Room-temperature ferromagnetism in Sr_(1-x)Y_xCoO_(3-delta) (0.2 < x < 0.25)

0.32 MB·

by  W. Kobayashi

Room-temperature ferromagnetism in Sr1−xYxCoO3−δ (0.2 ≤ x ≤ 0.25)
∗
W. Kobayashi and S. Ishiwata
Department of Applied Physics, Waseda University, Tokyo 169-8555, Japan
I. Terasaki
Department of Applied Physics, Waseda University, Tokyo 169-8555, Japan
also CREST, Japan Science and Technology Agency, Tokyo 108-0075, Japan
6
0
0 M. Takano
2 Institute for Chemical Research, Kyoto University, Uji 611-0011, Japan

n
a I. Grigoraviciute, H. Yamauchi, and M. Karppinen
J Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan

8 (Dated: February 2, 2008)
1
We havemeasured magnetic susceptibility and resistivity of Sr1−xYxCoO3−δ (x= 0.1, 0.15, 0.2,
] 0.215,0.225,0.25,0.3,and0.4),andfoundthatSr1−xYxCoO3−δ isaroomtemperatureferromagnet
l with a Curie temperature of 335 K in a narrow compositional range of 0.2 ≤x≤ 0.25. This is the
e
highest transition temperature among perovskite Co oxides. The saturation magnetization for x=
-
r 0.225 is 0.25 µB/Co at 10 K, which implies that the observed ferromagnetism is a bulk effect. We
t
s attributethis ferromagnetism toa peculiar Sr/Y ordering.
.
t
a PACSnumbers: 75.30.−m,75.50.Dd,61.10.Nz
m
-
d I. INTRODUCTION cited carriers induce the double-exchange interaction to
n align the ferromagnetic chains, and stabilizes the ferro-
o magnetic state from 270 to 300 K [8, 9].
Transition-metal oxides with perovskite-based struc-
c
[ ture exhibit many fascinating phenomena such as colos-
sal magnetoresistance in Mn oxides [1], high tempera- Very recently, Withers et al. [12, 13] and Istomin et
3 ture superconductivity in Cu oxides [2], and ferroelec- al. [14, 15] independently reported a new A-site ordered
v
1 tricity in Ti oxides [3]. Perovskite Co oxides also show perovskite Co oxide Sr1−xRxCoO3−δ (R = Y and lan-
7 very rich physics such as spin-state transition [4, 5], fer- thanide). According to their structure analysis, Sr and
6 romagnetism [6], and thermoelectricity [7]. The ferro- R are orderedbecause ofthe different ionic radii. TheR
1 magnetismintheperovskiteCooxideLa1−xSrxCoO3has ions prefer to occupy at every two sites in the ab plane,
1 been explained in terms of the double-exchange interac- which doubles the lattice parameter of the a and b axes
4 tion between Co3+ and Co4+ in the intermediate spin fromtheprimitiveperovskitecell. Theorderedabplanes
0
state. The ferromagnetic transition temperature (T ) is arestackedalongthe c-axisinunitsoffourcells,andthe
/ c
t 270Kathighest,whichissignificantlylowerthanT =370 c axis length quadruples that of the primitive cell. This
a c
m K of perovskite Mn oxides at an optimum composition. will give an ideal composition of Sr0.75R0.25CoO3−δ, but
a particular composition near x =1/3 has been exten-
In recent years, the A-site ordered perovskite
-
sivelystudiedsofar. Aside fromthe cationordering,the
d RBaCo2O5.5 (R = rare earth element) has been inten-
n sively studied [8, 9, 10]. It shows a ferromagnetic tran- twogroupsproposeddifferentoxygenorderingstructures,
o and the complete structure is still controversial.
