Magnetism of Mn_2O_3 nanocrystals dispersed in a silica matrix: Size effects and phase transformations

摘要

Mn_2O_3 nanocrystals have been grown in a silica matrix following the sol-gel route and subsequent calcination at various temperatures in the range 700℃-900℃. These nanocrystals have been studied by x ray and various magnetic methods including electron paramagnetic resonance (EPR). X-ray studies have revealed the presence of single-phase cubic α-Mn_2O_3 nanocrystals, the mean sizes lying in the range 9-18 nm. A large increment in the intensity and asymmetry of the EPR signal in lowering the temperature from room temperature to liquid-nitrogen temperature suggest that Mn_2O_3 nanocrystals become ferromagnetic (FM) at low temperatures. This is in sharp contrast with the observation of antiferromagnetic (AFM) ordering in bulk α-Mn_2O_3 crystals below 90 K. From zero-field-cooled (ZFC) and field-cooled (FC) magnetization and magnetic hysteresis measurements in the temperature range 5-300 K it has been established that α-Mn_2O_3 nanocrystals present in the calcined samples are superparamagnetic above the blocking temperature TB and weakly ferromagnetic below T_B. Small-sized nanocrystals are observed to have larger uncompensated magnetic moments. This is consistent with the postulate that uncompensated spins are present in a greater number on the surface of the antiferromagnetic inner cores of the smaller-sized nanomagnets. ac susceptibility experiments at various audio frequencies in the low-temperature region down to 5 K, have confirmed the presence of magnetic-phase transitions in Mn_2O_3 doped silica samples (calcined at various temperatures) at low temperatures and have also provided evidence that α-Mn_2O_3 nanocrystals grown in the silica matrix are best described as an assembly of randomly frozen nearly noninteracting magnetic particles.