Structural, microstructural, transport, and magnetotransport properties of nanostructured La 0.7 Sr 0.3 MnO 3 manganites

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N.A. Shah Department of Electronics, Saurashtra University, Rajkot - 360 005, India (Received 29 January 2010; accepted 13 April 2010)

The structural, transport, and magnetotransport properties of single-phase, homogeneous nanostructured La0.7Sr0.3MnO3 (LSMO) manganites synthesized by the coprecipitation route were investigated and the effect of sintering temperature on the microstructure of LSMO compounds was studied. A strong dependence of transport and magnetotransport behavior on the microstructure and nature of grain boundaries has been observed in the single-phase LSMO sintered at various temperatures. High-field magnetoresistance (HFMR) at room temperature is found to increase [13% (LS6) to 25% (LS9)] while low temperature (5 K) magnetoresistance decreases [75% (LS6) to 46% (LS9)] under 9 T field with increase in sintering temperature, which has been attributed to the spin-polarized tunneling and spin-dependent scattering of charge carriers. I. INTRODUCTION

Synthesis and characterization of colossal magnetoresistance (CMR) nanomaterials has been a subject of intense research due to the unique transport, magnetotransport, and magnetic properties exhibited by them. The studies on CMR materials are interesting because of interrelated structural and physical properties exhibited by them and their possible technological applications in magnetic memory storage and faster READ-WRITE devices.1,2 In manganites, zener double exchange (ZDE) mechanism and the Jahn–Teller (JT) distortion of MnO6 octahedra play an important role and the resistivity is governed by the carrier hopping rate between the adjacent Mn sites.3,4 The drop in resistivity below the insulator–metal (I–M) (TP) and/or paramagnetic–ferromagnetic (PM–FM) (TC) transition temperature results because of the decrease in scattering of the charge carriers resulting from the ordering of the Mn-spins. Two types of magnetoresistance (MR), namely intrinsic MR and extrinsic MR, are exhibited by the mixed valent manganites. The intrinsic MR effect under high applied fields is observed around TP or TC and governed by ZDE and hence by eg electron spins. At low temperatures (below TP or TC), under low applied fields (1 T), the extrinsic MR exhibited by manganites, is governed by external factors such as synthesis parameters, sintering temperature, grain morphology, grain boundaries, etc.

II. EXPERIMENTAL

Nanostructured bulk polycrystalline LSMO manganites were synthesized using the coprecipitation method. High purity (99.99%) nitrates of La and Sr and

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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0229 J. Mater. Res., Vol. 25, No. 9, Sep 2010

http://journals.cambridge.org

Coexistence of competing phases results in intrinsic electronic inhomogeneities, which governs MR behavior of the mixed valent manganites.5 It is reported that the CMR effect is prominent in the vicinity of TP. The large MR exhibited by the optimally doped mixed valent rareearth manganites require relatively higher applied fields for the

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