Abstract
The magnetizations of a magnetite colloid prepared by a Water/Oil microemulsion method were measured. Magnetization curves are not represented by a simple Langevin function, but are given by a complex formula incorporating the surface condition of ultrafine particles. The temperature dependence of the spontaneous magnetization is different from the bulk and is explained by considering the existence of surface magnetism. The size distribution of colloidal particles is a log-normal type with a mean diameter of 
=3.6 nm and with a standard deviation of σ=1.28 (from an electron micrograph).
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