Synthesis and characterization of the core–shell Au covered LSMO manganite magnetic nanoparticles

In this work we report the synthesis and characterization of the nanomagnetic perovskite Sr manganites covered with Au shells having different thickness. La2/3Sr1/3MnO3 (LSMO) manganite nanoparticles were first prepared by a sol–gel procedure. The LSMO manganite nanoparticles were chemically covered with gold to produce the core–shell samples. TEM, HRTEM and atomic emission spectroscopy techniques were used to determine the morphology and structure of the LSMO@Au nanoparticles. The bare LSMO nanoparticles have a mean diameter of around 4.4 nm while LSMO@Au nanoparticles have mean diameters between 7.15 and 4.8 nm depending on the gold quantity involved in the capping process. XRD studies show that both core and shell systems have the expected crystalline structure. The formation of the core–shell structure is sustained by the shift of the plasmon resonance wavelength maximum observed in the UV–vis absorbtion spectra of the LSMO@Au samples depending on the gold concentration. The magnetization versus applied magnetic field of the bare LSMO nanoparticles and LSMO@Au samples shows no hysteresis loop indicating the superparamagnetic behavior of these systems. The analysis of the temperature dependences of FC and ZFC magnetizations shows that for all the samples the axial anisotropy energy barriers are increased due to the magnetic dipolar interactions between neighbor nanoparticles.