Effects of thickness on the cation segregation in epitaxial „ 001 ... and „ 110 ... La 2 / 3 Ca 1 / 3 MnO 3 thin films

La2/3Ca1/3MnO3 thin films S. Estradé, J. M. Rebled, J. Arbiol, F. Peiró, I. C. Infante, G. Herranz, F. Sánchez, J. Fontcuberta, R. Córdoba, B. G. Mendis, and A. L. Bleloch Departament d’Electrònica, MIND-IN2UB, Universitat de Barcelona, c/Martí i Franquès 1, 08028 Barcelona, Catalonia, Spain Institut de Ciència de Materials de Barcelona, CSIC, 08193 Bellaterra, Catalonia, Spain Instituto Universitario de Investigación en Nanociencia de Aragón (INA), c/Pedro Cerbuna 12, 50009 Zaragoza, Spain SuperSTEM Laboratory, STFC Daresbury, Daresbury WA4 4AD, United Kingdom

Mixed-valence ferromagnetic manganite films 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 observed 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. 4Whether the latter is a pure electronic effect or related to inhomogeneous chemical distributions can be suitably explored by electron energy-loss spectroscopy ͑EELS͒.6][7][8] On the other hand, mappings of the chemical composition can be achieved offering chemical information at the nanometric scale [5][6][7][8][9] We recently reported 8 a comparative characterization, both from the microstructural and chemical points of view, of relatively thick ͑ϳ80 nm͒ LCMO layers grown on STO in order to determine which differences accounted for the fact that ͑110͒ LCMO films display enhanced magnetic properties when compared to their ͑001͒ counterparts.While homogeneous chemical composition was associated to the contribution of plastic defects as strain-relieve mechanism in the more relaxed ͑110͒ films, a Ca 2+ ion migration toward free surface, and the concomitant Mn m+ oxidation state variation was observed in the more stressed ͑001͒ films.
In this letter, we have extended the characterization of the microstructure, local stoichiometry variations, and Mn oxidation state of ͑001͒ and ͑110͒ LCMO films within a range of layer thicknesses between 3.5 and 40 nm.The results confirm the same trend for LCMO films with t ϳ 40 nm as those with t ϳ 80 nm previously reported. 8owever, most importantly, we assess that for ultrathin LCMO films ͑t Ͻ 20 nm͒, a new scenario is revealed with significant differences regarding cation distribution across the film in comparison with thicker layers, while keeping an orthorhombic crystalline phase in contrast with structural changes reported by Qin et al. 10 The ͑001͒ and ͑110͒ LCMO films with thicknesses ͑t͒ ranging from 3.5 to 82 nm 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 for thicker samples ͑ϳ80 and ϳ40 nm͒ and ϳ0.3 nm/ min for the thinner ones.After growth, samples were in situ annealed at 800 °C for 1 h in O 2 atmosphere at 350 torr.Although already reported, 8 some results concerning the layers of t = 80 nm are also included for the sake of completeness.
Transmission electron microscopy ͑TEM͒ samples, were prepared for cross section view by Dual Beam system from FEI ͑focused ion beam ͑FIB͒/scanning electron microscope͔, and for plane-view ͑PV͒ geometry by mechanical thinning down to 25 m and Ar + bombardment at V = 5 kV and 7°u sing a PIPS-Gatan.They were observed in a Jeol J2010F scanning TEM ͑STEM͒ microscope, with a hot field emission gun at 200 keV, coupled with a GIF spectrometer with a 0.8 eV energy resolution.Additional EEL spectra and STEM images were obtained in a Nion UltraSTEM 100 aberration corrected dedicated STEM, with an energy resolution of 0.3 eV.Ca, La, and Mn normalized -weighted intensities were obtained from EELS spectra as I x / x , ͑where x is Ca, La, or Mn͒, normalized to the value of the mean Mn -weighted intensity along the layers.I x are the integrated intensities of background subtracted 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 DIGITALMICROGRAPH software͒.The Mn L 3 EELS edge position and Mn L 3 / Mn L 2 edge intensity ratio were determined using MANGANITAS software package. 7,8attice parameter obtained from x-ray diffraction ͑XRD͒ reciprocal space maps 11 show that for both ͑001͒ and ͑110͒ films, the out-of-plane cell parameters in thinnest films ͑t Ͻ 20 nm͒ are substantially larger than expected ͓Fig.1͑a͔͒ and the unit cell volume is also abnormally large ͓Fig.1͑b͔͒.In Figs.1͑c͒ and 1͑d͒ we show TEM PV bright field twobeam images of the t Ϸ 36 and 14 nm ͑110͒ LCMO films, respectively.Plastic relaxation was evident for ͑110͒ thicker ͑t Ͼ 20 nm͒ films ͓Fig.1͑c͔͒.In contrast, PV images of the thinnest ͑t Ͻ 20 nm͒ ͑001͒ and ͑110͒ films yields no evidence of misfit dislocations ͓Fig.1͑d͔͒.Thus, t Ͻ 20 nm LCMO layers are dislocation free irrespective of their orientation.Moreover, TEM characterization of these thin films shows no crystal symmetry change with respect to thicker films.The insets in Figs.1͑c͒ and 1͑d͒ display on-axis electron diffractions corresponding to the 36 and 14 nm, respectively, ͑110͒ films in PV geometry.
