In uence of grain size and grain boundaries on the properties of La 0.7 Sr 0.3 Co x Mn 12x O 3

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1 In uene of grin size nd grin oundries on the properties of L.7 Sr.3 Co x Mn x O 3 S. Ds, P. Chowdhury, *, T.K. Gundu Ro, D. Ds, D. Bhdur,1 Deprtment of Metllurgil Engineering nd Mterils Siene, Indin Institute of Tehnology, Bomy 476, Indi Regionl Sophistited Instrumenttion Center, Indin Institute of Tehnology, Bomy 476, Indi Astrt We hve synthesized L.7 Sr.3 Mn.9 Co.1 O 3 grnulr perovskites using ethylenediminetetreti id EDTA) s omplexing gent. The in uene of grin size nd grin oundry struture on the physil properties ws investigted. Our results show tht deresing grin size leds to lowering of the ferromgneti trnsition temperture, rodening of the trnsition, derese in mgnetiztion nd enhnement of resistivity nd mgnetoresistne t low tempertures. Also, eletron spin resonne linewidth nd g-vlue will derese. These results re explined y spin dependent sttering t the grin oundries nd wekening of mgneti intertion ner the surfe of grins, whih redues the internl eld in the system. Keywords: Mngnites; Resistivity; Mgnetoresistvity; Eletron spin resonne; Grin oundry 1. Introdution The in uene of the grin oundries or the grin size in mngnese perovskites on the mgneti nd mgnetotrnsport properties hs reently een investigted extensively [1,2]. One of the erliest reports on the effet of prtile size on the physil properties of mngnites ws y Mhesh et l. [3], where they hve vried the prtile size from.2 to 3. mm nd found signi nt hnges in the properties. It ws oserved y most of the reserhers tht the low temperture mgnetoresistne enhned sustntilly with the derese in grin size. Severl theoretil models hve een put forwrd to understnd the mehnism of inter-grin tunneling ondution. One of the rst models proposed y Hwng et l. [4] ws sed on the spin polrized tunneling SPT). In systems like mngnites, SPT my our, ut it is not neessrily the dominnt use of mgnetoresistne. Reently Zhng et l. [] hve studied the mgneti properties of LSMO systems nd proposed spin dependent intrgrnulr sttering in these systems. Aording to them, the intergrnulr sttering results from spin dependent surfe effet, relted to the differene in the mgneti struture etween the ore nd the surfe of the perovskite grins. This results in n enhned mgnetoresistne t low tempertures due to eld-indued hnge of interfil spin on gurtion. Previous eletron spin resonne ESR) studies on mngnities exhiiting olossl mgnetoresistne CMR) hve reported strong ESR lines oth in the prmgneti nd ferromgnetilly ordered phses [6]. Lrge vrition of the line prmeters like intensity nd linewidth were oserved s funtion of temperture. A rossover from ottleneked regime, where Mn 41 nd Mn 31 ions re strongly oupled, to n isotherml one ws proposed [7] to explin the oserved vrition of linewidth with temperture. In this ontext, ESR is n extremely useful tehnique to study mgneti orreltions on mirosopi level nd is well suited to the study of mngnites. In the present investigtions, we hve used low temperture sol±gel proessing to synthesize L.7 Sr.3 Co x Mn x O 3 LSCMO) with x ˆ :1;.3 nd. nd investigted the

2 692 Tle 1 Het tretment detils, revitions used for smples nd the estimted grin sizes Smple no. Dwell time h) Men grin size mm) LSMCO1..2 LSMCO1 2. LSMCO1.62 LSMCO1d 8.81 LSMCO1e 1.2 Mgnetistion (emu/gm) o C, 8 h 9 o C, h metl±insultor trnsition temperture through the resistivity mesurements. Mngnese nd olt ions re hemilly different with respet to ioni rdii nd eletroni on gurtions. Mn 31 t 3 2ge 1 g is Jhn Teller ion, wheres Co 31 n exist in high spin t 4 2ge 2 g nd low spin t 6 2ge g sttes. These my hve strong in uene on lttie distortion, mgneti exhnge s well s eletron trnsport ehvior. It is oserved tht the omposition with x ˆ :1is the est ompromise smple showing trnsition loser to room temperture with no hrmful effets on mgneti nd mgnetotrnsport properties. Thus, we hve investigted the in uene of the grin size