ORIGINAL RESEARCH published: 20 September 2016 doi: 10.3389/fphy.2016.00041 Formation of Self-Organized Mn3O4 Nanoinclusions in LaMnO3 Films Alberto Pomar 1*, Zorica Konstantinović 2 , Núria Bagués 1, 3 , Jaume Roqueta 3 , Laura López-Mir 1 , Lluis Balcells 1 , Carlos Frontera 1 , Narcis Mestres 1 , Araceli Gutiérrez-Llorente 4 , Maja Šćepanović 2 , Nenad Lazarević 2 , Zoran V. Popović 2 , Felip Sandiumenge 1 , Benjamín Martínez 1 and José Santiso 3 1 Instituto de Ciencia de Materiales de Barcelona-Consejo Superior de Investigaciones Científicas, Bellaterra, Spain, 2 Center for Solid State Physics and New Materials, Institute of Physics Belgrade, University of Belgrade, Belgrade, Serbia, 3 Catalan Institute of Nanoscience and Nanotechnology, Consejo Superior de Investigaciones Científicas and The Barcelona Institute of Science and Technology, Bellaterra, Spain, 4 ESCET, Universidad Rey Juan Carlos, Madrid, Spain We present a single-step route to generate ordered nanocomposite thin films of secondary phase inclusions (Mn3 O4 ) in a pristine perovskite matrix (LaMnO3 ) by taking advantage of the complex phase diagram of manganese oxides. We observed that in samples grown under vacuum growth conditions from a single LaMnO3 stoichiometric target by Pulsed Laser Deposition, the most favorable mechanism to accommodate Mn2+ cations is the spontaneous segregation of self-assembled wedge-like Mn3 O4 Edited by: Marin Alexe, ferrimagnetic inclusions inside a LaMnO3 matrix that still preserves its orthorhombic University of Warwick, UK structure and its antiferromagnetic bulk-like behavior. A detailed analysis on the formation Reviewed by: of the self-assembled nanocomposite films evidences that Mn3 O4 inclusions exhibit an Beatriz Noheda, epitaxial relationship with the surrounding matrix that it may be explained in terms of a University of Groningen, Netherlands Saeedeh Farokhipoor, distorted cubic spinel with slight (∼9◦ ) c-axis tilting. Furthermore, a Ruddlesden-Popper University of Cambridge, UK La2 MnO4 phase, helping to the stoichiometry balance, has been identified close to the *Correspondence: interface with the substrate. We show that ferrimagnetic Mn3 O4 columns influence the Alberto Pomar apomar@icmab.es magnetic and transport properties of the nanocomposite by increasing its coercive field and by creating local areas with enhanced conductivity in the vicinity of the inclusions. Specialty section: This article was submitted to Keywords: self-organization, nanocomposite, LaMnO3 , oxide thin films, strain effects Condensed Matter Physics, a section of the journal Frontiers in Physics INTRODUCTION Received: 15 June 2016 Accepted: 30 August 2016 The large choice in chemical elements in either A- and B-site of the perovskite structure Published: 20 September 2016 allows tailoring many parameters including lattice constants, electronic band structures, magnetic interactions, and more. In particular, manganese based oxides display a huge range of properties Citation: Pomar A, Konstantinović Z, Bagués N, from total spin polarization and colossal magnetoresistance to multiferroicity, with potential use Roqueta J, López-Mir L, Balcells L, from active electrodes in electronic devices to sensors and magnetic memories. Furthermore, Frontera C, Mestres N, nanostructured thin films widen their application range as their final functional properties Gutiérrez-Llorente A, Šćepanović M, are determined by the very active role of interfaces and surfaces which may be efficiently Lazarević N, Popović ZV, modified by strain relaxation mechanisms [1–4]. Specially in the case of undoped LaMnO3 Sandiumenge F, Martínez B and (LMO), the appearance of ferromagnetism in heterostructures of LaMnO3 /SrTiO3 [5] and Santiso J (2016) Formation of Self-Organized Mn3 O4 LaMnO3 /SrMnO3 [6], or the observation of exchange bias in LaMnO3 /LaNiO3 superlattices [7] Nanoinclusions in LaMnO3 Films. evidence the importance of interfacial phenomena. In addition to standard heterointerfaces in Front. Phys. 4:41. layered structures, new strategies have been developed to increase the surface to volume ratio and to doi: 10.3389/fphy.2016.00041 enhance interfacial effects. In this sense, vertically heteroepitaxial nanocomposites (VHN) provide Frontiers in Physics | www.frontiersin.org 1 September 2016 | Volume 4 | Article 41 Pomar et al. Self-Organized LaMnO3 :Mn3 O4 Films a vertical contact area between two immiscible compounds much of growth conditions [27]. In particular, AF behavior is only larger than substrate to films. Additionally, lateral strain may be obtained in films prepared at low oxygen pressures while the tuned to large film thicknesses [8]. right stoichiometry is maintained. In fact, deviations from Spontaneous organization of vertical heteroepitaxial [La]/[Mn] = 1 stoichiometry in LMO thin films may easily nanocomposite thin films has enabled the discovery of new change Mn oxidation state leading to ferromagnetic behavior physical phenomena at the nanoscale and their possible use [28, 29]. The secondary phase in films is the manganese oxide in magnetoelectric devices. For instance, large adjustable low- MnOx which could present mixed Mn oxidation states (Mn2+ , field magnetoresistance is observed in vertically aligned Mn3+ , or even Mn4+ ). In general, manganese may easily alter its La0.7 Ca0.3 MnO3 [9] or La0.7 Sr0.3 MnO3 (LSMO) based valence forming different oxide stoichiometries in response to the nanocomposites [10, 11]. A good example of this is the environmental conditions. Under thermodynamic equilibrium, system LSMO:Mn3 