Enhanced room-temperature magnetoresistance in high-temperature sintered La2/3Sr1/3MnO3 doped with ZrO2

Article abstract of DOI:10.1016/j.physb.2006.09.020

Granular metal/insulator (La_2/3Sr_1/3MnO₃ doped with ZrO₂) composites were prepared by high-temperature sintering. The effects of sintered time on structure and grain size, as well as on magnetic and electric properties have been investigated. The XRD patterns of all doped samples show that the crystal structure is predominantly a perovskite phase with R3?c symmetry, in addition to the minor secondary compounds (SrZrO₃, La₂Zr₂O₇ and ZrO₂). The microscopic studies show that the long-time sintered samples have excellent connectivity between grains. The analysis of XPS reveals that the loss of oxygen occurs during the high-temperature sintering in air. It is also found that with sintered time increasing the paramagnetic insulator-ferromagnetic metal transition temperature T_C (or T_P) decreases gradually and the room-temperature magnetoresistance increases significantly.

Full text of DOI:10.1016/j.physb.2006.09.020

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Physica B 391 (2007) 206–211
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Enhanced room-temperature magnetoresistance in high-temperature
sintered La2/3Sr1/3MnO3 doped with ZrO2
Ã
Qianxue Zhoua,b, Mingxing Daia, , Renhui Wanga, Lei Jina, Shijian Zhua, Liang Qiana,
Yanmei Wua, Jiwen Fengc
aDepartment of Physics, Wuhan University, Wuhan 430072, China
bTechnology Centre, Wuhan Iron and Steel (Group) Company, Wuhan 430080, China
cWuhan Institute of Physics and Mathematics, the Chinese Academy of Sciences, Wuhan 430071, China
Received 9 May 2006; received in revised form 16 September 2006; accepted 27 September 2006
Abstract
Granular metal/insulator (La2/3Sr1/3MnO3 doped with ZrO2) composites were prepared by high-temperature sintering. The effects of
sintered time on structure and grain size, as well as on magnetic and electric properties have been investigated. The XRD patterns of all
¯
doped samples show that the crystal structure is predominantly a perovskite phase with R3c symmetry, in addition to the minor
secondary compounds (SrZrO3, La2Zr2O7 and ZrO2). The microscopic studies show that the long-time sintered samples have excellent
connectivity between grains. The analysis of XPS reveals that the loss of oxygen occurs during the high-temperature sintering in air. It is
also found that with sintered time increasing the paramagnetic insulator–ferromagnetic metal transition temperature TC (or TP)
decreases gradually and the room-temperature magnetoresistance increases significantly.
r 2006 Elsevier B.V. All rights reserved.
PACS: 61.10.Eq; 68.37.Hk; 73.43.Qt; 74.25.Ha; 74.25.Jb
Keywords: Colossal magnetoresistance; X-ray diffraction; Sintering; Electron properties; Magnetic properties
1. Introduction
occurs only at a high magnetic field and within a narrow
temperature range around TC, much beyond the premise of
application [1,3]. By tuning the composition of these
compounds, e.g., doping A or B site with other ions of
different sizes or charges, one can control the competitions
among the double-exchange(DE), superexchange, and
Coulomb interactions between Mn ions, and thus modify
the magnetoresistive properties of these perovskite com-
pounds [4,5]. As reported previously, the MR effect of
polycrystalline compounds exhibit a low-field magnetore-
sistance (LFMR) in a broad temperature range [6,7]. The
LFMR has been explained by the following two possible
mechanisms. Firstly, the spin-polarized conduction elec-
trons tunnel across grain boundaries and interfaces
separated by an energy barrier, which is the so-called
tunnel magnetoresistance (TMR) [6]. The second mechan-
ism is the electrons spin-dependent scattering by barrier
layers, which leads to giant magnetoresistance (GMR) of
granular film [8]. Thus, tuning grain surfaces or barrier
The compounds Re1ꢀxAxMnO3 have recently attracted
much attention (where Re is a trivalent rare-earth ion and
A is a divalent ion) [1,2]. They show a transition from a
paramagnetic semiconducting to a ferromagnetic quasime-
tallic state at the Curie temperature (TC). Many of these
compounds also exhibit the colossal negative magnetore-
sistance (CMR), being most pronounced in the vicinity of
TC. So far many investigations on CMR materials have
concerned some aspects relative to their application. To
this aim, materials need to be tailored with great negative
magnetoresistance in lower magnetic field and proper
temperature range, since the magnetoresistance (MR)
effect of single crystals and epitaxial films commonly
Ã
Corresponding author. Tel.: +86 27 62825082; fax: +86 27 68752569.
E-mail addresses: zhouqianxue@163.com (Q. Zhou),
mxdai@whu.edu.cn (M. Dai).
0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2006.09.020

