Nano Science and Nano Technology

Biography

Biography: Dr. Masaru Matsuo,

Abstract

Superparamagnetism particles have been applied to magnetic fluids, microwave absorption materials, drug delivery systems and pigments. The high magnetization under external applied magnetic field could be obtained by using Zn-doped Fe₃O₄ nanoparticles. By the replacement of Fe³⁺ ions by Zn²⁺ ions in A-site of Fe₃O₄ crystal unit with inverse-spinel ferrite, the super-exchange interaction between spins of Fe³⁺ existing in A-site and B-site becomes weakened, while the double-exchange interaction between Fe³⁺ and Fe²⁺ in B-site remains. This indicates that the magnetization increases with Zn doping. Different from the general concept, however, the highest magnitude was x = 0.2, when Zn-doped Fe₃O₄ is represented as Zn_xFe_3-xO₄ (0x1). To investigate such contradiction, the crystallinity and crystal size of Fe₃O₄ by the doping were evaluated using X-ray and dielectric measurements. The crystallinity and crystal size decreased with increasing x up to 0.4 but they increased beyond 0.5 inversely. This indicated that Fe₃O₄ crystal can not accept further doping by Zn²⁺ to maintain Zn_xFe_3-xO₄ crystal. This is due to the fact that Zn doping resulted in the damage of the Fe₃O₄ crystal, since the atomic size (74 pm) of Zn²⁺ is bigger than that of Fe³⁺ (64 pm). As for Mg doping represented as Mg_xFe_3-xO₄, Fe₃O₄ crystal accepted Mg²⁺ up to x = 1 because of similar size of Mg²⁺ (65 pm). In this case, Fe³⁺ ion in A-site becomes zero and crystal structure of Fe₃O₄ crystal was collapsed. The crystallinity and crystal size were sensitive to their dielectric properties. The complex impedance for Mg_0.6Fe_2.4O₄ and MgFe₂O₄ with no crystal domain can be represented by Kramers-Kronig relation which has been utilized for amorphous materials, while that for Fe₃O₄ and Zn_0.2Fe_2.8O₄ can be represented by  the equivalent circuit model with three units relating to suppressed circle by Cole-Cole plots. The three units correspond to particle (grain) resistance, grain boundary resistance and interface resistance between electrode and grains indicating the existence of crystal particles. The dielectric behaviors of superparamagnetism particles were in good agreement with X-ray diffraction profiles. The DC component of conductivity was highest for Zn_0.2Fe_2.8O₄ with the highest magnetization.