M. H. Park

Synthesis and aging effect of spherical magnetite (Fe3O4) nanoparticles for biosensor applications

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 Mossbauer 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 Mossbauer 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 nanop...

Magnetization configuration and switching behavior of submicron NiFe elements: Pac-man shape

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.

Synthesis of nanosized (Li0.5xFe0.5xZn1−x)Fe2O4 particles and magnetic properties

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.

Spin orientation of hematite (/spl alpha/-Fe/sub 2/O/sub 3/) nanoparticles during the Morin transition

We have synthesized 40-nm-sized hematite particles by the forced hydrolysis of acid Fe/sup 3+/ solution to investigate the change in spin direction during the Morin transition. Mo/spl uml/ssbauer spectra were obtained at various temperatures ranging from 4.3 to 300 K. The Morin temperature (T/sub M/) was found to be 220 K, which is lower than 263 K of bulk hematite. Fe/sup 3+/ spins in the 40-nm-sized hematite particle flip from 90/spl deg/ to 28/spl deg/ with respect to the c-axis of hexagonal structure during the Morin transition, which corresponds to the [110] direction of rhombohedral structure. This is not in agreement with the spin flip from 90/spl deg/ to 0/spl deg/ observed in bulk hematite or sub- and micron-sized particles at T

Synthesis of nano-sized spherical barium-strontium ferrite particles

Magnetic recording media requires good particle dispersion, a smooth surface, and small interparticle interaction to make an adequate signal-to-noise ratio (SNR). Well dispersed 50-60 nm sized spherical barium-strontium ferrite (S-Ba/Sr-Fe) nanoparticles were successfully prepared with 40 nm sized hematite precursor particles and BaCO/sub 3//SrCO/sub 3/ colloid. The coercivity and saturation magnetizations of S-Ba/Sr-Fe nano-particles were 1568 Oe and 48.6 emu/g, respectively. In order to evaluate magnetic interaction, magnetic tape was prepared using an Eiger mill with binder and organic solvent. /spl Delta/M measurement showed the S-Ba/Sr-Fe nanoparticles in the tape had negative magnetic particle-to-particle interaction.

Difference in coercivity between Co/Fe and Fe/Co bilayers

A study of the deposition order and film thickness dependence on the coercivity of ferromagnetic bilayers, ∥Co/Fe and ∥Fe/Co, is presented (the sign, “∥,” is for indicating glass or Si substrate position). 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. ∥Fe/Co shows an isotropic and hard-magnetic behavior, whereas ∥Co/Fe shows an anisotropic and soft-magnetic behavior. A sudden drop of coercivity in ∥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.

Ex situ annealing method for c-axis oriented barium ferrite thick films

A barium hexaferrite (BaFe12O19, magnetoplumbite structure, BaM) multilayered film, with a total thickness of 0.63 μm, was deposited on a Si substrate by stacking several 0.09 μm layers using a rf sputtering technique. Each 0.09 μm layer was ex situ annealed at 800 °C for 10 min prior to deposition of the succeeding layer. A vibrating sample/torque magnetometer was employed to characterize the magnetic properties of both the BaM multilayered and a single layered film of the same thickness. The 0.63 μm thick BaM multilayered and single layered films show a squareness (SQ=Mr/Ms at 7 kOe) of 0.81 and 0.62, respectively. An anisotropy field (HA) was found to be 17 kOe for the BaM multilayered film and 14.5 kOe for the single layered film. A stacking of BaM layers, with ex situ annealing between each layer, improves the c-axis orientation and anisotropy field as compared to a single layered film with the same thickness. This is attributed to limiting the number of nucleation sites for randomly oriented BaM cry...

Effect of shape anisotropy on switching behaviors of Pac-man NiFe submicron elements

We investigated the shape anisotropy effect on switching the behaviors of submicron permalloy Pac-man (PM) elements with a thickness of 40 nm by a magnetic force microscope. The probability to form a vortex configuration at an as-patterned state was lower in an elongated PM type-I (EPM-I) (10%) than a PM-I element (47%). The switching process in PM-I, PM-II, and EPM-I elements was governed by a vortex-driven reversal, while the magnetization of a EPM-II element switches through a single-domain reversal. It was found that a PM element involving a vortex-driven switching process shows a nonreproducible reversal.

Microstructure and magnetic properties of hexagonal barium ferrite thin films with various underlayers

BaM (barium ferrite) thin films and underlayers (Fe, Cr, Al2O3, Fe2O3, ZnFe2O4, TiO2) were prepared by rf/dc magnetron sputtering on (100) oriented bare Si substrates. The effects of the underlayer on grain orientation, magnetic properties, and microtexture of BaM film were studied. All the BaM films, except BaM/Fe/Si film, attained nearly the same perpendicular and in-plane coercivities. The BaM/TiO2/Si exhibits the highest coercivity. However, regardless of the underlayer, BaM grains are randomly oriented. By adopting ZnFe2O4 as an underlayer, the interdiffusion of Si from substrate was prohibited to some degree. Elongated grains from the extinction of small platelet grains were grown with an increase in the BaM film thickness of BaM/Si. The microstructure of BaM in BaM/TiO2/Si was strongly dependent on both the microstructure of TiO2 underlayer and the total sputtering gas pressure.

Micromagnetic configurations and switching mechanism in Pac-man-shaped submicron Ni80Fe20 magnets

Micromagnetic modeling analysis and magnetic force microscopy studies were performed in order to characterize the magnetization configuration and magnetic switching behavior in two types, PM I and PM II, of submicron Pac-man-shaped Ni80Fe20 magnetic elements. It was found that a slight variation in the shape of the elements has a striking influence on the internal magnetic structures and switching field distribution. In particular, the vortex-formation driven switching is replaced by quasicoherent reversal by removing the central core part at the center of element. The sensitive dependence of remanent magnetic configuration and switching behavior on sample geometry is discussed in terms of the competition between the exchange and demagnetizing energy terms.