Magnetotransport and magnetoelastic effects in Co-doped La0.7Sr0.3MnO3 nanocrystalline perovskites

Abstract

La 0.7 Sr 0.3 Mn 1− x Co x O 3 ( x =0, 0.05, 0.1) nanoparticles, prepared by sol–gel method, were studied by means of X-ray diffraction, transmission electron microscopy , resistivity , magnetoresistance , thermal expansion and magnetostriction measurements. Results show that partial substitution of Mn by Co leads to a reduction in lattice parameters , enhancement of resistivity and room temperature magnetoresistance MR, decrease of metal-insulator transition temperature T MI and T C , an increase in thermal expansion coefficient, volume magnetostriction and anisotropic magnetostriction. The latter increases about one order of magnitude with 10% Co substitution. In comparison with Mn ions, the Co ions possess higher anisotropy energy, larger magnetostriction effect, smaller ionic size and spin state transitions with increase in temperature and magnetic field; this suggests that Co substitution leads to double-exchange interaction weakening, resulting in suppression of ferromagnetic long-range order and metallic state and increase of magnetic anisotropy . Furthermore, our samples have a relatively lower T MI and T C , higher resistivity and MR, compared with the reported values for similar compounds with larger particle sizes. This is attributed to the nanometric grain size and spin-polarized tunneling between neighboring grains.