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Physica B 350 (2004) 55–58

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doi:10.1016

Size effect in nanocrystalline manganitesLa1xAxMnO3 with A=Ag, Sr

A.E. Teplykha,*, S.G. Bogdanova, E.Z. Valieva, A.N. Pirogova, Yu.A. Dorofeeva,A.A. Ostroushkob, A.E. Udilovb, V.A. Kazantzeva

a Institute for Metal Physics, Ural Division of RAS, 620219 Ekaterinburg, RussiabUral States University, 620083 Ekaterinburg, Russia

Abstract

Nanocrystalline manganites La0.9Ag0.1MnO3, La0.7Ag0.3MnO3, and La0.7Sr0.3MnO3 have been synthesized by

pyrolysis and subjected to isothermal annealing. The atomic, magnetic, and mesoscopic structures of these manganites

were investigated using magnetic measurements as well as X-ray and neutron diffraction. Particles of sizes ranging from

30–40 to 600–700 nm coalesced with an increase in the annealing temperature from 600C to 1300C. All investigated

samples had a rhombohedral structure and were ferromagnetic. The Curie temperature of the samples containing silver

decreased while that of the samples containing strontium increased with the annealing temperature (i.e. with the growth

in particle size). The magnetization of the Mn ions increased with annealing in all three systems. It is reasonable to

assume that the growing size of the nanoparticles lead to an increased magnetization owing to the reduction of the angle

between the nearest magnetic moments of the Mn ions.

r 2004 Elsevier B.V. All rights reserved.

PACS: 61.12.Ex; 61.46.+w; 75.25.+z

Keywords: Nanocrystalline manganites; Crystal and magnetic structure; Size effect

1. Introduction

At present, manganites of the typeLa1xAxMnO3 which exhibit ‘‘colossal’’ magne-toresistance (e.g. in Ref. [1]) are intensivelystudied. A correlation between the parametersdescribing the crystal structure of the manganitesand those describing their magnetic properties hasbeen found. In particular, the Curie temperature

onding author. Tel.: +8-34377-31476; fax: +8-

3.

ddress: teplykh@uraltc.ru (A.E. Teplykh).

- see front matter r 2004 Elsevier B.V. All rights reserve

/j.physb.2004.03.252

(TC) has been shown to depend on the Mn–O–Mnbond angle (YMn–O–Mn).It is of interest to investigate nanocrystalline

samples with particle sizes (L) comparable withmagnetic domain sizes in order to better under-stand the mechanism for the ‘‘colossal’’ magne-toresistance. Indeed, nanocrystalline manganiteshave a higher magnitude of the low-field magne-toresistance [2] as compared to ceramic samples.The results of earlier investigations show that

the properties of nanocrystalline manganites canessentially change with decreasing particle size.For example, La2/3Sr1/3MnO3 samples withLp30 nm did not reveal [2] a metallic conductivity

d.

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A.E. Teplykh et al. / Physica B 350 (2004) 55–5856

as opposed to the corresponding ceramic samples.According to Zhang et al. [3], the spontaneousmagnetization (s) of La0.85Sr0.15MnO3 doubleswhen L decreases from 1000 to 20 nm. On theopposite, s is reduced by a factor of B2 inLa1xAxMnO3, where A=Ca, Sr and xE0.3 [3, 4].It was concluded in Ref. [3] that the growth in sand TC for nanocrystalline La0.85Sr0.15MnO3, andtheir decrease in La0.65Sr0.35MnO3, are associatedwith a change of the angle YMn–O–Mn. The valuesof this angle were calculated [3] for samplesannealed at different temperatures, taking onlyinto account the changes in the lattice parametersin function of the annealing temperature (TAn). Apossible variation of oxygen-atom coordinates wasneglected since neutron-diffraction measurementswere not available. A different reason wassuggested in Ref. [4], where a reduction of s inthe La0.67Ca0.33MnO3 could be attributed to anincreased volume fraction of the amorphous phasethat exists in nanocrystalline samples at lower TAn.In the present paper an attempt is made at

finding a dependence of the values of the magneticmoment of the Mn-ion (mMn) and of the angleYMn–O–Mn on L in nanocrystallineLa1xAgxMnO3 and in La0.7Sr0.3MnO3 usingneutron diffraction.

0.01 0.10.01

0.1

1

10

100

1000

dΣ/d

Ω, c

m-1

q, Å-1

Fig. 1. Small angle neutron-scattering scans for La0.7Sr0.3MnO3, annealed at 700

oC (solid symbols) and at 1200oC (open

symbols). The lines are calculated.

2. Experimental

The initial nanocrystalline La1xAgxMnO3 withx=0.1, 0.3 and the La0.7Sr0.3MnO3 were synthe-sized by pyrolysis. To change particle sizes all thesamples were annealed for 4 h at different tem-peratures, i.e. the silver-doped samples at 600–1200C and the strontium-doped ones at 700–1300C. In order to provide stable oxygenstoichiometry the samples were subjected toadditional annealing at 700C.To qualify the structure, X-ray patterns were

obtained with a diffractometer using Cu Ka-radiation. Magnetization was measured with asquid magnetometer MPMSR5-XL from Quan-tum Design. The Curie temperatures were deter-mined from ac-susceptibility measurements in afield of 10Oe and at a frequency of 1 kHz.

