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Cooperative enhancement of in-plane orbital ordering by oxygen deficiency and in-plane

tensile strain in La0.7Sr0.3MnO3−δ thin films

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2007 EPL 80 37003

(http://iopscience.iop.org/0295-5075/80/3/37003)

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November 2007

EPL, 80 (2007) 37003 www.epljournal.org

doi: 10.1209/0295-5075/80/37003

Cooperative enhancement of in-plane orbital ordering by oxygendeficiency and in-plane tensile strain in La0.7Sr0.3MnO3−δthin films

C. Aruta1(a), G. Balestrino

1, A. Tebano

1, G. Ghiringhelli

2 and N. B. Brookes3

1 CNR-INFM Coherentia and Dipartimento di Ingegneria Meccanica, Universita di Roma Tor VergataVia del Politecnico 1, 00133 Roma, Italy2 CNR-INFM Coherentia and Dipartimento di Fisica, Politecnico di Milanopiazza Leonardo da Vinci 32, 20133 Milano, Italy3 European Synchrotron Radiation Facility - BP220, 38043 Grenoble, France

received 13 July 2007; accepted in final form 5 September 2007published online 28 September 2007

PACS 75.47.Lx – ManganitesPACS 78.70.Dm – X-ray absorption spectra

Abstract – The development of in-plane orbital ordering with oxygen deficiency is investigated inepitaxially strained thin films of La0.7Sr0.3MnO3−δ grown under in-plane tensile strain on SrTiO3substrates. The orbital character of 3d states is directly probed by linear dichroism in X-rayabsorption spectroscopy at the Mn L2,3 and O K edges. The electrical transport measurementsshow a rapid decrease of the metal-insulator transition temperature and an increase of thepolaronic activation energy when reducing the oxygen content. The diffraction measurementsshow a contraction of the average out-of-plane lattice parameter due to an enhancement of theregular distribution of the Mn3+ distorted octahedra in oxygen-deficient films. We show thatthe oxygen deficiency and the in-plane tensile strain cooperate to stabilize the in-plane orbitaloccupation: the oxygen deficiency is responsible for the increased number of Jahn-Teller distortedoctahedra and the in-plane tensile strain drives the arrangement of such octahedra, favoring thein-plane confinement of the eg orbitals. Linear dichroism in X-ray absorption also demonstratesthat the Mn 3d/O 2p hybridization changes little with the oxygen content, thus confirming thatthe in-plane orbital ordering is the main factor influencing the transport properties.

Copyright c© EPLA, 2007

Doped manganites are particularly interesting systemsin the field of orbital physics because of the 3d-orbitalpolarization. Superexchange and double-exchange inter-actions are strictly related to the various arrangements ofspin and orbital structures, giving rise to the rich phasediagrams experimentally observed. The global featuresof the La1−xSrxMnO3 ground-state phase diagramwere reproduced by theoretical calculations of Maezonoet al. [1]. They observed a crossover around x= 0.3from the ferromagnetic (FM) spin state to the A-typeantiferromagnetic (AF) spin state with the in-plane(x2− y2) orbital structure. In the case of thin films grownon mismatched substrates, the biaxial strain is a source of

(a)Present address: CNR-INFM Coherentia and Dipartimento diScienze Fisiche, Universita di Napoli Federico II, Piazzale Tecchio80, 80125 Napoli, Italy. E-mail: aruta@na.infn.it

cooperative Jahn-Teller (JT)-like distortion which givesrise to different orbital orderings [2]. As a consequence, theanisotropy of the wave function of the 3d occupied orbitalfavors an anisotropic transfer integral and superexchangeinteractions and different spin-orbital coupled statesarise. FM metallic and AF insulating phases are thereforeobtained. In the FM metallic state, where the doubleexchange interaction occurs, no preferential occupationof the 3d orbitals is realized (orbital disordered phase).On the contrary, the AF insulating phases show orbitalordering which can be tuned by changing the doping levelor the strain effects [3]. Indeed, the out-of-plane (3z2− r2)orbitals occupation is observed with the chain-type (C)AF phase, while the in-plane (x2− y2) orbitals occupationis typical of the layer-type (A) AF phase. An example ofA-type phase is the insulating LaMnO3 (LMO) parentcompound. Orbital ordering in LMO was first explained

