Peculiarities in the electrical and magnetic properties of cobalt perovskites Ln1−xMxCoO3 (Ln3+: La3+, M2+: Ca2+, Sr2+, Ba2+; Ln3+: Nd3+, M2+: Sr2+)

Peculia i ies in he elec ical and magne ic p ope ies o cobal pe o ski es Ln1−xMxCoO3 (Ln3+: La3+, M2+: Ca2+, S 2+, Ba2+; Ln3+: Nd3+, M2+: S 2+) M.A. Seña ís Rod ígueza, , M.P. B eijob, S. Cas oa, C. Reya, M. Sáncheza, R.D. Sánchezb, J. Mi ac, A. Fondadoc, J. Ri asc a Dp . Química Fundamen al e Indus ial, Uni e sidad de A Co uña, 15071 A Co uña, Spain b U.A. Tecnología de Ma e iales y Disposi i os, Cen o A ómico de Ba iloche, 8400 San Ca los de Ba iloche, A gen ina c Dp . Física Aplicada, Uni e sidad de San iago de Compos ela, 15706 San iago de Compos ela, Spain In e na ional Jou nal o Ino ganic Ma e ials Volume 1, Issues 3–4, Sep embe –Oc obe 1999, Pages 281–287 A ailable online 29 Oc obe 1999 doi:10.1016/S1466-6049(99)00042-2 Abs ac We e e he e o he elec ical and magne ic p ope ies o he Ln1−xMxCoO3 sys ems (Ln3+: La3+, M2+: Ca2+, S 2+, Ba2+; Ln3+: Nd3+, M2+: S 2+), paying special a en ion o hose e omagne ic compounds ha display M–I ansi ions as empe a u e ises: La1−xMxCoO3 (M2+: Ca2+, S 2+, Ba2+) in he composi ional in e al x=0.2–0.3, and Nd1−xS xCoO3, wi h x=0.40. The magne o- anspo p ope ies o such ma e ials a e peculia and in e es ing: hey show diodic beha io and la ge elaxa ion e ec s — hese la e being specially impo an in he Nd compound — hey display © 1999 Else ie Science L d. This manusc ip e sion is made a ailable unde he CC-BY-NC-ND 4.0 license (h ps:// c ea i ecommons.o g/licenses/by-nc-nd/4.0/) magne o esis i e e ec s specially a he M–I ansi ion empe a u es, and hey age wi h ime. All hese esul s a e discussed on he basis o he inhomogeneous elec onic s uc u e o hese doped cobal pe o ski es and aking in o accoun he in luence o he lan hanide ion on hei magne ic and elec ical p ope ies. Keywo ds A. oxides; D. elec ical p ope ies; D. magne ic p ope ies 1. In oduc ion The in e play be ween magne ism and elec ical conduc i i y in pe o ski e- ype oxides has been a ma e o s udy in he las decades [1] and wi h enewed in e es since he disco e y o colossal magne o esis ance (CMR) in doped manganese-based compounds [2] and [3]. Wi hin his amily o compounds, cobal i es cons i u e ano he specially in e es ing sys em in iew o he peculia way hei magne ic and elec ical p ope ies change wi h empe a u e and upon doping [4], [5], [6],[7] and [8], and mo e ecen ly in iew o he magne o esis i e e ec s ound in La1−xMxCoO3 (M=S , Ba) sys ems [9], [10] and [11]. Ea ly wo k on he undoped LaCoO3 compound [4] es ablished he exis ence o a he mally induced spin ansi ion a he Co ions esponsible o i s peculia beha io [5]. Since hen many s udies ha e ied o elucida e in mo e de ail he cha ac e is ics o such ansi ion, no only in LaCoO3[12], [13], [14] and [15] bu also in o he LnCoO3 (Ln= a e ea hs) compounds [13], [16], [17] and [18]. Those ea ly s udies also showed ha while LaCoO3 shows high esis i i y and an i e omagne ic exchange in e ac ions, upon doping he La1−xS xCoO3 ma e ials e ol e owa ds a e omagne ic me allic beha io [6]. Since hen, he magne ic and elec ical p ope ies o he La1−xS xCoO3 sys em ha e been epea edly in es iga ed, inding ha he e olu ion akes place smoo hly, and ha a numbe o di e en magne ic and elec ical beha io s a e p esen o he di e en deg ees o doping: supe pa amagne ism, spin-glass/clus e -glass beha io s, e c., semiconduc ing/me allic beha io s, and e en me al–insula o ansi ions as a unc ion o empe a u e [7] and [19]. Those s udies