Soon Cheon Byeon

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.

Microstructural Optimization of Low-Temperature-Fired Ni–Zn–Cu Ferrites Using Calcination

The purpose of this work was to examine the effect of low-temperature sintering on the microstructural optimization of Ni–Zn–Cu ferrites. This behavior was evaluated with respect to calcination temperature. The amount of unreacted hematite produced during calcination was quantitatively analyzed using a new calibration curve obtained using X-ray powder diffractometry. The formation of a spinel phase was initiated above 550° C, and completed above 850° C. The microstructural optimization of Ni–Zn–Cu ferrite samples was achieved using powder calcined at 750° C, and these specimens showed a maximum permeability of 700. A variation in microstructure with the change of the calcination temperature is ascribed as the cause of this optimization. Two mechanisms for the optimization are also suggested. One is the formation of hard agglomerates at high calcination temperatures and the second is the inhibition of grain growth due to the residual hematite in powders calcined at low temperatures. Microstructural optimization was achieved through a trade-off between these two mechanisms.

Role of nitrogen concentration in the thermal stability of the anisotropy in FeTiN thin films

The thermal stability of the anisotropy of FeTiN films has been investigated. The films were prepared by DC reactive sputtering on glass substrates in a N/sub 2//Ar atmosphere, and the N flow rate, chamber pressure, sputtering power and film thickness were varied. Target-substrate distances of 6.7 cm and 4.1 cm were used. For films sputtered at the normal target-substrate distance of 6.7 cm, the anisotropy of FeTiN films rotated about 90 degrees after a 100/spl deg/C, 1 hour annealing in the presence of a 300-400 Oe field perpendicular to the original easy axis. When the 4.1 cm target-substrate distance was used, the anisotropy direction was stable with N concentrations of 6 at.% or less in the films. The anisotropy was unstable for higher N concentrations. X-ray data and stress measurements taken as a function of N concentration showed lattice and stress changes coincident with the stability changes. The dependence of the thermal stability of the film anisotropy on target-substrate distance and N flow rate will be presented and possible mechanisms will be discussed.

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.

Line width of manganese–zinc ferrite polycrystals with oxygen partial pressure

A systematic variation in line width at X band (9.78 GHz) with oxygen partial pressure was observed in Mn0.47Zn0.47Fe2.06O4 polycrystalline samples. The linewidth of the samples increased from 105 to 188 Oe with decreasing atmospheric parameters from 8.4 to 6.4. It was found that contribution of anisotropy and porosity to the linewidth was small compared to the variation in linewidth with oxygen partial pressure. Estimation of the Fe2+ concentration of samples by measuring their thermoelectric power revealed that an increase in the concentration from 1.88 to 2.44 wt % was accompanied by decreasing oxygen partial pressure. As the resistivity of grain does not vary with oxygen partial pressure, the contribution of eddy current will be the same irrespective of the oxygen partial pressure. Therefore, the systematic increase in linewidth observed in our present study was attributed to the increase in Fe2+ concentration with decreasing oxygen partial pressure.

Oxygen partial pressure dependent magnetic properties of manganese-zinc ferrite polycrystals

A systematic variation in initial permeability with oxygen partial pressure during post sintering cooling was observed in Mn{sub 0.47}Zn{sub 0.47}Fe{sub 2.06}O{sub 4} polycrystalline samples. The initial permeability increased from 6,300 to 8,600 when the atmospheric parameter decreased from 8.4 to 6.4. Here atmospheric parameter is the degree of oxygen partial pressure engaged in the cooling stage of the sample preparation. The origins of this systematic variation were investigated by measuring the saturation magnetization under high fields (10 kOe) and by observing microstructure changes as well as the magnetic properties under small applied fields (0.15 mOe). It was found that saturation magnetization of samples under high fields was almost unchanged in the range of oxygen partial pressures through which Fe{sup 2+} concentration varied by up to 0.5%. The systematic changes in saturated magnetization and saturation time under small applied fields suggest that the permeability is strongly dependent on domain mobility. This increase in domain mobility was attributed to increased grain growth with decreasing oxygen partial pressure.

The relationship between microstructure and field-induced anisotropy in cobalt-rich amorphous alloys after magnetic field annealing

Abstract We have made a comparative microstructural study of field annealed Co 95−x Fe 5 (BSi) x amorphous alloys using transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). Until recently, it was generally assumed that amorphous magnetic alloys respond to field annealing by a process of local directional ordering which leaves the amorphous structure intact. However, striking differences in the microstructural morphology after field annealing were revealed for different glass former ratios B/Si. For high B/Si ratios, the surface crystals are predominantly fcc Co and demonstrate a high density of oxygen faults. For low B/Si ratios, the surface crystals are predominantly hcp Co and almost free of faults. Response to field annealing is proportional to the B/Si ratio and correlates with the presence of oxygen faults in surface crystals. These observations appear to be related to those of Nesbitt and Heidenreich (E.A. Nesbitt, R.D. Heidenreich, J. Appl. Phys. 30 (1959) 1000–1003) in perminvar alloys where oxygen was found to be necessary for field annealing to be effective.

Direct current-induced bonding between single- and polycrystalline manganese-zinc ferrites

The application potential of dc electric fields to promote and optimize the ferrite bonding between single- and polycrystalline manganese-zinc ferrite was investigated. The ferrite bonding process was accelerated under the application of a dc electric field. The degree of bonding increased as the bonding time and bonding temperature were increased. It was also found that the direction of an applied current from the poly- to single-crystalline ferrite regions enhances bonding.