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We report the synthesis of phase pure, monodispersed Fe 3 O 4 nanoparticles of size ∼10 nm via chemical co-precipitation of ferrous and ferric ions, under controlled pH and temperature. The nanoparticles are oleic acid functionalized and hence dispersible in organic medium. The structure and morphology of nanoparticles are determined by analyzing XRD pattern and TEM micrographs, confirming the formation of phase pure Fe 3 O 4 nanoparticles. The magnetization studies reveal the superparamagnetic behavior of the nanoparticles at room temperature. The changes in blocking temperatures (T B ) of magnetic nanoparticles with applied magnetic fields (H ap ), noted from the cusp of the zero-field-cooled magnetization, the indicate effects of dipole interactions. A decrease in blocking temperature from 95 K to 15 K has been observed on varying the magnetic field from 50 Oe to 5000 Oe. T B versus H relation follows the equation T B (H ) = T o (1 − (H /H o )) m , i.e. the Néel-Brown model of magnetic relaxation in nanoparticles.

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