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Experimental Evidence of the Origin of Nanophase Separation in Low Hole-Doped Colossal Magnetoresistant Manganites R. Cortés-Gil, M. Luisa Ruiz-González, Daniel Gonzalez-Merchante, Jose M Alonso, Antonio Hernando, Susana Trasobares, María Vallet Regí, J. Rojo, and Jose M. Gonzalez-Calbet Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b04704 • Publication Date (Web): 18 Dec 2015 Downloaded from http://pubs.acs.org on December 21, 2015

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Experimental Evidence of the Origin of Nanophase Separation in Low Hole-Doped Colossal Magnetoresistant Manganites Raquel Cortés-Gil,1‡ M. Luisa Ruiz-González,1‡ Daniel González-Merchante, 1‡ José M. Alonso,2,3‡ Antonio Hernando,2,4‡ Susana Trasobares,5‡ María Vallet-Regí,6‡ Juan M. Rojo‡7 and José M. González-Calbet1,2‡* 1

Departamento de Química Inorgánica, Facultad de Químicas, Universidad Complutense (UCM), CEI Moncloa 28040-Madrid, Spain 2

Instituto de Magnetismo Aplicado, UCM-CSIC-ADIF, P. O. Box 155, 28230-Las Rozas. Madrid, Spain 3

Instituto de Ciencia de Materiales, CSIC, Sor Juana Inés de la Cruz s/n, 28049-Madrid, Spain

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Departamento de Física de los Materiales, Facultad de Físicas, UCM, CEI Moncloa 28040Madrid, Spain 5

Departamento de Ciencia de los Materiales e Ingeniería Metalúrgica y Química Inorgánica, Facultad de Ciencias, Universidad de Cádiz, Campus Rio San Pedro, 11510-Puerto Real, Cádiz, Spain 6

Departamento de Química Inorgánica y Bioinorgánica, Facultad de Farmacia, UCM, CIBER de Bioingeniería, Biomateriales y Nanomedicina (CIBER-BBN), 28040-Madrid, Spain 7

IMDEA Nanoscience, C/Faraday 9, Ciudad Universitaria de Cantoblanco, 28049- Madrid, Spain ‡These authors contributed equally * [email protected]

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ABSTRACT While being key to understanding their intriguing physical properties, the origin of nanophase separation in manganites and other strongly correlated materials is still unclear. Here, experimental evidence is offered for the origin of the controverted phase separation mechanism in the representative La1-xCaxMnO3 system. For low hole densities, direct evidence of Mn4+ holes localization around Ca2+ ions is experimentally provided by means of aberration-corrected scanning transmission electron microscopy combined with electron energy loss spectroscopy. These localized holes give rise to the segregated nanoclusters, within which double exchange hopping between Mn3+ and Mn4+ remains restricted, accounting for the insulating character of perovskites with low hole density. This localization is explained in terms of a simple model in which Mn4+ holes are bound to substitutional


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divalent Ca2+ ions. KEYWORDS: nanophase segregation, atomic resolution, magnetoresistance, manganites.

TEXT In spite of much effort devoted to their study, the behavior at atomic level of mixed oxidation state manganese perovskites is far from being satisfactorily explained. In particular, the interplay between divalent substitutional cations in La1-x(Sr,Ca)xMnO3,1,2 lattice vacancies and induced holes requires further insight. Often, the interpretation of the intriguing magnetic and transport properties in these colossal magnetoresistant (CMR) perovskites rely only on macroscopic measurements3 and direct imaging of the real space distribution of the different elements is lacking in bulk materials. It is generally believed that CMR in manganites can be understood on the basis of a coexistence and competence between different electrical and magnetic phases,4,5 which are usually ferromagnetic (FM)-metallic (M) clusters embedded in a charge and orbital ordered antiferromagnetic (AFM)-insulator (I) matrix. For low hole (x0.8)9-10 densities, nanometric FM clusters have been recognized. The clustering phenomena are highly dependent on the doping level, i.e., the charge carriers density and have been experimentally probed by means of different techniques.6-14 For x≤0.175, as for example La0.9Ca0.1MnO3, these M double-exchange FM clusters are isolated and embedded in a long-range insulating matrix. Taking into account only the double exchange mechanism, long range FM order and M conduction would be expected across the La1xCaxMnO3

series for 0 < x < 1. However, the real behaviour differs from this ideal description as

an insulating state is observed for x≤0.175. This can be understood by assuming that the


