Cationic and charge segregation in La2/3Ca1/3MnO3 thin films grown on „001... and „110... SrTiO3

grown on „001... and „110... SrTiO3 S. Estradé, J. Arbiol, F. Peiró, I. C. Infante, F. Sánchez, J. Fontcuberta, F. de la Peña, M. Walls, and C. Colliex EME/CeRMAE/INUB, Departament d’Electrònica, Universitat de Barcelona, Martí i Franquès 1, 08028 Barcelona, CAT, Spain Institut de Ciència de Materials de Barcelona-CSIC, 08193 Bellaterra, CAT, Spain Laboratoire de Physique des Solides, UMR CNRS 8502, Bâtiment 510, Université Paris Sud, 91405 Orsay, France

Mixed-valence ferromagnetic manganite films, such as La 2/3 Ca 1/3 MnO 3 ͑LCMO͒, have been the object of much attention in recent years due to their potential applications in spintronics. 1,2However, expectations have been lowered by the negligible room-temperature magnetoresistance in tunnel junctions. 3Although the reasons for this behavior are not yet fully known, it has been suggested that they may be linked to electronic phase separation. 4Electron energy-loss spectroscopy ͑EELS͒ allows direct determination of local Mn oxidation state and composition at the subnanometric scale.Several EELS studies on epitaxial ͑001͒ manganite thin films have been reported in literature.Early experiments by Pailloux et al. 5 observed no variation in Mn oxidation state at the STO͑SrTiO 3 ͒ / LSMO͑La 2/3 Sr 1/3 MnO 3 ͒ interface.Simon et al. 6 observed migration of Ca 2+ toward manganitefree surface in ͑001͒ textured LCMO/ STO͑SrTiO 3 ͒ / LCMO structures, whereas Maurice et al. 7 and Samet et al. 8 found no cation migration in the LSMO ͑La 2/3 Sr 1/3 MnO 3 ͒ / STO/ LSMO system but found a weak decrease in the Mn valence near the interfaces, attributed to a transfer of electrons from STO to LSMO.It has been observed that in LCMO films grown on LaAlO 3 substrates, which are under compressive strain, the La 3+ ions migrate toward the top layer surface. 9Little attention was received, so far, by the chemistry and electronic structure of ͑110͒ LCMO films, reported to display enhanced magnetic properties when compared to their ͑001͒ counterparts. 10,11In this work, we report detailed transmission electron microscopy ͑TEM͒ and EELS analyses of epitaxial LCMO layers grown simultaneously on ͑001͒ and ͑110͒ STO substrates.
The ͑001͒ and ͑110͒ LCMO layers were grown by rf sputtering at 800 °C, at 330 mtorr, with an O 2 / Ar pressure ratio of 1 4 and a growth rate of ϳ0.4 nm/ min.After growth, in situ annealing was performed at 800 °C for 1 h in O 2 atmosphere at 350 torr. 10The magnetic properties of these films have been previously studied. 10,11both magnetization ͓Fig.1͑a͔͒ and Curie temperature of the ͑110͒ films are much larger than those of their ͑001͒ counterparts.LCMO films, when grown on ͑001͒ and ͑110͒ STO substrates, have identical ͑tensile͒ lattice mismatch ͑ϳ1.1%͒.However, whereas in-plane orthogonal directions ͓100͔ and ͓010͔ of ͑001͒ films are equivalent, ͓001͔ and ͓11 ¯0͔ directions are not equivalent a͒ Electronic mail: sestrade@el.ub.es.for ͑110͒ films, and have different Young's moduli.