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C. Aruta et al.

by Goodenough [4] as a consequence of cooperativeoxygen-atom displacements within the a-b planes thatgives rise to long Mn-O-Mn bonds alternating with shortMn-O-Mn bonds ((JT) distortion). However, it is largelyaccepted that (x2− y2) orbital ordering can be realizedin the A-type spin structure in doped manganites, bothunstrained and tetragonally distorted [5], in contrast tothe alternate (3x2− r2)/(3y2− r2)-type orbital orderingobserved in LMO with the same A-type spin structure.In spite of the vast literature on the orbital ordering

in the manganese oxides, a study of the effect of theoxygen deficiency on the orbital occupation in strainedthin films is still lacking. Nevertheless, the detrimental roleof oxygen defects on the magnetotransport properties inmanganite thin films was reported [6–9]. In particular, thelattice strain was invoked as the main source of depressedmagnetotransport properties in oxygen-deficient mangan-ite films [6,8]. In this framework, the comprehension of thestrain and oxygen effects on the orbital ordering becomesparticularly important, because the anisotropic confine-ment of the eg electrons due to orbital ordering makesthe double-exchange FM metallic phase unfavoured andstabilizes paramagnetic or AF insulating phases.Unfortunately, orbital ordering remains difficult to

investigate experimentally especially because the orbitaldegree of freedom does not couple directly to mostexperimental probes. Among the scattering techniques,the enhanced sensitivity to valence states of ResonantX-ray Scattering (RXS) by tuning the photon energy to aMn absorption edge, enabled to measure orbital orderingin the doped layered-manganite La0.5Sr1.5MnO4 [10,11],and in the undoped LMO [12]. However, orbital orderfluctuations and short-range order can broaden thescattered peak width [13] and the scattering from defectsin thin films can be larger than the orbital-dependentscattering, making orbital ordering hard to detect byRXS. Additionally, it has been reported that two typesof Mn sites with different local geometric structures canalso explain the RXS data, without the need to invokethe orbital ordering [14]. On the other hand, electricaland magnetic measurements have been also employed butonly give indirect insight in to the orbital ordering [15].On the contrary, Linear Dichroism (LD) in X-ray Absorp-tion Spectroscopy (XAS) at the Mn L-edge has beendemonstrated to be a direct probe of the orbital characterof 3d states in manganites [16]. C-type phase accompa-nied by (3z2− r2) orbital ordering was observed by LDmeasurements carried out on various antiferromagneticorbitally ordered manganites of different compositions,including LaSrMnO4 layered manganites [17]. Further-more, LD measurements on strained La0.7Sr0.3MnO3films demonstrated the preferential occupation of in-planeor out-of-plane orbitals depending on the nature of thebiaxial strain [18]. Namely, preferential (3z2− r2) or(x2− y2) orbital occupation was detected in the caseof SrTiO3 substrate (in-plane tensile strain) or LaAlO3substrate (in-plane compressive strain), respectively.

Table 1: Samples grown by PLD at different growth rates arereported in the table, together with their distinctive transportand structural parameters.

