Multiple double-exchange mechanism by Mn2+ doping in manganite ...

PHYSICAL REVIEW B 82, 205122 共2010兲

Multiple double-exchange mechanism by Mn2+ doping in manganite compounds P. Orgiani,1,2,* A. Galdi,1,3 C. Aruta,4 V. Cataudella,4 G. De Filippis,4 C. A. Perroni,4 V. Marigliano Ramaglia,4 R. Ciancio,5 N. B. Brookes,6 M. Moretti Sala,7 G. Ghiringhelli,7 and L. Maritato1,2 1CNR-SPIN,

I-84084 Fisciano (SA), Italy of Mathematics and Informatics, University of Salerno, I-84084 Fisciano, (SA), Italy 3Department of Physics, University of Salerno, I-84084 Fisciano, (SA), Italy 4 CNR-SPIN and Department of Physics, University of Napoli, I-80126 Napoli, Italy 5CNR-IOM, TASC Laboratory, I-34149 Trieste, Italy 6 European Synchrotron Radiation Facility, F-38043 Grenoble, France 7CNR-SPIN and Department of Physics, Politecnico di Milano, I-20133 Milano, Italy 共Received 18 October 2010; published 23 November 2010兲

2Department

Double-exchange 共DE兲 mechanisms in RE1−xAExMnO3 manganites 共where RE is a trivalent rare-earth ion and AE is a divalent alkali-earth ion兲 relies on the strong exchange interaction between two Mn3+ and Mn4+ ions through interfiling oxygen 2p states. Nevertheless, the role of RE and AE ions has ever been considered “silent” with respect to the DE conducting mechanisms. Here we show that a new path for DE mechanism is indeed possible by partially replacing the RE-AE elements by Mn2+ ions, in La-deficient LaxMnO3−␦ thin films. X-ray absorption spectroscopy demonstrated the relevant presence of Mn2+ ions, which is unambiguously proved to be substituted at La site by resonant inelastic x-ray scattering. Mn2+ is proved to be directly correlated with the enhanced magnetotransport properties because of an additional hopping mechanism trough interfiling Mn2+ ions. Such a scenario has been theoretically confirmed by calculations within the effective single-band model. The use of Mn2+ both as a doping element and an ion electronically involved in the conduction mechanism reveals an unconventional phenomenon in transport properties of manganites. More important, such a strategy might be also pursed in other strongly correlated materials. DOI: 10.1103/PhysRevB.82.205122

PACS number共s兲: 75.47.Lx, 71.55.⫺i, 73.61.⫺r, 81.15.Hi

I. INTRODUCTION

Since its discovery, colossal magnetoresistance 共CMR兲 effect has undoubtedly been among the most studied phenomena in solid-state physics.1–4 CMR phenomenon has been explained within the framework of double-exchange 共DE兲 mechanism, based on a strong exchange interaction between Mn3+ and Mn4+ ions through intervening filled oxygen 2p states.5,6 The metallicity in these systems, as its strong dependence from an external magnetic field, comes from the mixed valence of Mn ions, which can transfer both charge and spin between their Mn3+ and Mn4+ states. Due to Hund’s rule, this transfer occurs only if the core spin of Mn3+ is aligned with that of Mn4+, thus explaining the colossal effects on the resistance values by the application of a spinaligning external magnetic field. CMR effect has been intensively investigated in RE1−xAExMnO3 共where RE is a trivalent rare-earth ion and AE is a divalent alkali-earth ion兲 manganites. The mandatory Mn3+ / Mn4+ mixed population is generally controlled by the chemical substitution of the trivalent RE3+ ion with a divalent AE2+ ion. Indeed, to satisfy the overall charge neutrality within the manganite unit cell, when RE3+ ions are substituted by AE2+, some of the Mn atoms are forced into a 4+ state.1–4,6 Nevertheless, with respect to the DE hopping mechanism, the role of AE-RE elements has always been considered as “silent,” their corresponding conduction band being too far from Fermi’s energy level. In order to make such an atomic site active within the transport mechanism, a possible strategy calls for a RE3+ substitution by using multiple-valence Mn ions themselves. Here we show that a partial substitution of Mn ions at the La site is indeed possible in La-deficient LaxMnO3−␦ 共LMO兲 1098-0121/2010/82共20兲/205122共6兲

