Mn-Zn ferrite, Mn1−xZnxFe2O4 nanoparticles encapsulated in amorphous SiO2 were prepared using our original wet chemical method. X-ray diffraction patterns confirmed that the diameters of these particles were within 7–30 nm. Magnetization measurements for various sample compositions revealed that the saturation magnetization (Ms) of 7 nm particles was maximum for the x = 0.2 sample. AC magnetic susceptibility measurements were performed for Mn0.8Zn0.2Fe2O4 (x = 0.2) samples with 13–30 nm particles. The peak of the imaginary part of the magnetic susceptibility χ″ shifted to higher temperatures as the particle size increased. An AC field was found to cause the increase in temperature, with the 18 nm particles exhibiting the highest temperature increase, as expected. In addition, in vitro experiments were carried out to study the hyperthermia effects of Mn1−xZnxFe2O4 (x = 0.2, 18 nm) particles on human cancer cells.

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.

CuFe1−xCrxO2 (0 ≤ x ≤ 1.0) nanoparticles surrounded by amorphous SiO2 with an average diameter of 30–50 nm were synthesized using a wet chemical method. The annealing temperatures were controlled to yield various sizes of single-phase CuFe1−xCrxO2 nanoparticles. CuFeO2 bulk crystal is known to have a multiferroic delafossite structure with two Néel temperatures of 11 and 14 K; however, the transition temperature shifted higher as the Cr–ion doping level increased. In addition, the lattice constants decreased in accordance with increased Cr-ion doping, which was confirmed by X-ray diffraction measurements. The magnetization curves showed weak ferromagnetic behavior and no coercivity was observed. Hence, frustration in the triangular lattice of the delafossite structure can be released by Cr–ion doping and higher magnetization can be expected. A fine structure analysis through X-ray absorption fine structure measurements was also conducted. It was found that the structure of the Cu ion is similar to that of Cu2O, and the c axis of the CuFe1−xCrxO2 should be shortened by the Cr–ion doping.

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.

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