O4 that has been often proposed as potential bulk oxide stoichiometries are directly determined by oxygen nanocomposite due to the chemical compatibility of Mn3 O4 partial pressure and temperature. In the vacuum conditions used with the whole manganite perovskite family [12]. Indeed, if the in the present work either MnO, Mn3 O4, or Mn2 O3 may be composition ratio [La+Sr]/[Mn] < 1, a spinodal decomposition formed depending on the residual oxygen partial pressure in mechanism in chemical solution deposited films may lead to the chamber [17]. At the growth temperature of 850◦ C, Mn2+ O the spontaneous formation of LSMO:Mn3 O4 VHNs [13]. The should be thermodynamically favored if P(O2 ) < 2 × 10−4 mTorr so-generated vertical lattice strain is found to be related to the while Mn3+ 2 O3 is only expected for oxygen pressures close to different domain and grain boundary structures and directly atmospheric conditions (α-Mn2 O3 being the usual polymorph impact their ferromagnetic properties [11]. In the particular case phase while γ-Mn2 O3 has been observed in nanometric form of the parent compound, i.e., undoped LaMnO3 , manganese [30]). At intermediate pressures, the mixed valence manganese segregation is expected to occur when La deficiency leads to oxide Mn3 O4 (or Mn2+ Mn3+ 2 O4 in spinel notation) is expected a non-stoichiometric ratio of La/Mn < 0.9 [14]. There, the to be the dominant phase [31]. The different oxides are quite appearance of randomly distributed segregates of manganese difficult to distinguish and share several characteristic features. oxides such as Mn3 O4 or MnO has been reported in bulk samples Manganese oxides as Mn3 O4 , or γ-Mn2 O3 are ferrimagnetic [15, 16] and thin films [17]. It is important, from a fundamental with transition temperature, Tc , around 40 K while MnO and α- point of view, to understand the formation of manganese oxide Mn2 O3 exhibit antiferromagnetic ordering below 118 and 80 K, segregates, as it is a characteristic secondary phase in several respectively [30, 32, 33]. Regarding their crystal structure, MnO REMnO3 (RE = Rare Earth) thin films and heterostructures, is a cubic rocksalt with a = 4.45 Å [34], α-Mn2 O3 is cubic bixbyite thus modifying their structural quality and functional properties (a = 9.42 Å) while Mn3 O4 and γ-Mn2 O3 exhibit spinel-like [18–20]. Furthermore, manganese oxides are extremely attractive structures. Mn3 O4 is hausmannite mineral with distorted spinel for potential applications due to their electrochemical, optical, structure with manganese ions placed on two non-equivalent or catalytic properties [21–24]. In spite of their abundance in tetrahedral (Mn2+ ) and octahedral (Mn3+ ) sites. The structure nature reproducible properties of manganese oxides require the is well described by tetragonal I41 /amd space group with cell fabrication of uniform structures and a large effort is devoted parameters of a = 5.76 Å and c = 9.47 Å [35, 36]. A very to produce MnOx nanostructures (for recent Reviews see, for similar spinel structure is reported for γ-Mn2 O3 with a = 5.79 example, [23, 24] and references therein). In the present study, Å and c = 9.40 Å [30]. As each of these oxides have cubic we report on the spontaneous formation of regular vertically or tetragonal symmetry they can epitaxially grow on cubic aligned nanocolumns composed of manganese oxide embedded perovskite substrates under suitable conditions. Furthermore, in antiferromagnetic LaMnO3 matrix grown by pulsed laser biaxial strain (either compressive or tensile) may modify not deposition (PLD) on (001)-oriented single crystal perovskite only their equilibrium phase diagram but it also may lead to the substrates of LaAlO3 (LAO) and SrTiO3 (STO). The formation appearance of other crystal structures as, for example, the high mechanisms of the manganese oxide secondary phase and their pressure Mn3 O4 cubic phase [37–39]. This epitaxial stabilization influence on functional properties of manganite thin films are reported on MgO substrates was not observed for Mn3 O4 films discussed in detail. grown on STO or LAO due to the large compressive mismatch of √ √ (aMn3O4 / 2-aSTO )/aSTO = −4% and (aMn3O4 / 2-aLAO )/aLAO = MATERIALS AND METHODS −7% [40]. However, in VHNs where lateral strain imposed by the surrounding oxide matrix may add a supplementary gain to LaMnO3 and Mn3 O4 the free energy balance, the formation of Mn3 O4 cubic phase has The primary phase i.e., the matrix in the studied films is been recently reported [13, 41]. LaMnO3 (LMO) which in its stoichiometric bulk state is an antiferromagnetic insulator with TN ∼140 K [25, 26]. The crystal Experimental Methods structure of the bulk LMO is described by an orthorhombic Thin films of LaMnO3 with self-assembled manganese oxide Pbnm space group with cell parameters a = 5.533 Å, b = nanocolumns have been prepared from stoichiometric LaMnO3 5.727 Å, and c = 7.668 Å at room temperature [26]. Achieving ceramic targets by PLD with a KrF excimer laser. A spontaneous the right LaMnO3 stoichiometry in thin films leading to assembly of manganese oxide nanocolumns has been observed an antiferromagnetic (AF) ground state remains challenging at 850◦ C under vacuum conditions (background pressure of regardless of the growth technique and it requires a fine control <5 × 10−4 mTorr). The laser fluency was kept below <1 J/cm2 Frontiers in Physics | www.frontiersin.org 2 September 2016 | Volume 4 | Article 41 Pomar et al. Self-Organized LaMnO3 :Mn3 O4 Films with 10 Hz pulse repetition rate. The films were grown on top of SrTiO3 (100) and