sition around 300 K (higher than Tc of La1−xSrxCoO3)
c
withgiantmagnetoresistance[8,11]. Inaddition,atran-
:
v sition from the ferromagnetic state to an antiferromag- Magnetic properties of Sr1−xYxCoO3−δ were mea-
Xi netic state is seen around 255 K in the magnetization sured only for x=0.1 and 0.33 [16, 17], and we expected
[10]. The complicated magnetism has been explained as thatthepeculiarSr/Yorderingnearx=1/4couldshow
r
a follows: TheCo3+ionsintheintermediatespin(IS)state a novel magnetism. We have found that polycrystalline
of S = 1 exhibit an orbital ordering at 350 K along the samplesofSr1−xYxCoO3−δ showaferromagnetictransi-
a axis [9], which induces ferromagnetic interaction along tionat335K ina narrowrangeof0.2≤x≤0.25,which
the a axis. On the other hand, antiferromagnetic inter- is the highest transition temperature among perovskite
actionisexpectedalongthebaxisbecauseoftheconven- Cooxides. Themagnetizationofthex=0.225sampleis
tionalsuperexchange,whichcausestheantiferromagnetic 0.25 µB/Co at 10 K, indicating a bulk ferromagnet. We
orderbelow270K.Athighertemperatures,thermallyex- suggestthat this ferromagnetismis drivenby the orbital
ordering, just like the ferromagnetism in RBaCo2O5.5.
However,T oftheformerisevenhigherthanthatofthe
c
latter,andferromagneticordersurvivesdownto5K.We
∗Electronicaddress: [email protected] will discuss a possible origin for the difference.
2
5
II. EXPERIMENT (a)
0.1P,ol0y.c1r5y,st0a.l2li,ne0.2s1a5m,pl0e.s225o,f 0S.r215−,xY0.x3CoaOnd3−δ0.4()xan=d emu/g) 4 M (emu/g)10500 at 01.0001 T 200 300 Sr1-xYaxtC 0.o1O T3-d xxxxxx======000000......112222512555
Sr0.775R0.225CoO3−δ (R =Dy, Ho, and Er) were pre- on ( 3 T (K) xx==00..34
pared by a solid-state reaction. Stoichiometric amounts ati
ofSrCO3,Y2O3,Dy2O3,Ho2O3,Er2O3,andCo3O4were etiz 2
mixed, and the mixture was calcined at 1100◦C for 12h gn
a
in air. The product was finely ground, pressed into a M 1
◦
pellet, and sintered at 1100 C for 48h in air, followed
by slow cooling down to room temperature in the fur- 0
0 100 200 300 400
nace. That the resultant oxygen content of the sample
Temperature (K)
represented an equilibrium value (in air) was confirmed
by means of thermogravimetric annealing experiments. 0.2 (b)
The oxygen content was determined through iodometric o)
titration[18]. A powdered sample of 20-40 mg was dis- /CB 0.1 Sr0.775Y0.225CoO3-d
solvedin 1 M HCl solution(ca. 30 ml) containing an ex- mn ( at 10 K
caegsasinosftKaI.sTtahnedtairtdrat0i.o0n15ofMthesofdoirummedthI2iowsualsppheartfeorsmoleud- zatio 0 1.5
tion using starch as an indicator. For each sample, the gneti-0.1 /Co)B0
analysiswasrepeatedthree times inminimumwithare- Ma mM ( at 10 K
producibility better than ±0.003 for δ. Throughout this -1.5
-0.2 -4 -2 0 2 4
paper, we will use the nominal composition x, although Magnetic field (T)
the third decimal point of x includes some uncertainty. -5 0 5
We evaluated through energy dispersive X-ray analysis Magnetic field (T)
(EDX) the real compositions for x= 0.215 and 0.225 to
be Sr : Y = 0.781:0.219 and 0.766:0.234, respectively, FIG. 1: (a) Magnetization versus temperature (M(T)) of
meaning that x is correct within an accuracy of ±0.01. Sr1−xYxCoO3−δ (x = 0.1, 0.15, 0.2, 0.215, 0.225, 0.25, 0.3
As a reference, Sr0.775Y0.225CoO3−δ powder was sealed and0.4),(b)magnetization versusmagneticfield(M(H))of
withKClO4 inagoldcapsule,andthentreatedat3GPa Sr0.775Y0.225CoO3−δ. The insets of Fig. 1(a) and (b) show
and 700◦C for 30 min in a conventional cubic anvil-type M(T) and M(H) of Sr0.775Y0.225CoO3, respectively.
high pressure apparatus.
