Novel size effect of LaMnO_3+δ nanocrystals embeded in SBA-15 mesoporous silica

LaMnO_3+δ nanocrystals with a mean diameter of 75 Å, were synthesized using SBA-15 mesoporous silica. The magnetic properties of the nanocrystals showed a novel size effect and were quite different from those of bulk crystal. The magnetic and electron spin resonance measurement results indicate that the existence of two transition temperatures, 278 and 235 K, which are higher than Néel and Curie temperature of bulk LaMnO _3+δ. The ESR spectrum was observed even at the lower transition temperature and consisted of the two broad lines whose shape changed at each transition temperature. The magnetization curve at 77 K was well reproduced by the summation of the two superparamagnetic magnetizations corresponding to antiferromagnetic and ferromagnetic nanoparticles, respectively. These results suggest that there are noninteracting ferromagnetic and antiferromagnetic nanoparticles with transition temperatures of 278 and 235 K, respectively.