Origin of intrinsic ferromagnetism in undoped antiferromagnetic NiO thin films

Thin films of nickel oxide (NiO) have been deposited on Si substrates using pulsed laser deposition technique. The number of laser pulses and substrate temperature were changed to vary the average particle size of different samples. The x-ray data show that all films are polycrystalline irrespective of deposition condition and the preferred texture of the thin film changes with temperature. A detailed magnetic characterization of the M-H loops at different temperatures as well as zero-field-cooled and field-cooled (ZFC-FC) curves has been carried out to show that NiO films having a particle size of 3.6 nm and 5.9 nm exhibit the transition from superparamagnetic to ferromagnetic as we decrease the temperature. Whereas for films having a larger average particle size (30.3 nm), the behavior is antiferromagnetic (AFM) at all temperatures similar to the bulk NiO. The exchange bias proves the presence of ferromagnetic contribution in addition to the AFM part in films having a small crystallite size. The linear correlation between susceptibility and the inverse of the particle diameter confirmed that our samples follow Neel's case for random distribution of uncompensated spins at the crystallite surface. To further verify this model, the value of the surface anisotropy constant is calculated and found to be in agreement with the reported values.