Sample Growth rate TMI (K) c-axis (A)(A/laser pulse)

S370 1.4× 10−2 >370 3.850S330 3.2× 10−2 330 3.844S200 6.4× 10−2 200 3.837S0 1.5× 10−1 – 3.830

In this paper we report on the LD signal in XASmeasurements at the Mn L2,3-edge and O K-edge asa function of the oxygen content in La0.7Sr0.3MnO3−δ(LSMO) thin films grown by pulsed laser deposition(PLD) on SrTiO3 (STO) substrates. The oxidation stateis directly related to the doping level, thus influencingthe electron interactions and consequently the electronanisotropy arising from the orbital degree of freedom inthe conduction eg electrons. In addition, the strain statecan be read as a collective JT distortion which producesthe orbital ordering. We can therefore inspect the effectof both electron localization and lattice distortion on theorbital ordering in LSMO.We modified the oxygen content in our films by chang-

ing the growth rate per laser pulse in the depositionprocess. Indeed, as demonstrated in a previous work [19],the oxidation state of LSMO films grown by low pres-sure PLD depends on the deposition rate, namely thedeposited thickness per each laser shot. As a matter offact, film oxidation occurs primarily as a result of theinteraction between plume and background gas. Thus,the plume oxidation process gets enhanced with decreas-ing the ablated material density from the target aftereach individual laser shot. In the present case the growthrate was carefully controlled by in situ Reflection HighEnergy Electron Diffraction (RHEED) through the inten-sity oscillations of the RHEED specular spot. Several filmswere grown on (100) STO substrates at different deposi-tion rates keeping all the remaining growth parametersunchanged. As a consequence, film metal-insulator tran-sition temperatures (TMI) varied as reported in table 1,from sample S370 (TMI > 370K) with the highest TMI , tosample S0 (TMI = 0K) which does not show any metal-insulator transition. RHEED analysis allowed to verifythe 2D surface quality for all the films, regardless of thedeposition rate per laser pulse. All films had the samethickness of 100 unit cells (about 400 A). Further detailson the deposition technique are reported elsewhere [19].The crystal structure of bulk La0.7Sr0.3MnO3 is

rhombohedral with a pseudocubic lattice parameter ofaLSMO = arhomb/

√2 ∼ 3.87 A. Structural properties

of films were investigated by X-ray diffraction (XRD)measurements at Cu Kα wavelength in the Bragg-Brentano configuration. XRD diffraction spectra reportedin fig. 1 were indexed on the basis of the pseudocubic

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In-plane orbital ordering by oxygen and strain in LSMO films

46.0 46.5 47.0 47.5 48.0

ST

O

Inte

nsit

y (a

rb. u

nits

)

2Θ

S370 S330 S200 S0

0.96 0.98 1.00 1.02 1.04

2.98

3.00

3.02

3.04

3.06 LSMO

STO

S0

H (r.l.u.)

L (

r.l.u

.)

2.98

3.00

3.02

3.04

3.06LSMO

STO

S370

L (

r.l.u

.)

a) b)

Fig. 1: (Color online) (a) X-ray diffraction measurements ofthe (002) reflections of the four investigated LSMO films: S370(circles), S330 (triangles), S200 (squares) and S0 (diamonds).(b) Isointensity contour plot on a logarithmic scale of the (103)reciprocal space map measured at the Cu Kα wavelength ontwo LSMO thin films grown on STO substrates with differentgrowth rates: 1.4× 10−2 A/laser pulse (S370 sample, top panel)and 1.5× 10−1 A/laser pulse (S0 sample, bottom panel). Blackasterisks indicate the expected positions for the fully relaxedLSMO films.

unit cell. As a consequence of the reduction of theoxygen content in films grown at high deposition rate, anexpansion of the lattice parameters would be expectedbecause of the larger size of the Mn3+ ion compared toMn4+ [20]. On the contrary, XRD measurements at the(002) reflection reported in fig. 1(a), show a decrease ofthe c-axis parameter from 3.85 A, for the optimally dopedfilm (S370), to 3.83 A for the insulating film (S0). Allfilms have high crystallographic quality, as confirmed bythe rocking curves of the (002) reflection which have afull width at half maximum comparable with that of thesubstrate, smaller than 0.08. In addition, the reciprocalspace maps around the (103) asymmetric reflections offig. 1(b) confirm, within our experimental resolution, thatall films are in-plane matched with the STO substrate(cubic, with a= 3.905 A).Transport measurements reported in fig. 2 were

