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Effect of fuel on the formationstructure, transport and magneticproperties of LaMnO3+ δ nanopowdersB.M. Nagabhushana a , R.P.S. Chakradhar b , K.P. Ramesh c , V.Prasad c , C. Shivakumara d & G.T. Chandrappa ea Department of Chemistry , M.S. Ramaiah Institute ofTechnology , Bangalore 560054, Indiab Glass Technology Laboratory, Central Glass and CeramicResearch Institute (CSIR) , Kolkatta 700032, Indiac Department of Physics , Indian Institute of Science , Bangalore560012, Indiad Solid State and Structural Chemistry Unit, Indian Institute ofScience , Bangalore 560012, Indiae Department of Chemistry , Bangalore University , Bangalore560001, IndiaPublished online: 12 May 2010.

To cite this article: B.M. Nagabhushana , R.P.S. Chakradhar , K.P. Ramesh , V. Prasad , C.Shivakumara & G.T. Chandrappa (2010) Effect of fuel on the formation structure, transport andmagnetic properties of LaMnO3+ δ nanopowders, Philosophical Magazine, 90:15, 2009-2025, DOI:10.1080/14786430903341410

To link to this article: http://dx.doi.org/10.1080/14786430903341410

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Philosophical MagazineVol. 90, No. 15, 21 May 2010, 2009–2025

Effect of fuel on the formation structure, transport and magnetic

properties of LaMnO3Yd nanopowders

B.M. Nagabhushanaa*, R.P.S. Chakradharb, K.P. Rameshc, V. Prasadc,C. Shivakumarad and G.T. Chandrappae

aDepartment of Chemistry, M.S. Ramaiah Institute of Technology, Bangalore 560054,India; bGlass Technology Laboratory, Central Glass and Ceramic Research Institute(CSIR), Kolkatta 700032, India; cDepartment of Physics, Indian Institute of Science,Bangalore 560012, India; dSolid State and Structural Chemistry Unit, Indian Institute ofScience, Bangalore 560012, India; eDepartment of Chemistry, Bangalore University,

Bangalore 560001, India

(Received 9 June 2009; final version received 15 September 2009)

Single-step low-temperature solution combustion (LCS) synthesis wasadopted for the preparation of LaMnO3þ� (LM) nanopowders.The powders were well characterized by powder X-ray diffraction(PXRD), scanning electron microscopy (SEM), energy dispersive spectros-copy (EDS), surface area and Fourier transform infrared spectroscopy(FTIR). The PXRD of as-formed LM showed a cubic phase but, uponcalcination (900�C, 6 h), it transformed into a rhombohedral phase.The effect of fuel on the formation of LM was examined, and its structureand magnetoresistance properties were investigated. Magnetoresistance(MR) measurements on LM were carried out at 0, 1, 4 and 7T between 300and 10K. LM (fuel-to-oxidizer ratio; ¼ 1) showed an MR of 17% at 1T,whereas, for 4 and 7T, it exhibited an MR of 45 and 55%, respectively,near the TM-I. Metallic resistivity data below TM-I showed that the doubleexchange interaction played a major role in this compound. It wasinteresting to observe that the sample calcined at 1200�C for 3 h exhibitedinsulator behavior.

Keywords: combustion synthesis; nanomaterials; manganites; magneto-resistance; PXRD; SEM; FTIR

1. Introduction

Nanocrystalline manganites show different resistivity behavior from those reportedin single crystals [1] and samples prepared via the ceramic route [2]. In fact, nanosizemanganites have a higher magnitude of low-field magnetoresistance (LFMR)compared to samples obtained through the high temperature ceramic process [3].A number of detailed investigations have shown that synthesis temperature is themain criterion in controlling the Mn3þ/Mn4þ ratio in LaMnO3þ � [4–6]. Hightemperature synthesis leads to LaMnO3þ � samples with low-stoichiometry, whereaslow temperature synthesis yields high cationic vacancies. With this in mind, an

*Corresponding author. Email: bmnshan@yahoo.com

ISSN 1478–6435 print/ISSN 1478–6443 online

� 2010 Taylor & Francis

DOI: 10.1080/14786430903341410

http://www.informaworld.com

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attempt was made to synthesize nanocrystalline LaMnO3þ � by a low temperaturesolution combustion method, which can synthesize many crystalline oxide materialswithin a few minutes [7]. Phase purity and homogeneity are achieved at temperaturesas low as 300�C versus a temperature of 1300�C for the solid-state method [8].The basic idea behind the combustion preparative technique originates from thethermo-chemical concept used in propellant chemistry. In the present study, thecombustion-derived products were well characterized by various spectroscopictechniques. Metal–insulator transition and magnetoresistance measurements werecarried out on sintered pellets using a four-probe method down to 10K. The effect offuel quantity on particle size and crystallinity of the end-product has also beeninvestigated and discussed.

