  We investigate slow dynamics of collection of a few antiferromagnetic NiO
nanoparticles. The zero-field cooled magnetization exhibits size dependent
fluctuations. We find memory effects in field cooled magnetization, as well as
aging effects in thermoremenant magnetization of antiferromagnetic
nanoparticles. The antiferromagnetic nanoparticles show a stronger memory
effect than the corresponding effect in the ferromagnetic particles, when the
distribution of particles include very small sizes. The situation reverses for
bigger sizes. The relaxation of the magnetization after a sudden cooling,
heating and removal of fields reiterate the memory effects. We also see a weak
signature of size-dependent magnetization fluctuations in aging effect of
antiferromagnetic nanoparticles. We find a two-step relaxation of
thermoremenant magnetization in antiferromagnetic case, which differs
qualitatively from relaxation of ferromagnetic nanoparticles.

Mesoscale and Nanoscale Physics