Composition profiles across film thickness were obtained by EELS from FIB lift-off cross section foils.In order to evaluate the suitability of the FIB lamellas for EELS experiments, foil thicknesses ͑t s ͒ were estimated from low loss spectra via log-ratio calculations.All t s were found to fall within the ͑20Ϯ 2͒ -͑32Ϯ 3͒ nm range.General spectra ͑in the range of 325-870 eV͒ were acquired along the direction perpendicular to the substrate/film interface.Concentrations of the different elements at different positions were determined from these spectra.Ca and La and -weighted intensities along the layers are depicted for ͑001͒ and ͑110͒ films of various thicknesses, in Fig. 2. Depending on the thickness, two distinct regimes are evident.For t Ͼ 20 nm, ͑110͒ and ͑001͒ layers present a clearly different behavior.While in ͑110͒ films the Ca, and La concentrations are found to be constant and close to the expected values-0.33and 0.66across the whole film, this is not the case of the ͑001͒ films where a monotonous reduction of the La intensity across film thickness is found, the interface being La enriched and the free surface, La deficient.Accordingly, the Ca intensity increases with distance to interface.In sharp contrast, for t Ͻ 20 nm, irrespective of layer orientation, the free surface gets progressively La rich ͑Ca deficient͒ as thickness is re-duced.The effect is somewhat more pronounced in the ͑001͒ films than in ͑110͒.It is worth noticing that the total ͓͑Ca +La͒ / Mn͔ ratio remains constant, around the expected proportion of ϳ1, for all the considered samples.No overall cation deficiency is thus observed.
EELS spectra in the 400-750 eV energy-loss range, where Mn L 2,3 edges ͑640 and 651 eV, respectively͒ and O K edge ͑532 eV͒ occur, were also recorded.In Fig. 3͑a͒, representative individual EEL spectra are given for illustration purposes.3][14] In Fig. 3͑b͒ we report the Mn L 3 edge position along the layer ͑from interface to free surface͒ for ͑001͒ and ͑110͒ layers of different thicknesses.Dotted lines in Fig. 3͑b͒ indicate the Mn L 3 edge positions reported in literature for Mn 2+ , Mn 3+ , and Mn 4+ ions. 12Two thickness regimes are observed once more.For thicker ͑110͒ films, no significant variation is found to take place along the layer.Yet, the oxidation state of Mn m+ ions gradually changes in ͑001͒ thick films, with a progressive reduction in the Mn m+ species near the interface while free surface is overoxidized.This observation is consistent with the La 3+ enrichment of the free surface described before ͑Fig.2͒ and it is in agreement with early results. 8In contrast, in Fig. 3͑a͒ it can be appreciated that for the thinnest samples the Mn m+ ions get progressively more reduced when approaching the free surface.Local Mn oxidation state as determined by L 3 / L 2 edge intensity ratio ͑not displayed here͒ is coherent with the one obtained through L 3 onset position data for all samples.The observed variation in Mn m+ oxidation state in the thinnest ͑001͒ and ͑110͒ films is fully consistent with the detected La enrichment near the free surface.Yet, for thinner films, Mn seems to be more reduced than it should be expected from the Ca/La proportion data.
In Fig. 4, the O -weighted intensity along the layers is depicted, for ͑001͒ and ͑110͒ films of various thicknesses.For thicker layers, the O intensity remains constant across the film; the ͓O/Mn͔ concentration ratio is found to be close to the expected value ͑ϳ3͒.In contrast, for the thinner films, the oxygen concentration is not constant but a significant decrease is observed close to the substrate/film interface In view of TEM, EELS, and XRD results two different regimes are revealed.Data of samples of t ϳ 40 nm are in very good agreement with those of thicker layers ͑t ϳ 80 nm͒ previously reported, 8 thus supporting the identified stress accommodation mechanisms for ͑001͒ and ͑110͒ LCMO films grown on STO, 8 namely, Ca cation migration toward free surface ͑and concomitant Mn oxidation state variation͒ for ͑001͒ LCMO films and plastic relaxation for ͑110͒ LCMO orientation.The self-adapting chemical variation related to strain-induced elastic energy is the prevalent mechanism for chemical-phase separation only for ͑001͒ films thicker than a critical value which is about ϳ20 nm for LCMO/STO.For thicknesses below this threshold, a different behavior emerges: La migration toward free surface occurs in absence of plastic relaxation irrespective of LCMO orientation.Both this cation distribution and the apparition of O vacancies, mostly near the interface, determine the Mn m+ oxidation state.The presence of O vacancies would account for the abnormally large unit cell volume detected by XRD for the thinnest layers.For this ultrathin regime for which elastic energy considerations should not be the determinant, kinetic factors such as cation incorporation rate, i.e., sticking coefficient and growth parameters, would determine the local chemistry of the layers.

FIG. 2 .
FIG. 2. ͑Color online͒ Ca and La and -weighted intensities as a function of normalized distance to interface for several film thicknesses for ͑a͒ ͑001͒ and ͑b͒ ͑110͒ LCMO films.Dashed lines indicate the outmost 3.5 nm of each film.

Financial
support from the Ministerio de Ciencia e Innovación of the Spanish Government Projects ͑Grant Nos.MAT2008-06761-C03 and NANOSELECT CSD2007-00041͒ and from the European Union ͓Project MaCoMuFi ͑Grant No. FP6-03321͒ and FEDER͔ is acknowledged.We acknowledge the TEM facilities of Science and Technical Services of Universitat de Barcelona ͑SCT-UB͒.We also acknowledge Nanoaracat Aragon-Catalonian cooperation program and ESTEEM.

FIG. 3 .
FIG. 3. ͑Color online͒ ͑a͒ Representative individual EEL spectra obtained at the STO substrate ͑left panel͒ and at the LCMO layer.͑b͒ Mn L 3 peak onset as a function of normalized distance to interface in representative ͑001͒ and ͑110͒ LCMO films.