2. Experimental

2.1. Calculation of stoichiometry and preparation of LaMnO3þ � sample

The stoichiometry of the redox mixture for combustion is calculated based on thetotal oxidizing (O) and reducing (F) valencies of the oxidizer and fuel, keeping theO/F ratio at unity, using the concepts of propellant chemistry. For the preparation ofLM nanopowders, lanthanum nitrate (La(NO3)3�6H2O), manganese nitrate(Mn(NO3)2�4H2O) and oxalyal dihydrazide (ODH; C2H6N4O2) (1 : 1 : 1.25, moleratio) were dissolved in double-distilled water. The resulting solution was transferredto a 300-ml cylindrical Petri dish and heated over a hot plate to boil off the excesswater and obtain a wet powder. The Petri dish containing the wet powder was placedin a muffle furnace at 300� 10�C. The reaction mixture underwent dehydration andignited at one spot with the release of gaseous products, such as oxides of nitrogenand carbon. The combustion propagated throughout the reaction mixture withoutfurther need of external heating, as the heat of reaction released during combustion ismore than the heat required for decomposition of the redox mixture. The formationof LM through LCS using ODH as a fuel can be represented by the followingreaction:

4LaðNO3Þ3ðaqÞþ4MnðNO3Þ2ðaqÞ þ 10C2H6N4O2ðaqÞ þ 6O2ð gÞ ! 4LaMnO3þ�ðsÞ

þ 20CO2ð gÞ þ 30H2Oð gÞ þ 30N2ð gÞ

20 moles of gas produced=mole of LM:

ð1Þ

In general, it is not possible to obtain LaMnO3 in stoichiometric form undernormal preparation condition due to the tendency of Mn to form tetravalent ions.The formula is usually written LaMnO3þ �, taking into consideration the presenceof Mn4þ ions.

2.2. Instrumentation

Phase purity and crystal structure was studied using a Philips X-ray diffractometerwith Cu K� (�¼ 1.5418 A) radiation. The surface morphology of the grains of thesample was examined on a JEOL (JSM-840A) scanning electron microscope. FTIR

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spectral studies were performed on a Perkin-Elmer spectrometer (spectrum 1000)with KBr pellets. A Quanta Chrome Corporation NOVA 1000 gas sorption analyzerwas used to find the surface area of the powder samples. Thermal studies (TG-DTA)on lanthanum manganites were performed using a NETSCH simultaneous thermalanalyzer STA 409. The Rietveld method and Debye–Scherrer’s formula were used tofind the lattice parameters and crystallite sizes, respectively. Density measurementswere carried out via the Archimedes’ principle using xylene as reference solvent.Resistivity measurements were carried out on sintered pellets down to 77K using astandard four-probe technique. Magnetoresistance (MR) measurements wereperformed on these samples in a liquid helium cryostat (Janis SupervaritempCryostat) from 300 to 10K at magnetic fields of 1, 4 and 7T, which was obtainedusing a super-conducting magnet. The sample contacts were made using fineenameled copper wires and silver paint. The magnitude of MR is defined as

%MR ¼½D��½ �ð0Þ�

� 100 ¼½ �ðHÞ � �ð0Þ�

�ð0Þ� 100, ð2Þ

where �(H) and �(0) are the resistivities at a given temperature in the presence andabsence of a magnetic field H, respectively.

3. Results and discussion

3.1. Powder X-ray diffraction (PXRD)

Figure 1 shows PXRD patterns of as-formed and calcined (at 450, 600, 800 and900�C for 6 h) LM samples. The PXRD patterns of as-formed and calcined (up to800�C, 6 h) lanthanum manganites exhibit completely crystalline cubic phase.The phase transformation into a rhombohedral phase occurs after 800�C (Figure 2).The structure of the LM was refined by the Rietveld technique using theFULLPROF suite program [9]. The as-formed LM sample is refined in a cubicstructure with the Pm-3m space group, whereas the sample calcined (900�C for 6 h) isin rhombohedral symmetry (hexagonal setting) with the R-3c space group. Thecomposition, refined structural and lattice parameters are summarized in Table 1.The crystallite size of the as-formed and calcined powders were determined by usingScherrer’s formula [10] and were found to be in the range 30–40 nm. The physicalproperties of the combustion-derived LM samples have also been evaluated and aredisplayed in Table 2.