The X-ray diffraction (XRD) of the sample was mea-
sured using a standard diffractometer with Fe K radi- The range of 0.2 ≤ x ≤ 0.25 is quite singular, be-
α
ation as an X-ray source in the θ-2θ scan mode. The low which or above which the magnetization rapidly
resistivity was measured by a four-probe method, below fades away. This makes a remarkable contrast with
room temperature (4.2-300 K) in a liquid He cryostat, La1−xSrxCoO3, where Tc and magnetization continu-
andaboveroomtemperature(300-400K)inanelectronic ously change with the Sr content. Thus, the observed
furnace. The magnetization M was measured from 5 to ferromagnetism should not be attributed to the double-
400 K by a commercial superconducting quantum inter- exchange mechanism.
ference device (SQUID, Quantum Design MPMS). We The inset of Fig. 1(a) shows M(T) of
applied 0.01-0.1 T for the measurement of M (T) data, Sr0.775Y0.225CoO3 annealed in high-pressure oxy-
and -7 to 7 T for M (H) data. gen. We expect no oxygen vacancies (δ = 0) for this
sample. Thus the formal valence of Co is more than 3.7,
andthemagnetismseemstobedominatedbythedouble
III. RESULTS AND DISCUSSION exchange interaction between Co4+ and Co3+. In fact,
the observed ferromagnetic state is quite different from
Figure 1(a) shows the magnetization of that in the oxygen-deficient samples: Tc is remarkably
Sr1−xYxCoO3−δ at 0.1 T as a function of temper- lower, but the magnetization is larger. These features
ature. A tiny trace of a ferromagnetic transition is seen are essentially similar to M(T) of La1−xSrxCoO3
at 300 K for x = 0.1 and 0.15, and a weak and broad [6]. These differences clearly reveal that oxygen defi-
peak is seen near 300 K for x = 0.3 and 0.4, which ciency is essential to the peculiar room-temperature
is consistent with the literature [12, 19]. Surprisingly, ferromagnetism.
the samples only in a narrow compositional range of Figure 1(b) shows the magnetic-field dependence of
0.2 ≤ x ≤ 0.25 show large magnetization. The ferro- the magnetization (M(H)) of Sr0.775Y0.225CoO3−δ at
magnetic transition temperature (T ) is 335 K, which is, 10 K. At 7 T, the magnetization reaches 0.25 µ /Co.
c B
to our knowledge, the highest temperature in perovskite Although this value is somewhat smaller than that of
Co oxides. La1−xSrxCoO3(typicallymorethan1µB/Co),itislarge
3
105
ounts)111000234 (a) S(002)r0*. 7: 7Y52YO03.2(103)25C(200)(004)oO(202)3-d*(204)(220) (301) (215)(224)(206) (305)(008)(400)(226) (325)(420)(109) *(228)(424)(219) TSSrtAx1a−BnxdLYaErxdICa:doC(eO˚Avr3yi)−asttδiao(lnlxcogi=(sr˚Aag0)pi.vh1e,iScn0p.ai1ann5cd,epa0igor.r2edo,nout0mph.2ee2tsr5eis,3c.0-tδ.i2t5r,at0Ci.oo3nvadanaldetna0c.f4eo)r.
