Effects of Particle Size on the Magnetic Properties of Maghemite Nanoparticles*

The effects of particle size on the magnetic properties of oleic-acid-coated maghemite ($\gamma $-Fe$_{2}$O$_{3})$ nanoparticles (NPs) with average diameters of 3.2 nm and 7.0 nm are reported. These samples were prepared by identical procedures and characterized by x-ray diffraction, transmission electron microscopy, FTIR spectroscopy and temperature-dependent ac and dc magnetometry. The zero field-cooled and field-cooled magnetization M vs. T data under H $=$ 100 Oe yield the blocking temperature T$_{\mathrm{B}} \approx$ 21 K (35 K) for the 3.2 nm (7.0 nm) NPs. Changes in T$_{\mathrm{B}}$ with changes in the measuring frequency f$_{\mathrm{m}}$ (10 Hz to 10 kHz) are used to determine the Neel-Brown relaxation time and the strength of inter-particle interaction. Above T$_{\mathrm{B}}$, the data of M vs. H up to H $=$90 kOe are used to determine magnetic moment per particle and to understand the effects of size distribution on the measured properties. Below T$_{\mathrm{B}}$, the plots of M vs. H show surprisingly negligible hysteresis with coercivity H$_{\mathrm{C}} \approx$ 20 Oe for both NPs. Interpretation of these results will be presented along with comparison with results obtained from bulk maghemite.