# Fabrication and Transport Properties of Manganite-Polyacrylamide-Based Composites

Authors: Viorel Sandu; Stelian Popa; Ion Ivan; Carmen Plapcianu; Elena Sandu; Camelia Mihailescu; Florica Doroftei
Journal: Journal of Nanomaterials
 (2009)

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## Abstract

We present the fabrication and transport properties of a series of composites made ofLa2/3Sr1/3MnO3 and acrylamide-based copolymers. The most important result is the very narrow transition, of only 27 K, displayed by the peak that appears around the metal-insulator transition of the composites made with poly(acrylamide-vinylacetate). Although the amount of polymer is rather low,
different copolymers change drastically the electric transport characteristics.


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## Body

## 1. Introduction

Polycrystalline structures based on perovskitic manganites which display colossal
magnetoresistance (CMR) have been the subject of intense research during the
last decade because of the great expectations for application of their low
field magnetoresistance (see [1–4] and therein references). Although the
mechanism leading to the colossal magnetoresistance in polycrystals is not
completely understood, the key role of grain borders is unanimously accepted in
the transport process. Therefore, a wealth of data reports on experiments
trying to control and improve the grain border/intergrain area and further to
decipher the underlying physics. It is considered that the magnetic disorder at
the interface enhances the spin dependent scattering in that area. As the
charge and spin transport occur by tunneling and hopping, many attempts to
improve the barrier were made by dilution with a second non-CMR material [5–12]. The dilution not only modifies
the barrier geometry but also produces a kind of conditioning by
changing the magnetic structure and interactions at the grain border.An interesting way of managing the grain surface is the dressing of the
manganite grains with a very thin polymer layer. Consequently, the sintering
involves only the polymeric interface, hence, requires much lower
temperatures and sintering time as compared to the use of oxides. The low processing temperatures are also beneficial for the grain cores since they are
no more influenced by interdiffusion. In addition, the use of polymers increases
the resistance of the interface, hence, of the whole sample, with orders of
magnitude. There are few data concerning the use of polymers in conjunction
with CMR manganites  by our knowledge, only polyparaphenylene [13, 14], poly(methyl
methacrylate) [15], and polyphenylene sulfide [16] have been reported in
combination with manganites.In this paper,  we present the use of an acrylamide-vinylacetate
copolymer to obtain manganite-polymer composite. The reason is that polyacrylamide
and its copolymers are amongst the most used polymers in composite materials
due to their outstanding capacity of wrapping the grains. In addition, they confer
good mechanical properties to the composite samples.


## 2. Experimental

The manganite La2/3Sr1/3MnO3 (LSMO) was prepared starting from La2O3, SrCO3, and
MnO2 mixed together in warm citric acid (80C°) in the
appropriate molar ratio (La:Sr:Mn = 0.67:0.33:1). A transparent brownish gel was obtained which further  was dried for 5 hours at 150C° in the oven, when a glassy solid oxide precursor
could be separated. The precursor was further annealed at 1200C° in air
for 12 hours. The complexes formed in both cases were characterized by
elemental chemical analysis, TGA, IR spectroscopy, and nitrogen absorption (BET).Acrylamide-vinylacetate copolymer
(PAAm-VAc) was prepared by free radical polymerization of the comonomer mixture (feed molar
ratio = 1/1). Aqueous medium copolymerization, initiated by the redox system K2S2O8/Na2S2O5,
was carried out at 65C° following the reaction illustrated in Scheme 1.Scheme 1At the
end of the reaction, after 120 minutes, the reaction product was precipitated
in methanol, filtered off, and washed several times. Subsequently, the polymer
was dried to constant weight at 50C°. The polymer yield, gravimetrically
determined, was over 50% (54.47%).Differential scanning calorimetry (DSC) date showed a softening temperature of 157C°.For the synthesis of poly(acrylamide–co–methyl methacrylate)  (PAAm-MMA), was applied the microwave assisted copolymerization.The reaction was carried out in
glass vessels without stirring, with 1% AIBN as radical initiator and
dimethylsulfoxide solvent for microwave absorption. Microwaves have the
advantage of a rapid and uniform heating so that the radical initiator decomposes
almost instantaneously. Consequently, the concentration of the primary radicals
is very high; hence, an increased efficiency is expected. In addition, the thermal
degradation process is very limited. The copolymerization with methyl
methacrylate followed the reaction illustrated in Scheme2. Finally, the
copolymer was conditioned in the same way as described for PAAm-VAc. Nevertheless,
in this case the polymer yield is rather low 22.7% for five minutes reaction
time. DSC data of this copolymer shows a softening temperature of 178C°.Scheme 2The composites were prepared by dipping the
manganite powder within appropriate solutions of copolymer in chloroform. The
powder obtained after drying at room temperature was pelletized in bar shape at
20 kgf/cm2 and sintered in an oven at 150C° for one hour. We
prepared three types of La2/3Sr1/3MnO3-PAAm-VAc
composites with 2%, 4%, and 6% copolymer, respectively. For transport
measurements, four copper leads were attached to each pellet by ultrasonic
soldering with Cerasolzer.The electrical resistance was measured using
the four probes technique with currents between 1 nA and 1μA. Because of the polymer matrix, the
temperature for measurements was limited at 325 K; therefore, it was difficult
to reach the full paramagnetic state of the manganites that was reported at
high temperatures.


