Spinel-structured manganese oxide (M Mn2O4: M = Fe, Ni, Co, Zn) nanoparticles encapsulated in amorphous SiO2 were produced using our original wet chemical method. From the X-ray diffraction patterns, the diameters were estimated to range from 4 nm to 23 nm, depending on the annealing temperature. Bulk manganese oxide crystals are known to have the spinel structure, and a tetragonal structure with Jahn-Teller distortion is a stable and normal phase. This Jahn-Teller distortion is induced by Mn 3+ ions at the octahedral B sites. However, we have successfully produced cubic structures consisting of FeMn2O4, CoMn2O4, and NiMn2O4 nanoparticles in which the local distortion is suppressed. In order to clarify the local structure, X-ray absorption fine structure (XAFS) and magnetization measurements were performed. The cubic and tetragonal structures were selectively obtained, and their magnetic properties were investigated.

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CoMn2O4 (cobalt manganite) nanoparticles encapsulated in amorphous SiO2 cages were prepared by a wet chemical method. Two different structures of CoMn2O4 nanoparticles, cubic and tetragonal, were observed in the X-ray diffraction patterns. CoMn2O4 crystals usually form a tetragonal structure with Jahn-Teller distortion, and the cubic structure is unstable. X-ray absorption fine structure (XAFS) measurements were performed on the obtained samples. From the X-ray absorption near-edge structure spectra of the Mn K-edge, it was concluded that Mn ions had a valence of 3+. This ion distribution of cobalt manganites should easily induce Jahn-Teller distortion. The magnetization of each particle was measured using a superconducting quantum interference device magnetometer, and obvious differences between cubic and distorted tetragonal structures were observed in the magnetization curve.

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Bi 1-x Gd x FeO 3 (0≤ x ≤1.0) nanoparticles with an average diameter of 18 to 47 nm were synthesized by a wet chemical method. The annealing temperatures were controlled to obtain single-phase Bi 1-x Gd x FeO 3 nanoparticles. The crystal structures changed from rhombohedral to orthorhombic as Gd ions increased to around x =0.2. The behavior of magnetization curves of x =0.1 suggested canted antiferromagnetism and magnetization drastically increased compared with x =0 (BiFeO 3 ). This suggests that the canting angle increased with the increase of Gd ion. The dielectric properties of x =0.1 showed that dielectric loss (tan δ) has been improved compared with that of x =0 (BiFeO 3 ) to about 90%, while real dielectric constant ε' decreased about 15%. The reason is considered to be that impurities are restrained and leakage current decreased.