Structure and magnetic properties of Fe nanoparticles in amorphous silica implanted with Fe ions and effect of subsequent energetic heavy ion irradiation

Amorphous silicon dioxide (hereafter SiO₂) samples were implanted with 380 keV Fe ions at room temperature. After the implantation, some samples were irradiated with 16 MeV Au ions. The magnetic property was investigated by using a SQUID magnetometer, and the morphology of Fe-implanted SiO₂ samples was examined by using a transmission electron microscope and x-ray absorption spectroscopy (extended x-ray absorption fine structure and x-ray absorption near edge structure). The size of Fe nanoparticles increases with an increase in the amount of Fe implantation. A part of Fe nanoparticles consists of Fe oxides, and with an increase in the amount of Fe implantation, the valence state of Fe atoms and the structure of Fe nanoparticles gets close to those of metallic α-Fe. The room temperature magnetism was observed in Fe-implanted SiO₂ samples. The magnetization–magnetic field curves for samples implanted with a small amount of Fe are reproduced by the Langevin equation, implying that Fe nanoparticles present the superparamagnetic behavior. For a large amount of Fe implantation, the magnetization–magnetic field curve shows the ferromagnetic state. Such a result of magnetic property is consistent with the results of the x-ray absorption. By the subsequent 16 MeV Au irradiation, the Fe nanoparticles were fragmentated, resulting in the decrease in magnetization. The optical absorption property of the SiO₂ samples is briefly discussed.