Physica C 317–318 Ž1999. 464–470
Magnetic anomaly of Y1yx Sr xVO 3yd
Kazuo Sakai ) , Shinji Migita, Katsunobu Yamada, Taisuke Shindo, Hidetaka Fujii,
Hironaru Murakami
Department of Electrical Engineering, Osaka UniÕersity, 2-1 Yamada-oka, Suita-shi 565-0871, Osaka, Japan
Abstract
YVO 3 –SrVO 3 system, in which the valence of vanadium ion varies depending on the contents of the oxygen and
strontium ion, has been investigated magnetically and electronically to know the behavior of the strongly correlated electron.
YVO 3y d , a Mott-type insulator with a perovskite structure, is found to exhibit an anomalous magnetic behavior and
interpreted to be based on the parasitic ferromagnetism proposed by Dzyaloshinsky and Moriya wT. Moriya, Phys. Rev. 117
Ž1960. 635x. The energy gap and the intensity of the anomalous peak in the field cooling process are reduced with the
increase of Sr molar fraction in Y1yx Sr xVO 3y d . q 1999 Elsevier Science B.V. All rights reserved.
Keywords: Perovskite structure; SQUID; M–H curve; Resistivity; Strong correlation
1. Introduction
It is well known that the high-Tc oxide superconductor comes out between an antiferromagnetic insulator and a metal with the hole doping. Such phase
transitions in the oxides have been often elucidated
using a Mott–Hubbard type or charge-transfer-type
model. Mott has pointed out that the wide energy
gap characterizing an insulator gradually becomes
narrower as the distance among layers is shortened,
and the number of free electron abruptly enhances
when the distance among layers reaches a certain
distance, then the insulator transfers to the metal w1x.
At that critical point the anomalous magnetic andror
electric variation, the so-called metal–insulator tran-
)
Corresponding author. Tel.: q81-6-879-7686; Fax: q81-6875-0506; E-mail: sakai@gene.pwr.eng.osaka.u.ac.jp
sition, takes place cooperatively depending on temperature andror impurity content.
We took notice of the vanadium oxides with the
perovskite structure as one of such compounds. The
reason is as follows: Ž1. the vanadium ion can easily
take the various valences depending on the cations
occupying the A site of the perovskite cell and the
coordination number. Ž2. YVO 3 is a Mott-type insulator, while SrVO 3 is a metal with a Pauli paramagnetic behavior. The valences of vanadium ion in
YVO 3 and SrVO 3 are formally q3 and q4, respectively. An extra electron emitted from V 4q may be
able to play a similar role as the hole for the Cu-type
superconducting oxide although V as M forms the
Mott-type gap in LaMO 3 system, while Cu forms the
charge-transfer ŽCT. type gap in LaMO 3 system w2x.
Thus, we have investigated the magnetic and electric properties in this system as functions of temperature and the hole carrier content, and found an
intriguing, anomalous behavior in the magnetic property of the YVO 3 –SrVO 3 system.
0921-4534r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 1 - 4 5 3 4 Ž 9 9 . 0 0 1 0 0 - 8
K. Sakai et al.r Physica C 317–318 (1999) 464–470
2. Experimental
YVO 3y d are prepared from the mixture of Y2 O 3
Žpurity: 99.99%. and V2 O5 Žpurity: 99.99%. via
several repetitions of a conventional solid state reaction at 14508C under the H 2 gas flow. In this process
a little amount of YVO4 easily remains under the
worse reduction atmosphere. YVO4 synthesized as
a precursor in air is so stable that pure YVO 3y d
cannot be obtained via the reduction of YVO4 .
SrVO 3y d is synthesized by sintering repeatedly at
14508C under the H 2rAr gas flow after the mixture
of SrCO 3 and V2 O5 was calcined. The partial oxygen pressure is checked by a PtrY-stabilized zirconiarPt sensor. Then Y1y x Sr xVO 3y d is synthesized
by sintering the mixture of YVO 3y d and SrVO 3y d
at appropriate ratios until there is no trace of the raw
materials.
The compositional ratio is checked by an inductively coupled plasma ŽICP. photoemission spectroscopy ŽSeiko Electronics. and the oxygen content
dissolved in the synthesized specimen was analyzed
using the N2rO 2 analyzer ŽHoriba.. Temperature
dependence of dc-susceptibility and resistivity were
measured by SQUID susceptometer ŽHoxan. and
four-probe method, respectively. Magnetic field dependence of magnetic moment, the so-called M–H
465
curve, was investigated by a vibrational sample magnetometer ŽVSM-5-18S; Toei Industry.. Temperature
dependence of the lattice parameter was inspected by
X-ray diffractometer ŽXRD: RINT-6; Rigaku. using
a cooling attachment at a certain temperature from
room temperature to about 80 K, controlling the
liquid nitrogen flow and the heater. The sample
temperature was compensated by pasting on the sample holder a CurConstantan thermocouple.
