Magnetism of Mn2O3 nanocrystals dispersed in a silica matrix: Size effects and phase transformations

Mn2O3 nanocrystals have been grown in a silica matrix following the sol-gel route and subsequent calcination at various temperatures in the range 700°C–900°C. 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‐Mn2O3 nanocrystals, the mean sizes lying in the range 9–18nm. 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 Mn2O3 nanocrystals become ferromagnetic (FM) at low temperatures. This is in sharp contrast with the observation of antiferromagnetic (AFM) ordering in bulk α‐Mn2O3 crystals below 90K. From zero-field-cooled (ZFC) and field-cooled (FC) magnetization and magnetic hysteresis measurements in the temperature range 5–300K it has been established that α‐Mn2O3 nanocrystals present in the calcined samples are superparamagnetic above the blocking temperature TB and weakly ferromagnetic below TB. 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 5K, have confirmed the presence of magnetic-phase transitions in Mn2O3 doped silica samples (calcined at various temperatures) at low temperatures and have also provided evidence that α‐Mn2O3 nanocrystals grown in the silica matrix are best described as an assembly of randomly frozen nearly noninteracting magnetic particles.