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electronic itinerancy is not spread across the whole sample but restricted to the individual FM-M nanoclusters embedded in the matrix. To explain the origin of that clustering, in earlier publications,7,15-17 we proposed a model based on the assumption that, in the low calcium region of La1-xCaxMnO3, divalent Ca2+ substituting ions, in addition to providing holes to the system, act as effective attractors for these holes. This localization effect allows explaining the coexistence of Mn3+ and Mn4+ around the doping cation, leading to double exchange interactions and, consequently, to locally FM and M nanoclusters. In order to validate our model and clarify the origin of the clusters, a direct observation of holes and the spatial distribution of those clusters is critically needed. In earlier work elsewhere, scanning tunneling microscopy has been used to identify the spatial hole distribution in the surface of a manganite thin film and a process of short-range hole correlation leading to hole clustering disclosed.18 However, it is not clear whether this clustering is connected with the distribution of divalent cations18 as this would have required a compositional analysis at atomic level. In the following, we report direct experimental evidence that holes and divalent cations tend to be closely correlated on the basis of a set of measurements using aberration-corrected scanning transmission electron microscopy (STEM). This technique combined with electron energy loss spectroscopy (EELS) is state-of-art tools able to provide unprecedented levels of information at the nanoscale with atomic resolution.19-23 For low Mn4+ hole densities, we show that manganese holes are not statistically distributed in the sample, their positions lying instead in the close neighborhood of Ca2+. To study the local cation distribution and the Mn oxidation state of La0.9Ca0.1MnO3, once the nominal composition was checked (see SI, Fig. SI-1, Fig. SI-2, Table SI-1 and Table SI-2), a probe aberration corrected electron microscope was used. Z contrast atomic resolution images


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along [100] and [10 1 ] are depicted in figure 1 in which atomic columns of different brightness are clearly observed. Under high angle annular dark field (HAADF) experimental conditions (see SI), the scattered intensity scales with the square of the atomic number, Z2, of the elements in the sample. In this sense, the intensity differences among columns must be related to the different cationic composition, i.e. La (Z=57), Ca (Z=20) and Mn (Z=25). Along [100]c (figure 1a), the basic ABO3 perovskite unit cell (marked in yellow in the image) consists of four atomic columns of brighter contrast defining a 0.39 nm x 0.39 nm square and one less bright in the middle. In this arrangement, according to the atomic number and the normal occupation of A and B sites in perovskite, the brighter dots should be related to La/Ca columns while the less brighter to Mn, as schematically represented in the lower part of the figure (see unit cell marked in yellow both in the image and in the schematic model with La/Ca and Mn as green and red spheres, respectively). Along [10 1 ] (figure 1b), a similar situation is found in a cell of typical 0.39 nm x 0.28 nm dimensions in this projection. At this point, let us emphasize that subtle brightness differences are also distinguished among the A columns. These A columns of different intensity have been also arrowed in the enhanced detailed image of the lower part of figure 1 for both projections. According to Z contrast, brighter A columns (see green arrows in the enlarged image) must correspond to the dominant effect of the heaviest A cation, that means La, while the less bright to an increase of the concentration of the lighter one, Ca (see yellow arrows in the magnified image). The observed differences do not follow any ordered pattern. Since the nominal composition involves 90% La (Z=57) and 10% Ca (Z=20) the brighter dots must be, in average, controlled by the heaviest La cation even when one Ca cation also occupies this A site. Nevertheless, both images clearly reflect differences in the intensity level of the atomic columns corresponding to the A site. This situation can be understood on the


