The nature and enhancement of magnetic surface contribution in model NiO nanoparticles

We report an alternative synthesis method and novel magnetic properties of Ni-oxide nanoparticles (NPs). The NPs were prepared by thermal decomposition of nickel phosphine complexes in a high-boiling-point organic solvent. These particles exhibit an interesting morphology constituted by a crystalline core and a broad disordered superficial shell. Our results suggest that the magnetic behavior is mainly dominated by strong surface effects at low temperature, which become evident through the observation of shifted hysteresis loops (approximately 2.2 kOe), coercivity enhancement (approximately 10.2 kOe) and high field irreversibility (>or=50 kOe). Both an exchange bias and a vertical shift in magnetization can be observed in this system below 35 K after field cooling. Additionally, the exchange bias field shows a linear dependence on the magnetization shift values, which elucidate the role of pinned spins on the exchange fields. The experimental data are analyzed in terms of the interplay between the interface exchange coupling and the antiferromagnetically ordered structure of the core.