performed as a function of temperature by the standardfour probe technique. The measurements in fig. 2(a)show that, as expected, TMI decreases when the oxygencontent is reduced. The high temperature films resistivity(fig. 2(b)) follows the behavior predicted by small polaronhopping, ln(ρ/T ) =A+Ea/KBT , with Ea, hoppingenergy, which increases while TMI drops. A higherhopping energy corresponds to an increase of the Mn3+

content. This is expected in less oxidized samples wherethe Mn3+ ions content increases in order to preservecharge neutrality. JT distortion around the Mn3+ iongives rise to small polarons associated with local latticedistortions which localize the charge carriers [21]. Inthe case of the insulating sample, the hopping energyis lower than the value found in slight off-stoichiometry

0 100 200 300 400

10-2

100

102

104

ρ ( Ω

cm

)

T (K)

0.003 0.004 0.005 0.006-12

-9

-6

-3

0

3

134 meV

128 meV

45 meV

ln( ρ

/T)[

Ω c

m K

-1]

1/T (K-1)

a)

b)

Fig. 2: (Color online) (a) Resistivity vs. temperature behaviorin logarithmic scale for the four investigated LSMO films: S370(circles), S330 (triangles), S200 (squares) and S0 (diamonds).(b) ln(ρ/T ) vs. (1/T) with Holstein’s model curves (continuousstraight lines) calculated with polaron hopping energies of45meV, 128meV and 134meV, for the S330, S200, and S0samples, respectively.

LMO films [22], i.e. 355meV with about 95% of Mn3+.Although, even the resistivity curves of the samples withhigher TMI (S370 and S330) show lower hopping energiesif compared with values reported in the literature foroptimally doped La0.7Ca0.3MnO3 films [21]. Of course,no cooperative long-range ordering of the JT distortionis expected in bulk manganites above TMI . On theother hand, the epitaxial strain in thin films can favourthe ordering of the local JT distortions over a longrange, which can finally lead to electrically insulatingphases [23].XAS measurements were performed at the Mn L2,3-

edge and O K -edge with linear polarized synchrotron lightat the ID08 beamline of European Synchrotron RadiationFacility at grazing incidence with two orthogonal lightpolarizations: E ‖ ab-plane (V-polarization) and E ‖ c-axis(H-polarization). LD was obtained by subtraction of thetwo XAS signals as reported in ref. [18]. The results arereported fig. 3. XAS measurements at the Mn L2,3-edge

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C. Aruta et al.

635 640 645 650 655 660-9

-6

-3

0

V-H

(%

)

Photon Energy (eV)

4

8

12

(V+

H)/

2

S370 S330 S200 S0

525 530 535 540

-10-505

10

V-H

(%

)

Photon Energy (eV)

0

1

2

3

(V+H

)/2

S370 S330 S200 S0

b1

a1

b)

Mn 3dLa 5dSr 4d

egeg t2g

a)

Fig. 3: (Color online) (a) XAS and LD measurements at theMn L2,3-edge for the four investigated samples. Top panel:average between experimental XAS signals taken in V andH polarizations (V+H)/2, offset for clarity. Bottom panel:LD=V−H in percent of the absorption signal shown in thetop panel (a). (b) XAS and LD measurements at the O K -edgefor the four investigated samples. Top panel: average betweenexperimental XAS signals taken in V and H polarizations(V+H)/2, offset for clarity. Bottom panel: LD=V−H inpercent of the absorption signal shown in the top panel (b).

are reported in the top panel of fig. 3(a). The distin-guishing feature of the different XAS spectra is the mainshoulder at 640.5 eV which becomes more pronouncedwhen TMI decreases. Such a feature has been associatedto the Mn3+ high spin contribution [24]. Therefore, it isa clear indication of the enhancement of Mn3+ contentin less conductive samples. In the LD spectra reportedin the bottom panel of fig. 3(a), the feature at 640.5 eValso develops with decreasing the oxygen content. SinceMn4+ (3d3), unlike Mn3+ (3d4), does not tend to distortits octahedral environment by JT effect, the enhancementof LD intensity is in agreement with the increase ofMn3+ content, provided that distorted octahedra arecooperatively oriented, giving rise to preferential orbital

0 100 200 300 400

200

300

400

O K-edge

Inte

grat

ed L

D (

arb.

u.)

