Structure, Magnetization and Resistivity of La$_{1-x}$M$_x$CoO$_3$ (M = Ca, Sr, and Ba)

0.48 MB·English

by  M. Kriener

Structure, Magnetization and Resistivity of La1−xMxCoO3 (M = Ca, Sr, and Ba)
M.Kriener,C.Zobel,A.Reichl, J.Baier,M.Cwik,K.Berggold,H.Kierspel,O.Zabara,A.Freimuth, andT.Lorenz
II. Physikalisches Institut, Universit¨at zu K¨oln, Zu¨lpicher Str. 77, 50937 K¨oln, Germany
(Dated: February 2, 2008)
We present an investigation of the influence of structural distortions in charge-carrier doped
La1−xMxCoO3 bysubstitutingLa3+ withalkalineearthmetalsofstronglydifferentionicsizes,that
4 is M=Ca2+, Sr2+, and Ba2+, respectively. We find that both, the magnetic properties and the
0 resistivity change non-monotonously as a function of the ionic size of M. Doping La1−xMxCoO3
0 with M = Sr2+ yields higher transition temperatures to the ferromagnetically ordered states and
2 lower resistivities than doping with either Ca2+ or Ba2+ having a smaller or larger ionic size than
Sr2+, respectively. From this observation we conclude that the different transition temperatures
n
a andresistivitiesofLa1−xMxCoO3 fordifferentM(ofthesameconcentrationx)donotonlydepend
J on the varying chemical pressures. The local disorder due to the different ionic sizes of La3+ and
M2+ play an important role, too.
1
2

] I. INTRODUCTION arise fromdifferent preparationtechniques.16,17,18 More-
l over, it is not clarified which spin states are present and
e
- whetherornotspin-statetransitionstakeplaceasafunc-
r Among transition-metal oxides with perovskite struc- tion of temperature in the Sr-doped systems.19,20,21
t
s ture ABO3 the compound LaCoO3 is of particular inter- The electronic and magnetic properties of doped
t. est because it shows a spin-state transition as a func- LaCoO willnotdependonthechargecarrierconcentra-
a 3
tion of temperature. In its ground state LaCoO is
m 3 tionalonebutalsoonstructuralparametersresultinge.g.
a non-magnetic insulator, but with increasing temper-
fromchemicalpressure. Duringthelastdecadetheinter-
- ature a paramagnetic insulating state continuously de-
d playbetweenchargecarrierdopingandchemicalpressure
velops above about 50K and around 500K an insulator-
n has been intensively studied in doped manganites and
o to-metaltransitionisobserved. Thespin-statetransition a broad variety of fascinating physical phenomena has
c maybe attributedto thefactthatCo3+ with3d6 config-
been observed (for recent reviews see e.g. [22,23,24]).
[ uration can occur in different spin states depending on
For doped cobaltates there are only a few studies on
1 the ratio of Hund’s rule coupling and crystal field split- La1−xBaxCoO3 [25,26,27] and La1−xCaxCoO3 [28,29,
v ting. The ground state of LaCoO3 is usually attributed 30,31,32,33,34,35]. These data indicate qualitative sim-
0 to the low-spin configuration of Co3+ (LS: t6 e0; S=0).
2g g ilarities between Ca, Sr, and Ba doping with respect to
0 However,the question whether the paramagneticbehav-
the suppressionofthe non-magneticgroundstate andto
4 ior above 100K arises from a thermal population of the
the occurrence of some kind of magnetic order. In this
1 high-spin state (HS: t4 e2; S=2) or of the intermediate-
0 2g g paper we present a comparative study of the structural
4 spin state (IS: t25ge1g; S=1) is subject of controversial data as well as magnetization and resistivity measure-
0 discussions. Earlier publications1,2,3,4,5 often assume a mentsoftheseriesLa1−xCaxCoO3,La1−xBaxCoO3,and
t/ population of the HS state whereas more recent investi- La1−xSrxCoO3.
a gations6,7,8,9,10,11,12 often favor a LS/IS scenario.
m
Another aspect of cobaltates that is controversially
d- discussed in literature is the influence of charge carrier II. PREPARATION AND STRUCTURE
n doping which can be obtained by partial substitution
o of three-valent La3+ by divalent alkaline earth metals Our samples were prepared by a standard solid-state
c such as Ca2+, Sr2+, or Ba2+. Most studies concern the reaction using La O , Co O and MCO (M = Ca, Sr,
: 2 3 3 4 3
v La1−xSrxCoO3-series, where it is found that the non- Ba) as starting materials. The materials were mixed
i magneticgroundstateisrapidlysuppressedwithincreas- in the prescribed ratio and calcined in a Pt crucible at
X
◦
ing x. For small x a spin-glass behavior is observed at 850 C for 48h in air. Then the material was repeat-
r ◦
a low temperatures whereas larger doping leads to a fer- edlyregroundandsinteredat1200 Cfor40hinair. For
romagnetic order.3,13 The resistivity strongly decreases a bare polycrystal preparation the material was finally
with x and above x 0.2 metallic behavior is observed. pressed to cylindrical pellets (Ø 13mm; h 1mm) by
∼ ◦ ∼ ∼
These qualitative features are always found in studies 7.5kbar and sintered at 1200 C for 40h in oxygen flow.
on La1−xSrxCoO3, but the detailed phase diagrams pre- For a single crystal growth the polycrystalline material
sented so far are contradictory.3,14 In some samples the was pressed to a cylinder (Ø 8mm; h 10cm) by
∼ ◦ ∼
transitiontemperaturestoaspinglassoraferromagnetic 2kbar and sintered once more at 1200 C for 20h in air.