manganite thin films. By combining polarization-dependent x-ray absorption spectroscopy 共XAS兲 and resonant inelastic x-ray spectroscopy 共RIXS兲, the relevant Mn2+ content is demonstrated, and it is unambiguously assigned its crystallographic site 共namely, the La site兲. Similarly to AE2+-doped manganites, the La3+ / Mn2+ substitution induces the required Mn3+ / Mn4+ mixed population. However, differently from the AE2+ doping, the Mn2+ ions at La site are electronically involved in the transport mechanisms, having their electronic bands crossing the Fermi energy. Such an energetic configuration favors the hopping of electrical charge through that site 共usually silent兲, in addition to the traditional Mn3+ / Mn4+ hopping path, thus contributing to the ferromagnetic and metallic state. Such a multiple-DE phenomenon has never been reported and it opens unexplored perspectives in both fundamental studies on transport mechanism in strongly correlated manganites as in possible application by using these new class of materials. II. THIN FILM GROWTH AND STRUCTURAL PROPERTIES

LMO thin films, with different values of the La/Mn ratio, were fabricated by molecular-beam epitaxy on SrTiO3 共STO兲 substrates. Oxygen content was systematically varied by postannealing all the LMO samples, by varying the duration and the temperature of the process 共thus providing LMO films at different stages of oxygenation, i.e., oxygen content兲.7 X-ray diffraction 共XRD兲 analyses show that all the investigated LMO samples are in-plane matched with STO substrates. However, the out-of-plane lattice parameter

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2Θ (degree) FIG. 1. 共Color online兲 ␪-2␪ scan around the 共002兲 Bragg reflection of four LMO films with La/Mn ratio 0.66 共black兲, 0.88 共red兲, 0.98 共green兲, and 1.07 共blue兲, respectively 共asterisks indicate the STO peaks兲. In the inset, a cartoon showing the expected reduction in the unit cell by the Mn substitution 共blue spheres兲 at the La site 共orange sphere兲 is also sketched.

monotonically depends on the La/Mn stoichiometric ratio. For each annealing batch 共i.e., oxygen content兲, such a systematic trend in c-lattice parameter has always been observed within the LMO samples with different La/Mn stoichiometric ratio, which appears to be the major factor in c-lattice variation within each set of samples. XRD spectra for optimized LMO series 共i.e., highest metallicity兲 are reported in Fig. 1. All the spectra only shows the 关00l兴 peaks, indicating the preferential orientation of the film along the 关001兴 substrate crystallographic direction, and no secondary phase is detected. The out-of-plane lattice parameters vary from 3.859 共measured in the sample with La/ Mn= 0.66兲 to 3.878 Å 共sample with La/ Mn= 1.07兲. Considering that Mn ionic radius 共which varies from 0.53 to 0.89 Å, depending on its electronic configuration兲 is sizably smaller than those measured for the La ions 共ranging from 1.17 to 1.5 Å兲, the monotonic decrease in the out-of-plane lattice parameter values is compatible with the gradual substitution of Mn atoms at the La site, which was confirmed possible by neutrondiffraction investigations of bulk samples.8 It is worthful to remark that, in polycrystalline LMO bulk samples, it was shown a La-vacancies limiting value of 0.125 共i.e., La0.875MnO3兲, while further excess of Mn in the structure favors the formation of spurious phases, mainly Mn3O4.9 Such a scenario is sustained by XRD investigation of those samples, showing no variation in the LMO XRD peaks positions 共i.e., saturation in structural and/or chemical composition of La-deficient LMO兲 and, more important, the appearing of Mn3O4 diffraction peaks. On the contrary, heavily Ladeficient LMO thin films were proved to be structurally stable when grown on suitable substrates.7,10 In such a specific form, XRD investigation shows a monotonic and continuous change in structural lattice parameters, and no sign of diffraction peaks assessable to any spurious phase, mainly Mn3O4. It is clear that the substrate plays a crucial role in

FIG. 2. 共Color online兲 Temperature dependence of the resistivity 共normalized to the residual resistivity兲 for a set of four LMO samples with La/Mn ratio 0.66 共black兲, 0.88 共red兲, 0.98 共green兲, and 1.07 共blue兲, respectively 共color code is the same as in Fig. 1兲. TMI are 387K, 360K and 260K for LMO samples with x⫽0.88, 0.98 and 1.07, respectively 共T MI for x = 0.66 sample is not indicated, being ⌬␳ / ⌬T ⬍ 0 at any temperature兲. In the inset, the metal-insulator transition temperature T MI for different annealing process 共keeping fixed the duration at 54 h and by changing the temperature Ta兲 is also reported. Data refer to three set of LMO samples, with La/Mn stoichiometric ratio of 0.66 共black squares兲, 0.88 共red circles兲, and 0.98 共green triangles兲, respectively. In the graph, the blue line corresponds to a value of 430 K.

