We report electrical transport experiments on the colossal magnetoresistance compound (La,Ca)MnO3 over a wide range of composition and temperature. Comparison of thermopower and electrical resistivity measurements above the metal-insulator transition indicate a transport mechanism not dominated by spin disorder, but by small polaron formation. Additionally, we find that in the high-temperature limit the thermopower corresponds to backflow of spin entropy, expected from motion of positively charged particles in a rigid S=2 system, showing a remarkable independence of S=3/2 particle density

Cheong, S.W.

Palstra, T.T.M.

Ramirez, A.P.

Schiffer, P.

Zaanen, J.

Zegarski, B.R.

We report electrical transport experiments on the colossal magnetoresistance compound (La,Ca)MnO3 over a wide range of composition and temperature. Comparison of thermopower and electrical resistivity measurements above the metal-insulator transition indicate a transport mechanism not dominated by spin disorder, but by small polaron formation. Additionally, we find that in the high-temperature limit the thermopower corresponds to backflow of spin entropy, expected from motion of positively charged particles in a rigid S=2 system, showing a remarkable independence of S=3/2 particle density.</p

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Transport mechanisms in doped LaMnO3:Evidence for polaron formation

University of Groningen

Leiden University Scholary Publications

erlandsPHYSICAL REVIEW B 1 SEPTEMBER 1997-IVOLUME 56, NUMBER 9Transport mechanisms in doped LaMnO3: Evidence for polaron formationT. T. M. PalstraDepartment of Chemical Physics and Materials Science Center, State University of Groningen, 9794 AG Groningen, The Nethand Bell Laboratories, Lucent Technologies, Murray Hill, New Jersey 07974A. P. Ramirez, S-W. Cheong, and B. R. ZegarskiBell Laboratories, Lucent Technologies, Murray Hill, New Jersey 07974P. SchifferUniversity of Notre Dame, Notre Dame, Indiana 46556J. ZaanenLorentz Institute, University of Leiden, 2300 RA Leiden, The Netherlands~Received 18 November 1996; revised manuscript received 9 April 1997!We report electrical transport experiments on the colossal magnetoresistance compound (La,Ca!MnO3 overa wide range of composition and temperature. Comparison of thermopower and electrical resistivity measure-ments above the metal-insulator transition indicate a transport mechanism not dominated by spin disorder, butby small polaron formation. Additionally, we find that in the high-temperature limit the thermopower corre-sponds to backflow of spin entropy, expected from motion of positively charged particles in a rigidS52system, showing a remarkable independence ofS53/2 particle density.@S0163-1829~97!09334-X#entheinrarisriaia.eththuspeacrercaporendlewtoattoe ofe-lla-thepo-thein-r-ithisizednalther atfornentsre/80instfe-m-The recent observation of large magnetoresistance effin thin films of doped LaMnO3 has renewed the interest ithe metal-insulator transition in these materials.1–3 Since themetal-insulator transition temperature,TMI , can be tuned toabove room temperature, this opens possibilities to usematerial not only for recording media but also for other typof magnetic switching applications. However, the emergnotion that magnetoresistance due to a metal-insulator tsition decreases with increasingTMI , necessitates a betteunderstanding of the underlying basic transport mechanto exploit the magnetoresistive properties of these matefully. The microscopic nature of the transport mechanismcondensed into the description of double exchange, themultaneous electron transfer of an electron on a Mn atoman O atom of the surrounding oxygen octahedron, andother electron from this O atom to a neighboring Mn atom4This indirect exchange mechanism establishes, becausspin conservation in the exchange, a direct relation ofmetallic state with the ferromagnetic coupling betweenMn spins. However, there is remarkably little known abothe transport mechanism in the insulating state. Whereasdisorder can contribute to the variations in the resistivity nTc , only more recently the role of electron-phonon interations on the localization has been put in a cleaperspective.5–7 This theoretical work was based on reseaon magnetic semiconductors like EuO, and only recentlyplied to doped LaMnO3. Still, it is not known if the metalinsulator transition in doped LaMnO3 is driven by changes inthe carrier density or the mobility.In order to gain insight into the nature of the transpmechanism, we have performed thermopower measuremon doped LaMnO3 over a wide range of concentrations atemperatures in addition to the basic characterization by etrical resistivity and magnetic measurements. Thermopo560163-1829/97/56~9!/5104~4!