Low-temperature polymer precursor-based synthesis of nanocrystalline particles of lanthanum calcium manganese oxide (La
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We report a simple modified polymeric precursor route for the synthesis of highly crystalline and homogenous nanoparticles of lanthanum calcium manganese oxide (LCMO). The LCMO phase formation was studied by thermal analysis, x-ray powder diffraction, and infrared spectroscopy at different stages of heating. These nanocrystallites (average particle size of 30 nm) possess ferromagnetic–paramagnetic transition temperature (Tc) of 300 K, nearly 50 K higher than that of a single crystal. The Rietveld analysis of the powder x-ray diffraction data of the nanopowders reveals significant lattice contraction and reduction in unit cell anisotropy-these structural changes are correlated to the enhancement in Tc.
I. INTRODUCTION
Doped rare-earth manganites, R1−xAxMnO3, where R is a trivalent cation of the type La3+, Pr3+ etc., and A is a divalent cation of the type Ca2+, Sr2+ etc., well known for their colossal magneto-resistance (CMR) properties, are technologically important materials.1–3 The change in their electrical resistance upon the application of a magnetic field is most prominent near their ferromagneticparamagnetic transition temperature (Tc). The relatively low Tc of most CMR oxides is a major bottleneck in the practical application of these materials in devices that operate at room temperature. For example, the Tc of lanthanum calcium manganese oxide, La0.67Ca0.33MnO3 (LCMO), one of the most thoroughly studied CMR materials, is about 250 K for a bulk single crystal. LCMO is preferred over other members of the family such as La0.67Sr0.33MnO3 (LSMO) because it has higher MR in the vicinity of transition temperature. However, it is essential to raise the Tc of LCMO by more than 50 K to exploit it in a practical device working at room temperature. Our research is focused on optimizing the properties of LCMO for applications in low field magnetic sensors. We have been exploring methods of synthesizing nanocrystalline LCMO with a high level of chemical (compositional) homogeneity because compositional
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0031 J. Mater. Res., Vol. 21, No. 1, Jan 2006
inhomogeneities could degrade MR drastically. We focused on synthesis methods that are versatile, easy to scale up, and energy efficient. Chemical methods such as sol-gel processing are known to yield chemically homogeneous nanoparticles with superior properties. Recent studies have pointed out that in addition to the chemical, thermodynamical, and structural parameters, the extrinsic parameters such as microstructure and particle sizes play an important role in modifying the magnetic properties of these materials.4–6 The principal motivation of the work is to synthesize nanocrystalline LCMO through a low-temperature route and to investigate the effect of size reduction on its magnetic properties. It was interesting to observe that the transition temperature in nanocrystalline LCMO is inversely proportional to the particle size and in particles of 30 nm, Tc is as high as 300 K, whi
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