LaAlO3 (100) substrates and their thickness are kept between 30 and 100 nm as determined by X-ray reflectometry. The structural characterization was carried out by X-ray diffraction using a four-angle diffractometer with a Cu- Kα radiation source (X’Pert Pro MRD-Panalytical). All the structural drawings and schemes were produced by using VESTA program [42]. The detailed microstructure and the chemical composition was investigated by Transmission Electron Microscopy (TEM) observations of cross section specimens of LMO/STO on a FEI Tecnai G2 F20 S-TWIN HR(S)TEM operated at 200 kV. Cross section specimens were prepared by conventional cutting, gluing and grinding procedures followed by an Ar-ion milling step down to electron transparency by Precision Ion Polishing System (PIPSTM )—Gatan. High annular dark field (HAADF) images, energy dispersive X-ray spectroscopy (EDS) profiles and electron energy loss spectroscopy (EELS) spectrum—images (SI) were obtained to identify the composition of the nanopillars. EELS-SI of cross-section samples was also carried out in DualEELS mode with a Titan3TM G2 60-300 operating at 300 kV, equipped with a Dual-EELS spectrometer. The principal components analysis (PCA) provided by Lucas et al. [43] for Digital Micrograph was used to remove the random noise. The power law was used before energy peak to remove the background and then data was deconvolved using the Fourier Log method. The La/Mn ratio was obtained from EDS of areas in the matrix tilting the sample out-of-zone axis in order to minimize the electron channeling. High resolution TEM images obtained from selected regions in the micrograph (the matrix, the nanopillar, and the interface between them) were used to identify the matrix orientation and the manganese oxide phase of the inclusions using the Eje-Z software [44]. The surface morphology was studied by Scanning Electron Microscopy (SEM) using a QUANTA FEI 200 FEG-ESEM microscope. Measurements of the local electrical conductivity were performed by Conducting-Atomic Force Microscopy (c- AFM) using a MFP-3D Asylum and ORCA module. A conductive tip with PtIr coating was used as a top electrode and the voltage was applied directly to the sample. A constant nominal force of 3 N/m was used in tapping mode. The topography and current maps were acquired simultaneously. The magnetic properties were measured in a commercial SQUID magnetometer (Quantum Design). The magnetic field was applied parallel to the substrate in all measurements. The substrate diamagnetic contribution is evaluated from the field dependence of the magnetization at room temperature and systematically subtracted from the measurements. Raman scattering measurements were performed using a TriVista 557 Raman system equipped with a nitrogen-cooled CCD detector, in backscattering micro-Raman configuration. The 532 nm line of the Verdi G laser was used as an excitation source. A microscope objective with 100 magnification was used FIGURE 1 | Surface morphology of 40 nm thick LMO nanocomposite for focusing the laser beam. All measurements were carried out films grown on top of STO substrate (A) SEM image (B) AFM image (C). Statistical distribution analysis of size and shape of the Mn3 O4 inclusions at low laser power, in order to minimize local heating of the observed in SEM images. sample. Raman scattering measurements at temperatures lower Frontiers in Physics | www.frontiersin.org 3 September 2016 | Volume 4 | Article 41 Pomar et al. Self-Organized LaMnO3 :Mn3 O4 Films than room temperature were performed in the back-scattering geometry in a cryostat system using 488-nm line of an Ar+ laser, Jobin–Yvon U1000 double monochromator, and photomultiplier as detector. The experimental results presented in this work were obtained on the exact same samples, either on LAO or STO substrates with the only exception of temperature dependent Raman measurements where, to enhance signal to background ratio, a thicker sample grown on LAO (∼100 nm) was used. RESULTS AND DISCUSSION Films grown in vacuum conditions exhibit a clear ordered nanocomposite surface, as evidenced by SEM images either on STO or LAO substrates (see Figures 1A, 2A, respectively). It is observed that the inclusions of the secondary phase (manganese oxide) are uniformly distributed in the LMO matrix (primary phase) with typical rectangular-based dimensions of 40 ×150 nm (on STO substrate) and 40 × 90 nm (on LAO substrate). It is worth reminding here that this spontaneous regular formation of manganese oxide in LMO thin films has been obtained by laser ablation of single target and it is linked to the low oxidation conditions during the PLD process. Films grown at higher oxygen pressures exhibit flat surfaces with no sign of phase segregation [27]. More information about the arrangement of the secondary phase may be obtained from atomic force microscopy images as the shown in Figure 1B (on STO) and Figure 2B (on LAO). While the manganite LMO phase appears to be very flat with low roughness (rms < 0.5 nm), MnOx nanopillars appear as large inclusions exceeding the surface of the matrix by several nm. From this preliminary characterization we may estimate that secondary phase occupies around 10–15% of the total surface, as determined by quantifying black areas (different chemical composition) in the SEM images. An interesting observation is that MnOx inclusions are mainly oriented with their rectangular sides aligned with the main axes of the substrates. Statistical analysis of the inclusions is presented in the histograms of Figures 1C and 2C. We