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basis of an inhomogeneous distribution of Ca along the La rich columns. The brightness decrease of some columns, typically observed along both projections, must be related to a higher Ca occupation compared to the brighter columns which must be La richer. Thickness analysis (see SI) indicates similar values in the crystal (near 5 nm), confirming that the contrast differences come from a heterogeneous occupation of A sites. The above description is representative of the bulk material as corroborated by other HAADF images, shown in the SI, corresponding to different crystals of the sample. In addition, to better visualize the subtle contrasts differences in La0.9Ca0.1MnO3, a representative image of an undoped LaMnO3 has been also included in the SI (figures SI-3 and SI-4). To confirm these compositional differences, an atomically resolved EELS study was performed. A representative HAADF image [100] is shown in figure 2a. The HAADF image recorded simultaneously to EELS acquisition is depicted in figure 2b. To enable simultaneous analysis of Ca–L2,3 (346 eV), Mn–L2,3 (640 eV), and La–M4,5 (832eV) signals, an spectrum imaging (27 x 38 spectra) was recorded by using a dispersion of 0.5 eV per channel. The sum spectra obtained over the area marked in figure 2a, 2.21 nm x 1.55 nm, is depicted in figure 2c. The chemical maps were obtained by analyzing of every individual spectra of the Ca–L2,3 (346371 eV), Mn–L2,3 (640-680 eV) and La–M4,5 (832-872 eV) signals. Figure 2d-f shows La, Ca and Mn compositional maps, respectively. Note that grey scale has been used to better visualize contrast differences at the A site. These chemical maps unambiguously indicate that Mn is always located at the B site of the perovskite lattice while suggest, in agreement to HAADF intensity contrast, a heterogeneous La and Ca cationic arrangement at the A sites. It should be noticed that the chemical maps (figure 2d-e) reflect an increase of the Ca signal where HAADF signal is less bright (see arrows in figures 2b, d and e). A more detailed study of the spectrum


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imaging evidences the presence of either rich-La columns (figure 3a) or La/Ca coexisting columns as evidenced by clear appearance of the Ca-L2,3 signal in figure 3b. This allows us to localize at atomic level richer Ca columns in La0.9Ca0.1MnO3. This situation, characteristic for the whole sample, is evidenced in another representative example shown in Figure SI-5. At this point, it should be remembering that the nominal La/Ca ratio is 9/1. If we consider one A perovskite column, in a crystal of around 5 nm thickness, it would comprise, in average, 10 unit cells, that means, 9 La atoms and just one Ca. Under this assumption, the presence of Ca would be almost unnoticeable, as experimentally observed in figure 3a. Nevertheless, if we consider a heterogeneous La/Ca distribution, keeping La/Ca = 9/1, columns with higher and lower Ca content could be present. These different columns have been experimentally detected as observed in figure 3a and b, respectively. In order to carry out a statistical analysis of Ca content distribution we have plotted the corresponding histogram (figure 4a) from the Ca intensities for each column shown in the compositional map in figure 2e. According to the visual inspection of this map, a heterogeneous distribution is clearly observed at the histogram as a consequence of the existence of different events with different intensities. To expand the statistical analysis of Ca we have included additional intensity values recorded in others areas of of the HAADF image shown in figure 2a. Actually, as observed in figure 4b, a random Ca atoms distribution is, once again, observed. According to the hole attractor model, this local distribution of Ca2+ would imply a correlated distribution of Mn4+ holes. In order to get information about the Mn4+/Mn3+ distribution in La0.9Ca0.1MnO3, a spectrum imaging study (27 x 34 spectra) using lower energy dispersion (0.1 eV per channel) was carried out in an area in the same region shown in the HAADF image of figure 2a (see also figure SI-6). This energy dispersion provides more accuracy for detecting the