3.2. Effect of Mn4þ concentration on phase formation

LM can only accommodate a large amount of Mn4þ with oxygen over-stoichiometry. Wollan and Koehler synthesized LaMnO3þ � samples with 35 at%Mn4þ [11] and recent studies also indicate that these over-stoichiometric LaMnO3þ �

compositions can accommodate up to 52% Mn4þ [4,12]. Van Roosmalen et al.[13,14] reported that the crystallographic structure of oxygen-excess perovskitephases is a random distribution of cationic vacancies equally distributed on the Aand B sites.

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The ratio of Mn3þ/Mn4þ is a key component in understanding the metal–insulator effect, ferromagnetism, as well as the metallic conduction mechanism[11,15]. Varelst et al. [16] reported that LaMnO3þ � is orthorhombic up to �20%Mn4þ and becomes rhombohedral at higher Mn4þ content. When the Mn4þ contentis greater than 30%, the rhombohedral angle becomes close to 180� and attains cubicphase. Samples of LaMnO3þ � with 0 � 0.18 were prepared via a precipitationmethod and the temperature dependence of resistance at various � values reported.LaMnO3þ � with � 0.13 was semiconducting, while, at values of �¼ 0.14 and�¼ 0.18, a resistance maximum at 130K was reported [16].

In the present study, the Mn4þ content was determined using a modifiediodometric titration technique, which involves separated processes of sampledissolution in concentrated HCl and chlorine absorption by KI solution [17]. Itwas estimated that the Mn4þ content in as-formed LM was 34� 4% and the cubicphase was favored. Atmospheric oxygen oxidized Mn2þ to Mn3þ/Mn4þ in thelanthanum manganites samples during the combustion reaction and Mn4þ stabilizedthe cubic phase [18]. On the other hand, in the calcined LM (900�C, 6 h) sample, theMn4þ content decreased to 28� 4% and attained a rhombohedral phase. Thesephase changes were clearly exhibited by the PXRD patterns.

20 30 40 50 60

(b)

(c)

(d)

(214

)

(122

)(024

)

(006

)

(202

)

(e)

(012

)

(110

)

(104

)

(211

)

(018

)2θ

(a)

(100

) (110

)

(111

)

(200

)

(210

) (211

)

Inte

nsity

(ar

b. u

nits

)

(degrees)

Figure 1. Powder X-ray diffraction patterns of combustion-derived LaMnO3þ�

powders: (a) as-formed and calcined at (b) 450�C, 6 h (c) 600�C, 6 h (d) 800�C, 6 h and(e) 900�C, 6 h.

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3.3. Thermal analysis

The thermal analysis (Figure 3) of as-formed LM was carried out between roomtemperature and 1300�C (at a rate of 10�C/min). DTA curve revealed noendothermic peak, indicating that there is no dehydration or decomposition of theproducts. A broad exothermic peak around 800�C in the DTA curve correspondedto the reduction of Mn4þ to the Mn3þ state. The exothermic peak at 800�C was aclear indication of phase change of structure from cubic to rhombohedral, asconfirmed by PXRD studies (Figure 1). According to the TG results, there is a slightweight loss around 650�C, which continued due to the reduction in Mn4þ ions withincreasing temperature.

3.4. Microstructural studies

The microstructure, morphology and homogeneity of the samples were studied byscanning electron microscopy and energy dispersive spectroscopy (EDS) analysis.

O–2Mn+3La+3

a

b

c a

b

O–2Mn+3La+3

(a)

(b)

Figure 2. (Color online). Packing diagram of LaMnO3þ � (C¼ 1) powders: (a) cubic(as-formed) and (b) rhombohedral, (calcined at 900�C, 6 h).

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Table 1. Refined structural parameters, unit cell volume, R-factors and bond angles ofas-formed and calcined LaMnO3þ � ( ¼ 1.0) samples.

Compounds LaMnO3þ � as-formedLaMnO3þ � calcined

(900�C, 6 h)

Crystal structure Cubic RhombohedralSpace group Pm-3m (221) R-3c (167)Lattice parameter (A) a¼ 3.880(5) a¼ 5.502(3)

c¼ 13.293(7)Unit cell volume/ 58.62 58.07formula unit (A)3

La 1(a) 6(a)x 0.000 0.000y 0.000 0.000z 0.000 0.250

Mn 1(b) 6(b)x 0.500 0.000y 0.500 0.000z 0.500 0.000

O 3(c) 18(e)x 0.000 0.455y 0.500 0.000z 0.500 0.250

R-factorsRP 6.64 2.66RWP 9.39 3.46RBragg 1.63 2.70RF 1.52 4.05

Bond anglesMn–O–Mn (��) 180 165.43Bond (Mn–O) 1.940 1.952Length (A)

Table 2. Physical properties of LaMnO3þ� ( ¼ 1.0).