may see that size and aspect ratio is enhanced in the case of films grown on STO substrates. This suggests that the different mismatch with substrate plays a role in the statistical size distribution of the segregates. As mentioned above, epitaxial growth of orthorhombic stoichiometric LMO films has been studied by several groups in the past to elucidate the controversial appearance of ferromagnetic behavior. Either strain or oxygen content has been claimed to be at the origin of this anomalous magnetic behavior [28, 29, 45–49]. In LMO bulk state, oxygen off- stoichiometry (in fact, cation vacancy) is accommodated by modifying the Mn-O octahedral environment mainly resulting in a strong change in the orthorhombic lattice parameters that also influences the cooperative Jahn-Teller effect and, as a result, the magnetic ordering. Quite recently we have shown that a careful epitaxial analysis is required to fully understand the growth of LMO films under different oxygen conditions FIGURE 2 | Surface morphology of 40 nm thick LMO nanocomposite [27]. For this reason, we have performed reciprocal 2θ–φ area films grown on top of LAO substrate (A) SEM image (B) AFM image (C). Statistical distribution analysis of size and shape of the Mn3 O4 inclusions scans at grazing incidence around 220 and 004 orthorhombic observed in SEM images. in-plane reflections (corresponding to 200 reflections in Frontiers in Physics | www.frontiersin.org 4 September 2016 | Volume 4 | Article 41 Pomar et al. Self-Organized LaMnO3 :Mn3 O4 Films FIGURE 3 | (A) In-plane 2θ–φ area scan of (200) reflection of LMO sample grown on LAO (right) substrates. (B) Schematic representation of the (110) epitaxial growth of orthorhombic LMO thin films on cubic perovskite substrates. (C) Phi-integrated intensity of 2θ–φ area scan for LMO/STO (blue) and LMO/LAO (red). (D) Schema of the in-plane accommodation of LMO films onto STO substrates showing the matching along [110]STO direction. pseudocubic notation). Figure 3A shows a typical area scan (a2 +b2 )1/2 /2. We may see that εarea [110]STO = −0.1% and for LMO sample grown on LAO. Similar results were obtained εarea [001]STO = −3.7% for STO and εarea [110]LAO = −5.9% and previously for films grown on STO [27]. We observe the εarea [001]LAO = −9.3% for LAO. In both cases, (110) orientation presence of two peaks occurring at different 2θ angles that are shows clearly a better matching which very likely favors this the clear signature of twinned domains with two in-plane lattice orientation. In this case, when LMO c-axis is contained in the parameters. To understand this result, we need to consider that plane of the substrate, we have two different in-plane Mn-Mn growth temperature (Tg = 1123 K) is well above Jahn-Teller matching distances, d[110]LMO = 3.982 Å and d[001]LMO = transition of LMO (TJT = 750 K) and, although still exhibiting cLMO /2 = 3.834 Å (see schema in Figure 3B) leading to twinned Pbnm crystallographic structure, all orthorhombic LMO domains and the two peaks in Figure 3A. A reliable quantitative √ matching distances √ are nearly equal (aLMO798K / 2 = 3.9468 analysis of the in-plane cell parameters has been performed Å, bLMO798K / 2 = 3.9478 Å, cLMO798K = 3.9448 Å) [50]. by integrating the intensity of the respective area scans along Thus, LMO structure is metrically cubic and a cube-on-cube the azimouthal angle phi and fitting the corresponding peaks epitaxial relationship with substrate is expected. During cooling, (see Figure 3C). We have obtained values of 3.952 and 3.875 cooperative Jahn-Teller transition takes place in LMO and its Å (on STO) and 3.947 and 3.857 Å (on LAO) for d[110]LMO MnO6 octahedral framework is extremely distorted (Mn-O bond and d[001]LMO , respectively. The resulting effective strains are distances being 1.903, 1.957, and 2.184 Å). To accommodate approximately −1% compressive and 1% tensile for STO and this structural distortion to the underlying substrate lattice, film −4 and −2% compressive for LAO substrate. These values of may adopt either (110) or (001) orientation. In such situation, residual strain and in-plane lattice parameters indicate that the it is useful to consider  the area mismatch in the basal plane, LMO thin films grown under vacuum conditions are almost in a 2 i.e., εarea (110) = d[110]f − a2s /a2s for (110)-orientation and relaxed state. In the case of the films grown on STO, such relaxed   state may be understood if we calculate the corresponding εarea (001) = d[110]f cf /2 − a2s /a2S for (001) orientation, where diagonal basal distances of STO, d[110]STO = 5.523 Å and film, f and s indicate film and substrate, respectively, and d[110] = dLMO = 5.534 Å which show a mismatch of 0.3%. This suggests Frontiers in Physics | www.frontiersin.org 5 September 2016 | Volume 4 | Article 41 Pomar et al. Self-Organized LaMnO3 :Mn3 O4 Films that LMO tends to accommodate to the underlying substrate compatible with those of orthorhombic LaMnO3 reported for along STO[110] direction, i.e., in-plane LMO c-axis is slightly stoichiometric single crystals [51]. Temperature dependence of rotated with respect to substrate principal axes (see scheme in Raman response for LMO samples grown on LAO substrates Figure 3D) which is characteristic of the formation of LMO is presented in Figure 5B. The most pronounced features in twined domains being the twin boundary plane aligned with the the Raman spectra of LMO/LAO sample in the range of [110]STO direction. The formation of these twin domains takes 450–660 cm−1 correspond to the Eg Raman mode of LAO place