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shift on the Mn signal, evidencing the presence of Mn3+ and Mn4+ in our case. The HAADF image recorded simultaneously to EELS acquisition is depicted in figure 5a. The sum spectra obtained over all area (1.99 nm x 1.56 nm) is depicted in figure 5b. This experimental spectrum shows the characteristic EELS data for the O-K and Mn-L2,3 signals after alignment and calibration from Dual EELS data. Simultaneous acquisition of the Mn signal and the zero loss peak ensures the correctness position for the different Mn signals. Standards for Mn4+ (CaMnO3) and Mn3+ (LaMnO3) have been included in the inset of figure 5b. The presence of Mn3+ and Mn4+ with a higher ratio of Mn3+, in agreement to the nominal composition, is evident. To analyse the Mn4+/Mn3+ distribution in the sample, a comparative study of the fine structure of the Mn signal in different columns has been carried out. The spectra extracted for Mn columns in different areas, using a window corresponding to 0.06 nm per pixel, are shown in figure 5c. An energy shift on the Mn signal depending on the position of the HAADF online image is observed. Areas in which the Mn signal is shifted towards Mn3+ or Mn4+ have been detected (marked at figure 5c). By locally comparing the Mn4+ distribution with the La/Ca composition according to the previous HAADF-EELS study, one obtains the crucial result that Mn3+ is localized in a rich-La environment while a higher Mn4+ concentration is placed in a richer Ca one. These results are reproducible as it is shown in the SI (figures SI-6 to SI-8 and Table SI-3). Even more, as we have done for the Ca distribution, a more statistical study of the Mn oxidation state distribution has been performed by estimating the Mn oxidation state at each Mn column by analyzing the O-K fine structure (figures SI-9 to SI-12). At room temperature, chemical disorder induced by inhomogeneous La/Ca distribution is present in bulk insulator-paramagnetic La0.9Ca0.1MnO3. Below Tc, this intrinsic chemical disorder induces magnetic and electric phase separation through the hole attractor model. In this


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model, Ca confines the itinerancy of the Mn holes and delays the setup of metallic conductivity till the onset of percolation among the Ca containing cells. As the threshold of percolation among Ca clusters requires an x value around 1/8, this explains the observed nonmetallic behavior in our perovskite for x= 0.10 as this Ca concentration is below the percolation threshold.16 These experimental results are also supported by a simple analysis of the binding energy between the hole and the Ca atom. Considering the basic perovskite cell, a hole hopping process leading to the double exchange mechanism is expected to take place between the eight Mn placed at the cube corners. However, hopping between any corners of the first neighborhood to any other corner of the second nearest neighborhood would require an extra electrostatic energy. When the hole jumps farther from the Ca site, the eight Mn at the first neighborhood become Mn3+ and the total electrostatic charge of the cube becomes negative. The hole is then attracted back to any of its eight initial positions. A simple argument customarily used for estimating the binding energy of electrons and holes in doped semiconductors can help to estimate the order of magnitude of the energy required to delocalize the hole or to carry it from the ground localized state to the conduction band. The maximum binding energy, EB, between the hole and the Ca atom is roughly given by the hydrogenic model, i.e., EB = (m*/mεr2)R∞ , where m is the electron mass, m* the electron effective mass, εr the electric constant and R∞ the Rydberg constant (13.6 eV). If one considers that m*/m is approximately 3 for the Mn compounds and εr for LaMnO3 is 18,24 one obtains EB = 0.12 eV. This value is consistent with the binding energy for a selftrapped magnetic polaron obtained by Meskine et al.25 The radius corresponding to the ground state, given by r = (mεr/m*) a0 (ao being the Bohr radius, 0.53 Å), becomes 3.18 Å. This value of EB ensures that, at room temperature, only a very small fraction of holes can be excited at the


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conduction band. Note also that the radius r is smaller than the distance of the Ca ion to the first sphere of Mn atoms. The restriction of the double exchange mechanism to the first neighborhood of Ca atoms leads to a localization of the ferromagnetic order, i.e. magnetic polaron, in the same nanoregion. This effect can be discussed on the context of a more generalized argument brought to light in dilute magnetic semiconductors oxides.26 In these oxides, the exceptional large ordered moments can be explained in terms of a magnetic polaron that is created as a consequence of the interaction between the magnetic cation and an electron trapped in a vacancy. Coey et al 26 propose that the electron associated with the defect is confined in a hydrogenic orbital in much the same way as we have considered for the itinerant electron in the eg orbital of manganese. The existence of a binding energy EB >> kTc (Tc, the Curie temperature)27 hinders the hole exchange with the Mn ions farther than the first cube. We conclude that the attraction between the divalent ion and the hole provides a simple mechanism that can be crucial to the understanding of the nanophase separation and nucleation of clusters and, consequently, of the electrical and magnetic behavior of these perovskites. This attractor effect has never been taken into account in previously reported theoretical approaches. ACKNOWLEDGMENT R.C-G. acknowledges a postdoctoral fellowship from Moncloa Campus of International Excellence (UCM). We thank the National Center for Electron Microscopy (UCM) for facilities.


