Large magnetoresistance in (La1−xCaxMnO3)1−y:ZrO2 composite
D. Das, A. Saha, S. E. Russek, R. Raj, and D. Bahadur
Citation: J. Appl. Phys. 93, 8301 (2003); doi: 10.1063/1.1556260
View online: http://dx.doi.org/10.1063/1.1556260
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Published by the American Institute of Physics.
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JOURNAL OF APPLIED PHYSICS
VOLUME 93, NUMBER 10
15 MAY 2003
Large magnetoresistance in „La1À x Cax MnO3 … 1À y :ZrO2 composite
D. Dasa)
Department of Metallurgical Engineering and Materials Science, IIT-Bombay, Mumbai, India
A. Saha
Department of Mechanical Engineering, University of Colorado at Boulder, Boulder, Colorado 80309
S. E. Russek
Magnetic Thin Films and Devices Division, National Institute of Standard and Technology (NIST), Boulder,
Colorado 80309
R. Raj
Department of Mechanical Engineering, University of Colorado at Boulder, Boulder, Colorado 80309
D. Bahadur
Department of Metallurgical Engineering and Materials Science, IIT-Bombay, Mumbai, India
共Presented on 15 November 2002兲
Colossal magnetoresistance 共CMR兲 composite materials have been synthesized to explore the
possibility of improving magneto-transport and structural properties in CMR systems. In this work
we describe (La1⫺x Cax MnO3 ) 1⫺y 共LCMO兲 (ZrO2 ) y (x⬇0.3 and 0.0⭐y⭐0.40 mole %) composites
that have been synthesized using a modified 共non Pechini type兲 sol–gel technique.
Magnetoresistivity of the composites was evaluated at 5 T field and in the temperature range 5–300
K. The composites show higher magnitude of MR compared to pure LCMO. The MR rises from a
base value 76%, for the case y⫽0, to a maximum value of 93.8%, obtained at y⫽0.05. dc
susceptibility measurements show a distinct ferromagnetic to paramagnetic transition in all
composites. The ferromagnetic transition temperature (T C ) drops from 225 K in pure LCMO
(y⫽0) to 121 K in y⫽0.05 and then slowly rises to 157 K as y increases. The plots of zero field
cooled susceptibility ␹ ZFC 共T兲 and field cooled susceptibility ␹ FC 共T兲 diverge clearly below T C ,
indicating magnetic irreversibility. The composite exhibits a clear metal–insulator transition (T MI)
at or just above the magnetic transition. The peak resistivity ␳ MI at the metal–insulator transition
also exhibits interesting changes. For pure LCMO polycrystals, ␳ MI⫽102 ⍀ cm, but it increases to
228 ⍀ cm for y⫽0.05 and then gradually decreases to 1.94 ⍀ cm for y⭓0.10. The phase evolution
in the LCMO:ZrO2 composites was studied by x-ray powder diffraction and correlated to the
magnetic and electrical properties. © 2003 American Institute of Physics.
关DOI: 10.1063/1.1556260兴
There has been a renewed interest in the research of
manganese perovskite since the discovery of colossal magnetoresistance 共CMR兲 property in this compound in 1993.1
The parent compound LaMnO3 is a charge transfer Mott insulator with trivalent Mn in ⫹3 oxidation state having the
3 1
electronic configuration t 2g
e g (S⫽2). This insulating
LaMnO3 can be driven metallic through the partial substitution of trivalent La⫹3 by divalent metal ions like Ca, Sr, and
Pb or by tetravalent2,3 metal ions, thereby converting a corresponding number of trivalent Mn⫹3 ions into quadrivalent
Mn⫹4 共for bivalent substitution兲 with electronic configura3
(S⫽3/2). The magnetic and electronic transport
tion t 2g
properties of these hole doped manganites have traditionally
been examined within the framework of ‘‘double exchange,’’
which considers the magnetic coupling between spin aligned
Mn⫹3 and Mn⫹4 ions, through the hopping of an electron
between two partially filled d orbitals with strong on-site
Hund’s coupling as was proposed by Zener in 1950.4 Among
the various perovskite manganites, substitutions at the La
a兲
Author to whom correspondence should be addressed; electronic mail:
[email protected]
0021-8979/2003/93(10)/8301/3/$20.00
8301
site5,6 共A site兲 and at the Mn site7,8 共B site兲 have been extensively tried to enhance the CMR effect. Attempts have also
been made to increase the MR through the formation of
some composites.9–11
Recently our work on La0.67Ca0.33MnO3 共LCMO兲:SiO2
composite11 showed interesting results. The composite
showed a percolation threshold composition corresponding
to 90 vol % of LCMO, beyond which the resistivity shoots
up sharply to almost 6 orders of magnitude. Zr being in the
⫹4 state in ZrO2 共like Si⫹4 in SiO2 ) should produce interesting results in the LCMO:ZrO2 composite since ZrO2 is
also a refractory oxide like SiO2 . One major difference between these two ions is that while the Si⫹4 ion can not go
into the perovskite lattice due to its strong preference for
tetrahedral coordination, Zr⫹4 can be accommodated in the B
site of the perovskite lattice. The present investigation intends to study the magnetic and electrical transport properties of the CMR composite LCMO:yZrO2 and the results are
presented in this article.