TEM samples, prepared in cross section geometry by focused ion beam ͑FIB͒ and in plan-view ͑PV͒ geometry by mechanical thinning down to 25 m and Ar+ bombardment at V = 5 kV and 7°using a PIPS-Gatan, were observed in a Jeol J2010F scanning TEM ͑STEM͒ microscope, with a hot field emission gun at 200 keV.EELS spectra and STEM images were obtained in a VG 501 dedicated STEM.Ca, La, and Mn normalized -weighted intensities ͑nW intensities͒ were obtained from EELS spectra as I X / X , normalized to the value of the mean Mn -weighted intensity along the layers.I X , the integrated intensities of background substracted Ca L 3,2 , La M 5,4 , and Mn L 3,2 edges, and X is the cross section for each edge ͑calculated using Gatan Digital Micrograph software͒.The Mn L 3 EELS edge position and Mn L 3 / Mn L 2 edge intensity ratio were determined using MANGANITAS ͑Refs.9 and 12͒ software package.
X-ray reflectometry ͑XRR͒ measurements ͓Fig.1͑a͒, in-set͔ were used to determine film thickness.From the observed reflectivity oscillations, layer thickness was determined to be about 80 and 82 nm for ͑001͒ and ͑110͒ films, respectively.A faster decrease in the reflected intensity in the ͑110͒ film indicates 13 a relatively higher roughness.Exhaustive x-ray diffraction ͑XRD͒ and electron diffraction experiments showed 10 that the LCMO films grow on ͑001͒ and ͑110͒ STO substrates in a "cube-on-cube" mode with the following epitaxial relationships: LCMO͑001͒ ϫ͓100͔ ʈ STO͑001͓͒100͔ and LCMO͑110͓͒001͔ ʈ STO͑110͒ ϫ͓001͔, respectively.
PV TEM study of ͑001͒ sample showed no evidence of defects.This is in good agreement with XRD lattice parameter measurements, 10 which indicated that ͑001͒ film is fully and isotropically in-plane strained.Yet, for ͑110͒ film, g = ͑1-10͒ two beam observation ͓Fig.1͑b͔͒ shows a highly periodic distribution of misfit dislocations running perpen-dicular to the ͓11 ¯0͔ direction ͑which seems to be in good agreement with recent reports͒; 14 g = ͑001͒ two beam observation ͓Fig.1͑c͔͒ shows a much lower density of dislocations, running perpendicular to ͑001͒.XRD measurements of the ͑110͒ LCMO films indicated an anisotropic partial in-plane relaxation: strain ϳ + 0.34% in ͓11 ¯0͔ direction and ϳ + 0.62% in ͓001͔ direction. 10omposition profiles across film thickness were obtained by EELS from cross sections.First, spectra ͑in the range of 325 to 870 eV͒ were acquired along the direction perpendicular to the substrate/film interface.Concentrations of the different species at different positions were determined from these spectra.The Mn, Ca, and La -weighted intensities along the layers are depicted, for ͑001͒ and ͑110͒ films, in Fig. 2. In ͑110͒ film ͓Fig.2͑b͔͒, Mn, Ca, and La concentrations are found to be clearly constant, and Ca and La nW intensities are close to the expected values ͑ϳ0.33 and ϳ0.66͒ across the whole film.In contrast ͓Fig.2͑a͔͒, for ͑001͒ films, a monotonous diminishing of the La nW intensity across film thickness is found, with interface ͑La en-riched͒ and free surface ͑La deficient͒.It is also observed that the Ca nW intensity increases with distance to interface.