on this prtiulr smple y sintering it t 9 8C for different dwell times. These smples were hrterized y X-ry diffrtion XRD), snning eletron mirosopy SEM), eletril resistivity, mgnetoresistne nd mgnetiztion mesurements. ESR 9 o C,. h experiments hve een rried out to further understnd the mgneti orreltions on mirosopi level. The results on the smple L.7 Sr.3 Mn.9 Co.1 O 3 LSCMO1) re presented in this pper. 2. Experimentl Temperture (K) Fig. 2. Vrition of mgnetiztion of LSMCO1 smples sintered t 9 8C for ). h, ) h, nd ) 8 h.. µ m Fig. 1. SEM of LSMCO1 smples sintered t 9 8C for ). h, ) 2 h, nd ) 8 h. Mngnite hving the formul L.7 Sr.3 Mn.9 Co.1 O 3 LSCMO1) hs een synthesized y mixing stoihiometri mounts of the individul metl nitrtes to form the trget omposition. One mole of this trget omposition ws mixed with 4 mol of itri id nd 3 mol of ethylene glyol nd heted t 8 8C until gel ws formed. To remove the orgni prts, the gel ws then heted t 3 8C. Differentil therml nlysis DTA) nd thermo-grvimetri nlysis TGA) studies of the preursor powders were done to estimte the lintion temperture. Bsed on this, the resultnt powder ws lined t 8 8C in ir for 2 h. The lined powder ws then pelletized nd sintered t 9 8C for., 2,, 8 nd h, respetively, to otin different grin size. The smples were hrterized y powder XRD Philips PW 1729) using Cu K rdition. SEM Cme SU 3) ws used to see the mirostruture. The mgnetiztion with temperture ws mesured using virting smple mgnetometer Oxford VSM). The temperture dependene of resistivity r T ) ws mesured y four-proe tehnique in the temperture rnge 1±3 K using losed yle refrigertor. Mgnetoresistivity mesurements hve een rried out from 3 to 8 K using liquid N 2 gs- ow ryostt. ESR experiments were rried out on Vrin Century series E-Line E-1 ESR spetrometer whih utilizes khz eld modultion.

3 693 Tle 2 Different prmeters otined for smples with vrying grin sizes Men grin size mm) T p K) T K) r 8 K) V m) r T p) V m) MR% 8 K) ± 348 ± Results nd disussion Smple prepred y ermi route sintered t 8C for 2 h) shows tetrgonl struture with lttie prmeter ˆ ˆ 3:893 A nd ˆ 11:2 A: On the other hnd, the smples prepred y sol±gel route, sintered t the sme temperture, shows ui struture with lttie prmeter ˆ ˆ ˆ 3:883 A: All the smples sintered t 9 8C for different dwell times given in Tle 1 show ui struture. The derese in the linewidth with inrese in dwell time ws oserved in the XRD pttern of the smples, whih indites n inrese in grin size. The grin size nd shpe of the smples were noted from the photogrphs of snning eletron mirosope Resistivity (Ω m) K Temperture (K) 22.3 K K Fig. 3. Vrition of resistivity nd mgnetoresistivty with tempertures for LSCMO1 smples sintered t 9 8C for different times: ). h, ) 2 h, ) h nd d) 8 h. d Mgnetoresistne(%) Fig. 1 )± )). As given in Tle 1, the grin size inreses from out.2 to 1.2 mm s the dwell time inreses from. to h. A typil vrition of mgnetiztion with temperture mesured t 2 koe for the three grin-size vried smples is shown in Fig. 2. The Curie temperture T ws tken from the in exion point of M±T urve nd is given in Tle 2. With derese in grin size, the vlue of M lowers. But the derese in mgnetiztion is firly lrge for smple LSMCO1 where the grin size is estimted round.2 mm. Seondly, T dereses signi ntly for this smple. There re similr reports in the literture. For exmple, Hung et l. [8] oserved suh trend of M with grin size in pure LSMO smples sintered t different tempertures. Iss et l. [9] nd Snhez et l. [] showed tht the spins eome hevily disordered t the grin oundry due to strin with lowering of the grin size, whih my result in redution of the vlue of M s nd T. Gupt et l. [11,] explined the vrition in properties with grin size s the re etion of mgneti disorder in the grin oundry region nd disorder indued nting of Mn spins ner the surfe of the grins. The depression in T in grin size vried smples studied here re onsistent with these pitures. Fig. 3 )± d) shows the temperture dependene of resistivity with nd without mgneti eld for