Properties Values

Flame temperature during combustion(measured by thermocouple) 850�C

Foam density 0.0261 g/ccTap density 0.3125 g/cc

Powder density 4.7� 10�4g/ccSurface area:As-formed 24.0m2/gCalcined (900�C, 6 h) 12.5m2/gParticle size (Scherrer’s formula) 30–50 nm

Mn4þ concentration:As-formed 34%Calcined (900�C, 6 h) 28%

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SEM images of as-formed and calcined LM samples are shown in Figure 4.

SEM images revealed that the primary particles are uniform, circular in shapeand weakly agglomerated, with average grain size in the range 0.5–2.5 mm.

Upon calcination, the sample displayed a growth in size due to a congregationeffect, which was reflected in surface area measurements. SEMmicrographs show the

pores and voids in the compound, which can be attributed to the large amountof gasses escaping from the reaction mixture during combustion. The average

composition for LM was obtained from the EDS spectrum recorded at differentregions of the sample and the values agree well in terms of atomic ratio(La :Mn¼ 0.96 : 1.03) with the starting composition (La :Mn¼ 1.0 : 1.0).

3.5. Surface area measurements

The surface area is one of the important parameters used to characterize powder

samples. The surface area (m2/g) of powder samples is related to other parameters,such as particle size, surface textures, shape, size distribution and open porosity

within a crystalline or in agglomerated particles. Combustion-derived productsusually exhibit good surface area as the release of heat (exothermicity) during the

combustion reaction is long enough for nucleus formation but too short for graingrowth. The surface area of the LM was determined by the Braunauer, Emmett and

Teller (BET) method using nitrogen as adsorbent gas [19]. As gaseous products areliberated during combustion, the agglomerates disintegrate and more heat is carriedaway from the system, thereby hindering particle growth, leading to a high surface

area. As-formed LM exhibited a surface area of �24 m2/g, whereas, the calcined(900�C) sample was 12.5 m2/g. The surface area decreased with increasing calcination

10

–10

–20

–30

–40

0

Mas

s / %

0 200 400 600 800 1000 1200 1400

Temperature /°C

DTA

TG

180

140

100

60

20

0

DTA

/µV

Figure 3. Combined TG-DTA of combustion-derived LaMnO3þ� (C¼ 1) sample.

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temperature due to the growth in particle size. Figure 5 demonstrates the decrease insurface area of the LM sample upon calcination.

3.6. Fourier transform infrared spectroscopy

Fourier transform infrared (FTIR) spectroscopy is an important and appropriate

tool for studying CMR materials, where lattice parameters play an important role.

Both stretching (�s) and bending vibrating modes (�b) are sensitive to the octahedraldistortion produced by the lowered symmetry arising from Jahn–Teller effects.

Phonons and the electron–phonon interaction play an important role in thephenomenon of colossal magnetoresistance (CMR) observed in lanthanum

manganites [20–23]. The phonon energies fall in the range 20–100meV and, thus,are ideally suited to investigation by IR spectroscopy. Two characteristic IR bands

were observed: one around 600 cm�1 corresponds to the stretching mode �s of theMn–O–Mn or Mn–O bond and another band around 400 cm�1 attributed to

the bending mode �b, which is sensitive to the Mn–O–Mn bond angle. In the heat-treated (900�C, 6 h) LM sample, the �s (601–610 cm

�1) and �b (395–399 cm�1) shift

Figure 4. SEM micrographs of LaMnO3þ� (C¼ 1) powder: (a) as-formed and calcined at(b) 900�C, 6 h and (c) at 1200�C, 3 h.

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to the higher frequency side, which may be due to increased particle size and change

in lattice parameters.

