS results [15]. The lower spectra are the same simulations with a distorted octahedron and calculated with the electric field vector perpendicular to the expected film plane surface.

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4. Concluding remarks We presented here the results of a polarized X-ray absorption study of the local characterization of structural distortion around the manganese atoms in La0.7 Sr0.3 MnO3 films with strain effects induced by SrTiO3 and LaAlO3 substrates. The modifications of the XANES spectra were correlated to a modification in the average Mn–O distance and a distortion of the MnO6 octahedra. Ab initio calculations using the full multiple scattering approach confirm the structural model of distorted octahedra.

Acknowledgements Fig. 4. Near edge absorption spectra at the Mn L2 and L3 edges for the tensile (SrTiO3 ) LSMO film for some orientations between the in-plane and out-of-plane conditions.

octahedra may fully accommodate the strain, without modification of the Mn–O–Mn angle. A slight modification of the line shape is also observed in the XANES spectra. This feature can be associated to local distortion [13]. To investigate the exact origin of this effect we compared in the same geometrical conditions the XANES spectra obtained with the three types of substrates (Fig. 2) and performed ab initio simulation of the XANES spectra expected for various models of local distortion. The ab initio XAFS simulations were performed in a full multiple scattering approach, using Feff82 code [14]. The atomic potentials were calculated by a self-consistent field calculation method. The results from these calculations are summarized in Fig. 3. Based on the results obtained from the near edge measurements (Figs. 1 and 2) and EXAFS results [15], we propose the model of a distorted octahedron. By looking at the middle spectra on Fig. 3, and comparing these results with the ones shown on Fig. 2, we can confirm that the modification on the intensity of main line is correlated to a MnO6 octahedron distortion in the structure of the film. The unique model capable to account for the all measured data, is that of a distorted MnO6 octahedron (Fig. 3, lower spectra). L2,3 spectra for the film grown over the STO (tensile) substrate are shown in Fig. 4. Only very subtle differences are observed for different angle between the electric field vector and the film surface. Further measurements on the more strongly constrained compressive substrate are needed to conclude about any strain-induced orientation in the electronic distribution in the Mn 3d orbitals, proposed for a similar case [16].

This work is partially supported by LNLS/ABTLuS/MCT. NMSN acknowledges the grant from Capes/Unicamp. AYR acknowledges the grant from CNPq. The solid state detector was granted by multi-users proposal from FAPESP under the contract 1999/12330-6.

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