Superparamagnetic Transition in Ultrasmall Superparamagnetic Iron Oxide Nanoparticles

Abstract
We report the superparamagnetic transition for USPIO (ultrasmall superparamagnetic iron oxide) nanoparticles with an average diameter of 7 nm. This was observed in the zero field cooled/field cooled (ZFC/FC) and hysteresis curves. The field dependence of the blocking temperature followed the Almeida-Thouless linear behavior of a spin glass system below a crossover field of $H_{c}$ = 300~Oe. However, a more rapid change in the blocking temperature with increasing applied field was observed above $H_{c}$, which can be qualitatively explained by the dipole-dipole interactions in the powder sample. The temperature dependence of the coercivity followed a power law of $T^{1/2}$ in the intermediate temperature range.
Keywords: Superparamagnetic transition, Nanoparticles, Iron ferrite

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