Comparative study of ordered and disordered Y1-xSrxCoO3-d

0.44 MB·English

by  S. Fukushima

Comparative study of ordered and disordered Y Sr CoO
1−x x 3−δ
∗
S. Fukushima, T. Sato, D. Akahoshi, and H. Kuwahara
Department of Physics, Sophia University
Chiyoda-ku, Tokyo 102-8554, JAPAN
We have succeeded in preparing A-site ordered- and disordered-Y1/4Sr3/4CoO3−δ with various
oxygen deficiencies δ, and have made comparative study of their structural and physical proper-
ties. In the A-site ordered structure, oxygen vacancies order, and δ = 0.34 sample shows a weak
ferromagnetic transition beyond 300 K. On the other hand, in the A-site disordered structure, no
oxygenvacancyorderingisobserved,andδ=0.16sampleshowsaferromagneticmetallictransition
8 around150K.A-sitedisorderingdestroystheorderingsofoxygen-vacanciesandorbitals,leadingto
0 thestrong modification of theelectronic phases.
0
2 PACSnumbers: 75.30.-m

n
a
Cobalt oxides with perovskite-based structure ex- (a) (b)
J
hibit many attracting phenomena such as ferromag-
9 32% O
netism, metal-insulator transition, and spin-state tran-
2
sition. These phenomena are caused by the various spin
] states of Co ion such as low-spin (LS), intermediate- Y
el spin (IS), and high-spin (HS) states. La1−xSrxCoO3 is
- the most typical example: The ground state of x = 0 Sr
r
t compound is a nonmagnetic insulator with LS state of
s Co3+.[1, 2] With increase of Sr content x, a ferromag-
.
t netic metallic state emerges in x≥ 0.18, which is driven
a
m by the double-exchange interaction between Co4+ in LS
c
state and Co3+ in IS state.[3, 4]
-
d Among many perovskite cobaltites, Y1−xSrxCoO3−δ a b
n
attracts our interest, because of its characteristic struc-
o
tural features and high magnetic-transition tempera- ordered –Y Sr CoO disordered –Y Sr CoO
c 1/4 3/4 2.66 1/4 3/4 2.84
[ tures. A-site ordered-Y1−xSrxCoO3−δ has been first re-
ported by Withers et al.[5] and Istomin et al.[6] This
1 FIG. 1: Schematic structure of (a) A-site ordered-
compound has large oxygen deficiency δ. The struc-
5v ture ofordered-Y1/4Sr3/4CoO2.66 canbe regardedasthe YSm1/a4lSlrd3/a4rCkosOph2.e6r6esainnd(a)(bre)predsiesnotrdoexryedge-Yn1s/i4tSer33/04C%oOoc2.c8u4-.
4 alternate stacking of AO and CoO1.7 sheets, where A- pied. The ordered-Y1/4Sr3/4CoO2.66 has CoO2 layers at z =
4 sitecationsandoxygenvacanciesconcomitantlyorderas 1/4and3/4,andCoO1.3 layersatz=0and1/2withtransla-
4 shownin Fig. 1(a). Y1/4Sr3/4O sheet stacks along the c- tion h1/2, 1/2, 0i. In disordered-Y1/4Sr3/4CoO2.84, Y3+ and
1. axis with translation h1/2, 1/2, 0i. CoO1.3 and CoO2 Sr2+ occupyA-siteatrandom,andoxygenvacanciesareran-
0 sheets alternately stack along the c-axis with transla- domly distributed.