during the cooling down process below the Jahn–Teller substrate and to the modes of Ag and B2g symmetry of transition temperature. From standard θ–2θ x-ray diffraction orthorhombic LMO structure. Note that with lowering of the measurements we have also determined the pseudocubic LMO temperature, B2g Raman mode shows characteristic softening out-of-plane cell parameter, obtaining 3.985 Å for LMO on in the antiferromagnetic phase [52], as shown in the right STO and a larger value of 4.006 Å for LMO on LAO. In both inset of Figure 5B. Besides, a low-intensity peak at about cases, our estimation of pseudocubic unit cell volume leads 640 cm−1 , better discerned in the Raman spectra collected to ∼61 Å3 , close to the expected bulk value for orthorhombic at low temperatures (Figure 5B, left inset), can be attributed stoichiometric LMO [26]. The relative orientations of film and to the stretching vibration of Mn–O bond in the MnO6 substrate were also observed by X-ray diffraction reciprocal unit of manganese oxide, possibly nanocrystalline Mn3 O4 . space maps, as depicted in Figure 4A. The map shows the area Namely, due to phonon confinement effect, with decreasing around the main symmetrical 002 reflection of LaAlO3 substrate. mean crystallite size the A1g Raman mode in nanostructured The position of the main film peak close to the substrate peak Mn3 O4 gradually redshifts compared to its position in bulk is compatible with 220 LMO, which corresponds to c|| -oriented domains. The observation of 221 LMO reflection indicates the formation of orthorhombic Pbnm LMO (which corresponds to half order 2 0 ½ reflection in cubic primitive notation). The fact that half order reflection lies along Qx direction supports the c|| -orientation of the LMO. The two separate reflections at lower Qz values are consistent with 004 reflections from Mn3 O4 spinel with their c-axis tilted 9◦ at both sides of the vertical direction. The corresponding pole figure of this 004 Mn3 O4 reflection is depicted in Figure 4B showing the presence of four reflections at χ ∼9◦ separated by φ = 90◦ . This indicates the textured orientation of Mn3 O4 inclusions within the LMO matrix. Further confirmation that partially relaxed LMO films still preserve orthorhombic symmetry has been obtained by studying the Raman response. Figure 5A shows the Raman spectra of LMO nanostructured films at room temperature. In both spectra the same peaks are observed. Those found at frequencies of 257, 244, and 481 cm−1 are associated to Ag symmetry whereas those at 308 and 611 cm−1 correspond to Raman frequency of B2g symmetry. All of them are fully FIGURE 4 | (A) X-ray diffraction reciprocal space map around the 220 FIGURE 5 | (A) Raman shift at room temperature from corresponding samples LaMnO3 and 002 LaAlO3 main reflections showing the presence of 221 half of LMO samples grown on STO (blue) and LAO (red) substrates. (B) Raman order reflection of c|| -LaMnO3 , and the presence of 9◦ -tilted c∧ Mn3 O4 spectra of LMO/LAO sample at low temperatures. Right inset: Temperature domains. (B) Pole figure of the 004 Mn3 O4 reflection showing the four-fold dependence of B2g wavenumber. Left inset: Enlarged part of Raman symmetry of the Mn3 O4 inclusions. spectrum measured at 20 K. Frontiers in Physics | www.frontiersin.org 6 September 2016 | Volume 4 | Article 41 Pomar et al. Self-Organized LaMnO3 :Mn3 O4 Films crystal (∼656 cm−1 ) [53]. In the case of γ-Mn2 O3 , the main The microstructural analysis on the formation of the MnOx Raman feature is reported to appear at 628 cm−1 and, thus, it secondary phase ordered nanostructures, their correlation with cannot explain our results of Figure 5B [54]. So, the appearance substrate/film interface and the relaxation of strain was studied in of the Raman mode at ∼640 cm−1 could be related to detail by cross-section TEM (Figure 6). The TEM images reveal a the presence of Mn3 O4 nanocrystallites with mean size of wedge-like shape of the nanopillars. Surprisingly, MnOx columns about 30 nm. do not reach the interface with the substrate. However, there FIGURE 6 | TEM images of 40 nm LMO thick film grown on STO (A) TEM image around a MnOx inclusion (B) HAADF image of the film (C) EELS mapping: (c1) HAADF image where orange square indicate the zone of the mapping and yellow square is drift correction reference, (c2) Z-contrast image used as a reference for the maps: (c3) manganese energy windows, (c4) lanthanum energy windows, and (c5) oxygen energy window. (D) Oxygen K edge EELS corresponding to MnOx nanocolumn obtained from EELS-SI in DualEELS mode. Frontiers in Physics | www.frontiersin.org 7 September 2016 | Volume 4 | Article 41 Pomar et al. Self-Organized LaMnO3 :Mn3 O4 Films is a thin LMO matrix layer of around 5 nm (Figure 6). Taking that shape of the inclusions is completely determined during into account the huge lattice mismatch between manganese nucleation and growth at the high temperature stage, regardless oxide and substrate, it looks energetically unfavorable to nucleate of their relative orientation with the twinned LMO domains directly on top of the substrate and, instead, a thin LMO layer formed during cooling. The other relevant feature identified by could act as an efficient buffer layer for drastically decreasing HRTEM images and spectroscopy (EDS and EELS) analysis is the strain. HAADF images (Z- contrast images) presented in the presence of an epitaxial (001)-oriented layer of a La-rich Figure 6C show a different contrast between matrix (bright secondary phase. This layer is not directly at the interface with