Intensity (c111100002345 (b)S(002)r0*(101). 7: 7E5rE(110)2Or03.2(103)25C(200)(004)o(202)O(211)3-d*(213)(204)(220) * (215)(224)(206)(107)(305)(008)(400)(226) (325)(420)(109)**(228)(424)(219) 00000.....112225255 37377.....868665846735744(((((17188))))) 111555...333--724(((111))) III444PP///mmmmm33mmmmmmmm 22222.....666669954550510(((((31003))))) 33333.....221008313790035
101 0.3 7.647(7) 15.26(1) I4/mmm 2.643(2) 2.985
15 30 45 60 75
(c) 2q (deg) 0.4 7.649(7) 15.24(1) I4/mmm 2.634(2) 2.865
100 Sr1-xYxCoO3-d x=0.1
0
100 x=0.15 difference from Fig. 2(a) is that the (101), (110) and
(103) peaks are much stronger in Fig. 2(b). This should
0
100 (103) x=0.2 be associated with the different X-ray scattering cross
sections of Er and Y, directly indicating that these
s) 0 superstructure peaks come from the A-site ordering.
nt100 Y2O3 x=0.225
u In other words, if the A site cations were disordered
o
y (c 0 (but the Co or O ions were ordered), the two XRD
sit100 x=0.3 patterns should be identical. Although the possibility of
Inten1000 x=0.4 tahbeovoerdreersiunlgtsosftrtohnegClyosaunpdpoOrtitohnastisthneotAe-sxicteludoredde,rtinhge
is predominant [20].
0
400 Figure2(c)showsthe xdependence ofthe (103)peak,
200 Sr0.775Er0.225CoO3-d whichisadirectprobefortheA-siteordering. Clearly,it
0 appears abovex=0.2, which exactly correspondsto the
25 26 27
2q (deg) emergence of the room-temperature ferromagnetism in
this compound. As listed in Table I, the crystal symme-
tryforx=0.1and0.15iscubic(Pm3m)withoutanysu-
FIG.2: X-raydiffractionpatternsof(a)Sr0.775Y0.225CoO3−δ
and (b) Sr0.775Er0.225CoO3−δ. (c) X-ray diffraction patterns perstructurepeakslikethe(103)peakinFig. 2(a),which
around 26◦ of Sr1−xYxCoO3−δ (x= 0.1, 0.15, 0.2, 0.225, 0.3 is consistent with Ref. [15]. Thus we conclude that the
and 0.4) and Sr0.775Er0.225CoO3−δ. A-siteorderingdrivestheferromagnetismintheCoions.
This is the same situationas in the caseof RBaCo2O5.5,
where the ordering of R and Ba stabilizes the Co3+ or-
enough to be taken as an indication of bulk ferromag- dering. Thus we think that the ferromagnetism in the
netism, and is almost equal to that of polycrystalline presentmaterialwouldbedrivenbytheorbitalordering,
sample of GdBaCo2O5.5 at 250 K [11, 19]. The inset although the ordering pattern may be different because
of Fig. 1(b) shows M(H) of Sr0.775Y0.225CoO3 (the an- RBaCo2O5.5 shows adifferent XRD pattern. We further
nealedsample)at10KinwhichtheM(H)curveisquan- note that the peak is monotonically shifted to a higher
titatively different from that in the non-annealed sam- degree with increasing x, indicating that the lattice pa-
ples. These experimental facts further consolidate that rameter is systematically decreased by substitution of Y
the ferromagnetism in Sr1−xYxCoO3−δ is not caused by for Sr (See Table I). On first glance, it looks inconsis-
the double-exchange mechanism. tent that the x = 0.3 sample does not show the bulk
Figure 2(a) shows the XRD pattern of ferromagnetism in spite of having the same intensity of
Sr0.775Y0.225CoO3−δ, which is consistent with those the (103) peak as the x = 0.25 sample. We think that
of Ref. [12, 13, 14, 15]. All the peaks are indexed as the ferromagnetismisquite susceptibleto the Sr/Ysolid
a tetragonal cell of the space group I4/mmm with the solution in the Sr site: Y cannot help occupying the Sr
lattice parameters of a = 7.674 ˚A and c = 15.34 ˚A, site for x >1/4. In fact, our conjecture is verified by
except for small peaks of Y2O3 (at most 2%; note that the fact that Sr0.75−yCayY0.25CoO3−δ rapidly loses fer-
the intensity scale is logarithmic). Note that the (h00), romagnetismwithy [21]. Incontrast,theoxygencontent
(0k0), (00l)(h,k,l 6= 2n) peaks do not appear according is unlikely to play an important role, because it remains
to the extinction rule. essentially constant (3−δ =2.64)between x=0.25 and
Figure 2(b) shows the XRD pattern of 0.3.