statemonotonouslyincreasewithincreasingSrcontent,3 Single crystals of La1−xSrxCoO3 with 0 x 0.3 were
≤ ≤
whereas other samples show anomalies around 250K grown by the traveling zone method in a 4-mirror im-
which occur almost independent of the Sr content for agefurnace(FZ-T-10000-H-VI-VP,CrystalSystemsInc.)
x 0.1.14,15 It has been proposed that these differences underoxygenatmosphereof5barwithtypicalgrowthve-
≥
2
58 61.0
ensity (arb. units) La0.82Sr0.18CoO3 oca blsc R-3c La0.8Ba0.2CoO3 ocablsc R-3c 3me (Å / f.u.) 57 M = BaM = S r MM = = C Sar 60.5degree)
Intnits) La0.81Ca0.19CoO 3 ocablsc R-3c La0.8Ca0.2CoO 3 Volu 560.0 0.1 0.2M = C0.a3 0.0 0.1 0.2M = B0.a3 60.0a (
u
arb. ocablsc Pnma M content x M content x
ensity ( FIG. 2: Volume per formula unit (left) and rhombohedral
Int angle αR (right) as a function of doping for La1−xMxCoO3
32.5 33.0 33.5 32.5 33.0 33.5 with M = Ba, Sr, and Ca. In La1−xCaxCoO3 symmetry
2q (°) 2q (°) changes to orthorhombic (Pnma) for x≥0.2.
FIG. 1: Representative X-ray diffraction patterns of
La1−xMxCoO3 for M= Sr, Ba, and Ca with 0.18 ≤ x≤ 0.2
around 2θ = 33◦. The solid lines are calculated intensities. symmetry, but for x ≥ 0.2 there is a structural change
to orthorhombic symmetry (Pnma), which can be best
ThereisastructuralchangeofLa1−xCaxCoO3 fromrhombo- ◦
hedral(R¯3cforx≤0.19) toorthorhombicsymmetry(Pnma visualizedaround2θ =33 inthediffractionpattern. As
for x≥0.2). shown in Fig. 1 the patterns of the Sr- and Ba-doped
samples and for the Ca-doped sample with x = 0.19
are very similar and well described by R¯3c symmetry.
locitiesof3.5mm/h. Allcrystalsaresinglephaseascon- However, the pattern of La1−xCaxCoO3 with x = 0.2 is
firmed by powder X-ray diffraction and the Sr content, systematically different and a description within Pnma
determinedbyenergydispersiveX-raydiffraction(EDX), symmetry is clearly better than within R¯3c. In this re-
agrees within few percent to the nominal composition. spect our result differs from those of Refs.[28,29], where
Single crystallinity was confirmed by neutron diffraction the rhombohedral symmetry was used also for x > 0.2,
andbyLauephotographs. However,allcrystalsareheav- andofRef.33,where a two-phaseregionconsistingofan
ily twinned as it is usually the case for slightly distorted admixture of a rhombohedral and a pseudo-cubic phase
perovskites. was proposed for x>0.2.
In this way we also prepared single phase polycrystals In Fig. 2 we compare the doping dependencies of the
of Ba- and Ca-doped La1−xMxCoO3 with 0 x 0.3. volume per formula unit (f.u.) and of the rhombohe-
However, a single crystal growth turned out t≤o be≤much dral angle αR for all three series. The volume increases
more difficult in these cases. From La1−xBaxCoO3 we stronglywithincreasingBaandmoderatelywithincreas-
preparedalargesinglecrystalwithx=0.1,butagrowth ingSrcontentwhereasCadopingcausesaslightdecrease
starting with x=0.2 broke down after 2cm and the ob- of the unit volume. For all three dopings the rhombohe-
tainedrodhadaBacontentofonlyx 0.13. Thisprob- dral angle systematically decreases with increasing con-
lemwasevenmoresevereforLa1−xCax≃CoO3. Growthef- tentofM.ForCadopingatransitiontoanorthorhombic
fortsstartingwithx=0.05and0.1yieldedcrystalswith lattice takes place, whereas for Ba and Sr cubic symme-
◦
Ca contents of only x 0.03, whereas the EDX analysis try (αR =60 ) is approachedand from a linear extrapo-
of the remaining melts≃of these efforts revealed Ca con- lation of our data is expected to occur around x = 0.37
tentsuptox 0.4. Thereforewesuspectsolubilitylimits and0.67,respectively. Thisvalueagreeswellwiththeob-
ofCaandBa≃aroundx 0.03and 0.13forourgrowth served cubic symmetry in La1−xBaxCoO3 for x=0.4,38
conditions,36 whereassi≃gnificantlyh≃igherdopingconcen- but is too large for La1−xSrxCoO3 where a cubic struc-
trationcanbereachedbyapuresolid-statereaction. The ture was reported already for x=0.5.39
measurements presented below were performed on poly-
crystals of La1−xBaxCoO3 and La1−xCaxCoO3 whereas
in the case of La1−xSrxCoO3 single crystals have been III. MAGNETIZATION
investigated.