contrast) and nanopillar (dark contrast) indicating different the substrate but embedded in the LMO matrix (see Figure 7). cation composition. Specifically, the chemical analysis, EDS This secondary phase, of around 2–4 nm thick, has interplanar profiles and EELS maps, reveals that the dark zones in HAADF distances of 3.94 Å in-plane and 13.5 Å out-of-plane and may image correspond to a pure manganese oxide whereas the bright correspond to the rarely observed Ruddlesden-Popper La2 MnO4 zone corresponds to LMO matrix with ratio La/Mn∼1. It is phase [14]. noticeable the absence of La inside the inclusions. The analyses The presence of twin domains in LMO has been also of HRTEM images of these nanopillars indicate that manganese confirmed by cross-section analyses as two different orientations oxide has a spinel crystal structure (compatible with Mn3 O4 ) of LMO were identified, i.e., if zone axis is [001]LMO (noted as with interplanar distances of 5.2 Å in-plane and of 9.2 Å LMOB ) or [110]LMO (noted as LMOA ). As the Mn3 O4 inclusions out-of-plane. Further confirmation that nanoinclusions may be are significantly elongated, especially for LMO grown on STO associated to Mn3 O4 phase was obtained by studying the oxygen (aspect ratio ∼1.7) it is interesting to determine the relative coordination by EELS. Figure 6D shows the oxygen K edge EELS orientation of their long side with respect to the LMO matrix spectrum obtained at the nanoinclusions. By comparing this axis. Surprisingly, we have only observed Mn3 O4 inclusions result with the EELS spectrum seies of the manganese oxides with their long side aligned with [001]LMO (see Figure 8) or reported in literature, the best agreement is found for the Mn3 O4 at the interface between [001]LMO or [110]LMO domains (see phase [55, 56]. Figure 9). We have not identified inclusions with long axis fully With respect to the perovskite substrate, all precipitates embedded in [110]LMO domains. The epitaxial orientation of exhibit a similar crystallographic orientation that may be the precipitates was determined by analyzing electron diffraction understood in terms of a distorted cubic spinel. Epitaxy may FFT images (middle panels in Figures 8, 9) and by taking be defined as (001)-oriented cube-on-diagonal (i.e., in-plane hausmannite I41 /amd crystal structure for Mn3 O4 . Although rotation of 45◦ ) but with a slight c-axis tilting of 9◦ , in not strictly parallel (disorientations angle between planes are agreement with the above x-ray diffraction results. HRTEM ∼1.5–2◦ ) the most accurate description for the corresponding images (Figure 6) also show that interfaces between LMO and epitaxial relationships are the following: Mn3 O4 inclusions are not flat, making difficult to define any contact plane. We should note that zig-zag interfaces between LMOA (−11 − 2)[110]//Mn3 O4 (−202)[010]//LMOB (100)[001] Mn3 O4 films and manganese perovskite films (in that case La0.7 Sr0.3 MnO3 ) have been previously reported [57], suggesting LMOB (010)[001]//Mn3 O4 (01 − 1)[111] a tendency in this system to avoid flat interfaces. Nevertheless, statistical analysis shows a tendency to form angles of 35 and 10◦ The lower panels in Figures 8, 9 are schematized representations with the vertical at both sides of the inclusion. These angles are of both epitaxial relationships where, for clarity, the defining independent of the surrounding LMO orientation. This suggests planes are painted in green. FIGURE 7 | HRTEM images close to the interface between nanocomposite and substrate showing the presence of epitaxial Ruddlesden-Popper La2 MnO4 phase. Frontiers in Physics | www.frontiersin.org 8 September 2016 | Volume 4 | Article 41 Pomar et al. Self-Organized LaMnO3 :Mn3 O4 Films occurrence of a three phase oxide manganese nanocomposite similar to our present case [58–60]. In those works, stoichiometry unbalance caused by the spontaneous outcropping of La-Sr-O islands during the growth of La1–x Srx MnO3 (LSMO) films is compensated by the appearance of a Sr3 Mn2 O7 Ruddlesden- Popper phase. The main difference in our present study is that LMO films are almost relaxed while formation of La-Sr-O islands helped to maintain LSMO fully strained state. Thus, in LMO films, the energy gain associated with reducing the overall elastic energy of the nanocomposite should be less important than the thermodynamic tendency to form Mn2+ ions. It is worth noting that outcropping of secondary phase islands toward the surface of the film is governed by misfit relaxation mechanisms and the resulting topological distribution depends on film thickness and kinetic effects [59]. As a consequence, secondary phases well embedded in the matrix or outcropped toward the surface could just reflect different evolution stages of a similar segregation mechanism. We have mentioned above that phase diagram of manganese oxides indicates a strong tendency of manganese to adopt an oxidation state of Mn2+ in the vacuum conditions (residual oxygen) used to grow the LMO films. The crystal radius of Mn2+ in octahedral coordination is 0.97 Å and, attending to the cation radius and the Goldschmidt tolerance factor for perovskites, it could be feasible for Mn to occupy the La site (La3+ radius being 1.172 Å) in the LMO perovskite up to the solubility ratio [14]. This mechanism has been reported for La-deficient thin films [61, 62] but, in stoichiometric films should lead to the formation of La-based oxides as La2 O3 [14] not present in our films. Furthermore, TEM and Raman measurements suggest that LMO matrix preserves [La]/[Mn] ∼ 