  In this paper we have tried to understand the precise nature of magnetism in
ZnO nanoparticles. Cooling field dependence of magnetic hysteresis and coercive
field was observed for high temperature annealed sample indicating cooperative
magnetic correlation and ordering within the agglomerated nanograins. The
increasing induced internal magnetic field along the direction of external
field for the high temperature annealed sample has been fitted with the
modified Weiss-Brillouin model indicating emergence of long range intergrain
interaction among the superparamagnetic grains. We propose a simple idea that
explains the reduction of magnetisation due to vortex state like flux closure
situation.