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ASSOCIATED CONTENT Supporting Information. Chemical synthesis and microscopy characterization techniques are described. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * José M. González-Calbet e-mail address: [email protected] The authors declare no competing financial interest Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. Funding Sources This work was supported by the Spanish Ministry of Innovation, Science and Technology and Spanish Ministry of Economy and Competitiveness through Research Projects CSD200900013, TSI-020100-2011-280, MAT2011-23068, and MAT2012-37109-C02-01.

ABBREVIATIONS CMR, colossal magnetoresistance; FM, ferromagnetic; M, metallic; AFM, antiferromagnetic; I, insulator; STEM, scanning transmission electron microscopy; EELS, electron energy loss spectroscopy; HAADF, high angle annular dark field.


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References (1) Schiffer, P.; Ramirez, A. P.; Bao, W.; Cheong, S.-W. Phys. Rev. Lett. 1995, 75, 33363339. (2) Urushibara, A.; Moritomo Y.; Arima, T.; Asamitsu, A.; Kido, G.; Tokura, Y. Phys. Rev. B 1995, 51, 14103-14109. (3) Tokura, Y. Rep. Prog. Phys. 2006, 69, 797. (4) Millis, A. J. Theory of CMR manganites in colossal magnetoresistance oxides: advances in condensed matter science, Edited by Tokura, Y. Gordon & Breach Science Publishers, Singapore 2000. (5) Dagotto, E. Science 2005, 309, 257-262. (6) Hennion, M.; Moussa, F.; Biotteau, G.; Rodríguez-Carvajal, J.; Pinsard, L.; Revcolevschi, A. Phys. Rev. Lett. 1998, 81, 1957-1960. (7) Alonso, J.; Herrero E.; González-Calbet; J. M.; Vallet-Regí, M.; Martinez, J. L;. Rojo, J. M.; Hernando, A. Phys. Rev. B 2000, 62, 11328-11331. (8) Algarabel, P. A.; De Teresa, J. M.; Blasco, J.; Ibarra, M. R.; Kapusta, Cz.; Sikora, M.; Zajac, D.; Riedi, P. C.; Ritter, C. Phys. Rev. B 2003, 67, 134402 1-6.


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(9) Granado, E.; Ling, C. D.; Neumeier, J. J.; Lynn, J. W.; Argyriou, D. N. Phys. Rev. B 2003, 68, 134440 1-6. (10) Neumeier, J. J.; Cohn, J. L. Phys. Rev. B 2000, 61, 14319-14322. (11) Fath, M.; Freisem, S.; Menovsky, A. A.; Tomioka, Y.; Aarts, J.; Mydosh, J. A. Science 1999, 285, 1540-1542. (12) Uehara, M.; Mori, S.; Chen, C. H.; Cheong, S.-W. Nature 1999, 399, 560-563. (13) Loudon, J. C.; Mathur, D. N.; Midgley, P. A. Nature 2002, 402, 797-800. (14) Murakami, Y.; Yoo, J. H.; Shindo, D.; Atou, T.; Kikuchi, M. Nature 2003, 423, 965968. (15) Alonso, J. M.; Arroyo, A.; González-Calbet, J. M.; Vallet-Regí, M.; Martínez, J. L.; Rojo, J. M.; Hernando, A. Phys. Rev. B 2001, 64, 172410 1-4. (16) Alonso, J. M.; Arroyo, A.; González-Calbet, J. M.; Hernando, A.; Rojo, J. M.; ValletRegí, M. Chem. Mater. 2003, 15, 2864-2866. (17) Cortés-Gil, R.; Alonso, J.M.; Rojo, J. M.; Hernando, A.; Vallet-Regí, M.; RuizGonzález, M. L.; González-Calbet, J. M. Prog. Solid State Chem. 2010, 38, 38-45. (18) Ma, J. X.; Gillaspie, D. T.; Plummer, E. W.; Shen, J. Phys. Rev. Lett. 2005, 95, 237210 1 -4. (19) Varela, M.; Oxley, M. P.; Luo, W.; Tao, J.; Watanabe, M.; Lupini, A. R.; Pantelides, S. T.; Pennycook, S. J. Phys. Rev. B 2009, 79, 085117 1-13.