stabilizing the LMO structure, which otherwise would be no longer stable for large La deficiency. III. TRANSPORT PROPERTIES

Transport properties were also investigated as a function of the La/Mn stoichiometry. Temperature dependence of the resistance for a series of four LMO films with different values of the La/Mn ratio is reported in Fig. 2 共data refer to the same LMO samples which XRD structural characterization is reported in Fig. 1兲. As the La deficiency increases, more and more Mn ions are expected to be pushed into a 4+ state. According to the general phase diagram of manganites, TMI is expected to be maximum for a 33% doping level of Mn4+ and subsequently to drop to zero for higher Mn4+ concentration.3 In LaxMnO3 where the Mn partial substitution at La site does not occur, such an optimal doping occurs for a 10% deficiency of La. A further decrease in the La content should force more and more Mn ions into a 4+ state, finally reaching a full Mn4+ population for a La/Mn ratio of 0.66. Mn4+-doping phase diagram is confirmed only for a small La deficiency, in which a monotonic increase in the T MI is indeed observed. However, the highest metallicity 共⌬␳ / ⌬T ⬍ 0 at any temperature兲 is measured in La0.66MnO3−␦, which is otherwise expected to be insulating and antiferromagnetic. Similarly, a monotonic increase in the Curie temperature 共TC兲 has been measured within the LMO series, reaching the highest value of 350 K for the La0.66MnO3−␦ sample. It is evident that the

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IV. X-RAY ABSORPTION SPECTROSCOPY MEASUREMENTS

To probe the electronic structure close to the Fermi level, which is dominated by the Mn 3d and O 2p states, XAS at

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metallic and ferromagnetic state in heavily La-deficient manganite films cannot be explained by the self-doping mechanism. We also investigated the possibility that the observed phenomena could be due to a different oxygen diffusion into the various samples. Such a diffusion can be affected by many extrinsic factors such as chemical composition, strain, oxygen content, film thickness, and so on. In order to rule out possible effects related to the only oxygen content, we systematically investigated the structural and the transport properties of our samples, by changing the time and the temperature of a postannealing process, therefore probing different oxygen content in the LMO films.11 In the inset of Fig. 2, the metal-insulator transition temperature TMI for different postannealing process 共keeping fixed the duration at 54 h and by only changing the temperature Ta兲 is reported. The temperature and the duration of the annealing process directly affected the oxygen content in LMO films, and, as a matter of fact, the T MI and the resistivity values. As the annealing temperature increases, the metallicity of the LMO samples also increases, thus confirming the incorporation of more and more oxygen within the structure. In this respect, by increasing the annealing temperature 共i.e., oxygen content兲 we are exploring the manganite T MI vs doping phase diagram, which foresees both an underdoped and an overdoped regime.2,3 Nevertheless, if we look, for instance, at the La/ Mn= 0.88 series, T MI show a maximum for an annealing temperature of 1000 ° C and subsequently a reduction in that value for an annealing temperature of 1030 ° C. Therefore, it should be concluded that the La0.88MnO3−␦ sample annealed at 1000 ° C is the optimally doped samples, showing the optimal Mn3+ / Mn4+ mixed population. However the highest T MI among the La0.66MnO3−␦ samples is well above to such a value 共in the inset of Fig. 2, we have indicated a value of T MI for this samples as high as 430 K; such a value corresponds to the destroying of any possible polaronic insulating state by the thermal energy12兲, while it should have been equal to that measured in La/ Mn= 0.88 series, even if obtained with a different annealing process 共i.e., oxygen content兲. From these data it is clear that the physical phenomena at play in heavily La-deficient LMO samples is not the one at play in conventional manganites. Finally, it is well known that chemical and structural inhomogeneities 共such as spurious phases, oxygen stoichiometry, grain size, and others兲 can play an important role in transport properties of manganite thin films.13–16 In this respect, as for general inhomogeneities, strong structural and chemical disorder, spurious insulating phase, impurities cooperate to worsen the transport properties of manganite thin films 共for instance, because of low-resistance percolation paths, T MI usually decreases and the resistivity increases兲. However, this is clearly not what we observed, thus strongly supporting the hypothesis that an unconventional mechanism is at play in heavily La-deficient thin films.