8 tion h1/2, 1/2, 0i to form the four times periodicity. For
0 0.75 ≤ x ≤ 0.8, A-site ordered-Y1−xSrxCoO3−δ under-
: goes a weak ferromagnetic transition at 335 K.[7] Ishi- tions exceed room temperature, and a bicritical feature
v
i wata et al. propose that the weak ferromagnetism is re- canbeseeninthe phasediagram. Therandompotential
X
lated to the orbital ordering.[8] In addition, the physical arising from A-site disordering suppresses long-rangeor-
r properties are sensitive to the variation of oxygen defi- ders,andgivesrisetothe colossalmagnetoresistivestate
a
ciency δ. For example, in x = 0.33, the ground state near the bicritical region. In the cobalt oxide system
is transformed from an antiferromagnetic insulator to a justlikethemanganeseoxidesystem,theA-sitearrange-
ferromagnetic metal with a slight decrease of δ.[9] ment probably affects the physical properties through
The high magnetic transition temperature is due to Co spin state and orbital arrangement. However, there
the A-site ordered structure free from chemical disor- are few reports on randomness effect of A-site ions in
der. The A-site arrangement in perovskite-based oxides the cobalt oxides.[11] In this study, we prepared A-site
plays an important role in determining their physical ordered-anddisordered-Y1/4Sr3/4CoO3−δ withvariousδ
properties.[10] In A-site ordered-R1/2Ba1/2MnO3 (R = and have made a comparative study of them in order to
rare earth ion), ferromagnetic or charge-ordering transi- investigate the A-site randomness effect on their struc-
tural and physical properties.
A-site ordered- and disordered-Y1/4Sr3/4CoO3−δ were
prepared in a polycrystalline form by solid state reac-
∗Electronicaddress: [email protected] tion. Mixed powders of Y2O3, SrCO3, and CoO were
2
heated at 1073 K in air with a few intermediate grind-
iwnagss,aannndeasliendtearetd11a7t314K23inKAinr aaitrm. oTsphheenret.heTshaemprele- Co site) 4 1o rkdOereed - FYC1/4Sr3/4CoO3-d (a)
sulting product has A-site ordered form. On the other 2m / B 3 d = 0.34
hand, A-site disordered form is obtained by quenching -0
1
the sample from 1473 to 77 K. We prepared ordered- n ( 2
o
anddisordered-Y1/4Sr3/4CoO3−δ withvariousδ through ati 0.30
z
different annealing conditions. The detailed conditions eti 1
n
will be described elsewhere. The values of δ were deter- g 0.44
a
minedthroughiodometrictitrationwithanaccuracyof± M 0
0p.l0es1.wTahsepcerryfosrtmalelodgrbayphXi-craayn-adliyffsrisacotfiotnhe(XobRtDai)nemdestahmod- o site) 8 disorderde d= -0 Y.115/4Sr3/4CoO31- dkOe F(Cb)
C
asnPttehatryniocsdoipacmarrdoltpPefeomrruotprpie-eepsrrratwotyuebrMreeeme.maTesetuahhrseoeudmrreeefsdrinsotutmisSvinyi5tsgytteaowmQ3a5su(0PmanPKetMa.usmTSu)rhDeefdremosbmiayggna5-, -1mon (10/ B 46 0.16 M (mol/emu)120000 d = 000...112652
ati H/ 0
to 350 K. etiz 2 0 100T (2K0)0 300
From the XRD measurements, we confirmed that gn
a 0.22
Y1/4Sr3/4CoO2.66 annealed in Ar has the A-site or- M 0
dered structure as shown in Fig. 1(a). A-site ordered- 0 100 200 300
Y1/4Sr3/4CoO3−δ with δ = 0.44 and 0.30 have the sim- Temperature (K)
ilar A-site ordered structure as that of δ = 0.34 sam-
ple, but the arrangement of oxygen vacancies are dif- FIG.2: Temperaturedependenceofthemagnetization of(a)
ferent from each other. On the other hand, quenched A-site ordered-Y1/4Sr3/4CoO3−δ (δ = 0.30, 0.34, and 0.44)
Y1/4Sr3/4CoO3−δ (0.15≤δ ≤0.27) samples have a sim- a0n.2d2)(.b)Tdhiesoirndseerteds-hYow1/s4Strh3e/4iCnovOer3s−eδsu(δsce=pti0b.i1l5it,y0o.1f6A, -asnitde
ple cubic perovskite structure. Bragg peaks arising from
A-siteand/oroxygen-vacancyorderingsarenotobserved, disordered-Y1/4Sr3/4CoO3−δ.