3.7. Effect of fuel on the formation of LaMnO3þ � sample (t¼ 1)

We also examined the effect of the fuel-to-oxidant molar ratio (i.e. the molar

ratio ( ) of ODH-to-(lanthanum nitrateþmanganese nitrate) on the nature of thecombustion reaction and characteristics of as-formed products. The combustion

reaction can be influenced by parameters, such as the nature of the fuel, the value

of , water content of the reaction mixture and ignition temperature. If excess wateris left at the time of ignition, it may decrease the flame temperature, causing

incomplete crystallization of the product. Moreover, if the ignition temperature is

too low, the combustion reaction may not be initiated. The fuel-to-oxidizer ( ) ratioplays a critical role in influencing the reaction or flame temperature. The flame

temperature of the reaction can be controlled by varying ; the value is calculated

by the following equation, as proposed by Jain et al [24]:

¼n 2� 4ðCÞ þ 6� 1ðHÞ þ 4� 0ðNÞ þ 2��2ð0Þ� �

x 1� 3ðLaÞ þ 3ð0ðNÞ þ 3��2ð0ÞÞh i

þ y 1� 3ðMnÞ þ 2ð0ðNÞ þ 3��2ð0ÞÞh i , ð3Þ

where n, x and y are the moles of fuel, lanthanum nitrate and manganese nitrate,

respectively. For ¼ 1, the reaction is perfectly stoichiometric; a lean fuel ( 5 1)mixture is used to lower the exothermicity and the mixture does not have enough fuel

0 200 400 600 800 1000 12000

5

10

15

20

25

Sur

face

are

a (m

2 /g)

Calcination temperature (°C)

Figure 5. Effect of calcination temperature on surface area of LaMnO3þ� ( ¼ 1) powder.

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for the reaction to undergo complete combustion. In a fuel-rich ( 4 1) mixture, the

combustion temperature is higher than in the lean case.To understand the effect of fuel on combustion products, stoichiometric, fuel-

lean and fuel-rich LMmanganites are prepared by keeping the quantity of precursors

fixed and varying the amount of fuel (Table 3). Figure 6 shows the PXRD patterns of

LaMnO3þ � samples obtained at different values. It is important to note that the

10 20 30 40 50 60 70 800

20

40

0

200

4000

200

400

0

60

120

0

200

400

600

Y = 0.5

Y = 1.0

(100

) (110

)(1

11)

(200

)

(210

)

(211

)

Y = 1.25

Y = 0.75

2θ (degrees)

Inte

nsity

(a.

u)

Y = 1.5

Figure 6. Powder X-ray diffraction patterns of lanthanum manganites powders prepared atdifferent values. Inset shows the shift in (110) peak with .

Table 3. Effect of fuel quantity on foam density and crystallite size.

La (NO3)3

(g)Mn(NO3)2

(g)ODH(g)

Weight ofLaMnO3þ� (g)

Foamdensity g/cm3

Crystallitesize* (nm)

0.50 5.0 2.898 1.70 2.078 0.1067 Poorlycrystalline

0.75 5.0 2.898 2.55 1.920 0.0348 321.00 5.0 2.898 3.40 1.910 0.0261 361.25 5.0 2.898 4.25 1.913 0.0280 401.50 5.0 2.898 5.10 1.920 0.0308 44

Note: *Measured by Scherrer’s formula.

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fuel-lean mixture ( ¼ 0.5) undergoes decomposition (not combustion), producingpoorly crystalline lanthanum manganite material. The incandescent combustion withthe appearance of flame is observed only with ¼ 0.75, 1.0, 1.25 and 1.5, which arenearly stoichiometric ( ¼ 0.75), perfectly stoichiometric ( ¼ 1.0) and fuel-rich( ¼ 1.25 and 1.5). As values increase, the length of the incandescent flame(exothermicity) increases. It is clear from the PXRD patterns that the intensity of thepeaks increases with increasing values. It is also observed that crystallinity andparticle size increase with value. The duration of the combustion period alsoincreases with increasing fuel quantity. From Table 3, it can be observed that thefoam density of the fuel-lean ( ¼ 0.5) product is high. This may be due toincomplete combustion of the starting materials, which leads to the formation ofpoorly crystalline product, as confirmed by PXRD (Figure 6, ¼ 0.5).

It was also found that with increasing fuel content, the number of moles of gasesliberated during the combustion reaction increases (Table 4), thereby, increasing thefire retention time and exothermicity of the reaction. This results in the decrease ofthe surface area of LM powders (Figure 7) due to the growth of particles size asobserved form SEM images (Figure 8).

3.8. Transport and magnetic properties of LaMnO3þ � (t¼ 1)

The phenomenon of giant magnetoresistance (GMR) in manganites has been thesubject of intense study over the last decade. GMR is generally encountered in theregion near the Curie temperature (Tc). The magnitude of the MR and otherproperties, such as Tc and TM-I, are dependent on the Mn4þ content and become

Table 4. Number of moles of gases liberated during combustion reactions at various values.

