Enhancement of magnetization in silica composite of ZnFe2O4 nanoparticles induced by femtosecond-laser irradiation

Room-temperature magnetization of ZnFe 2 O 4 nanoparticles (NPs) dispersing in silica was successfully enhanced by a femtosecond laser irradiation. This drastic increase in magnetization can be explained in terms of the metastable phase that exhibits ferrimagnetic high magnetization. The silica composite material was prepared by heat treatment of xerogels made by a sol–gel reaction of TEOS containing zinc nitrate and iron nitrate. When normal spinel ZnFe 2 O 4 NPs precipitated inside silica are irradiated with a femtosecond laser, a random cation distribution in spinel structure is instantly formed due to a local temperature rise, and it would be frozen as a metastable phase, resulting in an enhancement of magnetization at room temperature. When the irradiated sample was reheated at 800 °C, a decrease in saturation magnetization was observed by approximately 40%. This is ascribed to the cation rearrangement from the random spinel to the stable normal spinel structure. The size of the change region was estimated from the saturation magnetization, assuming that the cation distribution of ZnFe 2 O 4 NPs in the focal centered sphere was almost completely randomized. The calculated size was in close agreement with the laser spot size, suggesting that the induced magnetization increase is due to the formation of a metastable phase of ZnFe 2 O 4 in the local region. The femtosecond laser process using a silica/ZnFe 2 O 4 nanocomposite material enables three-dimensional patterning of magnetic materials. In the near future, it can be expected to be applied as waveguide-type magneto-optical devices and microphotonics based on magnetic materials.