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.

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.

Two types of submicron permalloy element, namely Pac-man, were investigated by a magnetic force microscope for magnetization configuration and switching behaviors. Two distinct domain configurations, bidomain for Pac-man type I and single domain for Pac-man type II, were observed in arrays of Pac-man elements. The domain configuration depends on the slot angle for the Pac-man type I, but is independent of the slot angle for the Pac-man type II. Array of Pac-man elements with a slot angle of 180° shows the highest switching field and the narrowest switching field distribution, as compared to rectangular and hexagonal elements of the same overall dimensions.

Magnetization configuration of vortex head-to-head ͑HTH͒ domain walls and the wall-formation process in Ni 80 Fe 20 ring elements were investigated using magnetic force microscopy ͑MFM͒ and micromagnetic simulation. At remanence, two types of vortex HTH domain walls were observed to be stable in the onion configuration, depending on the film thickness: single-and double-vortex HTH domain walls for 40 and 65 nm thick ring elements, respectively. As the vortex core nucleated during formation of the HTH domain wall, exchange energy began to decrease, accompanied by an increase in the width of the wall. Vortex nucleation in the 65 nm thick ring was found to be much faster than in the 40 nm thick ring element. This effect can be attributed to the higher initial magnetostatic energy density in the thicker ring.