LCMO:yZrO2 composites containing 5, 10, 20, and 40
mol % of ZrO2 have been synthesized by a modified 共non
Pechini type兲 sol–gel technique.11 The precursor on subse© 2003 American Institute of Physics
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8302
Das et al.
J. Appl. Phys., Vol. 93, No. 10, Parts 2 & 3, 15 May 2003
TABLE I. Measured electrical and magnetic transport parameters of the
composites.
FIG. 1. X-ray diffractogram of LCMO:yZrO2 composites.
quent calcination at 1000 °C for 5 h in static air produced
composite powders. The powders after cold pressing in a
uniaxial hydraulic press followed by sintering at 1350 °C for
24 h in air produced sintered pellets for characterization.
Crystallographic structure of the specimen was examined by a Scintag-V x-ray powder diffractometer using
Cu K ␣ radiation with a step size of 0.02° in the range 20°
⭐2 ␪ ⭐80°. Phases and cationic stoichiometry of the phases
were confirmed by a scanning electron microscope 共RJ Lee
Instruments兲.
Magnetization measurements were performed using a
commercial superconducting quantum interference device
共SQUID兲 共Quantum design MPMS 7兲 magnetometer in the
temperature range 5–300 K. Field cooling 共FC兲 and zero
field cooling 共ZFC兲 susceptibility were measured at a low
magnetic field of 20 Oe. Transport properties of the sintered
samples were measured using the same SQUID magnetometer interfaced with an additional lock-in amplifier in the same
temperature range using the standard four-probe technique.
Field data were taken at 5 T.
The room temperature x-ray diffraction patterns of
LCMO:yZrO2 sintered 共1350 °C for 24 h in air兲 pellets are
shown in Fig. 1. ZrO2 reacts with LCMO to form CaZrO3 in
the lower doping range, i.e., up to y⫽0.10 and beyond that it
forms La2 Zr2 O7 . These phases (CaZrO3 and La2 Zr2 O7 ) are
insulating in nature. The shifting of LCMO reflections in the
patterns indicates the occurrence of the reactions. The pattern
shows CaZrO3 and m-ZrO2 lines along with LCMO lines for
y⭐0.10. For y⭓0.20 it shows La2 Zr2 O7 and t-ZrO2 along
with LCMO lines. The lattice parameters of LCMO in different composites are calculated using Cohen’s12 least-square
method in the range 20°⭐2 ␪ ⭐80°. Cubic symmetry of
LCMO is observed in all the composites. The lattice parameters for these composites are presented in Table I. They tend
to increase indicating that some Zr⫹4 go to perovskite lattice
at the B site. The scanning electron microscope micrographs
show the grain growth of LCMO in y⫽0.05 compared to
pure LCMO but for y⫽0.40 the grains are smaller in size.
Micrographs show evidence of secondary phases.
Figure 2 presents the magnetic dc susceptibility of the
composites as a function of temperature. The temperature
dependence of magnetization M (T) was measured after
cooling the samples from 350 to 5 K in zero field 共ZFC兲 or in
the field of 20 Oe 共FC兲. The dependencies of ␹ ZFC 共T兲 and
Composition
LCMO:yZrO2
Lattice
parameter
a 共Å兲
T C 共K兲
␳ MIT 共⍀ cm兲
T MI 共K兲
MR%
y⫽0.0
y⫽0.05
y⫽0.10
y⫽0.20
y⫽0.40
3.878
3.880
3.882
3.887
3.881
225
121
125
133
157
102.64
228.75
116.74
10.88
1.94
216.8
115.0
169.0
148.0
195.9
76.3
93.8
86.3
87.6
86.6
␹ FC 共T兲 ( ␹ ⫽M /H) are presented in this figure. All the composites along with LCMO show a smooth transition from
ferromagnetic to paramagnetic state with a significant high
temperature tail. The plots of ␹ ZFC 共T兲 and ␹ FC 共T兲 diverge
clearly below the magnetic transition temperature T C showing magnetic irreversibility of the system. The ferromagnetic
transition temperature T C , calculated from the minimum in
the d ␹ /dT versus T curve, of all the composites are shown in
Table I. Initially T C drops sharply from 225 K for pure
LCMO to 121 K for y⫽0.05 followed by a gradual increase
up to 157 K for y⫽0.40. The shifting of T C towards lower
temperatures for a sample with y⫽0.05 and then subsequent
increase for y⭓0.10 can be explained on the basis of the
reaction products. For y⫽0.05, CaZrO3 is the second phase,
where the Ca stoichiometry in LCMO shifts from the nominal composition La0.67Ca0.33MnO3 towards the lower Ca
side. Formation of La2 Zr2 O7 , which starts at y⫽0.10 and
becomes prominent as y increases further up to y⫽0.40,
shifts the stoichiometry towards the higher Ca side and T C is
pushed up towards higher temperatures. The variation of T C
with mole % of ZrO2 共y兲 matches qualitatively the variation
of T C with Ca content 共x兲 in LCMO as reported by Laiho
et al.13 They have reported T C , which varies from around
175 K for x⫽0.0 to around 260 K for x⫽0.40. However our
pure LCMO (y⫽0.0) shows lower T C , which may be due to
the presence of nonstoichiometric defects such as oxygen ion
excess or La⫹3 , Ca⫹2 vacancies etc., which results in an
increment of electron scattering centers as proposed by
Massa et al.14
FIG. 2. Low field magnetization behavior with temperature for
LCMO:yZrO2 composites.