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FIG. 3. 共Color online兲 Room-temperature XAS 共panel a, in V polarization and panel b in H polarization兲 and LD at Mn L2,3 edge of LMO films with different La/Mn ratio. Data refer to LMO film with La/Mn ratio 0.66 共black兲, 0.88 共red兲, 0.98 共green兲, and 1.07 共blue兲, respectively. XAS spectra for a MnO single crystal are also reported 共cyan兲. All the spectra in V and H polarization are shifted vertically for clarity 共data refer to x⫽0.66, 0.88, 0.98, 1.07 and pure MnO crystal, from the bottom to the top兲.

the Mn L2,3 and O K edges were performed at the ID08 beamline of ESRF in Grenoble. The anisotropy effects related to the different doping levels were investigated by varying the polarization of the grazing incidence synchrotron radiation from horizontal 共H polarization兲 to vertical 共V polarization兲 which correspond roughly to investigate the outof-plane and the in-plane directions, respectively. All Mn L spectra reported in Fig. 3 show two broad multiplet structures, L3 and L2, separated by spin-orbit splitting. The effect of doping can be seen more clearly in the L3 region where the spectra are similar to Mn3+ / Mn4+ manganites when La/ Mnⲏ 1 while the increasing of Mn2+ content in LMO films with La/ Mn⬍ 1 gives rise to typical pronounced features.17 The linear dichroism 共LD兲 obtained from the difference between the two XAS 共V-H兲 is also particularly interesting when the La/ Mn⬍ 1. Indeed, the spectra are similar to the theoretical calculations for the single 3d4 configuration 共Mn3+兲 in case of a tetragonal distortion of the octahedra, with an elongation of the out-of-plane Mn-Mn distance.18–21 Unlike Mn3+, it may be noted that Mn4+ 共3d3兲 and Mn2+ 共3d5兲 do not tend to distort their own octahedral environment by Jahn-Teller effect. Therefore, no contribution is expected to the orbital component of LD from Mn4+ and Mn2+. The enhancement of LD intensity can be a consequence of the elongation of the Mn3+ octahedra to compensate the reduced O-Mn2+-O distances, thus being an indirect demonstration of the increased Mn2+ content. It is worthwhile to remark that in LaMnO3 single-crystals Mn2+-Mn3+ coexistence was already observed, pointing to an intrinsic mixed valence state in insulating manganite, rather than Mn2+ signal generated by spurious MnO surface layers.22 However, such Mn2+-Mn3+ coexistence is expected to decrease by increasing the metallicity of the system. In our investigation, some films do not show any Mn2+ features 共ruling out the possibility of a MnO

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FIG. 4. 共Color online兲共Upper panel兲 Calculated RIXS spectra within a single-ion crystal-field model and tuned 10Dq and the rescaling of the interatomic Slater integrals; in the inset, the oxygen coordination in undistorted perovskite around 共a兲 the A site and 共b兲 the B site is also reported. 共Lower panel兲 Experimental RIXS spectra for an insulating MnO single crystal 共black兲, a Mn2+-doped LMO metallic film 共red, labeled LMO-M兲 and an insulating LMO 共blue, labeled LMO-I兲 thin films are reported.

surface layer due to postannealing process and/or surface reconstruction兲 and these last are observed in the most metallic samples, while they are absent in the insulating ones. Finally, XAS spectra at the Mn L edge undoubtedly proves the strict correlation between the La deficiency and the presence of Mn2+ in heavily La-deficient LMO samples. V. RESONANT INELASTIC X-RAY SPECTROSCOPY MEASUREMENTS

In order to fully comprehend the role of the Mn2+ in the transport mechanisms at play in La-deficient LMO films, it is mandatory to unambiguously determine its structural configuration. Manganites are mostly arranged in a perovskite structure, having the general formula ABO3, which can be seen as a stacking sequence of AO and BO2 planes. Mn atoms are usually placed at the B site while the RE3+ / AE2+ atoms lie within the AO planes. In this respect, it is crucial to assign to the Mn2+ ions either the perovskite A site or the B site. In order to do that, we exploited the almost unique capability of L-edge RIXS to measure the set of the local dd excitations to assign the crystallographic site of divalent Mn2+ ions and, possibly, to derive some indications on its direct or indirect involvement in the DE ferromagnetism in LMO films. The two possible cation sites have different coordination and ion-oxygen distances: site A 共panel a in the inset in Fig. 4兲 is cubo-octahedral 共12 O2− neighbors兲, with A-O distance ⯝2.7 Å, site B 共panel b in the inset in Fig. 4兲