The diffraction patterns of La1−xSrxCoO3 and In Fig. 3 we compare the magnetization of all three
La1−xBaxCoO3 can be indexed by a rhombohedral unit La1−xMxCoO3 series as a function of temperature. A
cell containing 2 formula units (space group R¯3c). We magnetic field of 50mT has been applied at 300K,
note that a recent high-resolution single crystal X-ray then the sample was cooled to 4K and the data were
study of the undoped LaCoO revealed a small mono- taken during the subsequent heating run. The undoped
3
clinic distortion, which is related to a Jahn-Teller effect LaCoO showsa pronouncedmaximumofM(T)around
3
of the thermally excited Co3+ ions in the intermediate- 90K arising from the spin-state transition of the Co3+
spin state,37 but the observed distortion in LaCoO is ions (Fig. 3a). In Ref. [12] we have presented a com-
3
too small to be observedin a usual powder X-ray analy- bined analysis of this M(T) curve and the thermal ex-
sis. La1−xCaxCoO3withx 0.19alsohasrhombohedral pansionofLaCoO3,whichgivesevidenceforalow-spinto
≤
3
x = La SrCoO (b) (c) 1
1-x x 3 x = 0.3 8 o)
4 0.15 x = 0.02 (a) 0.25 / CB 0
-2mM (10 / Co)B2 0000....100028425 m-3M (10 / Co)B2 000..00102 0.18 46 -1mM (10 / Co)B mM ( -21 o)x =1 0.1205.25 x = 0.25 La1-xCaxCoO3
0 00 10T0 (K)200 0.15 02 / Co)B 01 mM ( / CB-10 -0.30.18 B (T) 0.3 x = 000...1128255
0 100 200 3000 100 200 300 mM (
T ( K) T ( K)
-1
La BaCoO (d) La CaCoO (f) 6 La SrCoO
8 1-x xCo) 63 0.3 (e) 1-x x x =3 0.25 -22 1-x x 3
-3mM (10 / Co)B46 0.15m-1M (10 / B 0240 10000..225 200 000000......3211105255 4-1mM (10 / Co)B / Co)B 01 mM ( / Co)B-000...202 x- 4= 0.1B (T) 4 x = 00..125
T (K) 2 mM (
2 0.1 0.125
0.05 -1
0 La BaCoO
1-x x 3
0 0
0 100 200 3000 100 200 -2
T (K) T (K) -6 -4 -2 0 2 4 6
Magnetic Field (T)
FIG.3: Magnetization vs.temperatureof La1−xSrxCoO3 (a,
b, c), La1−xBaxCoO3 (d,e), and La1−xCaxCoO3 (f). All
curvesweretakenduringfield-cooledrunsinanappliedmag- FIG. 4: Magnetization curves of La1−xCaxCoO3 (top),
netic field of 50mT. Please note the different orders of mag- La1−xSrxCoO3 (middle),andLa1−xBaxCoO3 (bottom)mea-
nitudeof M for thedifferent panels. sured at T = 4K. The inset of the middle panel is an ex-
panded of the low-field region for the Sr-doped samples with
x = 0.18 and 0.25 showing that the hysteresis of the latter
sample is extremely small. The inset of the bottom panel is
intermediate-spin state scenario. A maximum of M(T) an expanded view of M(B) of La0.9Ba0.1CoO3 which has a
small hysteresis overthe entirefield range.
is also presentfor x=0.002,but not visible anymorefor
x 0.01. The sampleswith 0.04 x 0.15showa kink
≥ ≤ ≤
or a peak in M(T) indicating a spin-glass behavior at
low temperatures (Fig. 3b), whereas the crystals with for0.18 x 0.5.3 OurresultsofM(T)agreewithsuch
≤ ≤
x 0.18 show a continuously increasing spontaneous aninterpretation,butforthereasonsexplainedabovewe
ma≥gnetization with decreasing temperature as expected cannot confirm with ambiguity this conclusion. For the
for a usual ferromagnet (Fig. 3c). At low temperatures sakeofsimplicitywewillnottrytodiscriminatebetween
all La1−xSrxCoO3 crystals show differences between the a cluster spin glass and a ferromagnet in the following.
M(T)curvesobtainedintheFCrunandthecorrespond- The magnetization of the Ba-doped samples with x=
ing curves obtainedin a ZFC run (not shown),when the 0.05and0.1 continuouslyincreaseswith decreasingtem-
magnetic field is applied at the lowest temperature after peratureandbelowacertaintemperaturethecurvesflat-
the sample has been cooled in zero field. Such a differ- tenasisclearlyseenforthex=0.1sample(Fig.3d). Al-
enceisexpectedforaspinglass,butthisdifferencealone thoughmoredifficulttosee,theflatteningisalsopresent
doesnotprovespin-glassbehavior,since itcanalsoarise for x = 0.05 as we have identified from a plot of 1/M
from the domain structure of a ferromagnet. In order to vs. T (not shown). Insofar the Ba-doped samples with
distinguish between spin glass freezing or ferromagnetic x=0.05andx=0.1showaverysimilarbehaviortothe
order, one has to perform time dependent studies, such Sr-doped ones with x = 0.04 and 0.08. With increasing
as AC-susceptibility measurements of different frequen- Ba doping the flattening of M(T) becomes more pro-
cies or relaxation studies of the magnetization. From nounced and another clear kink in M(T) occurs around
such studies it has been concluded that La1−xSrxCoO3 200K. It is remarkable that all Ba-doped samples with
shows a spin-glass behavior for x < 0.18 and the occur- 0.1 x 0.15 show almost the same low-temperature
≤ ≤
rence of a so-called cluster spin glass has been proposed value of M. This is in contrast to the corresponding
4
La Sr CoO
Sr-doped samples, where M at the lowest temperatures 1-x x 3 500 K 100 K 50 K
continuouslyincreaseswithx. Inaddition,theBa-doped 109 x = 0 109
(a) (b)
