  We report detailed studies on non-equilibrium magnetic behavior of
antiferromagnetic Co_3O_4 nanoparticles. 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 system
shows 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 the 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. We
estimate the N\'eel temperature by using Fisher's relation as well as directly
by measurement of specific heat, thus testing the validity of Fisher's relation
for nanoparticles. We show that Co3O4 nanoparticles constitute a unique
aniferromagnetic system which develops a magnetic moment in the paramagnetic
state because of antiferromagnetic correlations and enters into a blocked state
above the N\'eel temperature.