TMI

(K)

400

600

800 Mn L-edge

Fig. 4: (Color online) Integral of the LD curves reportedin the bottom panels of fig. 3. The integral of the LD atthe Mn L-edge (right y-axis) is carried out in the wholeL2,3 range (635–660 eV), while at the O K-edge (left y-axis)it is performed in the range where the contribution of thehybridization with Mn 3d (525–535 eV) is considered to beimportant. Right and left Y -ranges are chosen in order tooptimize the comparison between the two behaviors.

occupation. In addition, the general behavior of the LDspectra is in agreement with theoretical calculations forMn3+ ions with preferential in-plane orbital occupation,namely either (3x2− r2)/(3y2− r2) or (x2− y2) [16]: LDsignal does not allow to discriminate between the twopossibilities.A significant LD signal can be also observed in fig. 3(a)

in the optimally doped film (S370), demonstrating thata preferential occupation of in-plane orbital ordering ispresent even in the case of metallic samples. Such aneffect was already reported by us in refs. [18] and [23].It was explained in terms of nanometric phase separationbetween the AF orbital-ordered insulating phase and theFM orbital-disordered metallic phase.A general increase of the LD intensity is also observed

together with the TMI decrease, as shown in fig. 4.Therefore, the lower oxygen content seems to be associatedto a higher in-plane orbital occupation.XAS measurements at the O K-edge give information

on the Mn 3d states hybridized with the O 2p states. Inparticular, the XAS spectra region between 525 and 535 eVwas associated to orbital overlap with the Mn 3d and thehigher-energy component was assigned to the band withLa 5d/Sr 4d character [25], as indicated in the top panel offig. 3(b). In addition, the second structure at about 533 eVwas associated in ref. [25] to a band of minority eg ↓ char-acter which especially develops in the case of the insulatingsample. The LD spectra in the bottom panel of fig. 3(b)shows an increase of the same eg ↓. Higher LD intensity isassociated to higher octahedral distortion and/or higherMn3+ content. Moreover, in the bottom panel of fig. 3(b)the optimally doped sample shows the lowest LD signal inagreement with a small octahedral distortion. The feature

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In-plane orbital ordering by oxygen and strain in LSMO films

associated to the majority eg ↑ band is almost zero in theoptimally doped film and starts to increase when the TMIstarts to decrease. The LD at O K-edge general behav-ior is in agreement with the results reported in ref. [18].Indeed, the positive-negative sign of the LD of the eg ↑band demonstrates that the intrinsic width of the Mnstates projected onto the O 2p band is larger than thestrength of the crystal field tetragonal distortion. More-over, the band width stays almost the same for all thesamples indicating the same degree of hybridization. Thisis a quite unexpected result, because the Mn 3d/O 2phybridization is directly related to the band dispersion andtherefore to the metallicity of the sample. On the otherhand, the integrated LD signal reported in fig. 4 increaseswhen TMI decreases, at the O K-edge just as at the MnL-edge. This is a clear signature of the development ofin-plane orbital ordering in less oxidized films, in contrastto an almost unchanged band width. Both Mn L-edge andO K-edge XAS-LD measurements reveal that when theoxygen content decreases the in-plane orbital occupationincreases, because of the increased Mn3+ content with anin-plane elongation of the distorted octahedra. Transportmeasurements of fig. 2 are in agreement with the scenarioof a Mn3+/Mn4+ charge disproportion with a local Mn3+