/$10.00ctsissgn-mlsssi-ton-ofeetinr-rh-tntsc-eri in contrast to electrical resistivity relatively insensitiveeffects of grain boundaries and disorder. We will show thfor this material, thermopower provides unique insight inthe transport mechanism. First, we show that the changign in the thermopower is not related to competition btween electron and hole conduction, but accidental cancetion of the entropic term and the energy transport term ofthermopower. Secondly, the transport behavior aboveTc isnot dominated by spin disorder scattering, but by smalllaron formation. From our measurements we can estimatepolaron binding energy and the energy scale for polaronteractions. Finally, the entropic contribution to the themopower in the insulating state is in good agreement wthe expectation for a polaron gas if only the spin entropyconsidered. However, this state has to be more organthan a simple gas, because we find that the configuratioentropy is at an extremum with respect to changes innumber of carriers.Ceramic samples of (La,Ca!MnO3 were prepared by solidstate reaction. Starting materials La2O3, CaCO3, and MnO2were mixed in stoichiometric proportions and heated in ai1250 °C for 5 h, and 1380 °C for 13 h, and then 1390 °C20 h with intermediate grinding. Powder x-ray diffractioshows clean single phase patterns. Transport experimwere performed in a commercially available temperatumagnetic field platform~PPMS/Quantum Design! and a re-sistance bridge operating at a frequency of 17 Hz~LinearResearch LR400!. Thermopower was measured betweenand 475 K using a commercial setup~SB 100/MMR Tech-nologies! using proprietary software and copper leads agaa copper/constantan reference.In Fig. 1 we show the electrical resistivity o(La,Ca!MnO3 between 10 and 350 K for doping levels btween 10% and 60% Ca. The metallic behavior at low te5104 © 1997 The American Physical Societytubethcpbth-tebpeagdharmawstureofThfetinnaethectedtic-o-fornlym-on-esforbereac-wsowultmi-ithes ctric56 5105BRIEF REPORTSperatures and semiconducting behavior at high temperahave been reported much earlier.8–10 However, in contrast toearlier work, we concentrate in this paper solely on thehavior of the extrinsic semiconducting state aboveTc . Thisregime was shown to hold to about 900 °C above whichconduction becomes intrinsic. In the extrinsic semiconduing state the resistivity can be fitted tor(T)5r0exp(Dr/2kBT) to obtain an estimate of the transport gaThe transition into the ferromagnetic state is accompanieda transition into metallic behavior and is observed indoping range 0.2<x<0.45. The maximum Curie temperature is obtained forx50.33 at 250 K. For lower dopinglevels (x50.10) there is no transition into a metallic stadespite a ferromagnetic component in the magnetizationlow 100 K. Probably the double exchange is here in comtition with the superexchange which favors an antiferromnetic spin state, rendering a canted magnetic structure aninsulating ground state. Also for doping concentrationsx.0.5 no metallic ground state is obtained. It was shown tfor these concentrations a charge ordered ground state fowhich is insulating, exhibiting antiferromagnetic orderinglower temperature.11We show the temperature dependence of the thermopoS(T) of (La,Ca!MnO3 in Fig. 2. Also the thermopower habeen reported previously over a much broader temperaregime, but no convincing quantitative models weprovided.8,12Again, in this paper we focus on the behaviorthe extrinsic semiconducting state aboveTc . The Curie tem-perature is here reflected by a jump in the thermopower.metallic state exhibits a small thermopower less than amV/K. The thermopower changes sign nearx50.3 from ap-parent holelike behavior to electronlike transport, suggesthat the maximumTc is somehow related to symmetry ielectron and hole conduction. However, we will show thFIG. 1. Inverse temperature dependence of the electrical retivity of ceramic samples La12xCaxMnO3 for 0.1<x<0.6.re-et-.yee---ants,terreewgtthe change of sign inS(T) is fortuitous and does not havthis physical significance. Our data indicate instead thatentire doping range is hole transport dominated, as expefrom replacing trivalent lanthanum with divalent calcium.The magnetic field dependence of the ferromagnesemiconducting transition has been studied13 extensively be-fore in compounds like EuO. For this material it was prposed that electron-phonon interactions were responsiblethe localization in the semiconducting state.6 For the manga-nates the insulating state is still poorly understood, and orecently the role of the electron-phonon interaction was deonstrated theoretically7 and experimentally.14 In this paperwe focus therefore on the behavior of the extrinsic semicducting regime. It was not realized that this regime provida simple handle on the origin of localization. The basisdiscussion of the transport mechanism in (La,Ca!MnO3 isthe experimental observation that the thermopower candeconvolved simply into two contributions. The temperatudependence in the extrinsic semiconducting state can