3. Results and discussion
3.1. YVO3 y d
The oxygen content of YVO 3y d prepared by the
above method is evaluated between 2.84 and 2.90 by
the N2rO 2 analyzer, which consists of a GdFeO 3type orthorhombic primitive cell w3x with lattice pa˚ b s 5.59 8 " 0.01
rameters of a s 5.277 " 0.005 A,
˚A and c s 7.58 2 " 0.015 A˚ at room temperature.
The four monoclinic pseudo-cell with aX s 3.73 "
˚ bX s 3.85 " 0.01 A,
˚ cX s 3.79 " 0.015 A˚
0.005 A,
and b s 86.6 " 0.18 Žthe angle between ab-plane
and c-axis. are contained in a formal primitive cell
as shown in Fig. 1Ža.. The a X- and bX-axes approximately rotates by 458 from a- and b-ones, respec-
Fig. 1. Ža. The structure of Y1y x Sr xVO 3 ŽGaFeO 3 type.. Žb. A model for the antiferromagnetic correlation among each spin in Y1yx Sr xVO 3 .
466
K. Sakai et al.r Physica C 317–318 (1999) 464–470
tively and cX is cr2. The tolerance factor of YVO 3
˚ for Y 3q
is calculated at 0.924, employing 1.25 A
1
˚ for V 3q
coordinated by the 12 oxygens, 0.64 A
˚ for O 2y
coordinated by 6 oxygens and 1.35 A
according to Shannon’s table w4x. This value suggests
the existence of an extra vacant space, therefore the
wVŽO I .4ŽO II . 2 x-octahedral ion is buckling along the
direction of each axis since the angle of O–V–O is
different from 1808. The two different types of magnetic interactions would be considered; the magnetic
interaction is closely related with the conduction
mechanism within a sheet layer, while only the
magnetic interaction predominantly acts between interlayers. Fig. 1Žb. shows schematically the antiferromagnetic arrangement between the vanadium ion
within the ac-plane based on the superexchange
interaction generated through oxygen ion and will be
discussed hereafter.
Temperature ŽT . dependence on the susceptibilities Ž x . was observed under the magnetic field Ž H .
of 10 Oe via three different processes as shown in
Fig. 2Ža.: Ž1. T was gradually increased up to about
200 K after the specimen was cooled down to 4.2 K
under the zero field ŽZFC: open circle. and applied
at H s 10 Oe, Ž2. T was lowered from 200 K to 4.2
K under H s 10 Oe ŽFC: closed circle., then Ž3. T
was again raised keeping H constant Žtriangle.. The
remarkable difference of the x value was observed
between ZFC and FC processes in T F 140 K. In the
FC process YVO 2.84 exhibited an anomalous magnetic behavior although the x value is very weak,
10y4 emurg Oe order. After the strongest magnetization is displayed at about 110 K, it was weaken
rapidly down to about 80 K and repeatedly recovered
to some extent at about 60 K, then approximately
stayed at a constant value. Although the anomaly at
about 110 K was consistently observed in both processes, this specimen was hardly magnetized with
the increase of T in ZFC process.
Up to date these magnetic characteristics, to our
knowledge, have not been reported yet. The various
interpretations for this anomalous magnetic behavior
can be considered as follows: Ži. the freezing of
domain wall in the ferromagnetic material, Žii. the
1
This data is extrapolated using the relationship between ionic
radii and the coordination number.
spin glass-like behavior and Žiii. the parasitic ferromagnetic one based on the antiferromagnetic interaction and so on. The following studies have been
carried out for clarifying this characteristic behavior.
The comparison between ZFC and FC processes
denotes that the spin on the vanadium ion cannot be
arranged with the external field Ž H . in the lower T
regime, but each spin must be independently frozen.
Moreover, the characteristic reduction of the x value
from 80 to 100 K in the process Ž3. was depressed
compared to that in the process Ž2.. This hysteresis
generated by the difference between the T sweep of
heating up and down as well as between ZFC and
FC processes would be closely related to the relaxation mechanism of the spin arrangement. The x
values when applied to H s 10, 100 and 1000 Oe in
FC process are inserted at the right, upper side in
Fig. 2Ža.. The x Žs MrH . value becomes weaker
with the enhancement of H, and the anomalous
behavior in 120 G T G 80 K is also depressed. These
characteristics are consistently displayed independently from the synthesized specimens. Thus, the
external field dependence of magnetization, that is,
the M–H curve was measured in ZFC and FC
processes at room temperature and 80 K. Fig. 2Žb.
shows a result of the magnetic field sweep at 80 K
after FC process was carried out with H s 5000 Oe.
