Effect of finite size on the magnetization behavior of nanostructured nickel oxide

Nanostructured nickel oxide samples having different average particle sizes are synthesized through a wet chemical route. Room temperature magnetic hysteresis of the samples are recorded using a vibrating sample magnetometer. The magnetic properties of the samples are found to be markedly different from those of single crystalline nickel oxide. The sample with an average particle size of 2-3 nm showed superparamagnetism with magnetization curves defined by the Langevin function. Anomalously large uncompensated magnetic moment associated with this sample is attributed to the multisublattice magnetic structure. Interestingly, samples with larger average particle sizes of 13 and 18 nm exhibited superantiferromagnetism with the magnetization curves varying linearly with applied field and susceptibility values larger than that of bulk nickel oxide. The results highlight the importance of surface atoms and surface driven spin rearrangements in determining the magnetic properties of nanostructured nickel oxide.