Tae Young Byun

Factors affecting initial permeability of Co-substituted Ni-Zn-Cu ferrites

Iron deficient compositions of (Ni/sub 0.2/Cu/sub 0.2/Zn/sub 0.6/)/sub 1.02x/Co/sub x/Fe/sub 1.98/O/sub 4/ (0/spl les/x/spl les/0.05) were prepared to investigate their initial permeability dependence on cobalt contents. Extrinsic factors such as grain size and sintered density change little in samples sintered at 900/spl deg/C, so their effects on permeability can be neglected. Intrinsic factors such as saturation magnetization, magnetocrystalline anisotropy (K/sub 1/) and magnetoelastic anisotropy (K/sub /spl sigma//) can not account for the variation of initial permeability with Co content. Measurement of thermoelectric power shows that the concentration of cation vacancies increases with Co content. Therefore, the local induced anisotropy increases by the ordering of Co ions via increased cation vacancy concentration. This increase in induced anisotropy results in the decrease of initial permeability.

Crystallization of amorphous alloy Co68Fe4Cr4Si13B11

Crystallization of amorphous alloy Co68Fe4Cr4Si13B11 was analyzed by X-ray diffraction, transmission electron microscopy and differential scanning calorimetry. The polymorphic crystallization of cobalt and eutectic crystallization of cobalt and Co3B were observed during the crystallization of the alloy. The Avrami constant was found to be 1.65, indicating that crystallization of the glass is dominated by three-dimensional diffusion controlled growth with a near zero nucleation rate. The onset of crystallization of Co68Fe4Cr4Si13B11 alloy was found to be above 773 K which was higher than those of Cr-free Co-rich amorphous alloy. The magnetic property of crystallized samples was also correlated to the microstructure during crystallization.

Microstructure and crystallization kinetics of amorphous metallic alloy: Fe54Co26Si6B14

Abstract Magnetic properties and microstructural evolution of the amorphous Co26Fe54B14Si6 alloy during thermal annealing were characterized using transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). TEM demonstrated that, similar to the Co-free iron glass system, the amorphous Co26Fe54B14Si6 alloy crystallizes through two stages: initial development of α-(Fe,Co) dendrites and eutectic reaction due to the solute enrichment. Through the DSC analysis, the activation energy for the initial dendritic crystallization was determined to be 208 kJ/mol, which is considerably lower than that of Co-free system, indicating that the addition of Co in the glass system destabilize the amorphous structure and promotes devitrification. The magnetic property of the material was also correlated to the microstructure developed during the crystallization process. The coercivity of the material was observed to increase steadily as the crystallization process proceeded due to increasing density of α-(Fe,Co) crystallites providing domain wall pinning sites.

Domain structure of polycrystalline MnZn ferrites

Abstract The magnetic domain structure of sintered MnZn ferrites was observed using the Bitter method, Lorentz microscopy and Colloid-SEM method. The Bitter method revealed that the domain structure of the surface is stripe-like. Under an external magnetic field, the Bitter method showed the domain wall motion during the initial magnetization process and the irregular motion of domain wall near the saturation magnetic field (90∼120 Oe). Using Lorentz microscopy, the closure domain, which is characteristic of the soft magnetic materials, and the bulging of the domain wall were directly observed. In the Colloid-SEM method, the domain structure was observed as a function of the oxygen partial pressure during the sintering process. As atmospheric parameter, a , increased, the roughness of the domain wall increased, while the initial permeability decreased.

Origin of line broadening in Co-substituted NiZnCu ferrites

A systematic variation in linewidth with Co concentration at the X band (9.38 GHz) was observed in polycrystalline Co-substituted NiZnCu ferrites. Also, the temperature dependence of the linewidth and complex permeability were measured for two different Co concentrations. The linewidth shows a minimum with Co concentration. The contribution of porosity, magnetic anisotropy, and eddy current to line broadening was calculated. The line broadening due to eddy current was negligible and the line broadening due to porosity and magnetic anisotropy explain well the variation of linewidth with Co concentration. The temperature at which the linewidth increases rapidly increases with Co concentration. This temperature is consistent with the temperature of the second maximum peak in the temperature dependence of permeability. Therefore, the rapid increase in linewidth with temperature is attributed to the rapid increase in magnetocrystalline anisotropy of divalent cobalt ions.

Oxidation of Amorphous Co75.26−xFe4.74(BSi)20+x Magnetic Alloy

AbstractThe oxidation characteristics of the Co-rich amorphous magnetic alloy,

Mechanism for Ferromagnetic Resonance Line Width Broadening in Nickel-Zinc Ferrite

{\text{Co}}_{75.26 - x} {\text{Fe}}_{{\text{4}}{\text{.74}}} \left( {{\text{BSi}}} \right)_{20 + x}

Synthesis and thermal expansion behavior of liquid phase sintered CaZr4(PO4)6–Li2O

were investigated. A TEM study of the microstructure revealed a complex oxidation behavior of the alloy depending on composition, especially the boron and silicon concentrations. It was determined that the critical concentration of the metalloid to be 21 at.% above, which a continuous layer of an amorphous borosilicate phase formed on the surface. Phase separation of the surface oxide was also observed when the composition is rich in boron. The metalloid (boron and silicon) concentration was critical in determining the surface-oxide morphology, which in turn, affected the subsurface microstructure. As the magnetization behavior of the Co-rich amorphous alloy depends upon the surface oxide and the internal-oxide precipitates, the guidelines are provided by which one can engineer the microstructure of the alloy to optimize the magnetic properties.