SolidStateCommunications145(2008)

197–200www.elsevier.com/locate/ssc

HalfmetallicityinPr0.75Sr0.25MnO3:Afirstprinciplestudy

MonodeepChakraborty∗,PrabirPal,BijuRajaSekhar

InstituteofPhysics,SachivalayaMarg,Bhubaneswar751005,India

Received15October2007;accepted20October2007byA.H.MacDonaldAvailableonline25October2007

Abstract

InthiscommunicationwepresentafirstprinciplestudyofPr1−xSrxMnO3withx=0.25.Whiletheparentcompoundsofthissystemareantiferromagneticinsulatorswithdifferentstructuralandmagneticgroundstates,thex=0.25isinthecolossalmagnetoresistanceregimeofthePr1−xSrxMnO3phasediagram[C.Martin,A.Maignan,M.Hervieu,B.Raveau,Phys.Rev.B60(1999)12191].Ourbandstructurecalculationsfortheend-pointcompoundsmatcheswellwiththeexistingtheoreticalandexperimentalresults[C.Martin,A.Maignan,M.Hervieu,B.Raveau,Phys.Rev.B60(1999)12191;RuneSondena,P.Ravindran,SveinStolen,TorGrande,MichaelHanfland,Phys.Rev.B74(2006)144102].Interestingly,ourcalculationsshowthatthePr0.75Sr0.25MnO3hasahalf-metalliccharacterwithahugebandgapof2.8eVintheminorityband.Webelievethisresultwouldfuelfurtherinterestinsomeofthesespecialcompositionsofcolossalmagnetoresistivemanganitesastheycouldbepotentialcandidatesforspintronicdevices.Wediscussthehalf-metallicityofthePr0.75Sr0.25MnO3inthelightofchangesintheorbitalhybridizationasaresultofSrdopinginPrMnO3.Further,wehighlighttheimportanceofhalf-metallicityforaconsolidatedunderstandingofcolossalmagnetoresistanceeffect.c󰀃2007ElsevierLtd.Allrightsreserved.

PACS:75.47.Gk;72.25.-b;71.20.-b

Keywords:A.Colossalmagnetoresistance;C.Half-metallicity;D.Electronicstructure

1.Introduction

Thecolossalmagnetoresistance(CMR)materialshave

attractedalotofattentionofthecondensedmattercommunity

owingtotheirspectacularinsulator-metaltransitionwith

magneticfield.FerromagneticA1−xBxMnO3(A=rareearth,

B=alkalineearth)exhibitsCMRpropertiesatparticular

concentrationsofxintheirrespectivephasediagrams.Half-

Metallicity(HM)hasbeenobservedinafewofthese

compoundsboththeoreticallyandexperimentally[3,4].Incase

ofhalf-metalsoneofthespinbands(generallythemajority

band)isconductingwhereastheotherband(generallythe

minorityband)isinsulatingattheFermilevel(EF).This

facilitates100%spinpolarization.Thispropertyofthehalf-

metalsmakethempotentialcandidatesforapplicationin

spintronicdevisesandmagneticsensors.TheCMReffectalong

withhigh-spinpolarizationaddtothetechnologicalimportance

∗Correspondingauthor.Tel.:+916742301058;fax:+916742300142.E-mailaddress:monodeep@iopb.res.in(M.Chakraborty).oftheCMRmanganites.Apartfromtheirgreatpotentialin

technology,thestronginterplayofthespin,orbitalandcharge

degreesoffreedomofthechargecarriersinvolvedinthis

insulator-metaltransition,holdsoutapromiseforrichphysics.

InthispaperwehavedoneafirstprincipleTight

Binding-LinearizedMuffinTinOrbital(TB-LMTO)[5,6]

calculationoftheend-pointcompositionsofPr1−xSrxMnO3andwithx=0.25doping.ForSrMnO3wehavedonethe

calculationwithlocalspindensityapproximation(LSDA).

ForPrMnO3andPr0.75Sr0.25MnO3wehadtoincorporate

theelectron–electroncorrelation(LSDA+U)toaccountfor

thebandgapinPrMnO3andtomatchourresultswiththe

availablespectroscopicdata.Moreover,thechargeandorbital

orderobservedindopedmanganitesalsomeritsaLSDA+

Utreatmentinordertoaccountfortheintra-shell(dand

f)Coulombinteraction[7].Allthethreecalculationshave

beendonewithVosko–Ceperley–Alderparametrizationforthe

exchangecorrelationenergyandpotential.Wehaveincluded

Langreth–Mehl–Hugradientcorrectionstotheexchange

correlation.Thek-meshusedforalltheseself-consistent

calculationswas10×10×10.Although,SrMnO3cantake

0038-1098/$-seefrontmatterc󰀃2007ElsevierLtd.Allrightsreserved.doi:10.1016/j.ssc.2007.10.025198M.Chakrabortyetal./SolidStateCommunications145(2008)197–200

bothcubicaswellashexagonalstructures[2],herewe

haveconsideredonlythecubic(distorted)polymorphofthis

perovskitesinceourmainmotivationistostudytheHMin

Pr0.75Sr0.25MnO3.Wehavealsocomparedthebandstructure

resultsofPr0.75Sr0.25MnO3withtheend-pointcompounds.

CrystalstructureofthePrMnO3systemistakenfroma

publishedneutrondiffractiondata.ForPr0.75Sr0.25MnO3we

havetakenthesamestructureasPrMnO3withonePratom

replacedbyadivalentSr.