samples do not show a peak in the FC M(T) curves. x = 0 0.01 106m)
FspoornhtaignheeorusBmaacgonnecteinzatrtaiotnionoscc(uxrs≥as0.i2n)aacpornovneonutinocneadl 106 00..0012 00..0042 110003W (c
ferromagnet and both, the absolute value of M and the 00..0048 10-r3
t(rFaing.si3tieo)n. temperatureTc systematicallyincrease with x cm) 103 0.125 51/T 1(01 0-3/1K5) 20
0.18
The M(T) curves of La1−xCaxCoO3 (Fig. 3f) differ rW ( 100 0.25 (c) 1.5m)
from those of the Ba- and Sr-doped series because the 0.3 x = 0.18 1.0c
pasrinacifpuanlcttieomnpoefraCtaurceondteepnetn.deAnlcreeaddoyesthheasradmlyplcehawnitghe 10-3 00..235 0.5-3W (10
x=0.05showsaspontaneousmagnetizationwithatem- 0 200 400 600 0 100 200 300 0.0r
perature dependence typical for a ferromagnet and the Temperature (K) T (K)
absolute value of M(4K) is about one order of magni-
tude larger than those of the corresponding Sr- and Ba- La1-xBaxC oO3 La1-xCa xCoO3
doped samples. With increasing Ca concentration both, 106 (d) 1.5 (e) (f) x = 1.5 (g)
cTstrcaeanastnedfowtriht0eh.2axb≤usoplxutto≤exv0a=.l3u.0e.2oTfahnMedd(r4iffeKmer)aeinsnytsetMsesme(n4attKiiac)lalylvlayclouinnes-- 103 x00 ..=015 W-3 (10cm)01..50 x = 000...3225 0000....1112255W-3 (10cm)01..50 x = 000...2325
which depend non-monotonously on x cannot be taken m) r 0.0 0.25r 0.0
seriously, since for technical reasons we had to use ex- c 0.125 0 100200 0.3 0 100200
tremely small samples. Therefore the error in weighing W ( 100 0.15 T (K) T (K)
r
the samples is of order 10%, which is larger than the
differences in M(4K). 0.2
10-3 0.25; 0.3
Fig. 4 gives an overview of the magnetization curves
measured at 4K for all three series. La1−xCaxCoO3 0 50 100 150 200 250 0 50 100 150 200 250 300
shows the typical behavior of a ferromagnet with a Temperature (K) Temperature (K)
large hysteresis region. With increasing x the hys-
teresis narrows and the absolute value of the magne-
tization increases slightly. For both samples the mag- FIG.5: Resistivityvs.temperatureofLa1−xSrxCoO3 (a,b,c),
netization strongly flattens in the higher field range La1−xBaxCoO3 (d,e), and La1−xCaxCoO3 (f,g). Panel (b)
shows Arrhenius plots log(ρ) vs. 1/T for the low-Sr-doped
but the saturation is apparently not yet reached. For
samples(x≤0.04) andpanels(c),(e),and(f)giveexpanded
the La1−xSrxCoO3 series the form of the magnetization views of the low-temperature resistivities on a linear scale of
curves changes as a function of x. The x = 0.25 sam-
the(nearly) metallic samples (x≥0.18).
ple has only a very small hysteresis (see Inset of Fig. 4)
and the magnetization is essentially saturated in a field
of 6T. For x = 0.18 the magnetization becomes smaller
anomalous behavior is well reproducible and occurs also
and the hysteresis larger, but the M(B) curve still has
forx=0.3. Tothebestofourknowledgethisanomalous
the form of a typical ferromagnet. The form of M(B)
virgin curves are neither expected for a ferromagnet nor
changes when the Sr content is reduced to x = 0.125.
for a spin glass.
The hysteresis region becomes very broad and there is
no steep change of M around the coercive field. This
difference can be interpreted as further indication of a
IV. RESISTIVITY
transition from spin-glass behavior to a ferromagnet (or
a cluster spin glass) around x 0.18. A very broad hys-
≃
teresisisalsopresentinLa1−xBaxCoO3. Asshowninthe Fig. 5 compares the resistivity data of the
lower Inset of Fig. 4 the Ba-doped sample with x = 0.1 La1−xMxCoO3 series. In agreement with previous
has a small hysteresis that extends even up to the maxi- studies the undoped LaCoO is a good insulator at low
3
mum field of 6T. Similar M(B) curves are found in the temperatures and showsan insulator-to-metaltransition
entire range 0.05 x 0.15 of Ba doping. The samples around 500K. Below 400K ρ shows an activation type
≤ ≤
with x 0.2 haveagainmore conventionalM(B) curves behavior ρ exp(∆ /T) down to 50K with activation
act
≥ ∝
with hysteresis widths which are somewhat smaller and energy ∆ 1240K. The weakly doped samples
act
≃
larger than those of the corresponding Ca and Sr-doped (x 0.04)show qualitatively similar resistivity behavior
≤
samples, respectively. However, a closer inspection of as the pure compound, but the log(ρ) vs. 1/T curves
Fig. 4 reveals another anomaly: La Ba CoO has are not linear (Fig. 5b). With further increasing Sr
0.75 0.25 3
a virgin curve of M(B) that is located outside of the content the low-temperature ρ rapidly drops and the
subsequent hysteresis loop over a large field range. This samples above x = 0.18 show a metallic temperature
5
dependence of ρ over the entire temperature range
La Ca CoO
(Fig.5a). Thecrystalswithx=0.18,0.25,and0.3show 1-x x 3
200 PM
distinct anomalies in ρ(T) at the critical temperatures
of ferromagnetic order (Fig. 5c). K)
A drastic decrease of ρ with increasing concentration T ( 100
is also found for Ca and Ba doping, but there are no FM