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207
layers by altering the size of the ferromagnetic grains,
making artificial boundaries or diluting with a nonmag-
netic insulator will significantly influence the transport
process [9–11]. In recent years, it was reported that
introduction of a second phase into the ferromagnetic
manganite matrix had resulted in an enhancement of MR
[12–14]. These secondary phases include insulating oxide
[11,12]; half-metallic ferromagnet [13] and polymer com-
posite [14], etc. It was further pointed out that the
conduction behavior and MR effect of the perovskite
manganites are significantly influenced by the grain
connectivity [15,16].
measurements were performed by standard four-probe
method in the temperature range from 380 to 20 K. The
MR was defined as MR ¼ (r0ꢀr3 T)/r0, where r3 T and r0
are the resistivities measured at 3 T magnetic field and zero
field, respectively.
3. Results and discussions
Fig. 1 shows a series of powder XRD patterns of
LSMO(1400 1C), Zr1h, Zr4h, Zr8h, Zr24h, and Zr48h,
recorded at room temperature. The XRD pattern for the
undoped sample shows a single-phase perovskite structure
¯
with R3c symmetry. But the impurity phases including
In this paper, we introduce ZrO2 as the second phase
into the manganite and investigate the effect of sintered
time on magnetotransport properties and MR in La2/3Sr1/
3MnO3 (LSMO)+0.1 (the mole fraction) ZrO2 granular
composites. Firstly, the LSMO manganite is nearly 100%
spin polarization [6], which may boost magnetoresistance
associated with spin-dependent tunneling; and LSMO has
high Curie temperature (TCE360 k) [2], being available to
attain a large MR at room temperature. Secondly, due to
the low solid solubility of Zr ions in La2/3Sr1/3MnO3, only a
small amount of Zr4+ ions go into the lattice of La2/3Sr1/3
MnO3 [17] and most Zr4+ ions segregate as secondary at
grain boundaries insulating compounds at higher Zr
content [17,18], which will influence the transport process.
Thirdly, the grain connectivity of samples can be improved
by high-temperature sintering with long time [15,16].
SrZrO3, La2Zr2O7 and ZrO2 occur in all Zr-doped samples
investigated here. These impurity phases are still retained
even after 48 h sintering at 1400 1C, indicating that only
limited amount of Zr can go into the lattice of La2/3Sr1/
3MnO3. Although these impurity phases appear in all our
doped samples, their contents are quite low (see the inset of
Fig. 1). The lattice parameters of these samples are shown
in Table 1. The lattice parameters of LaMnO3.15 in
˚
the JCPDSICDD (No. 32-0484) are a ¼ 5.523 A and
˚
c ¼ 13.324 A in a hexagonal system. The lattice parameters
of our sample for LSMO are close to these values. The
lattice parameters of LSMO+0.1(ZrO2) sets of samples
increase slightly with the sintered time. This can be
explained in terms of the different ionic radii of Zr4+
and Mn4+ ions (they are 0.72 and 0.53 A, respectively [19]),
˚
or in terms of the oxygen vacancies as reported by Rojas et
al. [20].
2. Experimental
Fig. 2 shows the typical SEM micrographs for several
samples. It can be seen that the size of grains gradually
increases and the connectivity between grains becomes
excellent with prolonging sintered time. One hour sintered
sample is very porous with a small grain size of about 5 mm
and contains a large number of weakly linked grain
boundaries. By contrast, the 48 h sintered sample appears
very dense with an excellent connectivity and melting
LSMO+0.1(ZrO2) samples were fabricated by three
steps. Firstly, the LSMO powders were prepared by sol–gel
method [17] and sintered at 1300 1C for 8 h. Secondly, the
ZrO2 particles were pre-prepared by a thermal decomposi-
tion method. Zr(NO3)4 ꢁ 5H2O was sintered in air at 650 1C
for 8 h. Thirdly, the above two pre-prepared powders in an
appropriate mole ratio were completely mixed, ground
carefully, and subsequently pressed into discs under
400 MPa. The final sintering process was carried out at
1400 1C in air for 1, 4, 8, 24, 48 h, respectively, and then
furnace cooled in air, to obtain a series of granular samples
(these samples are called Zr1h, Zr4h, Zr8h, Zr24h, Zr48h,
respectively), as well as LSMO was carried out at 1300 and
1400 1C for 8 h in air. X-ray diffraction (XRD) was used to
characterize the structure of the samples with Bruker D8-
ADVANCE. The morphology was probed by FEG SEM
Sirion. An X-ray photoelectron spectroscopy (XPS)
spectrum was obtained with Kratos XSAM800. Mg Ka
(1253.6 eV) X-ray source was used for excitation. The
resistivity r and saturated magnetization Ms measurements
were carried out by physical property measurement system
(PPMS). The DC magnetization measurements were carried
out with the vibrating sample magnetometer (VSM) of the
IDE Corporation in the temperature region of 150–400 K.
The paramagnetic–ferromagnetic transition temperature
(TC) was taken as the minimum dM/dT. The resistivity
Zr48h
Zr24h
Zr8h
Zr4h
Zr1h
26
27
28
29
2θ (deg.)
30
31
32
Zr48H
Zr24H
Zr8H
Zr4H
Zr1H
LSMO
20
30
40
50
60
70
80
2θ (deg.)
Fig. 1. X-ray powder diffraction patterns of LSMO(1400 1C), Zr1h, Zr4h,
Zr8h, Zr24h and Zr48h. The inset shows the relative enlarged diffraction
patterns.