Amount of fuel Reaction

No. of moles ofgases/mole ofLaMnO3þ�

Fuel lean 0.50 4La(NO3)3 (aq)þ 4Mn(NO3)2 (aq)þ 5C2H6N4O2 (aq)þO2 (g)!4 LaMnO3þ� (s)þ 10CO2 (g)þ 15H2O (g)þ 15N2 (g)

10

Nearstoichimetric

0.75 4La(NO3)3 (aq)þ 4Mn(NO3)2 (aq)þ 7.5C2H6N4O2 (aq)þO2 (g)!4LaMnO3þ� (s)þ 15CO2 (g)þ 22.5H2O (g)þ 22.5N2 (g)

15

Stoichiometric 1.00 4La (NO3)3 (aq)þ 4Mn(NO3)2 (aq)þ 10C2H6N4O2 (aq)þO2 (g)!4LaMnO3þ� (s)þ 20CO2 (g)þ 30H2O (g)þ 30N2 (g)

20

More thanstoichiometric

1.25 4La(NO3)3 (aq)þ 4Mn(NO3)2 (aq)þ 12.5C2H6N4O2 (aq)þO2 (g)!4LaMnO3þ� (s)þ 25CO2 (g)þ 37.5H2O (g)þ 37.5N2 (g)

25

Fuel rich 1.50 4La(NO3)3 (aq)þ 4Mn(NO3)2 (aq)þ 15C2H6N4O2 (aq)þO2 (g)!4LaMnO3þ � (s)þ 30CO2 (g)þ 45H2O (g)þ 45N2 (g)

30

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optimal around 30% Mn4þ. The combustion-derived LM samples exhibit a broadmetal–insulator transition and GMR property. The presence of a sufficientconcentration of ferromagnetic clusters (even in small grains/particles) seems to besufficient for the observation of GMR near TM-I [25]. Low-field magnetoresistance,oriented from spin-polarized electron tunneling, is observed in polycrystallinemanganites [26]. The crystallization and homogeneity of the compound play a veryimportant role in electrical conductivity of the manganites. Better crystallized,

0.8 1.0 1.2 1.4 1.6

10

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20

25

30

35

Sur

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are

a (m

2 /g)

y

Figure 7. Effect of fuel on surface area of LaMnO3þ� powder.

y = 0.5(Amorphous)

y = 1.0(40 nm)

y = 1.5(44 nm)

(a) (b) (c)

Figure 8. SEM micrographs of LaMnO3þ� at (a) ¼ 0.5, (b) ¼ 1.0 and (c) ¼ 1.5.

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uniform samples suppress magnetic scattering at grain boundaries, and elevate

the TM-I and lower the resistance.Figure 9 shows the temperature dependence of the resistivity at different

magnetic field in LM ( ¼ 1) pellet sintered at 900�C for 6 h. MR measurements were

carried in variable magnetic fields 1–7T. The sintered pellet (900�C, 6 h) exhibited a

distinct metal-to-insulator transition (TM-I) at 223K at zero field. It is interesting to

note that the pellet sintered at 1200�C for 3 h exhibited insulator behavior (Figure

10), which may be due to decomposition of nanocrystalline LaMnO3 into La2O3 and

Mn2O3. Figure 11 shows a PXRD of the LM sample calcined at 1200�C for 3 h,

which exhibits a rhombohedral phase along with some unidentified impurity peaks.

Verelst et al. [16] prepared LaMnO3þ � samples by a sol–gel route with heating to

different temperatures. The sample heated at 1220K with 26% Mn4þ content

exhibited a rhombohedral phase with a TM-I of 233K. This value is slightly greater

than the TM-I (223K) value of combustion-derived samples. The higher TM-I is due

to size effect, as it is known that as particle size increases with increasing TM-I [27].

0.4

0.6

0.8

1.0

1.2

1.4

0.4

0.8

1.2

1.6

2.0

0.4

0.6

0.8

1.0

50 100 150 200 250 300

0.8

1.2

1.6

2.0

4T

1T

7T

Res

istiv

ity (

ohm

.cm

)

Temperature (K)

0T

Figure 9. (Color online). Temperature dependence of resistivity of LaMnO3þ� in absence andpresence of magnetic field of 1, 4 and 7T. Solid line gives the best fit to equation �¼ �1þ �2T

2.

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10 20 30 40 50 60 70 80–100

0

100

200

300

400

500

600

700

Inte

nsity

(ar

b. u

nits

)

2θ (degrees)

* **

*

* * ** * * * *

Figure 11. Powder X-ray diffraction pattern of LaMnO3þ� ( ¼ 1.0) powder calcined at1200�C, 3 h. *Indicates impurity peaks.