Ca-orBa-dopedsamplesshowingmetallicresistivitybe-

havior over the entire temperature range. From the Ba- 0
doped series only the samples with x 0.25 have a de-
La Sr CoO
≥ PM
creasingρwithdecreasingtemperaturefrom300Kdown 1-x x 3
200
to about 100K with clear slope changes around 200K
(Fig. 5e), where ferromagnetic order sets in. Insofar the K)
ρ(T) curves of the higher Ba-doped samples are similar T ( 100 FM
to those of higher Sr-doped crystals, but with further
decreasing temperature the resistivities of the Ba-doped SG
samples increase again. From the Ca-doped series even 0
thesampleswithx=0.25and0.3showaweaklyincreas- La Ba CoO PM
ing ρ with decreasing temperature from 300K down to 1-x x 3
200
about 150K (Fig. 5g). Then there is a slight decrease
of ρ(T) below about 150K, which is again the ferromag- K)
neticorderingtemperature. Finally,bothCa-dopedsam- T ( 100 FM
ples show a low-temperature increase of ρ below about
SG
80KthatisevenmorepronouncedthanintheBa-doped
samples. Onemightsuspectthatthelow-temperaturein- 0
0.00 0.05 0.10 0.15 0.20 0.25 0.30
creaseofρarisesfromthefactthattheCa-andBa-doped
samples are polycrystals whereas the Sr-doped ones are Concentration x
single crystals. However, this is not the only reason for
the different low-temperature behavior. We have mea-
suredρofpoly-crystallineLa Sr CoO andfoundthat FIG. 6: Phase diagrams of La1−xMxCoO3 with M=Ba, Sr,
0.7 0.3 3 and Ca (from bottom to top). PM, FM, and SG stand
it also shows a low-temperature increase, but this in-
for paramagnet, ferromagnet, and spin glass, respectively.
crease is weaker than in the Ba-doped and much weaker
The solid lines are to guide the eye. We mention, however,
than in the Ca-doped polycrystal. that an unambiguous distinction between a SG and a FM
(or the so-called Cluster Glass Phase proposed in Ref. 3 for
La1−xSrxCoO3 with x>0.18) is not possible from our mea-
V. DISCUSSION surementsof thestatic magnetization alone.
In Fig. 6 we summarize the phase diagrams ob-
qualitatively similar M(T) curves as the Sr-doped sam-
tained from the magnetization measurements. For
La1−xCaxCoO3 we find transitions from a paramagnetic ples. Thus, we conclude that La1−xBaxCoO3 shows a
spin-glass behavior for x < 0.2 and a ferromagnetic or-
to a ferromagnetic phase for the entire concentration
der for larger x with a saturation of T around 200K.
range. The transition temperature T monotonously in- c
c
creases with x for x 0.2 and tends to saturate around Forx 0.2allthree seriesofLa1−xMxCoO3 showfer-
T 150K for larger≤x. Our results of the Sr-doped se- romagne≥ticorderandforagivenxtheSr-dopedsamples
c
ries∼nicely agree with those obtained by Itoh et al..3 For have the largestTc values ( 220K), the Ba-dopedsam-
∼
x 0.18themagnetizationhasfeatureswhicharetypical pleshavesomewhatsmaller( 200K)andtheCa-doped
for≤the proposed spin-glass behavior and the character- samples significantly smaller∼Tc’s ( 150K). Obviously,
∼
istic temperature Tc strongly increases when x = 0.18 Tc depends non-monotonously on the ionic size of M.
is approached. For larger x the M(T) curves resem- Since the less conducting La1−xCaxCoO3 samples have
ble those of typical ferromagnets and T increases up thelowesttransitiontemperaturesonemaysuspectacor-
c
to about 230K for x = 0.3. We emphasize that addi- relationbetweenmetallicconductivityandferromagnetic
tional anomalous features in the M(T) curves around order as it would be expected within a double-exchange
240K, which have been frequently observed in polycrys- model. However, the Ba-doped samples have lower Tc
talline La1−xSrxCoO3 for a large doping range 0.025 valuesthantheSr-dopedonesalthoughtheirresistivities
x 0.5,14,15 do not occur in our single crystals. Thu≤s arecomparableorevenslightlysmallerthanthoseofthe
we≤conclude that these features are not intrinsic proper- Sr-doped samples.
ties of La1−xSrxCoO3 but rather arise from the different In doped manganites La1−xMxMnO3 Tc also depen-
preparation technique of those polycrystals as has been dends non-monotonously on the ionic sizes of M = Ca,
proposed in Refs.[16,17,18]. The Ba-doped series has Sr, and Ba for x 0.15.40,41,42 In the manganite case it
≥
6
is argued that the transition temperature is mainly de- of order 10−2˚A2), since Ba2+ is much larger than La3+.
termined by two kinds of structural distortions.43 The In the case of Ba doping the T reduction due to the de-
c
first is a global distortion arising from the deviation of viation from cubic symmetry is of only order 15K, but
the structure from the cubic perovskite. This distortion thereductionduetoA-sitedisorderincreasesfromabout
can be described by the deviation of the tolerance fac- 30Kto95Kwhenxincreasesfrom0.2to0.3andexplains
tor t = ( rA +rO)/(√2(rMn +rO)) from t = 1 where whytheTc valuesofLa1−xBaxCoO3 donotexceedthose
h i
rO and rMn denote the ionic radii of the Mn and O of La1−xSrxCoO3.