Although the symmetry of the M–H curve is strictly
distorted due to the liquid N2 cell attachment, a
semiquantitative discussion would be possible. Quite
a little hysteresis with the coercive force lower than
40 Oe was certified, while little hysteresis was observed even at 80 K in ZFC process as expected, and
at room temperature in both processes, the hysteresis
was not observed but M was approximately proportional to H. Judging from the small susceptibility of
10y4 emurg Oe order, these phenomena in YVO 3y d
cannot be related to the domain wall generated in the
ferromagnetic compounds.
Here, let us estimate the valence of the vanadium
ion according to the Curie–Weiss law in T G 140 K.
The effective Bohr magneton, meff , of YVO 3y d is
estimated at 2.59 " 0.02 m B . This value means the
mixture is between V 4q and V 3q because the spinonly values of V 4q Ž d 1 . and V 3q Ž d 2 . are 1.73 and
2.83, respectively. Some electrons coming from V 4q,
which is calculated to mix formally by about 20%,
move to the neighboring vanadium ion by the hop-
K. Sakai et al.r Physica C 317–318 (1999) 464–470
467
Fig. 2. Ža. Temperature dependence of x when applied at H s 10 Oe through the different paths in YVO 3y d . The number indicates the
sweeping order of H. The inset displayed x under each H in FC process. Žb. The relationship between M and H.
468
K. Sakai et al.r Physica C 317–318 (1999) 464–470
ping conduction through the bonding orbital made of
V–O 1 –V, but since the Mott–Hubbard gap generates
because of the strong electron–electron correlation,
consequently these extra electrons would be localized. Therefore, the magnetic exchange integral, J,
becomes larger than the transferring integral term, t,
and a magnetic arrangement would be brought about
in the vicinity of 110 K. The appearance temperature
of the ferromagnetic interaction in both processes of
ZFC and FC corresponds to a just phase transition
temperature, that is Neel
` temperature ŽTN .. Since the
arrangement of V 3q and V 4q would not bring the
ferromagnetic phase transition, it is considered that
this interaction is caused by the parasitic ferromagnetism based on antiferromagnetic interaction as illustrated in Fig. 1Žb.. The obtained data give our
consent to that explanation, but the anomalous temperature dependence of x cannot be solved yet.
Next, the spin relaxation has been examined, fixed
at the characteristic temperature, 4.5 K, 87 K and
110.2 K on account of investigating whether the spin
frozen behaves cooperatively as a spin-glass. The
relaxation of magnetization, however, has never been
observed within 5 h. This denotes that this anomaly
does not originate from the spin glass-like behavior
in spite of the fact that the spin must be frozen in the
lower temperature.
Fig. 3 shows temperature dependence of each
lattice parameter in a unit cell estimated from XRD
measurement. The lattice parameters along a Žor aX .and b Žor bX .-axes gradually shorten with the reduction of temperature in 120 G T G 80 K, while those
along the c Žor cX .-axis get longer. This compression
along aX-axis and the expansion of the cX-axis imply
that the spin on the vanadium ion interacts strongly
with each other. However, the magnetism on 80 F T
F 120 K is inversely weakened. It must be considered that the superexchange through the oxygen ion
may interact not ferromagnetically but antiferromagnetically.
In terms of the crystal field, the five degenerated
orbitals separated at the d´ and dg orbitals, the one
electron from V 4q settles in the d´ orbital. Since
this orbital is not directed toward the pz orbital of
the O 2y ion, in a simple Heitler–London approximation the transfer integral term t in the perovskite
structure is very small. It must be considered that the
buckling of the bonding among O 2y and vanadium
Fig. 3. Temperature dependence of each lattice parameter.
ions is added on the nonlinear overlap of each linear
orbital.
Each spin in YVO 3y d cannot be completely arranged antiparallelly among the interlayers along
each a- and c-axes, and the interlayers along the
b-one because of the worse symmetry, a monoclinic
structure even at best. Thus, the non-vanishing constituent remains in tensor as D P Ž M1 = M2 . as shown
in Fig. 1Žb.. This consideration is supported from the
lowering of the symmetry in YVO 3y d , thereby the
weak ferromagnetic phenomenon would be observed
according to the Dzyaloshinsky–Moriya ŽD–M. theory w5x. Since this effect is originally based on the
antiferromagnetic interaction, the sample is cooled
down to the low temperature by the ZFC process, the
interaction between each Mi exhausts, whereas in
FC process each vanadium ion is easily magnetized
by the external magnetic field when the specimen is
cooled down via the high temperature region in the
vicinity of 110 K.
If the magnetic anomaly is brought by two factors, the intra- and inter-spin correlation, temperature
dependence described above could be qualitatively
explained. Namely, this anomaly would arise from
the parasitic ferromagnetic effect in the ac-plane and
the weaker one along the b-axis, and these effects
begin separately at different temperatures.