ha e also been ex ended o Ca- and Ba-doped La1−xMxCoO3 (M=Ca, Ba) ma e ials[8] and [11], and o o he Ln1−xMxCoO3 sys ems (Ln= a e ea hs) [8] and [20] ha simila ly show in e es ing elec ical and magne ic p ope ies. Al hough he gene al end in hese Ln1−xMxCoO3 se ies o compounds (Ln=La, a e ea h, M=Ca, S , Ba) is again ha he ma e ials end o me allic and e omagne ic beha io as x inc eases, he cha ac e is ics o ha e olu ion depends on he na u e o he lan hanide ion [8] and [20] and he na u e o he dopan [8]. And while he p ope ies o he end membe s o he sys ems seem o be well es ablished, di e en magne ic and elec ical beha io s ha e been epo ed o he in e media e membe s o he se ies [7], [8] and [19]. I is in ha in e media e composi ional ange ha we ha e ocused ou s udies. And in his pape we epo peculia magne o- anspo p ope ies exhibi ed by ce ain Ln1−xMxCoO3 (Ln3+: La3+, M2+: Ca2+, S 2+, Ba2+; Ln3+: Nd3+, M2+: S 2+) samples wi h x in he ange 0.2–0.4 o which we ind e omagne ic beha io and me al-insula o ansi ions as empe a u e ises. 2. Expe imen al La1−xMxCoO3 (M=Ca, S , Ba) and Nd1−xS xCoO3 we e p epa ed by decomposi ion o he co esponding mix u e o ni a es. S oichiome ic amoun s o d y La2O3/Nd2O3, CaCO3/S CO3/BaCO3 and Co(NO3)2 we e dissol ed in ni ic acid. The esul ing solu ion was gen ly wa med up so as o slowly e apo a e he sol en . The so-ob ained mix u e o ni a es was decomposed a 600°C. The esul ing p oduc s we e p essed in o pelle s and annealed a empe a u es anging om 970°C, o he Nd1−xS xCoO3 se ies, up o 1175°C, o he La1−xCaxCoO3 samples. In all cases, a e he he mal ea men a high empe a u es, he samples we e cooled slowly o oom empe a u e (0.7°/min). The p oduc ma e ials we e examined by X- ay powde di ac ion wi h a Siemens D- 5000 di ac ome e and Cu Kα=1.5418 Å adia ion. The mo phology and size o he pa icles we e s udied in a scanning elec on mic oscope (SEM) Jeol 6400. The mog a ime ic analyses (TGA) and iodome ic i a ions unde an a gon a mosphe e we e ca ied ou o s udy he oxygen con en o he samples. Magne ic p ope ies we e s udied in a DMS-1660 Vib a ing-Sample Magne ome e . Ze o- ield cooled (ZFC) and ield cooled (FC) magne ic suscep ibili y da a we e ob ained in a ield o 1000 Oe om 77 o 330 K. ZFC magne iza ion cu es M(H) we e ob ained wi h ields±10 kOe a 77 K. The elec ical esis i i y ρ was measu ed as a unc ion o empe a u e in he ange 77<T≤300 K in a ze o magne ic ield (H=0) and wi h a cons an ield (H=5 kOe) wi h a dc ou -p obe me hod using sil e pain con ac s. These con ac s we e enewed o e en eplaced by ou new indium con ac s o gold spu e ed on o he samples, and he measu emen s epea ed when de ec ing anomalous changes in he esis i i y o he samples, in o de o disca d e ec s be ween sample and elec ical con ac s. The esis i i y da a we e ob ained, as commonly done, by calcula ing he a e ages ρ+ and ρ− (+ and− s and o he di ec ions o he cu en ) ou o 10 alues, espec i ely, and ge ing an a e age esis ance ρ=(ρ++ρ−)/2. Bu in some cases, as i will be shown below, only one pola i y o he cu en was used o he expe imen s, hus measu ing ei he ρ+ o ρ−. The magne o esis ance o he samples was measu ed be ween 0 and 9 kOe, specially a hose empe a u es a which he ρ(H) s. T cu es, co esponding o H=0 and H=5 kOe, showed bigge sepa a ion. 3. Resul s 3.1. Sample cha ac e iza ion Acco ding o hei X- ay di ac ion pa e ns, he ollowing compounds we e ob ained as single-phase c ys alline ma e ials: La1−xCaxCoO3 and La1−xBaxCoO3 o 0≤x≤0.30; La1−xS xCoO3 o 0≤x≤0.50; Nd1−xS xCoO3 o 0≤x≤0.40. These esul