smples sintered t 9 8C for different dwell times: ). h, ) 2 h, ) h nd d) 8 h. These gures lso show the vritions of MR with temperture. All the resistivity urves show only rod pek t temperture T p : The rod trnsition is hrteristi of the smll grin size. With the grin growth, the temperture T p shifts to higher temperture, wheres the mgnitude of resistivity redues y two orders of mgnitude. All the prmeters otined from mgneti nd mgnetotrnsport mesurements re given in Tle 2. The vlue of MR is mximum 2%) for the smple with smllest grin size. The mgneti inhomogenity t the grin oundries serves s strong sttering enters. The more the grin oundry region, the more should e the MR, s oserved in our present smples. Also, s the oundries of the grins onsist of more mgneti disorder thn the ores, the resistivity redues drstilly with the grin growth. For porous grnulr perovskite systems, Zhng et l. [] proposed tht the resistivity hs two ontriutions: 1) the

4 694 ρ sur (8 K) / ρ(8 Κ) w/D Grin size (µm) Grin size (µm) Intensity (Aritrry Units) g vlues Temperture (K) 2 Mgneti field (mt) Fig. 6. ESR spetr of the smple L.7 Sr.3 MnO 3 t tempertures: ) 2 8C, ) 6 8C nd )14 8C. Inset shows the vrition of g s funtion of tempertures for the sme smple. 4 Fig. 4. Grin size dependene of the interfil resistivity ontriution to totl resistivity r sur =r for grin size vried LSMCO1 smples. Inset shows the vrition of surfe volume frtion 6w/D with grin sizes. ody phse nd 2) the surfe phse. The surfe ontriution to resistivity with omptness hs een given y r sur ˆ fs r t where r t is the tunnel resistivity, f s is volume frtion oupied y the surfe nd is equl to 6w/D. Here, w is the verge thikness of the grin surfe nd D is the verge grin size. The vrition of r sur 8 K)/r 8 K) nd tht of 6w/D versus D is shown in Fig. 4. Here, r 8 K) is the ore resistivity nd hs een tken from the smples prepred y Intensity (Aritrry Units) g=2.277 Mgneti field mt H Fig.. Room temperture ESR spetr of the smples: ) LSMCO1, ) LSMCO1, ) LSMCO1, d) LSMCO1d nd e) LSMCO1e. d e 1 sol±gel nd sintered t 8C. r sur 8 K) is estimted from the differene etween the smple resistivity nd the ore resistivity [r 8 K)]. The verge thikness of the grin surfe is tken roughly s 2 nm s proposed y Zhng et l. [13]. Sine the surfe resistivity results from the sttering t the grin oundries, its mgnitude my e higher thn the ore resistivity. Therefore, s the surfe volume frtion 6w/D dereses with grin growth, the sttering t the oundries lso dereses resulting in redued r sur. Thus one should expet tht r sur /r nd 6w/D follow similr ehvior s lso reported y Zhng et l. [14]. The differene etween them in Fig. 4 n e ttriuted due to the omptness of grins s the sintering time inreses. Fig. shows the room temperture ESR spetr oserved in the smples LSMCO1, LSMCO1, LSMCO1, LSMCO1d nd LSMCO1e. In the system LSMCO1, single ESR line with gyromgneti ftor g, 2 is oserved t room temperture. With inresing grin size, the resonne line is oserved to shift to lower elds. The oserved ESR lines in LSMCO system hve to e exmined in the ontext of the ndings in L.7 Sr.3 MnO 3 system. Fig. 6 shows the ESR spetr of L.7 Sr.3 MnO 3 t 2, 6 nd 14 8C. The g vlue vrition, whih in effet re ets the resonne line position, is shown in the inset of Fig. 6. The g vlue is found to derese with inresing temperture nd remins onstnt ove the ferromgneti trnsition temperture of 8C oserved in L.7 Sr.3 MnO 3 system. Signi ntly high intensity ESR lines re oserved in doped mngnese perovskites investigted in the present study. Severl erlier investigtions hve onsidered the origin of the ESR signl in these perovskites [6,7]. These mngnese perovskites re mixed vlent systems ontining Mn 31 nd Mn 41 ions. Mn 31 3d 4 with S ˆ 2) exhiits lrge zero eld splitting nd strong spin±lttie relxtion [1] nd is unlikely to exhiit n ESR signl. On the other hnd, Mn 41 3d 3 with S ˆ 3=2), in n othedrl eld, hs wek spin lttie relxtion mking it oservle t room

