La0.7Sr0.3Mn1xCoxO3 (x¼0, 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 TMI and TC, 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 TMI and TC, 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.

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