Sr0.775Er0.225CoO3−δ, where all the peaks except Figure 3 shows the M(T) of Sr0.775R0.225CoO3−δ (R
for small peaks of Er2O3 (at most 4%) are again = Dy, Ho and Er). Not only the Y-substituted sam-
successfully indexed as the tetragonal cell. The only ples but also the other compounds exhibit a similar fer-
4
4 Sr0.775R0.225CoO3-d mu) 60 Y m)00..68
mu/g) 3 at 0.1 T 1/M (g/e 2400 101 rW (mc00..24
on (e 2000Temp3e0r0ature (K4)00 cm) 00Tem10p0era2t0u0re3 (0K0)
netizati 2 Wvity (
Mag 1 YEr sisti 10-1
HDoy Re xx==00..115
0 x=0.2
0 100 200 300 400 x=0.225
x=0.25
Temperature (K) xx==00..34 Sr1-xYxCoO3-d
10-3
FIG. 3: Magnetization versus temperature of 0 100 200 300 400
Sr0.775R0.225CoO3−δ (R = Dy, Ho, and Er). The inset Temperature (K)
shows 1/M of Sr0.775Y0.225CoO3−δ.
FIG. 4: The resistivity of Sr1−xYxCoO3−δ (x = 0.1, 0.15,
0.2, 0.225, 0.25, 0.3, and 0.4). The arrow indicates the kink
romagnetism below 335 K. This means that the mag- temperature in the inset of Fig. 3. The inset shows the re-
netic moments of the lanthanides are irrelevant to the sistivity of Sr0.775Y0.225CoO3 The arrow indicates the Curie
temperature.
335 K transition. The inset of Fig. 3 shows 1/M of
Sr0.775Y0.225CoO3−δ, in which a kink is observed at 360
K. A similar kink was observedin 1/M of TbBaCo2O5.5
at the orbital-ordering temperature [9]. listed in Table I, the oxygen content 3-δ for x = 0.25 is
The low-temperature magnetization is affected by the ∼2.64,correspondingtotheformalCovalenceof∼3.03.
magnetic moment of Dy3+, Ho3+, and Er3+. The low- This is in between the oxygencontent of 2.69 for x=0.2
temperaturesusceptibilityoftheEr-andDy-substituted by James et al. [13] and the oxygen content of 2.62 for
samples obeys the Curie law below about 50 K. The ef- x =0.3 by Istomin et al. [14]. Very recently, Maignan
fective magnetic moment is evaluated to be 6.7 µ /Er et al. [17] found a similar oxygen content of 2.66 for
B
for the Er-substituted sample, corresponding to 70 % of x=0.33, and reported that the formal valence of Co is
thatexpectedfromEr3+ ofJ =15/2andg =6/5,where 2.99. FromthesedatawebelievethattheCoionsarees-
J is the total angularmomentumand g is Lande’s g fac- sentiallytrivalentforx=0.25. Thustheferromagnetism
tor. Theeffectivemagneticmomentis5.3µ /Dyforthe mostlikelyarisesfromtheorbitalorderingoftheISCo3+
B
Dy-substituted sample, corresponding to 50 % of that ions, as in the case of RBaCo2O5.5, which naturally ex-
expected from Dy3+ (J =15/2 and g =4/3). Since the plains the several similarities to the ferromagnetism in
orbitalangularmomentumissometimesquenchedinthe RBaCo2O5.5 mentioned above.
