MAGNETIC PROPERTY OF ANTIFERROMAGNETIC MNO ULTRAFINE-PARTICLE

Superparamagnetism was observed in antiferromagnetic MnO ultrafine-par ticles with the size range of 3.7 similar to 5.4 nm. The decrease of N eel temperature in these finite systems was confirmed by measuring the temperature dependence of the magnetic susceptibility. The size depen dence of a magnetic moment per atom and per particle was estimated by fitting the magnetic susceptibility to the Curie-Weiss law at higher a nd lower temperature than the Neel temperature, respectively. The form er increased with decreasing size due to the surface effect, and the l atter increased in proportion to the square root of the number of spin s in a particle as expected in antiferromagnetic particles. The size d ependence of the coercive force and the blocking temperature was remar kable in this narrow size range due to the magnetic interaction betwee n particles.