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Table 1
The experimental data of partial samples
˚
a (A)
˚
c (A)
Samples
Temperature (1C)
Time (hours)
TC (K)
TP (K)
CMR (%)
LSMO
Zr1h
Zr4h
1400
1400
1400
1400
1400
1400
8
5.523
5.5.510
5.515
5.519
5.520
5.528
13.332
13.389
13.391
13.389
13.391
13.406
377
330
323
323
297
297
373
358
354
337
336
347
8.5
18.7
21.5
28.9
32.3
34.4
1
4
Zr8h
Zr24h
Zr48h
8
24
48
Fig. 2. SEM morphologies of samples: (a) Zr1h, (b) Zr4h, (c) Zr8h, (d) Zr48h.
occurs in the some parts of the surface, similar to the
observed previously in partially melted polycrystalline
[15,16].
The Curie temperatures are 377, 330, 323, 323, 297 and
297 K for LSMO(1400 1C), Zr1h, Zr 4h, Zr 8h, Zr 24h and
Zr 48h, respectively, as shown in Table 1, Obviously, the
TC values for LSMO+0.1(ZrO2) samples decrease gradu-
ally with increasing sintered time. It is also noticed that the
pure LSMO sample shows the sharpest transition, but the
transition for the LSMO+0.1(ZrO2) samples becomes very
broader and a residual tail is observed until 365 K. The
magnitude of low-temperature magnetization under a
magnetic field of 0.02 T for the short-time sintered samples
is not remarkably different, while it decreases for long-time
sintered samples. One of the possible reasons for the lower
magnetization of 48 h is due to the presence of a large
number of strong-link boundaries between domains. The
domain walls at the strong-link grain boundaries enhance
the interaction between domains in each strong-link grain.
Therefore, alignment of spins in each domain is hardly
enhanced at a low field than other samples below TC [16].
The inset shows the magnetic field dependence of
magnetization (M–H) of all samples recorded at 10 K.
For the undoped sample, the experimental saturated
magnetization of 89.3 emu/g agrees well with the theoreti-
cally expected the Ms of 91 emu/g for pure La2/3Sr1/3MnO3
(assuming spin-only moment). For the samples doped with
Zr, the saturated magnetization Ms is almost identical, say,
e.g., Msꢂ83 emu/g.
Since the Mn4+ content is a crucial factor in controlling
the magnetic and transport properties [21,22], we examined
the valence states of the manganese ions in the
LSMO(1300 1C) and LSMO+0.1(ZrO2) samples with
XPS. The samples were ground with a mortar and pestle
before the XPS measurement. Fig. 3 shows the XPS spectra
and their corresponding fitting curves for the Mn 2p level
of LSMO, Zr1h, Zr8h and Zr24h. The peak positions of
Mn3+ are observed at 641.3, 641.6, 641, and 641.7 eV for
LSMO, Zr1h, Zr8h and Zr24h, respectively. They are close
to the binding energy value 641.9 eV of the Mn 2p3/2 level
in a-Mn2O3 [23]. The peak positions of the Mn4+ are
observed at 642.8, 643.3, 643 and 643.7 eV, respectively.
They are close to the binding energy value 643.35 eV of the
Mn4+2p3/2 level in La0.9Te0.1MnO3 [24]. It can be obtained
from their fitting areas in Fig. 2 that the ratios of Mn4+
/
Mn3++ Mn4+ are 36.7%, 36%, 34.5%, 32.4% for
LSMO, Zr1h, Zr8h and Zr24h, respectively, indicating
that some of Zr4+ into Mn4+ site, meanwhile, a certain
amount of the oxygen is lost during sintering at high
temperature in air.
Fig. 4 shows curves of the magnetization versus
temperature at applied field 0.02 T for the various samples.