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(20) Rossell, M. D.; Ramasse, Q. M.; Findlay, S. D.; Rechberger, F.; Erni, R.; Niederberger, M. ACS Nano 2012, 6(8), 7077-7083. (21) Li, X.; Ma, X.; Su, D.; Liu, L.; Chisnell, R.; Ong, S. P.; Chen, H.; Toumar, A.; Idrobo, J. C.; Lei, Y.; Bai, J.; Wang, F.; Lynn, J. W.; Lee, Y. S.; Ceder, G. Natur. Mater. 2014, 13, 586592. (22) Farokhipoor, S.; Magén, C.; Venkatesan, S.; Íñiguez, J.; Daumont, C. J. M.; Rubi, D.; Snoeck, E.; Mostovoy, M.; de Graaf, C.; Müller, A.; Döblinger, M.; Scheu, C.; Noheda, B. Nature 2014, 515, 382-386. (23) Ruiz-González, L.; González-Merchante, D.; Cortés-Gil, R.; Alonso, J. M.; Martínez, J. L.; Hernando, A.; González-Calbet. J. M. Chem. Mater. 2015, 27, 1397-1404. (24) Cohn, J. L.; Peterca, M.; Neumeier, J. J. Phys. Rev. B 2004, 70, 214433 1-6. (25) Meskine, H.; Saha-Dasgupta, T.; Satpathy, S. Phys. Rev. Lett. 2004, 92, 056401 1-4. (26) Coey, J. M. D.; Venkatesan, M.; Fitzgerald, C. B. Nature 2005, 4, 173-179. (27) Zener, C. Phys. Rev. 1951, 82(3), 403-405.


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FIGURES

Figure 1. HAADF images along (a) [100] and (b) [10 1 ] corresponding to La0.9Ca0.1MnO3. The basic perovskite (ABO3) unit cell has been marked in yellow in both images. At the bottom, schematic models for the cationic position and enhanced details of the HAADF images have been inserted. Among A columns, atomic columns of different brightness, better perceived from a distance, reflecting different La and Ca occupations are arrowed in green and yellow, respectively.


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Figure 2. (a) HAADF image along [100] corresponding to La0.9Ca0.1MnO3. (b) HAADF image simultaneously recorded to EELS acquisition (0.5 eV per channel). (c) EELS spectra corresponding to the sum of all the spectra acquired in area marked in (a) with a green square, showing the Ca-L2,3, O-K, Mn-L2,3, and La-M4,5 signals. (d) La chemical map, (e) Ca chemical map and (f) Mn chemical map.


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Figure 3. HAADF image along [100] simultaneously recorded to EELS acquisition (0.5 eV per channel) and resultant EELS spectra corresponding to two different atomic columns evidencing the presence of different cation occupation at the A site: (a) La and (b) La and Ca. The shown spectra were obtained using a window corresponding to the sum of 4 pixels.


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Figure 4. (a) Probability distribution of the Ca cations corresponding to figure 2e; (b) Probability distribution of the Ca cations considering different areas along the crystal in fig 2a.


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Figure 5. (a) HAADF image along [100] simultaneously recorded to EELS acquisition (0.1 eV per channel) and (b) EELS spectra corresponding to the sum of all the spectra acquired over the whole area; the inset shows an enhanced representation of the Mn signal compared to references for Mn4+ and Mn3+. (c) Individual EELS spectra corresponding to two different atomic Mn columns showing the energy shift of the Mn signal depending on the surrounding La/Ca cation ratios (Mn3+ and Mn4+ reference spectra have been included). The signal is shifted towards lower energy (Mn3+) when the environment is brighter, i.e. La richer, while for less brighter columns, i.e. Ca richer, towards higher energy (Mn4+).


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Experimental Evidence of the Origin of Nanophase ... - ACS Publications

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