ions and rA = (1 x)rLa + xrM is the average ra- Considering the global and local distortions in
h i −
dius of the ions on the A site, that is the average of La1−xMxCoO3 alsoallowstogivesomequalitativeargu-
the radii of La3+ and M2+=Ca2+, Sr2+, or Ba2+, re- ments for the different resistivity behavior for different
spectively. The second type is a local distortion aris- M. Since for the Ca-doped samples the deviation from
ing from the different ionic radii of La3+ and M2+, that cubic symmetry is most pronounced, there is a rather
canbe described by the varianceof the A-site ionic radii strong deviation of the Co-O-Co bond angle from 180◦
σ2 = (1−x)rL2a +xrM2 −hrAi2. In order to quantify and the hopping integral, or in other words the band-
thesetwophenomenathefollowingempiricalrelationwas width, is small. As a consequence the ρ(T) curves of
proposed:43 La1−xCaxCoO3 remain almost semi-conducting even for
Tc(hrAi,σ)=Tc(rA0,0)−p1Q20−p2σ2 . (1) x18=0◦0a.3n.dIncoLnas1e−qxuSernxtClyoOth3etbhaenCdow-Oid-tChoinacnrgelaesiessclloeasedrintog
Here, T ( r ,σ) is the real transition temperature and to metallic ρ(T) curves for x> 0.18. In La1−xBaxCoO3
c A
T (r0,0)his aihypothetical transition temperature of an the bandwidth will be even larger than in the Sr-doped
idcealAcubic perovskite with A-site ion radius r0 that ful- samples in agreement with the smaller resistivities we
A observe at temperatures above about 100K. Yet, the
fills t = 1 and σ = 0. The deviation from cubic sym-
cmoentsrtyanitss.givWene buyseQth0e=tarbA0ul−atehdrAiionaincdrapd1iia1n.d35p˚A2 aforer tρe(mT)pecruartvuersefionrcrBeaased.oTpihnigs mwaityhaxris≥e fr0o.m25ashlaorwgearlloocwa-l
O2− and 1.216˚A, 1.18˚A, 1.31˚A, and 1.47˚A for the A- disorder in La1−xBaxCoO3 making these samples more
site ions La3+, Ca2+, Sr2+, and Ba2+ in 9-fold coor- sensitiveto chargelocalizationatlow temperaturesthan
dination44,45 and for r0 = 0.6r + 0.4r 1.32˚A the Sr-doped samples.
sinceLa1−xBaxCoO3 beAcomescubLiacforx=B0a.4≃.25 These Next we will discuss the doping range below x = 0.2,
values yield for the remaining parameters p 6.1 where the Sr- and Ba-doped series show spin-glass be-
1
1fo0r3xK=/˚A02.,2.p426 T≃he3.c4o·rr1e0s3pKon/d˚Ain2,gavnadlueTsc(froA0r,x0)=≃0≃.22529arKe· fhoarvsioprinw-ghlaicshs bisehaabvsieonrtairneLcoam1−pxeCtianxgCfoeOrr3o.maOgnneetsicouarncde
10.3 103K/˚A2, 6.3 103K/˚A2, and 286K and those for antiferromagnetic interactions. Such a competition may
x=0·.3 are 11.4 103·K/˚A2, 7.4 103K/˚A2, and 306K. arise from local disorder which will be more pronounced
For all three s·eries the observ·ed T values are signif- in the Sr- and Ba-doped samples due to the larger A-
c
icantly reduced from those of the corresponding hypo- site disorder than for Ca doping. Thus the absence of
thetical,idealcubic perovskites. ForLa1−xCaxCoO3 the spin-glass behavior in La1−xCaxCoO3 may arise from
reduction is most pronounced although in this case the the similar ionic radii of La3+ and Ca2+. Another rea-
effect of the A-site disorder is negligibly small (σ2 is of son could arise from the structural phase transition of
order10−4˚A2) because the ionic radiiof La3+ and Ca2+ La1−xCaxCoO3. This transitionis located at room tem-
areveryclosetoeachother. Thereductionisalmostcom- perature for x = 0.2 and possibly shifts towards lower
pletelyaconsequenceofthedeviationfromcubicsymme- temperatures for smaller x. If one assumes that the fer-
try which is most pronounced in La1−xCaxCoO3. This romagnetic exchange is enhanced in the orthorhombic
global distortion is already rather large in the undoped structure, a further reason for the ferromagnetic order
LaCoO3 andincreaseswith increasingsubstitution ofLa in low-doped La1−xCaxCoO3 could be that these sam-
by the smaller Ca ion. This is consistent with our ob- ples are orthorhombic at low temperatures.
servation of a structural phase transition from rhombo- As mentioned in the introduction it is not clear which
hedraltoorthorhombicwhichisexpectedforperovskites spin states of the Co3+ and Co4+ ions are realized in
when the tolerance factor becomes smaller. For Sr dop- La1−xMxCoO3 samples, and it is not known whether
ing the A-site disorder is larger (σ2 is of order 10−3˚A2) there are spin-state transitions as a function of temper-
than for Ca-doping, but the T reduction due to this ef- ature. From our present data we cannot unambiguously
c
fect remains small (of order 10K). The Sr-doped sam- resolve these puzzles. Clear indications of spin-state
pleshavesignificantlylargerT valuesthantheCa-doped transitions are only present in the M(T) curves of the
c
ones,sincethestructureapproachescubicsymmetryand La1−xSrxCoO3 sampleswithverylowdoping(x<0.01).