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Das et al.
J. Appl. Phys., Vol. 93, No. 10, Parts 2 & 3, 15 May 2003
FIG. 3. Variation of dc electrical resistivity with temperature of
LCMO:yZrO2 composites.
Temperature variations of resistivity between 5 and 375
K of the composites have been shown in Fig. 3. All the
composites show a distinct metal–insulator transition close
to their magnetic transition temperature T C . In the paramagnetic regime, at T⬎T C the composites show an insulator
behavior (d ␳ /dT⬍0) and below the transition a metallic
behavior (d ␳ /dT⬎0). The metal–insulator transition temperature T MI and peak resistivity ␳ MIT for all the composites
have been reported in Table I. Zr⫹4 being diamagnetic cannot participate in the electron transfer process between Mn⫹3
and Mn⫹4 ions via O⫺2 ions and in turn it dilutes the double
exchange 共DE兲 process. The dilution of the DE process in
effect breaks the long range ferromagnetic order in the system, which is reflected in lowering of T C to 121 K 共in y
⫽0.05) from 225 K 共in y⫽0.0) and increase of peak resistivity to 228.75 ⍀ cm for y⫽0.05 w.r.t LCMO ( ␳ MIT
⫽102.64 ⍀ cm). However, this peak resistivity value of pure
LCMO is higher than those reported in the literature for high
quality LCMO (T C ⫽260 K). The presence of excess electron scattering centers due to the nonstoichiometric defects
gives rise to higher resistivity. Zr⫹4 being a bigger ion than
Mn, should compress some Mn⫹3 – O– Mn⫹4 bonds, which
in turn would lead to bond angle distortion and would influence the transport and magnetic properties. But in a higher
doping range ZrO2 behaves oppositely. Zr⫹4 being bigger in
size than the Mn⫹4 ion, induces a higher residence time of
the higher valent Mn ion (Mn⫹4 , being smaller in size than
Mn⫹3 ) in its close vicinity, thereby causing a local charge
ordering. The local charge ordering in turn would straighten
out some Mn⫹3 – O– Mn⫹4 bonds in its immediate neighborhood, releasing the strain in the system, which will lead to
the improved quality of transport of the composite. This accounts satisfactorily for why the resistivity is low for y
⭓0.10. The low temperature resistivity rise of the composite
particularly for y⫽0.05 and y⫽0.20 may be due to the carrier localization in the system.
Magnetoresistivity behavior between 5 and 300 K and at
5 T field for the composites has been shown in Fig. 4. MR
shows a maximum near the magnetic transition temperature
for all the composites and a rising part at low temperatures.
Magnitude of MR is highest 共93.8% at 5 T兲 for y⫽0.05 and
then remains almost constant at around 87% for the remain-
8303
FIG. 4. Magnetoresistivity behavior with temperature for LCMO:yZrO2
composites.
ing composites. The MR value for pure LCMO is the lowest
at 76%. MR behavior of the composite with 5 mol % ZrO2 is
interesting. It peaks at T C and remains constant at around
93.8% up to 75 K and goes down slightly to around 85% and
maintains the same value as T→0. This near constancy below T C has also been reported in the CMR composite system
LCMO:SrTiO3 , 10 and could be useful from an application
point of view. The enhancement of MR and its constancy up
to low temperatures could be ascribed to the magnetically
disordered region near the grain boundary. Because of the
secondary phases the separation between conducting LCMO
grains may become comparable to spin memory length. If
the effect is only due to the magnetically disordered region
near the grain boundary, spin dependent scattering 共which
may be temperature dependent兲 could be essentially responsible for high MR and its near temperature independence
below T C .
In summary, an attempt to synthesize the LCMO:ZrO2
composite gives some secondary phases such as CaZrO3 and
La2 Zr2 O7 , which are insulating and nonmagnetic. However,
the important characteristics of LCMO are not lost. In fact
there is marked improvement in MR property at T C and below, especially for y⫽0.05. The highest MR of 93% is obtained for the y⫽0.05 composite. This, we assume, can be
further improved by an increased interfacial disorder by
changing the processing conditions.
Financial support from AFOSR, U.S. and DST, Government of India is gratefully acknowledged.
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