presence of ligand field, the magnetic moments are rea- Next, we will compare Sr1−xYxCoO3−δ with
sonably evaluated for the above samples. On the other RBaCo2O5.5. The ferromagnetic order is highly
hand, the Ho-substituted sample shows anomalous sus- robust down to 5 K in Sr1−xYxCoO3−δ, and is most
ceptibility that decreases with decreasing temperature likely the ground state of this system. According to the
below 50 K. It may indicate an antiferromagnetic cou- theory of orbital ordering [24], an antiferro-type orbital
pling between Co3+ and Ho3+ ions, and such a 3d-4f order favors a ferromagnetic order, and a ferro-type
coupling is actually observed in R2Cu2O5 [22]. orbitalorderfavorsan antiferromagneticorder. Suppose
Figure4showstheresistivityofSr1−xYxCoO3−δ. The the present compound be an insulator consisting of IS
temperature dependence of resistivity is nonmetallic, Co3+. Then an orbital order will be antiferro-type,
which is again incompatible with the double-exchange and prevent formation of the eg band, which drives a
mechanism. For 0.2 ≤ x ≤ 0.25 samples, a resistivity ferromagnetic order. Similar ferromagnetic states are
jump is seen at 360 K, which is close to Tc. A sim- seen in other orbital-ordered systems, such as K2CuF4
ilar but larger jump was reported in the resistivity of [24] and YTiO3 [25]. In this context RBaCo2O5.5
GdBaCo2O5.5 [8, 19, 23]. We further note that the re- is rather exceptional, in which the antiferromagnetic
sistivity for 0.2 ≤ x ≤ 0.25 is clearly distinguished from order is the ground state in spite of the anitiferro-type
that for x ≤ 0.15 or x ≥ 0.3. The inset of Fig. 4 shows orbital ordering. Thus the ferromagnetic state competes
theresistivityofSr0.775Y0.225CoO3 annealedinthehigh- with the antiferromagnetic state in RBaCo2O5.5, which
pressure oxygen. The resistivity shows metallic conduc- causes large magnetoresistance. Such a competition can
tion with a kink at 250 K, which is very different from explain why Tc is lower than Tc of Sr1−xYxCoO3−δ.
the resistivity of the non-annealed samples. Finally, we would like to add a few notes. (i) As a
Nowwediscusstheoriginoftheroom-temperaturefer- preliminary measurement, we measured the magnetore-
romagnetism in Sr1−xYxCoO3−δ (0.2 ≤ x ≤ 0.25). As sistance of Sr0.775Y0.225CoO2.65 at 300 K. The value is
5
−0.5 % at 7 T, which is very small compared with that 0.3 and 0.4), and have found that Sr1−xYxCoO3−δ for
of a typical giant magnetoresistance material. This sug- 0.2 ≤ x ≤ 0.25 can be regarded as a room-temperature
gests that the ground state is ferromagnetic and insu- ferromagnet. The ferromagnetic transition temperature
lating, which is consistent with our orbital-ordering sce- is approximately 335 K, which is the highest transition
nario. (ii)Weexaminedanimpurityeffect,andobserved temperature among perovskite Co oxides. The magneti-
that the ferromagnetism is quite susceptible against im- zation of the x = 0.225 sample at 10 K is 0.25 µ /Co,
B
purities, where only a 1%-substitution of Mn for Co de- which implies a bulk ferromagnet. We conclude that the
stroysahalfofthesaturationmagnetization. Suchresult ferromagneticorderisdrivenbythepeculiarY/Srorder-
cannot be understood from simple dilution effects, and ing. Weexpectthatthismaterialwillopenanewwindow
could be attributed to the impurity effect on the orbital to materials science of the A-site ordered Co oxides.
ordering. In a case of charge ordering, a tiny disorder
strongly suppresses the charge ordering state [26].
Acknowledgments
IV. SUMMARY
We thank B. Raveau and A. Maignan for fruitful dis-
Wehavemeasuredmagneticsusceptibilityandresistiv- cussion. This workwas partially supportedby Grant-in-
ity of Sr1−xYxCoO3−δ (x= 0, 0.1, 0.15,0.2, 0.225,0.25, Aid for JSPS Fellows.
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