5 69 temperture [16]. However, the Mn 41 ESR signl will e of lower intensity y ftor of out 7 less intense s ompred, for exmple, to the signl oserved in L.7 C.3 MnO 3 system) [6]. Hene, the intense signls seen t room temperture in the present system re inonsistent with either Mn 31 or Mn 41 ions. Oseroff et l. [6] were the rst to report the oservtion of n ESR signl in L x C x MnO 31d ompounds. The oserved strong ESR signls with unusul temperture dependene hs een ttriuted y them to omplex mgneti entity mde of olletion of Mn 31 nd Mn 41 ions. The present studies of the system L.7 Sr.3 Mn.9 Co.1 O 3, Mn 31 nd Mn 41 ions onstitute mjor omponent of mgneti ions. The oserved ESR signl in the present system rises lso from spin omplex formed y Mn 31 nd Mn 41 ions s in the system L x C x MnO 3 [6]. In our study on L.7 Sr.3 MnO 3, the pprent inrese in the g vlue elow T ould e due to n internl eld, whih rises from the onset of Mn±Mn exhnge. Within this frmework of oservtion, let us exmine the room temperture spetr oserved in smples with different grin sizes. It is found tht the g vlue generlly inreses in smples with lrge grin sizes. It thus ppers tht lrger grin size smples hve lrger internl elds, whih my e ttriuted to stronger Mn±Mn exhnge intertion. Mgnetiztion mesurements rried out in the present study on smples with different grin size support ESR oservtions. Mgnetiztion vlues re found to inrese with inrese in grin size. Smller grin size smples hve reltively lrger grin oundry region nd the disorder due to lrge strins present within the grin oundries n indue wekening of mgneti intertion etween Mn 31 nd Mn 41 ions. The wekening of the mgneti intertion is responsile for the suppression of T nd redution in mgnetiztion. 4. Conlusion The in uene of grin size on the mgnetotrnsport in L.7 Sr.3 Mn.9 Co.1 O 3 hs een investigted. The experimentl evidenes suh s mgnetiztion, resistivity, mgnetoresistne long with mirostruturl studies demonstrte tht spin dependent sttering t the grin oundry is responsile for low temperture mgnetoresistne. This enhnement in MR with derese in grin size is ttriuted to more re within the grin oundries. Progressive derese in linewidth nd g vlue of ESR signl with derese in grin size is inditive of redution of internl eld s is supported y mgnetiztion dt. For smller grins, lrger vlues of grin oundry region re ville, whih re ssoited with lrge strins disrupting the short-rnge mgneti intertion nd possily use nting of spins. This mehnism is presumly responsile for ll the oservtions of the mgneti nd mgneto trnsport ehvior. Aknowledgements The nnil support from Deprtment of Siene nd Tehnology, Government of Indi, is grtefully knowledged. Referenes [1] F. Dmmy, L.F. Cohen, J. Mmnus-Drisoll, R.S. Freits, L. Ghivelder, J. Mgn. Mgn. Mt ) L149. [2] L.E. Hueso, F. Rivdull, R.D. Snhez, D. Ceiro, C. Jrdon, J. Vzquez-Vzquez, M.A. Rivs, J. Mgn. Mgn. Mt ) 321. [3] R. Mhesh, R. Mhendirn, A.K. Ryhudhuri, C.N.R. Ro, Appl. Phys. Lett ) [4] H.Y. Hwng, S.W. Cheong, N.P. Ong, B. Btlogg, Phys. Rev. Lett ) 241. [] N. Zhng, W. Ding, W. Zhong, D. Xing, Y. Du, Phys. Rev. B ) [6] S.B. Oseroff, M. Torikhvili, J. Singley, S. Ali, S.W. Cheong, S. Shultz, Phys. Rev. B ) 621. [7] A. Shengely, Guo-Zhong, H. Keller, K.A. Muller, Phys. Rev. Lett ) 296. [8] Y.-H. Hung, Z.-G. Xu, C.-H. Yn, Z.-M. Wng, T. Zhu, C.- S. Lio, S. Go, G.-X. Xu, Solid Stte Commun ) 43. [9] S.P. Is, N.D. Mthur, J.E. Evets, M.G. Blmire, Appl. Phys. Lett ) 238. [] R.D. Snhez, J. Rivs, C. Vzquez-Vzquez, A. Lopez- Quintel, M.T. Cus, M. Tovr, S. Oseroff, Appl. Phys. Lett ) 134. [11] A. Gupt, G.Q. Gong, G. Xio, P.R. Dunome, P. Leoeur, P. Trouilloud, Y.Y. Wng, V.P. Drvid, J.Z. Sun, Phys. Rev. B ) R1629. [] A. Gupt, Clossl Mgnetoresistne, Chrge Ordering nd Relted Properties of Mngnese Oxides, World Sienti, Singpore, 1998 p [13] N. Zhng, S. Zhng, W. Ding, W. Zhong, Y. Du, Solid Stte Commun. 7 21) 417. [14] N. Zhng, W. Ding, W. Zhong, Phys. Lett. A ) 113. [1] A. Argm, B. Bleney, Eletron Prmgneti Resonne of Trnsition Ions, Clrendon press, Oxford, 197. [16] K.A. Muller, Phys. Rev. Lett ) 341.