0 50 100 150 200 250 300

0

2000

4000

6000

8000

0 T

1 T

4 T

7 T

Res

istiv

ity (

ohm

.cm

)

Temperature (K)

Figure 10. (Color online). Temperature variation of the resistivity at 0, 1, 4 and 7T ofLaMnO3þ� ( ¼ 1.0) powders calcined at 1200�C for 3h.

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From Figure 9, it is evident that the resistance decreases with increasing magneticfield and the TM-I shifts towards a higher temperature. This may be due to the factthat the applied magnetic field induces localization of charge carriers, which in turnmight suppress the resistance and also cause local ordering of the magnetic spins.Due to this ordering, the ferromagnetic metallic state may suppress the paramagneticinsulating regime. As a result, the conduction electrons ( e1g ) are easily transferredbetween the pairs of Mn3þ (t32g e

1g: S¼ 2) and Mn4þ (t32g e

0g: S¼ 3/2) via oxygen and,

therefore, the TM-I shifts to a higher temperature with application of the magneticfield. At a 1-T applied magnetic field, LM ( ¼ 1) shows a MR of 17%, whereas, for4 and 7T, the negative magnetoresistance is 45 and 55%, respectively, near the TM-I.Application of a magnetic field will give the field-induced ferromagnetic ordering.The magnetic fields tend to align the magnetic moments of Mn and reduce theresistance and shifting the TM-I to a higher temperature. Rao et al. [28] have reporteda 70% MR for LM with 33% Mn4þ content.

To understand the conduction mechanism, especially in a ferromagnetic low-temperature (T5TM-I) regime, an attempt was made to fit the resistivity data belowTM-I to an empirical equation [29,30]:

� ¼ �1 þ �2Tn, ð4Þ

where �1 represents the residual resistivity due to grain boundary effect. The fit toEquation (4) yields a value of n¼ 2. The residual resistivity (�1) in nano-structuredmanganites is more than in single crystals or large grain size samples. Jia et al. [31]discussed the role of the grain boundaries in manganites. According to their reporton polycrystalline manganite materials, the finite MR obtained for T5Tc is morelikely results from grain boundary scattering. It is believed that the difference inresistivity behavior between single crystal and polycrystalline manganites is due tothe grain boundary contribution and this is large for polycrystalline manganites. Theresidual resistivity (�1) in nano-structured manganites is an additive factor and,hence, the total resistivity (�) for nanocrystalline materials is always higher comparedto single crystals and large grain size samples.

The experimental data of a material (best fit) can be tested by evaluating astatistical term, known as the square of linear correlation coefficient (R2). As thevalue of R 2, in this case, is found to be 0.99. The term � 2T

2 indicates resistivity dueto the electron–electron scattering process in the ferromagnetic phase. It has beenconcluded that the variation in electrical resistivity data fits very well with thisequation. The above fit shows that the double exchange interaction plays a majorrole on the MR behavior of the sample. The solid lines in Figure 9 represent the fit.Furthermore, the parameter �1 is found to decrease (0.79503, 0.41694, 0.34088 and0.1982Ohm/cm, respectively for 0, 1, 4 and 7T) as the magnetic field increases, andthis may be due to enlargement of the magnetic domains.

4. Conclusions

LaMnO3þ � (LM) nanopowders have been synthesized by a simple, quick and novellow-temperature solution combustion method. This method produces homogenous,nanocrystalline powders having high surface area at low temperature (300�C).

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It is found that, as the amount of fuel increases, the reaction temperature alsoincreases, facilitating crystal growth. High crystalline LM powders could besynthesized by either in a nearly stoichiometric or fuel-rich combustion method.Below stoichiometric (fuel-lean) is enough to trigger the combustion of fuel andoxidizer. PXRD and scanning electronic micrographs (SEM) of the samplesprepared at different fuel-to-oxidizer ratios show that crystallite size increases withincreasing fuel quantity. LM exhibits a distinct metal-to-insulator transition (TM-I)at 223K. The applied magnetic field suppresses the resistance and shifts the TM-I tothe higher temperature side. The metallic resistivity data below TM-I shows that thedouble exchange interaction plays a major role in this compound.

Acknowledgements

The authors thank Professor K.C. Patil for his constant encouragement. The authorsacknowledge the Central Facility for recording PXRD and DST National Facility for LowTemperature and High Magnetic Field measurements of Physics Department, I.I.Sc.,Bangalore. We are also grateful to Professor B.S. Jaipraksh, Principal, BIT, Bangalore forhelping in surface area measurements.