s also show ha in he La1−xMxCoO3 samples, he in oduc ion o he di alen ions p og essi ely educes he hombohed al dis o ion p esen in he pa en LaCoO3 compound and inc eases he olume o he uni cell, in good ag eemen wi h da a epo ed in he li e a u e [6] and [8]. In he case o he Nd1−xS xCoO3 sys em, o 0≤x≤0.40 he compounds ha e all cubic pe o ski e s uc u es wi h a≈2ac, he cell pa ame e inc easing wi h x. As o he pa icle size and mo phology o hese samples, SEM mic og aphs show ha , while La1−xS xCoO3 and Nd1−xS xCoO3 consis o small sphe ical pa icles o a e age size d≈0.5 μm, in he Ca- and Ba-doped compounds, due o he highe empe a u es used in hei syn hesis, sin e ing be ween pa icles has al eady aken place, esul ing in plaque s 3.5 and 2 μm long, espec i ely. 3.2. Magne ic esul s Upon subs i u ion o he i alen La3+ o Nd3+ ions by he di alen M=S 2+, Ca2+, Ba2+ ions, he La1−xMxCoO3 and Nd1−xS xCoO3 ma e ials e ol e owa ds e omagne ic beha io . The c i ical x o achie e magne ic pe cola ion is x=0.20 in he case o he La1−xS xCoO3, La1−xBaxCoO3and Nd1−xS xCoO3 sys ems, while i inc eases o x=0.30 in he La1−xCaxCoO3 se ies. The Cu ie empe a u es a y om sys em o sys em, al hough wi hin each se ies Tc inc eases only e y slowly wi h x, so ha : Tc(La1−xCaxCoO3)≈145 K, Tc(La1−xBaxCoO3)≈175 K, Tc(La1−xS xCoO3)≈250 K,Tc(Nd1−xS xCoO3)≈225 K ( Fig. 1a–d). Fig. 1. Tempe a u e dependence o he ZFC and FC mola magne ic suscep ibili y and he elec ical esis i i y o (a) La0.75S 0.25CoO3, (b) La0.70Ca0.30CoO3, (c) La0.80Ba0.20CoO3, and (d) Nd0.70 S 0.30CoO3 samples . The χ(T) da a ollow a Cu ie–Weiss law o ≈250 K<T<330 K. F om he co esponding χ−1(T) i ings we can ob ain in o ma ion abou how he e ec i e magne ic momen pe cobal ion (μe -Co) and he Weiss cons an (θ) a e e ol ing wi h x in he di e en Ln1−xMxCoO3 se ies. In he case o he La1−xMxCoO3 (M=S , Ca, Ba) sys ems, μe -Co is di ec ly ob ained as μe -Co=μe = , while in he case o he Nd1−xS xCoO3 se ies he a e ea h con ibu ion o he ob ained μe has o be conside ed and subs ac ed (μe (Nd3+)=3.8 μB). The esul s ob ained o he di e en se ies show ha μe -Co a e in gene al a he simila o he La1−xS xCoO3 and La1−xCaxCoO3 se ies, while hey a e highe in he case o he La1−xBaxCoO3compounds and smalle in he Nd1−xS xCoO3 sys em. ( Fig. 2a). Fig. 2. (a) E ec i e magne ic momen pe cobal ion, and (b) Weiss cons an co esponding o he La1−xMxCoO3 (M=Ca, Ba) and Nd1−xS xCoO3 sys ems in he composi ional in e al 0.20≤x≤0.40. As o θ, i is seen o inc ease wi h x in all ou se ies ( Fig. 2b) so ha he nega i e alues ound o lowe doping deg ees change o highe posi i e alues as x ge s highe , in good ag eemen wi h li e a u e da a[6] and [8]. As o he M(H) cu es co esponding o he e omagne ic samples, hey show ha a 77 K none o hem eaches sa u a ion unde a ield o H=10 kOe ( Fig. 3), e en i he alues o maximum magne iza ion inc ease wi h x in all he s udied se ies. Fig. 3. ZFC magne iza ion M e sus applied ield H o x=0.30 La0.70 M0.30CoO3 (M: Ca, S , Ba) and Nd0.70 S 0.30CoO3 samples, measu ed a 77 K. I we compa e M(H) loops co esponding o he di e en Ln1−xMxCoO3 sys ems ( Fig. 3), we ind ha he alues o maximum magne iza ion, ex apola ed magne iza ion ( esul o he linea ex apola ion o M(H) o highe ields back o H=0) and emnan magne iza ion ollow he sequence: La1−xBaxCoO3>La1−xS xCoO3≫La1−xCaxCoO3>Nd1−xS xCoO3. As o he coe ci e ield, i is ema kably highe in he Nd1−xS xCoO3 sys em, whe e i achie es a maximum alue o ≈3800 Oe o x=0.30, and hen dec eases o 3200 and 2400 Oe o x=0.40 and o x=0.20, espec i ely. I should be no ed he e ha he magne ic p ope ies o hese samples a e no seen o expe ience empo al e olu ion, a di e ence wi h wha occu s wi h hei elec ical beha io (see below). 