Electron-latticecoupling(ELC)hasaveryimportantrolein

thephysicsofmanganites.Theyshowupintwoways.Firstis

thesocalled“tolerancefactor”[8]involvingthestaticeffect

ofcrystalstructureonelectronhopping,whichhasadirect

effectonconductivity.Theatomicsizedifferencebetween

therare-earthatomsandthedivalentdopantsresultsinan

internalstresswhicheffectstheMn–O–Mnbonds.Theelectron

hoppingbetweentheMnsitesisinverselyproportionaltothe

compressionoftheMn–O–Mnbonds.ThistypeofELCof

PrMnO3hasbeentakenintoaccountinourcalculations.

TheimportanceofdifferentJahn-Tellermodesandpolarons

inaccountingtheproperinsulatingA-typeantiferromagnetic

(AFM)groundstateofundopedLaMnO3hasbeendealtby

severalgroups[3,9,10].ThesecondtypeofELCisthedynamic

ELCwhichcouplesthelatticevibration(phonons)with

electronicdegreesoffreedom.SinceTB-LMTOcalculations

arebasedonadiabaticapproximationwhichdecouplesthe

electronicandthelatticedegreesoffreedom[8],accounting

forthedynamicELCisbeyondthescopeofthiswork.

Bandstructurecalculationscanprovideonlyaqualitative

descriptionofanysystemasthestructuralcomplications

thatexistinrealsystemsaredifficulttoaccommodateina

calculation.Although,weassumeproperstoichiometry,cation

vacancyandoxygennon-stoichiometryareacommoninreal

systems.AgaininPr0.75Sr0.25MnO3wehaveassumedthe

crystalstructureofPrMnO3.TheeffectofPr/Srdisorderand

localstrainsandrelaxationshavenotbeentakenintoaccount.

ButstillourpredictionofHMinPr0.75Sr0.25MnO3isrobust

enoughtomeritattention.

2.SrMnO3

CubicSrMnO3hasaG-typeAFMgroundstatewitha

latticeparameter[2]of3.824A.Ourcalculationswithdifferent

magneticconfigurationshaveshownthatthisG-typeAFM

stateturnsoutbethemoststable,inagreementwithearlier

reports[2].InthisworkwehaveusedonlytheG-typeAFM

structure.ResultsofourTB-LMTO(LSDA)calculationsare

showninFig.1.Thetotalandsite-projecteddensityofstates

(PDOS)clearlyrevealsabandgapof0.34eVwhichissimilar

tothatobtainedbyotherbandstructurecalculations[2]on

thissystem.ThenearEFstatesaredominatedbyMn3dand

O2p.ThenearEFvalancebandshowsastrongMn3d–O2p

hybridization.Although,Srstateshaveaverylittlepresence

nearEF,theyappeartobehybridizedwiththesignificantO

statesthroughoutthevalenceband.Itisclearfromthefigure

thattheconductionstatesaredominatedmainlybyMn.We

haveestimatedthemagneticmomentofindividualMn

atomFig.1.Thespinresolvedtotalandsite-projecteddensityofstatesofSrMnO3.Thetopleftpaneldepictsthetotaldensityofstatesforthemajorityandminorityband.Thebandgapis0.34eV.ThetoprightpanelshowsthespinresolvedMnPDOS.ThebottomleftpanelshowstheSrPDOSandthebottomrightshowstheOPDOS.

tobe2.48µβwhichisagainsimilarto2.47µβobtainedby

calculationsusingtheViennaAb-InitioSimulationPackage

(VASP)[2].Thisvalueofthemagneticmoment,whichis

lowercomparedtothatoffreeMn4+ion(3µβ,neglectingthe

orbitalcontribution),indicatesthestronghybridizationofthe

MnandOstatesincubicSrMnO3.Itshouldbenotedthat

theshapeoftheOPDOSandtheMnPDOSnearEFhave

strikingsimilarities,whichhintsatastrongcovalentbonding.

ThisinferenceissupportedbytheCrystalorbitalHamiltonian

population(COPH)analysisdonebyRuneSondenaetal.[2].

3.PrMnO3

LSDA+Uband-structurecalculationsforPrMnO3were

donefortheA-typeAFMphaseusingthecrystalstructure

(spacegroup:Pbnm(62))takenfromaneutrondiffraction

result[11]publishedearlier.TheLSDA+Umethodwas

employedmainlytoaccountforthestrongelectroncorrelation

whichisbehindtheinsulatingnatureofthismaterial[1].Unlike

inLaMnO3wheresimpleLSDAcanreproducethebandgap,

inPrMnO3theLSDA+Utreatmentisessentialtoderivea

realisticvalueofthegap.Inourcalculation,wehavetakenthe

exchangetermJandthecorrelationtermUforPr4fstateto

be0.95eVand7eVrespectivelyandthoseforMn3dtobe

0.87eVand4eVrespectively.

Fig.2showsthespinresolvedtotalandsiteprojected

densityofstatesofPrMnO3.OurLSDA+Ucalculation

showsaninsulatingA-typeAFMgroundstateforPrMnO3.

Wehaveestimatedthebandgaptobe0.11eV.Here,the

electron–electroncorrelationthathasbeenincorporated,was

foundtobecruciallyimportantforobtainingthebandgap.

AsimpleLSDAcalculationofthissystemdoesnotgivean