indicating that Y3+ and Sr2+ randomly occupy A-site,
and that oxygen vacancies are randomly distributed as
shown in Fig. 1(b). These results indicate that A-site low temperatures. The magnetization of δ = 0.44, 0.34,
disorderingmakestheoxygen-vacancyorderdestabilized. and 0.30 at 80 kOe are 0.127, 0.132, and 0.23 µB/Co,
The detailed results of the structural properties will be respectively.
published elsewhere. Figure2(b)showstheM-T curvesofA-sitedisordered-
Figure 2(a) shows the temperature dependence Y1/4Sr3/4CoO3−δ. The magnetization of δ = 0.15 ex-
of the magnetization (M-T) of A-site ordered- hibits anabruptjump around150K. TheM-H curveof
Y1/4Sr3/4CoO3−δ. The magnetizationof δ = 0.34 shows δ=0.15showsthetypicalferromagnetichysteresisbelow
an abrupt increase below 330 K. The onset of the mag- 150 K, in contrast to that of ordered-Y1/4Sr3/4CoO2.66.
netization corresponds to the weak ferromagnetic tran- The saturation magnetization of δ = 0.15 at 5 K is 1.2
sition. The magnetic field dependence of the magneti- µB/Co. With increase of δ, the ferromagnetic phase is
zation (M-H) shows a ferromagnetic behavior at 300 K, drastically suppressed. The magnetic transition temper-
but the magnetization remains unsaturated even at 80 atures of A-site disordered-Y1/4Sr3/4CoO3−δ are much
kOe (M = 0.19 µB/Co). This magnetic behavior is con- lowerthanthoseofA-site ordered-Y1/4Sr3/4CoO3−δ. As
sistentwiththatreportedbyKobayashiet al.[7]. Onthe shown in the inset of Fig. 2(b), the temperature depen-
basis of structural refinement, they propose two models dence of inverse susceptibility (H/M-T) of disordered-
forthemagneticorder. Oneisacantedantiferromagnetic Y1/4Sr3/4CoO3−δ (δ =0.15,0.16,and0.22)obeysCurie-
state, and the other is a ferrimagnetic state due to the Weiss law above 250 K with Weiss temperatures θ =
antiferromagneticcouplingbetweentheHSandISstates 190.3, 169.1, and 134.1 K, and effective moments esti-
of Co3+.[8] Slight increase or decrease of δ from 0.34 mated from the observed slopes in this region are Peff
suppresses the weak ferromagnetic phase around 325 K. = 3.06, 3.02, and 3.00 µB/Co, respectively. This result
The magnetization of δ = 0.30 gradually increases be- indicates that magnetic interaction among Co-spins is
low 250 K, and shows weak ferromagnetic behavior at ferromagnetic for disordered-Y1/4Sr3/4CoO3−δ with δ =
low temperatures. On the other hand, δ = 0.44 shows 0.15 to 0.22.
a magnetic transition around 230 K. The magnetization Figure 3(a) shows the temperature dependence of the
of δ = 0.44sample linearly depends onapplied magnetic resistivity(ρ-T)ofA-siteordered-Y1/4Sr3/4CoO3−δ. The
fields at 5K,while the M-H curvesofδ = 0.34and0.30 resistivities of all the samples show semiconducting or
indicatethattheyhaveferromagneticcomponents. With insulating behavior. With decrease of δ, the resistiv-
decrease of δ from 0.44, that is, with increase of Co va- ity abruptly drops over the whole temperatures. Taking
lence, the ferromagnetic component tends to increase at into account the results of the M-T curves, this implies
3
104 ordered - Y1/4Sr3/4CoO3-d rsiatneddoimso-rpdoetreendt-iYal1a/4nSdr3l/a4tCticoeO-d3−isδto(r0t.i1o5n≤duδe≤to0Y.2/7S)r,dthise-
(a)
m)103 ordering destroy the oxygen-vacancy order.