becurately fitted with a temperature independent termS0 and atemperature dependent termS52UkBe U• DS2T 1S0 ~1!for all doping concentrations 0.1<x<0.9. This simple de-convolution is unusual for doped semiconductors, and allointerpretation of the thermopower in more detail. We shin Fig. 3 the results of this parametrization:S0 , DS , Dr for0.1<x<0.6. Regardless of further interpretation, this resshows that the high temperature state is in an entropy donated regime. Equation~1! is in this case very general asfollows directly from the high-temperature expansion of tKubo formula:is- FIG. 2. Inverse temperature dependence of the thermoelepower of ceramic samples La12xCaxMnO3 for 0.1<x<0.9.e-riueqe-syersisfieiorgenehaatoti-rgepic-ntser,ilitythestillroniatedbu-yationaregasrItsosetryhasg ahiscan-ur, al-eticb-therm.r-n-d byeen-aal-he-yf5106 56BRIEF REPORTSS52S~2!/S~1!T2meT, ~2!where S(2) and S(1) are the energy-particle and particlparticle current correlation function andm the chemical po-tential. In the high-temperature limitS(2)/S(1)!m and thethermopower obtains its interpretation as entropy per car2m/eT. Hence the system is in a classical high-temperatstate, and by analyzingS0 andDS one can infer some crudcharacteristics of the liquid. The negative value ofDS for0.1<x<95 suggests hole conduction in the Mneg band, asobtained by replacing trivalent La by divalent Ca. From E~1! and Fig. 2 one can infer that the sign change in thmopower nearx50.33 is caused by cancellation of the temperature dependent and independent term, rather thanmetry of electron and hole conduction.From Fig. 3 we see that the transport gap from thmopower is much smaller than the gap derived from retivity measurements. Thermopower measures, in a simplipicture, the heat current associated with charge motwhich for usual semiconductors is the activation eneacross the~kinetic! gap. If transport is dominated by one typof carrier, this results in an equality of the transport athermopower gap. The vanishing value ofDS near x50.5can only be explained in a semiconductor model by symmtry of the holes and electrons, as expected, e.g., for afilled band. Theeg band is however only one quarter filledx50.5. This would suggest a splitting of theeg band, e.g., bya Jahn-Teller distortion of the 3d4 Mn31 ion. However, wediscount this explanation because it requires a rather gcancellation of both the prefactors~number of holes/electrons! and the respective gaps while still exhibiting acFIG. 3. Doping dependencex of the energy gaps derived btransportDr and thermopowerDS and the high-temperature limit othe thermopowerS0 .erre.r-m---dn,yd-lfodvated behavior over a large temperature interval and a larange of concentrations. Therefore, we think that such ature is not appropriate. Detailed Hall effect measuremecould provide more information on this issue. Howevthese measurements are difficult because of the low moband the large temperature dependence ofrxx . We have thusfar not yet been able to obtain experimental results forrxy oneither these ceramic samples or thin films.A more appropriate interpretation of the disparity ofDSandDr is in terms of lattice gas models, first discussed incontext of superionic solids and organic conductors.15,16 Inthese models the rate limiting step for charge transport isthe thermal activation of an electron above half the polabinding energyEP , which would exhibit only a modestxdependence. However, there is no heat transfer assocwith the electron transfer since the polaron energy contrition in the chemical potentialm cancels the polaron energterm in S(2)/S(1) @see Eq.~2!#. In this case one retainstemperature independent thermopower. This cancelladoes not, in general, occur if particle-particle interactionresponsible for the heat transport. For instance, in latticemodels the physical origin ofDS is the nearest neighbo~Coulomb! interactionV. More generally,DS should origi-nate from spin-polaron or polaron-polaron interactions.magnitude should give an estimate for the strength of thinteractions. In addition, the vanishing value forDS at x50.5 is consistent with the effect of particle hole symmein the interacting problem.17 The spin-polaron interactionterm was calculated by Liu and Emin18 for spin cluster mod-els. They obtained aT22 temperature dependence aboveTcwithin their model. Our experimental result of aT21 tem-perature dependence aboveTc cannot support this model.The description of colossal magnetoresistance effectsfocussed on the double exchange mechanism, providinlink between the magnetic and transport properties. Tleads to a description of scattering near and aboveTc interms of spin disorder. Whereas spin disorder scatteringreproduce the qualitative behavior nearTc , large quantita-tive discrepancies have been pointed out,7 such as the magnitude of the resistivity, and the doping dependence. Oexperiments show unambiguously that aboveTc a