References

[1] H.Y. Hwang, S.W. Cheong, N.P. Ong and B. Balogg, Phys. Rev. Lett. 77 (1996) p.2041.[2] R. Mahendiran, R. Mahesh, A.K. Rayachaudhuri and C.N.R. Rao, Solid State

Commun. 99 (1996) p.149.[3] M.B. Salamon and M. Jaime, Rev. Mod. Phys. 73 (2001) p.583.[4] I. Maurin, P. Barboux, Y. Lassailly, J.P. Boilot, F. Villain and P. Dordor, J. Solid State

Chem. 160 (2001) p.123.[5] C. Ritter, M.R. Ibarra, J.M. De Teresa, P.A. Algarabel, C. Marquina, J. Blasco,

J. Garcy a, S. Oseroff and S.W. Cheong, Phys. Rev. B 56 (1997) p.8902.[6] P.S.I.P.N. De Silva, F.M. Richards, L.F. Cohen, J.A. Alonso, M.J. Marty nez-Lope,

M.T. Casais, K.A. Thomas and J.L. MacManus-Driscoll, J. Appl. Phys. 83 (1998) p.394.[7] J.J. Kingsley and K.C. Patil, Mater. Lett. 6 (1988) p.427.

[8] G.H. Jonker and J.H. Van Santen, Physica. 16 (1950) p.337.[9] D.B. Wiles and R.A. Young, J. Appl. Crystallogr. 14 (1981) p.149.[10] H. Klug and L. Alexander, X-ray Diffraction Procedures, John Wiley, New York, 1962.

[11] E.O. Wollan and W.C. Koehler, Phys. Rev 100 (1955) p.545.[12] J.A. Alonso, M.J. Martinez-Lope, M.T. Casai and A. Munoz, Solid State Commun.

107 (1997) p.7.[13] J.A.M. Van Roosmalen, E.H.P. Cordfunke, R.B. Helmholdt and H.W. Zandbergen,

J. Solid State Chem. 110 (1994) p.100.

[14] J.A.M. Van Roosmalen and E.H.P. Cordfunke, J. Solid State Chem. 110 (1994) p.106.[15] R. Mahendiran, R. Mahesh, A.K. Rayachudhuri and C.N.R. Rao, Pramana J. Phys. 44

(1995) p.L393.[16] M. Verelst, R. Rangavittal and C.N.R. Rao, J. Solid State Chem. 104 (1993) p.74.

[17] Jeroen Spooren, Richard I. Walton and Franck Millange, J. Mater. Chem. 15 (2005)

p.1542.[18] G.H. Jonker, J. Phys. Chem. Solids. 9 (1959) p.165.[19] S. Brunauer, P.H. Emmett and E. Teller, J. Am. Chem. Soc. 66 (1938) p.309.

2024 B.M. Nagabhushana et al.

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201

4

[20] Xin Wang, Qiliang Cui, Yuewu Pan and Guangtian Zou, J. Alloys Compd. 354 (2003)p.91.

[21] F. Gao, R.A. Lewis, X.L. Wang and S.X. Dou, J. Alloys Compd. 347 (2002) p.314.[22] F. Gao, R.A. Lewis, X.L Wang and S.X. Dou, Physica C 341/348 (2000) p.2235.

[23] Xie Pingbo, Zhang Wieping, Yan Kuo, Jing Long, Zhang Wiewei and Xia Shangda,J. Alloys Compd. 311 (2000) p.90.

[24] S.R. Jain, K.C. Adiga and V.R.P. Verneker, Combust. Flame 40 (1981) p.71.

[25] R. Mahesh, R. Mahendiran, A.K. Raychaudhuri and C.N.R. Rao, Appl. Phys. Lett.68 (1996) p.2291.

[26] H.Y. Hwang, S.W. Cheong, N.P. Ong and B. Batlogg, Phys. Rev. Lett. 77 (1996) p.2041.

[27] J. Rivas, L.E. Hueso, A. Fondao, F. Rivadulla and M.A. Lopez-Quintela, J. Magn.Magn. Mater. 221 (2000) p.57.

[28] R. Mahesh, R. Mahendiran, A.K. Raychaudhuri and C.N.R. Rao, J. Solid. State. Chem.

114 (1995) p.297.[29] A. Banerjee, S. Pal, S. Bhattacharya and B.K. Chaudhuri, J. Appl. Phys. 91 (2002) p.5125.[30] G.J. Snyder, R. Hiskes, S. Dicarolis, M.R. Beasley and T.H. Geballe, Phys. Rev. B 53

(1996) p.14434.

[31] Y.X. Jia, Lu Li, K. Khazeni, V.H. Crespi, A. Zettl and M.L. Choen, Phys. Rev. B 52(1995) p.9147.

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