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Magnetic nanoparticles of magnetite (Fe3O4) coated with oleic acid (OA)
were synthesized by coprecipitation and investigated through, magnetization M, and electron spin resonance measurements (ESR) and thermal analysis. Among iron oxides, magnetite Fe3 O4 is a material that displays interesting magnetic properties, mostly when the particles are in the nanometric scale. Bulk magnetite is a ferrimagnetic compound with Curie temperature close to 860 K. Although many researches have focused in iron oxides nanoparticles for practical medical applications, there are still some controversial issues such as the role played by the surface magnetic order or polymer coated in the magnetic field dependence of the magnetization. The saturation magnetization Ms follows the Bloch’s law Ms(T)= Ms(0) (1-BT3/2) and values of Ms at room temperature as high as 62 emu/g were estimated, a result corresponding to 69 % of the bulk value. The ESR spectra were recorded using X-band JEOL JES- RES3X ESR spectrometer which is operated at 9–10 GHz frequency in the 77–300 K temperature range. The Bloch equation curve reveals a lower temperature To than Tc (860K) where Ms(T) is zero. Furthermore, TMA in magnetite nanoparticles in Ar atmosphere shows a phase transformation on surface of the particle only, because in cooling process measurements Fe3O4 are distinguished and Tc=858K. At low temperatures, the ferromagnetic phase showed low Hres , and asymmetric curve with an intensity ratio R= IHmax/IHmin lower than one. Interestingly, at 300K ESR also shows asymmetric curve linked to ferromagnetic order.