polaronictransport mechanism determines the electronic propertiesthough we cannot at this point distinguish between magnpolarons or lattice polarons. Similar conclusions were otained on thin film La0.67Ca0.33MnO3.19After discussing the temperature dependent term inentropy, we now turn to the temperature independent teThe constant termS0 has a very simple, unambiguous intepretation: in the high-temperature limit, when effects of iteraction can be neglected, it measures the entropy carriethe mobile particles.11 This quantity should know about thconfigurational entropy, thereby showing a strongx depen-dence, and the spin entropy. However, we find experimtally thatS0 is almostx independent. Assuming that each Cresults in a positively charged particle with aS53/2 spin inthe undopedS52 spin system, we can use Heike’s generized formula for configurational degeneracy including tspin degenaracy,16 by taking the derivative of the degeneracy,g, with respect to the number of carriersN3/2:ncyoiteftheisthhareinfobaraeingrrieeen-spe-ure.allisfe-de-ri-istheort-byofer-y,inggh-ofe-p-56 5107BRIEF REPORTSS5kBueu•] ln~g!]N3/25kBueu•F ln x~12x! 1 ln~4/5!G . ~3!The first term is the conventional configurational term, athe second term represents the change in spin degeneraintroducing carriers. For mobile electrons with spin51/2 oneobtains a spin contribution of2kB /ueu• ln(2)'260mVK.16Because we assume mobile positively charged particles ming in a rigid S52 system with a spin degeneracy per sgS54x•5(12x), we expect a backflow of spin entropy okB /ueu• ln(4/5)'220mVK, which is in good agreemenwith our experimental result. However, this implies that tconfigurational contribution to the thermopower, whichexpected especially at low doping to be much larger thanspin contribution, is effectively quenched. This means tthe configurational degeneracy is at an extremum withspect to changes in the carrier concentration. This caninterpreted either as the configurational entropy being mmized with respect to changes in number of carriers~chargeordering!, or as the entropy being maximized as expectedsystems near half filling. The latter state could possiblyrealized by microscopic phase separation in regions nex50 andx50.5. The extraordinary aspect of our data is ththe configurational entropic contribution to the thermopowis very small over the entire concentration range 0.1<x<0.6.With the above analysis we have made the simplifyassumption that each Ca adds one positively charge cainto the system. This does not take into account the carrinduced by ‘‘self-doping,’’ i.e., carriers introduced by thK.HWv.r.yaodbyv-et-bei-retrierrsexistence of vacancies on the La and Mn sites. Their conctration depends on the exact preparation conditions, ecially the sintering temperature and oxygen partial pressIt was shown by Tamuraet al.20 that for x50.2 the vacancyconcentration increases markedly only for extremely smoxygen partial pressures below 1022 Pa at 1473 K. Ma-hendiran et al.21 show that the vacancy concentrationbigger for smallerx. Nevertheless, for small amounts o‘‘self-doping’’ the spin entropy is unaffected, since it is dtermined by the ratio of the spin degeneracies and is inpendent of the doping concentration. Similarly, our expemental observation that the configurational termindependent of the doping concentration holds either fornominal doping or the actual doping concentration.In conclusion we have shown evidence that the transpmechanism aboveTc is not dominated by spin disorder, resulting from the double exchange mechanism, but insteadsmall-polaron formation, presumably involving distortionsthe Mn-O octahedra. The high-temperature limit of the thmopower is in agreement with backflow of spin entropexpected from positively charged mobile particles, with spS53/2, in a rigidS52 system. At the same time the dopinindependence of the Seebeck coefficient in the hitemperature limit hints at a remarkable collective naturethis fluid.We acknowledge stimulating discussions with P. Littlwood, A. Millis, and B. Shraiman. J.Z. acknowledges suport by the Dutch Royal Acadamy of Sciences~KNAW !. Wethank W. Bao for help with sample preparation.ki,a, J.op,S.-hys.1J. H. Jonker and J. 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Transport mechanisms in doped LaMnO3: Evidence for polaron formation

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T. T. M. Palstra

A. P. Ramirez

S-W. Cheong

B. R. Zegarski

P. Schiffer

J. Zaanen

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Transport mechanisms in doped<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">LaMnO</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mo>:</mml:mo></mml:math>Evidence for polaron formation

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Transport mechanisms in doped LaMnO3:Evidence for polaron formation

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University of Groningen