Effect of Femtosecond-laser Irradiation on Magnetic and Optical Properties in Au- and Fe-doped Transparent Glass

We have investigated an effect of near-infrared femtosecond-(fs-) laser irradiation on magnetic and optical properties in Au-and Fe-doped transparent glasses. A fs-laser beam with a wavelength of 775 nm was focused 1 mm below the surfaces of glass samples. This produces absorption peaks due to the formation of hole-trap centers in the irradiated region, which changes into different peaks originated from localized surface plasmon absorption after annealing at 450C. It is presumed that precipitation of Au nanoparticles occurs as results of photoreduction of Au ions and agglomeration by the thermal atomic diffusion. In addition, enhancement of magnetization in the localized area inside the glass was realized after laser irradiation and subsequent annealing. Ferrimagnetic behaviors were increased in the M-H curve even at room temperature, whereas the diamagnetic component dominated in the M-H curve of the as-prepared glass sample. The irradiated and annealed glass sample also exhibited superparamagnetic blocking in the temperature dependence of the magnetization with a blocking temperature higher than room temperature. These changes in magnetic properties are explainable in terms of the local crystallization of ferrimagnetic nanoparticles, such as magnetite, induced by fs-laser irradiation and annealing. Transmission-electron-microscopy observations support our considerations that Au nanoparticles and ferrimagnetic spinel-phase nanoparticles were simultaneously and separately crystallized in the focused region of the fs-laser beam.