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raw
raw
fitting curve
fitting curve
4+
Mn
Mn
4+
Mn
Mn
3+
3+
638 640 642 644 646 648
638 640 642 644 646 648
(a)
Binding Energy (eV)
(b)
Binding Energy (eV)
raw
raw
fitting curve
fitting curve
4+
4+
Mn
Mn
Mn
Mn
3+
3+
638
640
642
644
646
648
638 640 642 644 646 648
(c)
Binding Energy (eV)
(d)
Binding Energy (eV)
Fig. 3. The Mn 2p spectra and the fit curves (a) LSMO(1300 1C), (b) Zr1h, (c) Zr8h, (d) Zr24h.
0.028
0.026
0.024
0.022
0.020
LSMO
12
10
8
LSMO
Zr1h
Zr1h
Zr4h
Zr4h
Zr8h
Zr8h
Zr24
0.018
0.016
0.014
0.012
0.010
0.008
0.006
0.004
0.002
0.000
Zr24h
Zr48h
Zr48h
90
80
70
60
50
40
6
LSMO
Zr1h
Zr4h
4
Zr8h
Zr24h
Zr48h
2
0
10000 20000 30000 40000 50000
H (Gs)
0
50 100 150 200 250 300 350 400
150
200
250
300
350
400
T (K)
Temperature (K)
Fig. 5. Temperature dependence of resistivity of partial samples without
the field.
Fig. 4. The magnetization versus temperature curves for partial samples.
The inset shows the relative magnetization versus applied field measured at
10 K.
Fig. 5 shows the temperature dependence of the
resistivity r on cooling. For all samples, the resistance at
zero field increase with decreasing temperature until
insulator–metal transition temperature TP. Below TP, the
resistance decreases on further cooling down to 20 K. With
increasing sintered time, the resistivity r increases mark-
edly for all samples except for Zr48h one. Meanwhile, the
insulator–metal transition temperature TP decreases gra-
dually (seen Table 1), from 370 K for LSMO(1400 1C)
down to 336 K for Zr24h. The Zr48h sample exhibit a
decrease in resistivity and a slight increase in TP value
(see Table 1) by comparing with Zr24h. It is also noted
that the resistivity of Zr48h drops quickly as the
temperature is lowered and becomes smallest among all
LSMO+0.1(ZrO2) samples at To205 K. The lower
resistivity appearing in the Zr48h sample may result from
its good inter-grain connectivity produced by prolonging
sintered time. A good connectivity between grains may
open up many new conduction channels and thus reduce
the effective contact resistance between grains [25].