EELS spectra in the ͑500-675͒ eV energy-loss range, where Mn L 2,3 edges ͑640 and 651 eV͒ and O K edge ͑532 eV͒ occur, were also recorded.6][17][18][19][20] In Fig. 3 we show representative EELS spectra of the ͑001͒ and ͑110͒ LCMO films ͓Figs.3͑a͒ and 3͑b͒, respec-tively͔.The Mn L 2,3 doublet remains at constant energy across the ͑110͒ film.In sharp contrast, for ͑001͒ films ͓Fig.3͑a͔͒, it progressively shifts toward lower energies when approaching LCMO/STO interface ͓Fig.3͑b͔͒.Mn L 3 edge positions are collected in Fig. 3͑c͒.No significant variation is found for ͑110͒ films.Dotted lines in Fig. 3͑c͒ indicate the Mn L 2,3 edge energies reported in literature [18][19][20] for Mn m+ ions in various oxidation states.The oxidation state of Mn m+ ions is constant across ͑110͒ films but gradually changes in ͑001͒ films, with a progressive reduction in the Mn m+ species near the interface while free surface appears to be overoxidized.The variation in the Mn L 3 / Mn L 2 intensity ratio ͑the ratio decreases with oxidation state͒ 18,19 provides further confirmation of the reduction in Mn m+ close to the interface in ͑001͒ films, as it decreases from 3.7Ϯ 0.1 near interface to 2.3Ϯ 0.1 near free surface.The observed variation in Mn m+ oxidation state in ͑001͒ layer is fully consistent with the detected La enrichment near the interface.Present data cannot exclude additional Mn m+ reduction at the interface as a result of a possible transfer of electrons from STO substrate. 7,8n Fig. 1͑a͒ ͑main panel͒, we show the magnetization versus field at 10 K for ͑001͒ and ͑110͒ 80 nm films.The saturation magnetization value of the ͑110͒ LCMO film ͑Ϸ588 emu/ cm 3 ͒ is higher than that of the ͑001͒ LCMO film ͑Ϸ560 emu/ cm 3 ͒.The presence of La-rich and La-poor regions in ͑001͒ films leads to electronic phase segregation and occurrence of nonferromagnetic phases, as evidenced by nuclear magnetic resonance experiments. 4,11s the stressed ͑001͒ sample does not exhibit plastic relaxation, it seems that elastic strain accommodation has occurred via cation migration.These results are in good agreement with the findings of Simon et al., 6 and represent a solid experimental confirmation of the trends observed there.Yet, in the present work, the strain acting on the films has been changed without changing the composition of the film.We notice that due to the larger ionic size of La 3+ than Ca 2+ , La 3+ enrichment close to the STO interface ͑Fig.2͒ provides a natural mechanism for cell-parameter matching to STO substrate.Comparison between the two encountered stress accommodation mechanisms, namely, defect formation for ͑110͒ orientation and cation migration for ͑001͒ orientation, in two systems with the same layer/substrate mismatch and grown in the same conditions, strongly suggests that strain accommodation via cation migration is more detrimental to the magnetic performance than accommodation via defect creation, both because it is less effective ͑i.e., layers remain in higher strain conditions͒ and because it modifies the optimal hole doping ͑ ϳ1 3 ͒ in extensive regions of ͑001͒ films.
Support from European Union project ESTEEM allowed us to perform STEM measurements in the LPS ͑France͒.Financial support by the MEC of the Spanish Government Projects ͑Grant Nos.NAN2004-9094-C03, MAT2005-5656-C04, and NANOSELECT CSD2007-00041͒ and by the European Union ͓project MaCoMuFi ͑Grant No. FP6-03321͒ and FEDER͔ are acknowledged.The BRD grant program of Universitat de Barcelona is also acknowledged.We are very thankful to Dr. A. Romano-Rodríguez for FIB sample preparation.

FIG. 1 .
FIG.1.͑Color online͒ ͑a͒ Magnetization vs field curves in the −1 -20 kOe range for ͑001͒ LCMO ͑squares͒ and ͑110͒ LCMO ͑triangles͒ 80 nm films measured at 10 K. Magnetic field was applied in-plane of the samples, being parallel to ͓100͔ direction for the ͑001͒ films and to ͓001͔ direction for the ͑110͒ film.Inset: Normalized XRR spectra from ͑001͒ ͑squares͒ and ͑110͒ ͑triangles͒ 85 nm films.͑b͒ g = ͑1-10͒ and ͑c͒ g = ͑011͒ two beam images of ͓110͔ LCMO/STO in PV geometry.

FIG. 2 .
FIG.2.͑Color online͒ Mn, La, and Ca -weighted intensities along the layers, for ͑a͒ ͑001͒ and ͑b͒ ͑110͒ orientations.To get identical range of relative variations in all cation concentrations visible in the plot, data for La and Ca have been multiplied by ͑x3 / 2͒ and ͑x3͒, respectively.