Suppression of Ferromagnetic Properties for La_1-xSr_xMnO₃ Nanoparticles (x≥0.15)

抄録

Nanoparticles of La_1-xSr_xMnO₃ (x = 0, 0.05, 0.15, and 0.175) were prepared by calcination of the molecular sieve MCM-41 soaked in precursor solutions. An ultraviolet-visible spectrometer and a SQUID magnetometer were used for measurements of the diffuse reflectance spectra and the magnetization of the samples, respectively. The absorption peak energy of the nanoparticles was 0.5 eV larger than that of the bulk. At low temperatures, the nanoparticles in both the ferromagnetic and antiferromagnetic phases were in the superparamagnetic state. The superparamagnetic moments of the antiferromagnetic nanoparticles agreed well with the uncompensated moments estimated from the particle size. While the superparamagnetic moments of the ferromagnetic nanoparticles were so smaller than the moments derived from the particle size. For x = 0.15 and 0.175 samples, ferromagnetism in bulk was largely suppressed by the antiferromagnetic ordering at surfaces of the La_1-xSr_xMnO₃ unit cell.