Abstract
We have studied the temperature dependence of the spontaneous magnetization of ultrafine Fe particles by measuring the initial susceptibility χ∼M2s (T)/kBT in the superparamagnetic state. It was found that Bloch’s law Ms=M0(1−BT3/2) gives good description at temperatures much higher than the blocking temperature of the Fe particles. The value B is substantially larger than that of the bulk Fe, due to magnetic surface effect and finite size effect. Size distribution and magnetostatic interaction between particles have also been studied.

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