  We report magnetic studies on gelatin coated NiO nanoparticles of average
size 7 nm. Temperature and time dependence of dc magnetization, wait time
dependence of magnetic relaxation (aging), memory effects in the dc
magnetization and frequency dependence of ac susceptibility have been
investigated. We observe that the magnetic behavior of coated NiO nanoparticles
differs substantially from that of bare nanoparticles. The magnetic moment of
the particles is highly enhanced and the ZFC magnetization data displays a
sharp peak (Tp1 = 15 K) at a low temperature in addition to a usual high
temperature peak (Tp2 =170 K). We observe that this system exhibits various
features characteristic of spin glass like behavior and Tp2 corresponds to the
average freezing temperature. We argue that this behavior is due to surface
spin freezing within a particle. The nature of the low temperature peak is,
however, ambiguous as below Tp1 some features observed are characteristic of
superparamagnetic blocking while some other features correspond to spin glass
like behavior.