3.3. Elec ical p ope ies When measu ing he esis i i y as a unc ion o empe a u e ρ(T), he i s gene al obse a ion is ha he esis i i y o he samples dec eases as he doping deg ee inc eases, in ull ag eemen wi h esul s epo ed in he li e a u e [8] and [20] ( Fig. 4), and ha a smoo h ansi ion om semiconduc ing o me allic beha io akes place. The doping deg ee a which he elec ical pe cola ion is achie ed depends on he sys em: i is,x=0.30 o La1−xBaxCoO3 and La1−xS xCoO3; x>0.30 o La1−xCaxCoO3; and x>0.40 o Nd1−xS xCoO3. Fig. 4. Elec ical esis i i y e sus empe a u e o Nd1−xS xCoO3 samples (0≤x≤0.40). Ve y in e es ingly, o composi ions e y close o he elec ical pe cola ion h eshold, we ind samples ha expe ience M–I ansi ions as empe a u e ises. Among hem, he compounds La0.70 Ca0.30CoO3, La0.80S 0.20CoO3, La0.75S 0.25CoO3 and La0.80Ba0.20CoO3, ha show I–M–I ansi ions, also known as a ‘ een an semiconduc ing beha io ’ (RSB) [7], i.e., he samples a e semiconduc ing below a TMI1 and abo e a TMI2, and me allic o TMI1<T<TMI2 ( Fig. 1a–c); and he Nd0.60S 0.40CoO3 sample, which is semiconduc ing o T<TMI=160 K and me allic abo e i ( Fig. 1d). In his case, only a small change in he posi i e slope o he ρ(T) cu e akes place a Tc and he sample emains me allic up o oom empe a u e. These samples wi h M–I ansi ions a e e y sensi i e o ex e nal in luences. Al hough om he c ys allog aphic and magne ic poin o iew he samples seem o be s able and a e no seen o expe ience empo al e olu ion, hei elec ical esis i i y does indeed change wi h ime. Depending on he na u e and cha ac e is ics o he sample his aging akes place a a di e en speed, being slowe o he La0.80Ba0.20CoO3 and La0.70 Ca0.30CoO3 samples. In hese cases i is possible o see he esis i i y o he samples p og essi ely inc ease in he in e al o se e al weeks, un il he RSB disappea s gi ing ise o a pu ely semiconduc ing beha io in he whole empe a u e ange [21]. La1−xS xCoO3 (x=0.20 and 0.25) samples also a i e o his inal si ua ion, bu much as e . I is also in e es ing o no e ha elec ical cu en is seen o help o ha ‘aging’: a e making se e al uns o ρ(T) measu emen s he esis i i y o he samples inc eases ( Fig. 5), specially i he elec ical cu en is applied only in one di ec ion. This e ec is specially impo an in he Nd1−xS xCoO3 samples. Fig. 5. E olu ion o he elec ical esis i i y o La0.80S 0.20CoO3 when making se e al uns o ρ(T) measu emen s. Also, e y su p isingly, he alue and e en empe a u e dependence o he esis i i y a e g ea ly a ec ed by he pola i y o he applied elec ical cu en . Fo example, o La0.80Ba0.20CoO3: ρ+(T) shows a semiconduc ing beha io , ρ−(T) is smalle and shows a M–I ansi ion, ρa , measu ed in e ing he pola i y o he cu en , gi es in e media e alues and a RSB ( Fig. 6). Solid S a e Phys, 46 (1992), p. 147 23. E.L. Nagae Phys S a Sol (B), 186 (1994), p. 9 24. C.H. Ahn, R.H. Hammond, T.H. Geballe, M.R. Beasley, J.M. T iscone, M. Dec oux, Fishe , L. An ognazza, K. Cha Appl Phys Le , 70 (1997), p. 206 25. R.D. Sánchez, J. Mi a, J. Ri as, M.P. B eijo, M.A. Seña ı́s-Rod ı́guez J Ma e Res., 14 (1999), p. 2533 26. R. Ramesh, A. K ishnan, D. Keeble, E. Poindex e J Appl Phys, 81 (1997), p. 3543 27. S. Chikazumi Physics o magne ism, K iege , Malaba , FL (1986) chs. 7 and 10 Pape p esen ed a he Fi s In e na ional Con e ence on Ino ganic Ma e ials, Ve sailles, F ance, 16–19 Sep embe , 1998.