c102 d = 0.44 Such randomness strongly affects the physical prop-
Wy ( erties as well as the structural properties. As shown
stivit101 0.34 in Fig. 2, the magnetic transition temperatures of A-
Resi100 0.30 site disordered-Y1/4Sr3/4CoO3−δ are much lower than
10-1 those of ordered-Y1/4Sr3/4CoO3−δ. Furthermore, a de-
viation of δ from 0.34 as well as A-site disordering has
10-2 mucheffectonthemagneticstructure. InA-siteordered-
disordered - Y1/4Sr3/4CoO3-d Y1/4Sr3/4CoO3−δ, δ = 0.34 shows the highest magnetic
(b) transition temperature (the weak ferromagnetic transi-
m)
c10-1 d = 0.30 tion at 330 K). As described above, the oxygen-vacancy
(order)
Wvity ( 0.27 odredrienrgedpsattrtuecrtnurneedarepδen=ds0o.3n4δ.mTahyeboexyegsseenn-vtiaaclafnocrytohre-
sisti 0.22 weakferromagnetism. Therelationbetweenthearrange-
e
R ment of oxygen-vacancy and the magnetic structure is
10-2 0.16
now under investigation. The magnetic and transport
0.15 properties of A-site disordered-Y1/4Sr3/4CoO3−δ resem-
0 100 200 300 ble those of La1−xSrxCoO3.[3] In La1−xSrxCoO3, the
Temperature (K) magnetization and the magnetic transition temperature
increase continuously with increasing Sr content x or Co
FIG. 3: Temperature dependence of the resistivity of (a) A- valence, and the ferromagnetic metallic state is domi-
site ordered-Y1/4Sr3/4CoO3−δ (δ =0.30, 0.34, and 0.44) and nant for x ≥ 0.18. The ferromagnetic metallic state
0(b.2)7)d.isoTrdheererde-sYis1t/i4vSitry3/4oCfooOrd3−erδed(-δY1=/40S.1r53/,4C0.o1O6,2.700.22is, aalnsdo torfibAu-tseitdetdoisdoorudbelree-de-xYch1/a4nSgre3/i4nCteorOac3t−ioδnisjulsiktelliyketothbeecaaste-
shown (broken line) for comparison.
of La1−xSrxCoO3.[3]
In summary, we have prepared A-site ordered- and
theincreaseofferromagneticmetalliccomponentsexcept disordered-Y1/4Sr3/4CoO3−δ,andhaveinvestigatedtheir
for δ = 0.34 having large ferromagnetic component. δ structural and physical properties. Oxygen vacancy
= 0.34 shows a clear kink around 300 K, which corre- orders in the A-site ordered structure, but not in
sponds to the enhanced weak ferromagnetic transition. the A-site disordered structure. A-site disordered-
Figure 3(b) shows the ρ-T curves of A-site disordered- Y1/4Sr3/4CoO3−δ exhibits the magnetic transition be-
Y1/4Sr3/4CoO3−δ. δ = 0.27 shows insulating behavior. low 150 K, which is quite lower than those of ordered-
With decrease of δ, the resistivity is steeply decreased Y1/4Sr3/4CoO3−δ. The A-site randomness suppresses
similar to the case of A-site ordered-Y1/4Sr3/4CoO3−δ, magnetic order in cobalt oxide Y1/4Sr3/4CoO3−δ as re-
and δ = 0.15 sample shows almost metallic behavior. ported in manganese oxide R1/2Ba1/2MnO3.[10]
Then, we will discuss the effect of A-site disorder- ThisworkwassupportedbyIketaniScienceandTech-
ing on the structural properties of Y1/4Sr3/4CoO3−δ. nology Foundation, the Matsuda Foundation, the Asahi
In the A-site ordered structure (0.30 ≤ δ ≤ 0.44), GlassFoundation,andbyGrant-in-AidforScientificRe-
the periodic-potential and lattice-distortion associated search on Priority Areas (No.451) from the Ministry
with Y/Sr ordering are likely to stabilize the oxygen- of Education, Culture, Sports, Science and Technology
vacancy order and orbital order. In contrast, in A- (MEXT), Japan.
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