is octahedral 共six O2− neighbors兲, with B-O distance ⯝1.9 Å. In a simple point-charge crystal-field model one thus expects a much smaller splitting of eg to t2g states at A than B sites. Moreover the crystal-field parameter 10Dq = E共eg兲 − E共t2g兲 should be negative at A and positive at B. As a reference we use MnO, which has the NaCl structure: Mn2+ ion is at octahedral site with Mn-O distance ⯝2.2 Å, i.e., intermediate between sites A and B in LMO. Naming d the cation-ligand distance, and recalling that point-charge crystal field23 predicts 10Dq d−5, we expect 10Dq to be smaller 共greater兲 at site A 共B兲 than in MnO. We have measured the dd excitation spectra of Mn2+ in LMO and MnO using RIXS excited at the Mn L3 absorption edge. In particular, we have chosen the photon energy 640.0 eV, which is peculiar of Mn2+ sites 共see XAS spectra of Fig. 3兲. The RIXS spectra are shown in Fig. 4. In heavily La-deficient LMO metallic films, where XAS demonstrates a strong presence of Mn2+, the dd excitations are very different from those of MnO. To estimate quantitatively 10Dq we have calculated the RIXS spectra within a single-ion crystal-field model24,25 and tuned 10Dq and the rescaling of the interatomic Slater integrals so to fit the main experimental features. The result is shown in the top panel of Fig. 4. For MnO we essentially confirm the results of Ghiringhelli et al.:26 10Dq = 1.0 eV, Slater integrals rescaled to 70% of their Hartree-Fock value. For Mn2+-doped LMO films, we find a much smaller crystal field: 10Dq = −0.1 eV, Slater integrals rescaled to 64%. This indicates univocally that Mn2+ sits at site A and that the Mn-O bond has a nonnegligible covalent character 共indicated by the strong renormalization of Slater integrals兲. As further support to our interpretation of the RIXS spectra, we show an example of insulating LMO films, excited at the same energy as for the other cases: the spectrum is incompatible with dd excitations of Mn2+ and is rather typical of Mn3+ in magnanites.27 VI. THEORETICAL MODEL

With the Mn2+ ions indeed at the A site, one would expect that they could play the role of the divalent AE2+ ion in manganites by hole doping the MnO2 planes. In this respect, the substitution of La3+ ions with an atom having a smaller ionic radius 共namely, 0.89 Å and 1.17 Å for Mn2+ and La3+, respectively兲 should decrease the tolerance factor, thus pushing the system into a more and more insulating state.3 In this respect, an approach based on electron-hole Bose liquid28 has been recently proposed to describe metallic states in manganites which are supposed to be in an insulating and antiferromagnetic state. However, even though such phases are indeed metallic, they are described in terms of “poor metallic phase” with respect to the strong metallic and ferromagnetic state, which ultimately cannot described the enhanced metallicity that we observed in our samples. The observed enhanced metallicity can be understood by assuming that the Mn2+ ions at A sites contribute to the electron hopping via a new DE mechanism. The need for an alternative conducting mechanism stems from the fact that a smaller tolerance factor depresses the usual DE mechanism but favor a new hopping path involving Mn2+ ions. Beside

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Mn 2III+ Mn 4II+

Mn 3I+ Ox 2I-

Mn 3I+

a)

Ox

2II

Mn 2III+

Mn 4II+

b)

Ox 2IIIMn 4II+ c)

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FIG. 5. 共Color online兲 Possible hopping mechanisms in conventional and Mn2+-substituted manganites. In panel a, the traditional DE mechanism 共blue arrows兲. In panel b and c, two possible additional hopping paths in Mn2+-doped manganites are reported: a direct hopping 共panel b兲 between two Mn ions 共MnI and MnII, respectively兲 through intervening Mn2+ ion 共namely, MnIII兲 and a multiple-DE hopping path 共panel c兲 through the oxygen-Mn-oxygen ions 共OxII, MnIII and OxIII, respectively兲.