thereforethe reductionof T due to the globaldistortion For larger dopings the M(T) curves are dominated by
c
is less pronounced. For Ba doping the structure further the occurrenceofspin-glassorferromagneticallyordered
approaches cubic symmetry and consequently the influ- phases. The saturation values of the magnetization
ence of the global distortion on T is further reduced. in the ferromagnetic phases with x = 0.25 amount to
c
However, the A-site disorder is now pretty large (σ2 is M 1µ /Co for Ca- and to 1.65µ /Co for Sr- and
S B B
∼ ∼
7
Ba-doping. The latter value fits best to the expected an increasing bandwidth with increasing ionic radii of
spin-only value of 1.75µ /Co expected for a Co4+ LS the M2+ ions on the one hand side and by local disorder
B
state (t5 e0) and a∼Co3+ IS state (t5 e1). If these spin due to the different ionic radii of La3+ and M2+ on the
2g g 2g g
statesarerealizedonecaneasilyimaginetherelevanceof other. The M(T) curves indicate spin-glass behavior at
thedoubleexchangemechanismandunderstandthesim- lowtemperaturesforthe Ba-andSr-dopedsampleswith
ilaritytothe manganites,becausethesespinstatesdiffer x<0.2and0.18,respectively,whereasforlargerxferro-
onlybytwoadditionaldown-spinelectronsinthet2g level magnetic order occurs. In contrast, the La1−xCaxCoO3
from the corresponding high-spin states of Mn4+ (t3 e0) samples show ferromagnetic order over the entire dop-
2g g
and Mn3+ (t3 e1). In view of the much lower value of ing range. The reason for the absence of the spin glass
2g g
MS observedintheCa-dopedsamplethisverysimplified regioninLa1−xCaxCoO3remainstobeclarified. Thefer-
pictureofconsideringspin-onlyvaluesremains,however, romagneticorderoccursatthelargestorderingtempera-
questionable. tures Tc(x) for Sr doping, for Ba doping Tc(x) is slightly
smaller and for Ca doping significantly smaller. Similar
to the case of doped manganites43 this non-monotonous
VI. SUMMARY dependence of Tc on the ionic radii of M2+ can be phe-
nomenologically explained by assuming that the largest
We have presented a comparative study of the struc- Tc(x) would be obtained in a perfectly cubic perovskite
withoutdisorderandisreducedbyboth,adeviationfrom
ture, magnetization and resistivity of charge-carrier
doped La1−xMxCoO3 with M=Ca, Sr, and Ba (0 cubic symmetry and local disorder due to different radii
x 0.3). The Sr- and Ba-doped samples crystalliz≤e of La3+ and M2+. La1−xSrxCoO3 has higher Tc values,
≤ since T is strongly suppressed by a large deviation from
in a rhombohedral structure, which slowly (moderately c
fast)approachescubicsymmetrywithincreasingSr(Ba) cubic symmetry in La1−xCaxCoO3 and by pronounced
content. For La1−xCaxCoO3 there is a structural phase local disorder in La1−xBaxCoO3.
transition from rhombohedral to orthorhombic symme- Acknowledgments
try for x 0.2. For all three series the resistivity
≥
rapidly decreases with increasing concentration of M.
For Ca doping the ρ(T) curves do not reach a metal- We acknowledge fruitful discussions with M.Braden,
lic temperature dependence, whereas the Sr-doped crys- J.B.Goodenough, M.Gru¨ninger, D.Khomskii, and
tals become metallic for x > 0.18. The Ba-doped sam- L.H.Tjeng. We thank G.Dhalenne, P.Reutler, and
ples are metallic above about 100 K for x 0.25, but A.Revcolevschifor their help when we were growingour
≥
there are pronounced localizationeffects for low temper- first single crystals. This work was supported by the
atures. The different resistivities may be explained by Deutsche Forschungsgemeinschaftthrough SFB 608.
1 P.M. Raccah and J.B. Goodenough. Phys. Rev. 155, 932 T. Lorenz, P. Reutler, and A. Revcolevschi. Phys.Rev. B
(1967). 66, 20402 (2002).
2 K. Asai, O. Yokokura,N. Nishimori, H. Chou, J.M. Tran- 13 V.G.Sathe,A.V.Pimpale,V.Siruguri,andS.K.Paranjpe.
quada, G. Shirane,S. Higuchi,Y. Okajima, and K. Kohn. J. Phys. – Condens. Matter 8, 3889 (1996).
Phys. Rev.B 50, 3025 (1994). 14 M.A. Senar´ıs-Rodr´ıguez and J.B. Goodenough. J. of Sol.
3 M. Itoh, I. Natori, S. Kubota, and K. Motoya. J. Phys. State Chem. 118, 323 (1995).
Soc. Japan 63, 1486 (1994). 15 R. Caciuffo, D. Rinaldi, G. Barucca, J. Mira, J. Rivas,
4 M.A. Senar´ıs-Rodr´ıguez and J.B. Goodenough. J. of Sol. M.A. Senar´ıs-Rodr´ıguez, P.G. Radaelli, D. Fiorani, and
State Chem. 116, 224 (1995). J.B. Goodenough. Phys. Rev.B 59, 1068 (1999).
5 S. Yamaguchi, Y. Okimoto, H.Taniguchi, and Y. Tokura. 16 V. Golovanov, L. Mihaly, and A.R. Moodenbaugh. Phys.
Phys. Rev.B 53, 2926 (1996). Rev.B 53, 8207 (1996).
