The chemical coprecipitation process was used to synthesize about 7 nm, spherical magnetite nanoparticles to study magnetic properties and the aging effect. As-produced spherical magnetite nanoparticles have been aged in the atmosphere for 19 months. Magnetic properties and aging effect were studied by Mössbauer spectroscopy at a temperature ranging from 77 to 300 K, vibrating sample magnetometer, and x-ray diffraction. Saturation magnetization and coercivity were found to be 49 emu/g and nearly 0 Oe at room temperature, respectively. A singlet Mössbauer spectrum was observed at room temperature, implying superparamagnetic behavior of the particles, while a two-sextet spectrum was observed at 77 K. The particle size in this study is about 7 nm, which is smaller than the superparamagnetic size of 26 nm as calculated from Neel’s theory of single domain particles. After having aged these particles for 19 months, all magnetic properties and their original shapes were retained. Superparmagnetic magnetite nanoparticles synthesized in this study can be applied to microbead applications of a biosensor.

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In an attempt to synthesize nanosized (Li0.5xFe0.5xZn1−x)Fe2O4 (0⩽x⩽1) particles with high magnetic saturation and low coercivity, the energetic ball milling technique was employed. LiCO3, α-Fe2O3, and ZnO powders were used as starting materials. The ball milled, partially crystallized lithium zinc ferrite starts to crystallize at about 600 °C. This is much lower than the temperature of 1000 °C, which is used in conventional methods. Particle size of lithium zinc ferrite was in the range of 20 to 50 nm. Regardless of the annealing temperature, the saturation magnetization increases with increasing x and reaches the maximum (about 80 emu/g) at x=0.7 [(Li0.35Fe0.35Zn0.3)Fe2O4], followed by a decrease to 60 emu/g for x=1 [(Li0.5Fe0.5)Fe2O4]. On the other hand, the coercivity of x=0.7 composition decreases with increasing annealing temperatures. Saturation magnetization and low coercivity for x=0.7 annealed at various temperatures are discussed in terms of site occupation.

A study of the deposition order and film thickness dependence on the coercivity of ferromagnetic bilayers, ʈ Co/Fe and ʈ Fe/Co, is presented ͑the sign, '' ʈ ,'' is for indicating glass or Si substrate position͒. The magnetization of the Co layer is aligned with the in-plane direction during rf sputter deposition. The thickness is controlled in the range of 3-22 nm. Since there exists a strong exchange interaction between the two ferromagnetic layers, the magnetization reversal process occurs cooperatively. ʈ Fe/Co shows an isotropic and hard-magnetic behavior, whereas ʈ Co/Fe shows an anisotropic and soft-magnetic behavior. A sudden drop of coercivity in ʈ Fe/Co observed at the Fe layer thickness below 5 nm is caused by a decrease in the saturation magnetization of the Fe layer. Due to the surface roughness, the bilayer on the glass substrate possesses a higher coercivity than that of the bilayer deposited on the silicon substrate. The magnetization reversal process of the ferromagnetic bilayers is discussed.

Monodisperse hematite (α-Fe2O3) nanoparticles were synthesized by forced hydrolysis of acidic Fe3+ solution. Rietveld analysis was applied to the X-ray powder diffraction data to refine the lattice constants and atomic positions. The lattice constants for a hexagonal unit cell were determined to be a ∼ 0.50327 and c ∼ 1.37521 nm. High resolution transmission electron microscopy was employed to study the morphology of the particles. Atomic scale micrographs and diffraction patterns from several zone axes were obtained. These reveal the high degree of crystallinity of the particles. A series of observations made on the particles by tilting them through a range of ±45° revealed the particles to be micaceous with stacking of platelets with well defined crystallographic orientations. The Morin transition in these nanoparticles was found to occur at 210 K, which is lower temperature than 263 K of bulk hematite. It was ascertained from the previous Mössbauer studies that the spin orientation for nano-sized hematite particle flips from 90° to 28° with respect to the c-axis of the hexagonal structure during the Morin transition, which is in contrast to that observed in bulk hematite where spin orientation flips from 90° to 0°.