the hopping mechanism in the traditional DE framework 共panel a of Fig. 5兲, two extra possible paths for the added hole were considered: namely, a hopping mechanism between two Mn ions directly through an intervening Mn2+ ion 共panel b of Fig. 5兲 and a multiple-DE hopping path through the oxygen-Mn-oxygen ions 共panel c of Fig. 5兲. The effective hopping t of Mn3+-Mn2+-Mn4+ hopping mechanism can be estimated following Hasegawa-Anderson29 scheme, with the O2− replaced by Mn2+ via t = t21 / 共E1 − E2兲, where t1 is the resonance integral for the bond with the intermediate ion, E1 and E2 are the energy levels of the two atomic orbitals participating to the bond. In order to validate our proposal we estimate the overall hopping of an eg electron through Mn2+ ions in an undistorted cubical lattice. We adopt the values of ionic radii30 and the energy levels3 that are reported in the literature. Using as reference value the effective hopping Mn3+-Mn4+, through the px oxygen level along x direction, tx, we obtain an effective hopping parameter on the order of 10−3tx for both 3z2 − r2 and x2 − y 2. This analysis shows that a direct DE hopping through Mn2+ is very unlikely. On the contrary, the effective DE hopping of the added hole through the following path Mn3+-O2−-Mn2+-O2−-Mn3+ is relevant. It can be estimated extending the approach used above. By using a renormalization procedure that allows to decimate the three intermediate sites31 we get, t5 =

t22t21 共E1 − E2兲关E21 − E1共E2 + E3兲 − 2t22 + E2E3兴


levels E1, E2, and E3 are the energies of the orbitals involved in the ions Mn3+, O2−, and Mn2+, respectively. Assuming that the eg electrons are mainly in the 3z2 − r2 on Mn3+ 共based on XAS-LD analyses of orbital ordering兲 and that the only possible states on Mn2+ are xy and 3z2 − r2 共due to orbital symmetries32兲, our estimate is therefore t5 ⯝ 0.24tx. Such a value has been obtained by assuming that all the ions are structurally arranged in an undistorted perovskite. In fact, manganites, and, in particular, LaMnO3, generally show a distorted orthorhombic/rhombohedral perovskite unit cell, characterized by O ions slightly displaced out of the joining lines among the Mn ions 共see Fig. 5兲. In this respect, because of the buckling angle between the Mn plane and the Mn-O bonds in distorted perovskites, the superimposition of MnI-OxI-MnII orbitals is actually reduced, thus subsequently reducing the tx value. Moreover, the smaller size of Mn2+-ionic radius 共compared to La3+’s ones兲 should increase the cubic distortion of the perovskite cells, thus further reducing the tx value. On the contrary, because of the tilting of the oxygen octahedra, the distance between some oxygen ions and the central Mn2+ ion should be shortened with respect to the undistorted structure 共in which the A-O distance is 冑2 / 2 times unit-cell lattice parameter兲. As a consequence, the superimposition between O2−-Mn2+ orbitals actually increases, thus increasing the t5 value. Eventually, in real distorted perovskite, a substantial increment of the t5 / tx ratio is expected, thus pointing out toward a significative role of the proposed multiple-DE conducting path. VII. CONCLUSIONS

In conclusions, an unconventional charge-hopping mechanism ini manganites 共named multiple double exchange, or multiple DE兲 has been observed. We have shown that, in LaxMnO3−␦ manganite thin films, as La content decrease, a partial substitution of Mn ions at the La site is indeed possible. Polarization-dependent XAS characterization has demonstrated the relevant Mn2+ content in heavily La-deficient LMO samples. RIXS investigation has allowed us to unambiguously assign the crystallographic site 共namely, the perovskite A site兲 of the divalent Mn2+. This last is experimentally and theoretically proved to be more efficient with respect to traditional DE mechanism. Such a further increase in the Curie temperatures in manganites could possible allow the use of manganite-based devices at room temperatures, which nowadays is limited because of the depression of magnetic alignment at that specific temperatures. The very idea of using multiple-valence atoms both as a dopant and as an active element electronically involved in the conduction mechanism 共namely, the Mn2+ in manganites兲 opens unexplored scenarios in the study of physical properties of CMR materials. More important, such a strategy might be also pursed in other strongly correlated electrons materials. ACKNOWLEDGMENTS

,

where t1 and t2 are the resonance integrals between Mn4+-O2− and O2−-Mn2+ orbitals, respectively. The energy

R.C.’s research activity has received funding from the European Community’s Seventh Framework Programme 20072011 under grant Agreement No. 212348 NFFA.

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