1. Thus, in the present case, the main mechanism to accommodate Mn2+ ions is to form Mn3 O4 segregates (i.e., Mn2+ [Mn3+ ]2 O4 ) and the creation of a La-rich phase as the observed La2 MnO4 phase helps to compensate the stoichiometry unbalance. It is very likely that Mn3 O4 particles are formed on LMO films during growth process. At that stage, cube-on-cube nucleation of the Mn3 O4 spinel on the metrically cubic LMO is plausible. The crystallite Mn3 O4 shape may now be FIGURE 8 | HRTEM images of a Mn3 O4 embedded at inside a LMO seen as a reminiscence or truncated form of the typical octahedral domain with [001] zone axis. Middle panels show the corresponding FFT images obtained from HRTEM images. Schematic representation of the crystal shape of spinels where {111} surfaces (forming an angle epitaxial relationship is shown in lower panels. Planes used to define epitaxy of ∼35◦ with the vertical) have the lowest surface energy. It are painted in green. For clarity, coordination of Manganese cations (blue) is is not difficult now to argue that during cooling, when LMO shown in octahedral (purple) and tetrahedral (green) positions. Oxygen anions Jahn-Teller transition takes place, Mn3 O4 accommodates to the in red and Lanthanum cations in blue are also shown. surrounding LMO twins by distorting its own structure. The two epitaxial relationships of hausmannite Mn3 O4 with LMO at room temperature could be just the result of the loss of cubic Chemical phase segregation is a complex problem that symmetry, probably linked to distortions originated itself by the involves not only thermodynamic stability of the different phases Mn3+ ion in the octahedral positions of Mn3 O4 . Thus, to fully but also kinetic considerations. In general, segregation occurs understand this mechanism, it could be very interesting then to when the energy cost associated with the creation of new interface obtain structural information of these nanocomposites at high and contact area is overcome by the gain in free energy by temperature, namely above TJT . nucleating a stable secondary phase. Furthermore, in epitaxial Temperature dependence at 0.5 T (A) and magnetic field thin films, strain associated with the mismatch between lattice dependence at 10 K parameters of film phases and substrate play an important role The temperature and the magnetic field dependence of the and segregates with selected composition may occur to minimize in-plane magnetization were used to study the effects of the the overall elastic energy of the film. Previous investigations secondary phase on the magnetic properties (Figure 10). The have demonstrated that such mechanism may explain the temperature dependence of magnetization (in a magnetic field Frontiers in Physics | www.frontiersin.org 9 September 2016 | Volume 4 | Article 41 Pomar et al. Self-Organized LaMnO3 :Mn3 O4 Films FIGURE 9 | HRTEM images of a Mn3 O4 embedded at the interface between LMO domains with [001] or [110] zone axis. Middle panels show the corresponding FFT images obtained from HRTEM images. Schematic representation of the epitaxial relationship is shown in lower panels. Planes used to define epitaxy are painted in green. For clarity, coordination of manganese cations (blue) is shown in octahedral (purple) and tetrahedral (green) positions. Oxygen anions in red and Lanthanum cations in blue are also shown. of 0.1 T) revealed two transitions, the first one is the expected in vacuum conditions leading to a residual moment below antiferromagnetic transition of LMO at ∼140 K and the second 0.3 µB /f.u. at 10 K which approaches the bulk value reported one is the ferrimagnetic transition of manganese oxide secondary in the literature [63]. This residual magnetization is usually phase at around ∼40 K (Figure 10A). It is known that LMO associated to a canted antiferromagnetic structure (magnetic thin films may exhibit a broad range of magnetic behaviors, moments pointing along b-axis but at a small angle out of ab- from antiferromagnetic to strongly ferromagnetic, depending on plane). The hysteresis loop at 10 K (Figure 10B) exhibits typical the preparation conditions [27]. In our case, films are prepared features observed in loops of thin films composed of soft magnet Frontiers in Physics | www.frontiersin.org 10 September 2016 | Volume 4 | Article 41 Pomar et al. Self-Organized LaMnO3 :Mn3 O4 Films with small Hc and hard magnet with larger Hc [12]. However, The presence of disordered phase boundaries between LMO and pure antiferromagnetic LMO thin films shows almost closed Mn3 O4 and the larger vertical strain could additionally increase hysteresis loop with a very small coercive field Hc that is usually the pinning effects and coercivity in thin films [11]. associated to the existence of uncompensated spins at interfaces. Effects of secondary phase are revealed also by the local On the other hand, manganese oxide with spinel structure is transport measurements shown in Figure 11. Characterization a hard ferrimagnetic with large values of µ0 Hc ∼0.35 T [64], is performed by conductive AFM (cAFM) which senses the whose presence in LMO matrix enlarges loop shape in our current flowing from a metallic probe to Pt electrodes deposited system comparing with pure undoped manganite compound. on top of the sample. The Mn3 O4 secondary phase is highly insulating. However, we observe large spatial differences of the current distribution within the LMO matrix. The overall LMO primary phase in regions between inclusions shows some conduction of the order of few nA for an applied voltage of 0.5 