  We report detailed studies on magnetic properties of Co3O4 nanoparticles of
average size 12.5 nm. Temperature and field dependence of magnetization, wait
time dependence of magnetic relaxation (aging), memory effects and temperature
dependence of specific heat have been investigated to understand the magnetic
behavior of these particles. We find that the particles show some features
characteristic of nanoparticle magnetism such as bifurcation of field cooled
(FC) and zero field cooled (ZFC) susceptibilities and a slow relaxation of
magnetization. However, strangely, the temperature at which ZFC magnetization
peaks coincides with the bifurcation temperature and does not shift on
application of magnetic fields up to 1 kOe, unlike most other nanoparticle
systems. Aging effects in these particles are negligible in both FC and ZFC
protocol and memory effects are present only in FC protocol. Our results show
that Co3O4 nanoparticles constitute a unique system where superparamagnetic
blocking starts above the N\'eel temperature, in the paramagnetic state.