K. Sakai et al.r Physica C 317–318 (1999) 464–470
469
3.2. Y1y x Sr x VO3 y d
The solid solution between YVO 3 and SrVO 3 was
found to be formed in x F 0.3. In x ) 0.3 the unreacted material remained or some unknown phases
yielded. Fig. 4 is the variation of lattice parameter
with x. The tolerance factor increases gradually as
the A site is replaced by Sr 2q with the ionic radius
larger than Y 3q and approaches 1.00 Žcubic perovskite.. This replacement by Sr 2q displayed that
the lattice parameter of the b-axis is shorter whereas
that of the c-axis is longer, keeping the angle b
unchanged. It is strange, however, that the lattice
parameter of the a-axis is also shrunken. The lattice
parameters of a- and b-axes at room temperature
become shorter, while that of c-axis is longer with
x. This indicates that the distance of V–O II –V intralayer is shortened, while that of V–O I –V among the
layers is longer from each other. If the conduction
mechanism is based on the hopping one, this inclination implies the easier conduction as well as the
enhancement of hole carrier with the increase of x.
Fig. 5 shows temperature dependence of resistivity for Y1y x Sr xVO 3y d . In the case of YVO 3y d Ž x s
0., the energy gap is estimated at 560 meV, assuming an intrinsic semiconducting conduction mechanism, that is, log R Žs log s . vs. 1rT. The energy
gap Ž D E . is lowered with the increase of Sr content
Fig. 5. Ža. Activation type conduction mechanism in the high
temperature regime Žactivation type. for Y1y x Sr xVO 3y d . Žb. The
variable range hopping mechanism in the low temperature regime.
The relationship between 70 and 220 K is magnified in the
inserted view.
Fig. 4. The variation of each lattice parameter in Y1y x Sr xVO3y d
Ž x s 0, 0.1 and 0.2..
Ž x ., and D Es in x s 0.1 and 0.2 are 90 and 2 meV,
respectively, as shown in Fig. 5Ža.. This conduction
mechanism would be plausible in the higher temperature regime, but in the lower temperature regime the
hopping conduction mechanism will be predominant,
470
K. Sakai et al.r Physica C 317–318 (1999) 464–470
4. Summary
Fig. 6. Temperature dependence of x when applied at H s10 Oe
in FC process in Y1y x Sr xVO 3y d .
considering the parasitic ferromagnetic behavior.
Thus, Fig. 5Žb. shows the variable range of the
hopping mechanism, that is, log R vs. Ty1 r4 . In the
case of x s 0.1 the conduction mechanism seems to
be roughly divided at the three regions, that is,
T - 50 K, 50 - T - 100 K, and T ) 200 K. From the
linear-least-squares fitting, it would be dominated by
the variable range hopping mechanism in the lower
temperature, while in T ) 200 K the thermal activated type conduction would be predominant. As
shown in the inset, the linearity is intriguingly depressed in the middle region where the magnetic
anomaly comes out because of the spin-electron
correlation. The specimen with x s 0.2 shows only
the small gap, which appears near a metal, but notice
that the bending point is still displayed in the vicinity
of 50 K.
Fig. 6 shows temperature dependence of the susceptibility for Y1y x Sr xVO 3y d in FC process under
H s 10 Oe. The anomalous peak and x value in the
lower temperature are reduced with x. The reduction
of resistivity and susceptibility exhibits that the hole
carrier is introduced by the replacement of Sr 2q by
Y 3q.
YVO 2.84 is a Mott-type insulator. We have found
the anomalous magnetic behavior with temperature
when applied to different field paths. YVO 3y d has
the remarkable magnetic correspondence for the external magnetic field as follows. Ž1. The hysteresis
between FC and ZFC, Ž2. two peaks in FC process,
Ž3. the reduction of the peak intensity at 110 K by
the field sweep with the increase of temperature.
This anomaly would arise from the parasitic ferromagnetic effect caused by the D–M interaction along
each axis.
This YVO 3y d showed a semiconducting behavior
in the higher temperature region from 300 to 200 K
and the resistivity could not be measured in the
region lower than 200 K. This corresponds to the
470 meV band gap when estimating the intrinsic
semiconductor. The gap in Y1y x Sr xVxO 3y d is smaller
with the replacement of Sr 2q, and the anomalous
magnetic characteristics are deteriorated.
Acknowledgements
The authors would like to thank Professor S.
Kanamaru, Institute of Research and Industrial Science, Osaka University, for the measurement of the
low temperature X-ray diffraction.
References
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w4x R.D. Shannon, Acta Cryst. A 32 Ž1976. 751.
w5x T. Moriya, Phys. Rev. 117 Ž1960. 635.