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.Comment: 8 pages, 10 figure
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