## 3. Results and Discussion

The temperatureT dependence of
the resistance R follows a dependence
that roughly reminds the general behavior of fined grained manganites. However,
the type of polymer has strong influence on the particular characteristics of
each sample.In the case of the composites with PAAm-VAc, as the temperature
decreases, the resistance, of order0.1–0.9 MΩ, starts to increase very fast reaching a narrow
peak followed by a steep metal-insulator transition at lower temperatures. For
a further decrease of the temperature, the resistance shows a dip followed by
an increase toward a temperature-independent resistance. Figure 1 shows the
evolution of the resistance normalized to the peak resistance RMI as a function of the
temperature normalized to the metal-insulator transition temperature TMT, measured with a current
of 100 nA. What is outstanding is the width of the peak which is the narrowest
ever reported, by our knowledge, only 27 K at half width. A second special
feature is the wide T range, between
10 and 200 K, where the resistance keeps an almost constant value. In addition,
in this representation, all peaks collapse onto one curve. Magnetic
measurements have shown that the paramagnetic to ferromagnetic phase transition
occurs at temperatures higher than the limitations imposed by the material
composition (325 K); hence, the material is in ferromagnetic state in the
temperature range 269 ≤ T ≤305 K where TMI is located. A simple
model for the increase of the resistance in the ferromagnetic state was
presented by Yuan et al.  [17] that
consider the distribution of grain separation and an exchange coupling between
grains that can change the sign with the distance between grain borders. As a
result, the conductance has a spin dependent contribution due to the
ferromagnetic coupling that depends on the grain magnetization and a second
contribution which is almost constant  mirroring the antiferromagnetic coupling.
The latter term behaves like an activated transport; specifically the
conductance decreases fast with decreasing T while the conductance due to the ferromagnetically coupled grains increases and
becomes constant at low temperature where it is dominant. Thus, the interplay
of the conductances could explain both the peak and the constant resistance at
low temperatures.Figure 1
Temperature dependence of the relative
resistanceR/RMI for a series of composites made from La2/3Sr1/3MnO3 with 2; 4; and 6% poly(acrylamide–co–vinylacetate) (PAAm-VAc).The role of the copolymer is not very clear in this model, but
experimentally we found it is very important. For the sake of comparison, we
have tried a second copolymer, poly(acrylamide–co–methyl methacrylate)
(PAAm-MMA) with the same manganite. Although the amount of polymer was only 2%
in both cases, the differences between LSMO-PAAm-VAc and LSMO-PAAm-MMA
composites are noticeable (Figure2).  The TMI of the latter shifts to lower temperatures and the metal-insulator transition
is broad. Actually, the whole peak is four times broader than for LSMO-PAAm-VAc.
In addition, a low temperature peak appears around 100 K which is rather broad but
well defined.Figure 2
Comparison between temperature
dependence of two composites made from La2/3Sr1/3MnO3 and 2% poly(acrylamide–co–vinylacetate) (PAAm-VAc) and the same manganite with
2% poly(acrylamide–co–methyl methacrylate)  (PAAm-MMA).The SEM micrograph of the fracture of the LSMO-PAAm-VAc sample shows a
uniform covering of each manganite grain (Figure3). In the case of the LSMO-PAAm-MMA
sample, the polymer appears like a network which separates large clusters of
grains (Figure 4). Therefore, in the first case, the polymer acts only at the
level of the intergrain tunneling barrier. In the the latter case, the presence
of the polymer network-like structure, which encloses large clusters of grains,
leads to extra barriers. In addition,  we cannot rule out a temperature
dependence of the intergrain contacts due to the dramatic change of the elastic
properties of the polymer network as well as the difference in the thermal
expansion of the two constituents: mangnite and polymer.Figure 3
SEM micrograph of the composite
made of La2/3Sr1/3MnO3 and 2% poly(acrylamide–co–vinylacetate).Figure 4
SEM micrograph of the composite
made of La2/3Sr1/3MnO3 and 2%  poly(acrylamide–co–methyl methacrylate).Finally, we mention the history dependent resistance of the LSMO-PAAm-VAc
composite. There is a significant difference between theR–T dependence when the sample is warmed
up after the resistance was measured in a cooling down process. As Figure 5
shows, during the warming process after the first cooling down process, the dip
vanishes and the peak decreases with almost 40%. Most likely, this is an effect
of phase separation which is typical for manganites [18–22]. The border between
the conductive and dielectric phases is moved by the measuring current; hence,
the second run finds another phase arrangement with lower resistance at the peak.
The inset to Figure 5 shows the time increase of the resistance for a constant
current of 100 nA and at constant temperature close to the dip. A delay of several
hours at room temperature without any transport current running through the
sample reestablishes the initial state and the R–T curve recovers its initial
variation.Figure 5
History dependence of the
resistance of a composite made from La2/3Sr1/3MnO3 with 4% poly(acrylamide–co–vinylacetate)  PAAm-Vinylacetate (lin). Inset: time evolution of
the resistance at 290 K for a feeding current of 100 nA.In summary,  we have fabricated composites with La2/3Sr1/3MnO3-and polyacrylamide-based copolymers: poly(acrylamide–co–vinylacetate)
and, for comparison, poly(acrylamide–co–methyl–methacrylate). The samples with
PAAm-VAc display a transition from paramagnetic to ferromagnetic phase with a
very narrow breadth, of only 27 K at the half resistance peak, which is the narrowest
value, yet reported. An attempt of explanation was made based on the model of
oscillatory exchange coupling between grains. A history-dependent process and
its relaxation were also revealed.



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*Source: 429430-2009-04-13.xml*