increase with the oxygen deficiency. In parallel, LD spec-tra as a function of TMI demonstrate the developmentof larger regions with preferential occupation of in-planeorbitals, (3x2− r2)/(3y2− r2) or (x2− y2). The alternateoccupation of the (3x2− r2) and (3y2− r2) orbitals is typi-cal of the LMO compound, where only Mn3+ ions arepresent and the A-type phase is established. The alter-nation of long and short in-plane Mn-O bonds, typical ofthe A-type spin structure, is compatible with the in-planetensile strain induced by the substrate. As a matter of fact,in the A-type spin phase, all the longer bonds are in theplane and the average lattice parameter will result largerin the ab-plane than out of the plane. The appearance ofA-type phase ordered regions is indeed favored by thecooperative effect of the in plane tensile epitaxial strain.The optimally doped film (S370) is the least distortedwith a c/a ratio (ratio between the perpendicular andthe in-plane lattice parameters) equal to 0.986. The strainstate is not high enough to induce significant coopera-tive JT distortion. Therefore, a large amount of Mn4+

ions in the film is randomly distributed through a dis-ordered lattice of Mn3+ and Mn4+ ions, giving rise todouble-exchange mechanism in metallic-like bonds. Atthe same time, the smaller c-axis length in less oxidizedfilms grown at high deposition rates indicates an enhance-ment of the lattice distortion reaching its maximum value,c/a= 0.981 in the insulating film. Namely, as the extentof the ordered regions increases, a growing percentageof distorted octahedra arrange with shorter Mn-O-Mnbonds out of the plane and with longer bonds in theplane, in agreement with the increase of the tetragonaldistortion detected by XRD. In less oxidized films, havingreduced TMI values, the double exchange starts to be

inhibited in larger regions. In such regions the degen-eracy between the Mn3+-O-Mn4+ and Mn4+-O-Mn3+

state is removed and the development of regions withordered Mn3+-O-Mn3+ bonds is favored, thus resultingin a cooperative lattice strain. The correlation betweenthe LD spectra and the structural distortion is straight-forward: the crystallographic cell in the samples with lowerMn3+ content is less distorted since Mn4+ does not tendto deform its octahedral environment by JT effect. Theordered A-type phase regions expand with increasing theMn3+ content, namely with the reduction of the oxygencontent.In strongly reduced samples, with low Mn4+ content,

an ordering of long and short Mn-O-Mn bonds is expectedgiving rise to the A-type AF insulating phase [26]. Bothlattice distortion and electron-electron interaction effectscontribute to stabilize the in-plane orbital ordering asso-ciated with the A-AF spin ordering. The gradual increaseof the LD signal as the oxygen content is decreased infilms under tensile strain seems to point toward a phaseseparation phenomenon where the FM double-exchangeregions coexist with A-AF regions, the latter increasingat the expenses of the former as the oxygen content isdecreased. This scenario is in agreement with the recentlyproposed models about phase coexistence among almostdegenerate manganite phases in the presence of structuraldisorder [27].In conclusion, using LD in Mn L2,3 XAS, we have

observed an increase of in the in-plane orbital occupa-tion in strongly reduced films grown on STO. This is aconsequence of the in-plane tensile strain that energeti-cally favours the elongated Mn-O bonds to be in-plane;and of the oxygen deficiency that favors the increase ofMn3+ distorted octahedra. This means that the reductionin the oxygen content and the lattice distortion cooper-ate to stabilize the A-AF spin ordering associated withthe in-plane orbital ordering. On the other hand, theO K-edge XAS spectra show that the Mn 3d/O 2phybridized band does not significantly reduce its widthupon oxygen content reduction, suggesting a negligibleincrease of the electron localization.

∗ ∗ ∗We acknowledge stimulating discussions with

V. Cataudella, R. DeRenzi and A. Sidorenko.

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