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.