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Abstract

Polyethylene glycol (PEG) coated magnetic Fe3O4 nanoparticles with diameters of 12 nm, 15 nm, and 16 nm were synthesized by the usual co-precipitation method. The structure and morphology of the samples were characterized using X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). The ac magnetic susceptibility measurements were carried out using a vibrating sample magnetometer (VSM). The dc magnetic measurements were carried out using a commercial Quantum Design superconducting quantum interference device (SQUID). The XRD patterns indicated the sole existence of the inverse cubic spinel phase of Fe3O4 in all the samples. The histograms extracted from the TEM images show narrow size distributions with average sizes that are very similar to those obtained from the XRD images using the Scherrer's formula. The temperature dependence of both coercivity and saturation magnetization, which were determined from the magnetic hysteresis loops, were found to have considerable deviations from the Bloch's and Kneller's laws. The size-dependent coercivity and saturation magnetization were found to be non-monotonic at nearly all temperatures. These results are discussed and attributed mainly to the finite size effects in addition to the existence of inter-particle interactions and of spin-glass structures that resulted from frozen canted surface spins at low temperatures.