Neutron-diffraction measurements were per-formed with the diffractometers D-2 and D-3using neutron wave lengths l=1.80 and 2.43 (A,respectively. The X-ray and neutron diffractionpatterns were evaluated using the Fullprofprogram.The particle size was determined from small

angle neutron-scattering scans, obtained with thediffractometer D-6 at lE4.5 (A. The diffractometersD-2, D-3 and D-6 are placed on the IVV -2Mreactor (Zarechny, Russia).

3. Results and discussion

It was found from the calculation of the X-raypatterns that all the nanocrystalline samples understudy have a rhombohedral structure that belongsto space group R-3c. The increase in temperatureTAn induces an elongation of the lattice parameterin the basal plane (Da/aE0.2%) and a reductionalong the hexagonal axis (Dc/cE0.2%). On thewhole, it results in some increase in the volume ofthe unit cell with TAn (DV/VE0.5%).Fig. 1 shows small angle neutron-scattering

scans for two of the samples. For other samples,the d

P/dO curves exhibit differences in their

height, shape, and slope. This reflects differencesin L and in the concentration of particles (y) andalso in their surface properties.

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164

165

166

167

168

600 800 1000 1200

100

200

300

2.0

2.5

3.0

3.5

3

2

1

(a)

ΘM

n-O

-Mn,

deg

r

3

2

1

(c)

TC

, KTAn, C

3

2

1

(b)

µ Mn,

µB

Fig. 2. Changes with annealing temperature of (a) the bond

angle YMn–O–Mn, (b) mMn (the solid symbol are frommagnetization data), and (c) TC : curve (1) is for La0.9Ag0.1MnO3, (2) for La0.7Ag0.3MnO3, and (3) for La0.7Sr0.3MnO3.

A.E. Teplykh et al. / Physica B 350 (2004) 55–58 57

Results of the dP/dO curves computations are

presented in the Table 1. One observes that L

weakly changes with TAn from 600C to 900C

and dramatically increases with TAn from 900C

to 1000C.Fig. 2 shows the dependencies of TC on the

temperature TAn for the investigated nanocrystal-line La1xAxMnO3. One can see that TC increasesin La0.7Sr0.3MnO3 and decreases in La0.9Ag0.1MnO3 with increasing temperature TAn.To determine the magnetic structure and the

magnetic moment magnitudes, neutron powder-diffraction patterns were obtained at 4.2K. It wasfound that the wave vector of the magneticstructure k is zero. The magnetic moments of theMn ions are oriented ferromagnetically andparallel to the a–b plane. The dependence of mMnon TAn is show in Fig. 2. One observes that in allthe samples mMn monotonously increases with TAn,meaning that the magnetization increases with thegrowth of the nanoparticles.A similar behavior for s had been observed

earlier in nanocrystalline La0.65Sr0.35MnO3 [3] andin La0.67Ca0.33MnO3 [4]. On the opposite, areduction of s was found in La0.85Sr0.15MnO3[3]. According to Ref. [3], the behavior of s (as wellas of TC) for nanocrystalline samples was causedby the L-dependence of the angle YMn–O–Mn. Ifthis angle increases with L then TC is expected toincrease as well, as this is the case forLa0.65Sr0.35MnO3. The decrease of YMn–O–Mn withL reduces TC as in the case of La0.85Sr0.15MnO3.

Table 1

Characteristics of manganite nanoparticles in La0.9Ag0.1MnO3,

La0.7Ag0.3MnO3 and La0.7Sr0.3MnO3, determined from small

angle neutron-scattering data

TAn (C)La0.9Ag0.1MnO3 La0.7Ag0.3MnO3 La0.7Sr0.3MnO3

L (nm) y L (nm) y L (nm) y

600 36 0.16 36 0.16 — —

700 36 0.16 33 0.19 30 0.22

800 60 0.15 36 0.12 50 0.14

900 60 0.09 60 0.12 60 0.10

1000 400 0.14 400 0.15 140 0.11

1100 400 0.14 400 0.11 180 0.18

1200 600 0.17 700 0.14 200 0.16

1300 — — — — 300 0.12

L is the particle size and y their volume fraction in a solid phase.

From Fig. 2 one observes that the angleYMn–O–Mndecreases with TAn for all three groups ofsamples. There is a correlation between the valuesof TC and the angle YMn–O–Mn for samplescontaining silver: both values decrease with TAnfor La0.9Ag0.1MnO3 and they reveal a smallmaximum in La0.7Ag0.3 MnO3. On the other hand,an increase of TAn in the La0.7Sr0.3MnO3 system isaccompanied by growth of TC, although the angleYMn–O–Mn decreases. It seems that in the lattersystem the decreasing TC owing to the decreasingangle YMn–O–Mn is compensated by the growth ofTC owing to the decreased angle between nearestmoments of the Mn-ions when nanoparticlesgrow.

Acknowledgements

Work supported RFBR, government of Sver-dlovsk region (RFBR-Ural 01-02-96412), programSSTP (contract 40.012.1.1.11.50), program PSDRAS ‘‘NI MSFPM’’ and the SRC program ofKOSEF (Korea).

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