Electric and magnetic properties of assembled manganite nanograins constituting nanotubes and microspheres

M. E. SALETA; M. GRANADA; J. CURIALE; H. E. TROIANI; A. G. LEYVA; R. D. SÁNCHEZ

Conferencia; XI International Conference on Nanostructured Materials; 2012

The objects at the nanoscale, nanowires, nanotubes, nonobelts, nanoparticles are emerging materials with possible applications in the nanoelectronic field. In particular, some oxides as manganese perovkites are between the most studied materials in the last decade. In this work we compare the behavior of two systems constituted by assembled nanograins of La2/3Ca1/3MnO3 manganite. On one hand we have nanotubes (NT) with 800 nm external diameter, synthesized by the assisted filling of plastic templates followed by a microwave treatment; on the other hand, we have hollow microspheres (HMS) with a diameter of around 15 microns, grown by spray pyrolysis technique. Both systems were characterized by electron microscopy, X-ray diffraction and dc-susceptibility studies. The morphological results show that both assemblies consisto of nanograins with an average diameter of 25 nm, and crystallographic analysis confirms the perovskite structure of the samples. The magnetic studies reveal that both samples present ferromagnetic order below 258 K. The magnetism is governed by the size of the nanograins,regardless of the assembly?s morphology (tubular or spherical). TEM images show structural disorder on the surface of the nanograins, within a 2 nm thick layer. This disordered surface is responsible for a reduction the magnetization of about 40%, and also avoids a FM interaction between the nanograins in the magnetic ordered state. In addition, current ? voltage (I-V) curves were collected at room temperature from a single NT and a single HMS using a nanomanipulator with tungsten probes located inside a SEM chamber. The electrical conductivity (G = dI/dV) presents a quadratic dependence at low voltages that can be described by a simple model consisting of an array of conductances. In this scenario, the principal voltage drop is due to the tunnel barriers located at each nanograin boundary. These barriers are associated to the disordered surface observed by TEM. The arrangement of conductances is strongly dependent on the morphology of the isolated structure under study.