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57Fe-Mössbauer and Magnetic Properties of Iron Oxide Nanoparticles in Silica Matrix Prepared by Sol-gel method

In recent years, iron oxide nanoparticles have been interested in various applications, such as magnetic storage media, drug delivery, MRI contrast medium and hyperthermia. It is known that iron oxide nanoparticles are produced in the pores of the silica matrices, which is possible to control the size of iron oxide nanoparticles homogeniously, using sol-gel method. In this study, iron oxide nanoparticles in amorphous silica matrix (Fe2O3 :SiO2 = 40 : 60 in mass%, abbreviated as 40Fe60Si) were prepared by sol-gel method and investigated by powder X-ray diffractometry (XRD), transmission electron microscopy (TEM), 57Fe Mössbauer spectroscopy at room temperature (57Fe MS), and magnetic susceptibility. Only a halo was detected from the XRD patterns of 40Fe60Si heat treated at 400 and 600 oC for 2 h. Based on the fringes of the observed TEM images, the averaged crystalline size of iron oxide nanoparticles in 40Fe60Si heat treated at 400 and 600 °C were estimated to be 4 and 5 nm, respectively. Mössbauer spectra of 40Fe60Si recorded before (Fig. 1(a)) and after heat treatment at 400 oC (Fig. 1(b)) and 600 °C (Fig. 1(c)) for 2 h consisted of a doublet. The Mössbauer parameter of 40Fe60Si heat treated at 400 °C was similar to that heat treated at 600 °C. As the obtained value of the isomer shift was 0.32 mm s-1 at room temperature, it was determined that 40Fe60Si contained only Fe3+. From the temperature dependence of dc magnetic susceptibility under zero field cooling, the highest magnetic susceptibility of 40Fe60Si after heat treatment at 400 and 600 °C were recorded at the blocking temperature of 33 and 37 K, respectively. In ac magnetic susceptibility measurements, the blocking temperatures became higher with increase in magnetic field frequency. From the relationship between blocking temperature and relaxation time, in the case of 40Fe60Si heated at 400 °C, the value of microscopic relaxation time and the dynamical exponent were estimated to be 9.8 ns and 9.1, respectively, of which the magnetic property was ascribed to superparamagnetism.

Iron Oxide, Sol-gel method, Magnetic Properties, XRD, Fe-Mössbauer spectroscopy

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