Abstract
Superparamagnetism was observed in antiferromagnetic MnO ultrafine-particles with the size range of 3.7∼5.4 nm. The decrease of Neel temperature in these finite systems was confirmed by measuring the temperature dependence of the magnetic susceptibility. The size dependence of a magnetic moment per atom and per particle was estimated by fitting the magnetic susceptibility to the Curie-Weiss law at higher and lower temperature than the Neel temperature, respectively. The former increased with decreasing size due to the surface effect, and the latter increased in proportion to the square root of the number of spins in a particle as expected in antiferromagnetic particles. The size dependence of the coercive force and the blocking temperature was remarkable in this narrow size range due to the magnetic interaction between particles.
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