6 R.H.Potze,G.A.Sawatzky,andM.Abbate. Phys.Rev.B 17 P.S.AnilKumar,P.A.Joy,andS.K.Date. J.Appl.Phys.
51, 11501 (1995). 83, 7375 (1998).
7 M.A. Korotin, S.Y. Ezhov, I.V. Solovyev, V.I. Anisimov, 18 P.A. Joy and S.K. Date. J. Phys. – Condens. Matter 11,
D.I.Khomskii,andG.A.Sawatzky. Phys.Rev.B54,5309 L217 (1999).
(1996). 19 F. Munakata, H. Takahashi, Y. Akimune, Y. Shichi,
8 T. Saitoh, T. Mizokawa, A. Fujimori, M. Abbate, M. Tanimura, Y. Inoue, R. Itti, and Y. Koyama. Phys.
Y.Takeda,andM.Takano. Phys.Rev.B55,4257(1997). Rev.B 56, 979 (1997).
9 K.Asai,A.Yoneda,O.Yokokura,J.M.Tranquada,G.Shi- 20 A. Chainani, M. Mathew, and D.D. Sarma. Phys. Rev. B
rane, and K.Kohn. J. Phys.Soc. Japan 67, 290 (1998). 46, 9976 (1992).
10 S. Yamaguchi, Y. Okimoto, and Y. Tokura. Phys. Rev.B 21 K. Tsutsui, J. Inoue, and S. Maekawa. Phys. Rev. B 59,
55, 8666 (1997). 4549 (1999).
11 Y.Kobayashi,N.Fujiwara,S.Murata,K.Asai,andH.Ya- 22 M.B. Salamon and M. Jaime. Rev. Mod. Phys. 73, 583
suoka. Phys.Rev.B 62, 410 (2000). (2001).
12 C. Zobel, M. Kriener, D. Bruns, J. Baier, M. Gru¨ninger, 23 C.N.R. Rao, A. Arulraj, A.K. Cheetham, and B. Raveau.
8
J. Phys.– Condens. Matter 12, R83 (2000). 36 In Ref. [34] a La0.85Car0.15CoO3 single crystal was used,
24 J.M.D. Coey, M. Viret, and S. von Molnar. Adv. Phys. but unfortuantely no details of the single crystal growth
48, 167 (1999). are given.
25 S.B. Patil, H.V. Keer, and D.K. Chakrabarty. phys. stat. 37 G.Maris,Y.Ren,V.Volotchaev,C.Zobel,T.Lorenz,and
sol. (a) 52, 681 (1979). T. T. M. Palstra. Phys.Rev. B 67, 224423 (2003).
26 F. Fauth, E. Suard, and V. Caignaert. Phys. Rev. B 65, 38 S.B. Patil, D.K. Chakrabarty, M.V. Babu, and S.N.
060401 (2001).
Shringi. phys.stat. sol. (a) 65, 65 (1981).
27 J. Wang, Z. D. Wang, W. Zhang, and D.Y. Xing. Phys.
39 V.G.Bhide,D.S.Rajoria, C.N.R.Rao,G.RamaRao,and
Rev.B 66, 064406 (2002).
28 H.Taguchi,M.Shimada,andM.Koizumi. J.ofSol.State V.G. Jadhao. Phys. Rev.B 12, 2832 (1975).
Chem. 41, 329 (1982). 40 P.MandalandB.Ghosh. Phys.Rev.B68,014422(2003).
29 Ch. Zock, L. Haupt,K. B¨arner, B. M. Todris, K. Asadov, 41 H. Y. Hwang, S-W. Cheong, P. G. Radaelli, M. Marezio,
E. A. Zavadskii, and T. Gron. J. Magnetism Magnetic and B. Batlogg. Phys. Rev.Lett. 75, 914 (1995).
Materials 150, 253 (1995). 42 B.Dabrowski,K.Rogacki,X.Xiong,P.W.Klamut,R.Dy-
30 N.-C.Yeh,R.P.Vasquez,D.A.Beam,C.-C.Fu,J.Huynh, bzinski,J. Shaffer,and J.D.Jorgensen. Phys.Rev.B 58,
andG.Beach. J.Phys.–Condens.Matter9,3713(1997). 2716 (1998).
31 A.V. Samoilov, G. Beach, C.C. Fu, N.-C. Yeh, and 43 L. M. Rodriguez-Martinez and J. P. Attfield. Phys. Rev.
R.P. Vasquez. Phys.Rev.B 57, 14032 (1998). B 54, R15622 (1996).
32 R.Ganguly,I.K.Gopalakrishnan,andJ.V.Yakhmi. Phys- 44 R.D. Shannon. ActaCryst. A32, 751 (1976).
33 iKca.MBu2ta7,1Y, .11K6o(b1a9y9a9s)h.i,and K.Asai. J.Phys.Soc.Japan 45 We could also use the A-site ionic radii for 12-fold co-
ordination as is done e.g. in Ref. [42]. This would cause
71, 2784 (2002).
34 S.A. Baily, M.B. Salamon, Y. Kobayashi, and K. Asai. quantiatively different parameters, but the main qualita-
tiveconclusions remain unchanged.
Appl.Phys. Lett. 80, 3138 (2002).
35 S.A. Baily and M.B. Salamon. J. Appl. Phys. 93, 8316 46 SincewehavenotgrownasinglecrystalofLa0.8Sr0.2CoO3
(2003). we used the corresponding Tc =180K from Ref. [3].