Magnetic Behaviors of Surface Modified Superparamagnetic Magnetite Nanoparticles

The surface modified magnetite superparamagnetic nanoparticles were synthesized by using a co-precipitation method. The surfaces of magnetite nanoparticles were encapsulated by the poly-ethyleneglycole (PEG). The crystalline structure and the composition of core magnetite nanoparticles were partially changed from Fe₃O₄ to alpha -Fe₂O₃ after surface coating with a very thin outer shell of PEG. The magnetite and the surface-coated magnetite particles were well monodispersed. The average diameters of the nanoparticles were about 10.4 nm and 11 nm, respectively, determined by using transmission electron microscopy and dynamic light scattering method. The superparamagnetic behaviors were examined by measuring magnetization hysteresis loops and zero-field cooling and field cooling magnetizations as a function of temperature from 5 K to 300 K. The blocking temperatures of the magnetite and the surface coated magnetite were around 245 K and 130 K, respectively. The superparamagnetic behaviors of the magnetite and the surface coated magnetite nanoparticles were discussed in the context of the change of magnetizations with temperature.