V. Particularly, we observe that conduction is enhanced at the interface region of LMO in close proximity to the inclusions (up to a factor of 10 in these experimental conditions). Moreover, it seems that we observe that not all the four sides of each inclusion are equally conductive. This fact could be supported by TEM cross section images in Figures 6–9, showing that nanostructures exhibit a quite asymmetrical shape and thus, conduction could be differently affected at both sides of the inclusion. The driving force for the enhanced conductivity at these interfaces is far from being understood. Several mechanisms are known to increase electrical conductivity of LMO films as, for example, cationic vacancies or oxygen diffusion. Also, the effect on electrical conductivity of local strain fields is quite relevant in oxide thin films and it cannot be disregarded to explain the results in Figure 11. However, a deeper study of the transport mechanisms is beyond the scope of this manuscript. CONCLUSIONS In conclusion, we report the formation of self-organized Mn3 O4 nanocolumns in Pulsed Laser Deposited LaMnO3 films. We show that under vacuum growth conditions there is a strong tendency to form Mn2+ cations leading to the segregation of FIGURE 10 | Temperature dependence at 0.5 T (A) and magnetic field wedge-like Mn3 O4 inclusions aligned with the main axis of the dependence at 10 K (B) of the in-plane magnetization of a LMO:Mn3 O4 substrate. Formation mechanism at high temperatures (850◦ C) nanocomposite. may be understood by considering that Mn3 O4 nucleate directly FIGURE 11 | Topography (A) and conductive (B) atomic force microscopy measurements at the surface of a LaMnO3 :Mn3 O4 nanocomposite. (B) Image was taken by applying a voltage of 0.5 V. Frontiers in Physics | www.frontiersin.org 11 September 2016 | Volume 4 | Article 41 Pomar et al. Self-Organized LaMnO3 :Mn3 O4 Films on the nearly cubic LMO matrix. The final epitaxial relationship microstructural analysis. LL, LB, AP, ZK, NM, CF, and BM is determined by the subsequent Jahn-Teller transition that contributed to the initial structural characterization and to the takes place during cooling creating LMO domains and twins magnetic and c-AFM measurements. JR, AG, AP, and JS were and it may be defined as a distorted cubic spinel with c- responsible for sample preparation. AP was responsible for axis slightly tilted from the direction perpendicular to the film preparing the final version of the manuscript that includes the plane. Partial accommodation of off-stoichiometry and residual inputs and comments of all the authors. strain may be achieved by the creation of an epitaxial La-rich Ruddlesden-Popper La2 MnO4 phase close to the interface with the substrate. The magnetic and transport properties of the ACKNOWLEDGMENTS nanocomposite are strongly modified by the presence of the Financial support from the Spanish Ministry of Economy inclusions. In particular, an enhanced coercivity together with an and Competitiveness, through the “Severo Ochoa” Program increase of local conductivity close to the insulating inclusions for Centres of Excellence in R&D (SEV-2015-0496 and SEV has been observed. Our results demonstrate an easy single- 2013-0295), Projects MAT2011-29081 and MAT2015-71664-R step route for the preparation of nanocomposite thin films with and Ministry of Education and Science of Serbia (Grant— tuned functional properties consisting of self-assembled ordered III45018) is acknowledged. This work has received funding from nanostructures embedded in a matrix that still preserves its the European Union’s Horizon 2020 research and innovation structural characteristics. programme under the Marie Sklodowska-Curie grant agreement No. 645658 (DAFNEOX Project). AP thanks hospitality from AUTHORS CONTRIBUTIONS Senzor-INFIZ d.o.o. where the first draft of this manuscript was written. 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Orgiani P, Galdi A, Aruta C, Cataudella V, De Filippis G, Perroni CA, Conflict of Interest Statement: The authors declare that the research was et al. Multiple double-exchange mechanism by Mn2+ doping in manganite conducted in the absence of any commercial or financial relationships that could compounds. Phys Rev B (2010) 82:205122. doi: 10.1103/PhysRevB.82.205122 be construed as a potential conflict of interest. 62. Orgiani P, Aruta C, Ciancio R, Galdi A, Maritato L. Synthesis and properties of highly metallic orbital-ordered A-site manganites. J Nanopart Res. (2013) Copyright © 2016 Pomar, Konstantinović, Bagués, Roqueta, López-Mir, 15:1655. doi: 10.1007/s11051-013-1655-9 Balcells, Frontera, Mestres, Gutiérrez-Llorente, Šćepanović, Lazarević, Popović, 63. Topfer J, Goodenough JB. LaMnO3+δ revisited. J Solid State Chem. (1997) Sandiumenge, Martínez and Santiso. This is an open-access article distributed 130:117–28. under the terms of the Creative Commons Attribution License (CC BY). The use, 64. Guo LW, Peng DL, Makino H, Inaba K, Ko HJ, Sumiyama K, et al. distribution or reproduction in other forums is permitted, provided the original Structural and magnetic properties of Mn3 O4 films grown on MgO(0 0 1) author(s) or licensor are credited and that the original publication in this journal substrates by plasma-assisted MBE. J Magn Magn Mater. (2000) 213:321–5. is cited, in accordance with accepted academic practice. No use, distribution or doi: 10.1016/S0304-8853(00)00008-1 reproduction is permitted which does not comply with these terms. Frontiers in Physics | www.frontiersin.org 14 September 2016 | Volume 4 | Article 41