Chong Seung Yoon

Surface-plasmon resonance of Ag nanoparticles in polyimide

Ag nanoparticles were fabricated by sandwiching a Ag thin film (2.5–15nm thick) between two polyimide precursor layers. During imidization, depending on the initial Ag film thickness, 10–20nm sized Ag nanoparticles arranged in a monolayer were formed within the polymer matrix. The Ag particles exhibited a pronounced localized surface-plasmon resonance effect. The resonance wavelength (460–540nm) can be easily tuned by ±40nm by simply changing the Ag film thickness which alters the mean particle diameter. The classical Mie model was introduced to explain the size dependence of the plasmon peaks.

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.

Synthesis of ZnO Nanoparticles Embedded in a Polymeric Matrix; Effect of Curing Temperature

We studied ZnO nanoparticle formation in a polyimide (PI) film. Polyamic acid (PAA) was spin-coated onto Zn thin films and PAA/metal films were cured at various temperatures. Transmission electron microscopy was used to characterize oxide nanoparticles in the polyimide film. Nanosize ZnO particles were formed in the polyimide film regardless of the curing temperature. The size and the density of ZnO particles depended on the curing temperature. Th

Effect of microstructure on the magnetoresistive properties of NiFe/Co(CoFe)/Al(Ta)–oxide/Co(CoFe) tunnel junctions

The microstructure of the NiFe/Co(CoFe)/Al(Ta)-oxide/Co(CoFe) ferromagnetic tunnel junction was investigated using cross-sectional transmission electron microscopy (TEM). The effect of the insulating layer on the magnetoresistive (MR) properties of the junction was studied. The multilayer junction was formed using magnetron sputtering and the insulating layer was created by plasma oxidation of the deposited metal film. TEM analysis showed that the MR ratio was highly dependent on the insulating layer. For the NiFe/Co/Al-oxide/Co junction, when the Al2O3 layer was 13 A, the oxide layer was flat and the highest MR ratio of 15% was attained. As the Al2O3 thickness increased, the interface roughness rapidly increased, and the MR ratio also markedly dropped. In contrast, NiFe/CoFe/Al-oxide/CoFe junction showed a comparatively flatter interface and recorded a higher MR ratio. The Ta-oxide insulating layer remained flat regardless of the thickness; however, the largest MR ratio of only 9% was obtained within a n...

Impedance spectroscopic study on the magnetization of polycrystalline MnZn ferrite with very low magnetic anisotropy

Magnetization processes of polycrystalline MnZn ferrite with very low magnetic anisotropy were investigated in the Rayleigh region. Complex permeability spectrum of polycrystalline MnZn ferrite has typically one dispersion. However, two dispersions in complex impedance spectrum from which complex permeability are calculated were observed. Topological and magnetostrictive investigations showed that domain wall motion and domain rotation are responsible for the two dispersions. Calculation of individual contributions of domain wall motion and domain rotation showed that both mechanisms play a significant role in the magnetization process of this material. The role of domain wall in magnetic loss was also discussed.

Comparison of Metal-Oxide Nanoparticle Formation in the Cu and Sn Thin Films by the Reaction with Polyimide

We developed a simple method of producing metal oxide nanoparticles by reacting a polyamic acid (PAA) with Cu or Sn metal films. Respective particle size, distribution, and morphology were characterized by transmission electron microscopy (TEM). The morphology of metal oxides dispersed in the polyimide is different in Cu and Sn metal films. The Cu2O particles were formed by the dissolution reaction between the polyamic acid and the Cu films. During curing, PAA dehydrates and converts to polyimide, accompanied by precipitation of Cu2O particles. The synthesized Cu2O particles were randomly dispersed within the polyimide. And their particle size was relatively uniform, having a narrow distribution. Mostly nanosize Cu2O particles were formed in the specimen made from 10 nm thick Cu film and the mixture of nanosize particles and Cu layers were observed in the 30 nm thick Cu film. On the other hands, the Sn film undergoes surface reaction with the polyamic acid. Therefore, the synthesized SnO2 particles existed only at the surface of the substrate. SnO2 particle size distribution was not uniform in the polyimide. Although particles were not distributed uniformly in the polyimide, they were confined in a monolayer. The different particle distributions were attributed to the reactivity difference of PAA with Cu and Sn films.

Microstructure and electrical properties of magnetic tunneling junctions: Ta/NiFe/IrMn/M/Al-oxide/M/NiFe (M=Co, NiFe, CoFe)

Abstract The microstructure and electrical properties of the Ta/NiFe/IrMn/M/Al-oxide/M/NiFe (M=Co, NiFe, CoFe) ferromagnetic tunnel junctions with different Al-oxide thickness were investigated. The CoFe junction showed the highest magnetoresistance (MR) ratio of 32% at room temperature with the insulation layer of Al (15 A)-oxide, but also had the highest junction resistance of 34 kΩ (junction area of 200×200 μm) out of the three electrodes. The Co junction had the lowest resistance with reasonably high MR ratio of 24%. Cross-sectional transmission electron microscopy of the junctions showed that the increasing insulation thickness resulted in the rapid increased roughness at the top electrode/Al-oxide interface and the subsequent reduction of the MR ratio for all three electrodes materials. We have demonstrated that the MR effect is not only dictated by the intrinsic properties of the FM electrode materials, but also by the thickness and the microstructure of the oxide layer, which could be utilized to optimize the electrical properties of the ferromagnetic tunneling junction.

Oxidation mechanism of the insulation layer in NiFe/Co/Al(Ta)-oxide /Co magnetic tunnel junctions

Ferromagnetic tunneling junctions with Al-oxide and Ta-oxide as the insulating layer were fabricated using metal mask and Inductively Coupled Plasma (ICP) sputtering system. To interpret growth mechanism of the insulating layer during plasma oxidation process, the microstructure of the oxide layers was investigated with cross-sectional TEM. TEM analysis showed Al-oxide had different microstructures depending on the thickness of the layer. At 13 A, the Al-oxide layer was flat while the Al-oxide layer became progressively wavy with regular periodicity of 20 nm at increasing oxide thickness. Ta-oxide layer was partially oxidized under equal oxidizing conditions, but remained flat regardless of the thickness. Growth mechanism for the two different oxide layers is proposed in terms of oxidation kinetics and oxygen plasma.

Cu Oxide Nanoparticle Formation: Effects of Curing Time

Cu oxide nanoparticles were formed by reacting with Polyamic acid (PAA) with Cu during imidization. In this paper, we investigated the effect of holding time during curing on the Cu oxide nanoparticle formation. Cu thin films were deposited on SiO2/Si substrates by thermal evaporation. Polyamic acid was then spin-coated on the Cu thin film. The polyamic acid films were soft-baked at 135°C for 30 minutes and thermally cured at 350°C with various holding time in a nitrogen atmosphere. The size of the Cu2O nanoparticles formed in the polyimide (PI) matrix increased as the holding time increased. The size and distribution of Cu oxide particles were characterized using Transmission Electron Microscope (TEM). The degree of imidization of PI also increased proportional to the increase in holding time. The degree of PI imidization was analyzed by Infrared (IR) spectroscopy. Very uniform Cu2O particles less than 5 nm in size with particle density greater than 2×1012/cm2 were fabricated by controlling the holding time during curing.

Electron-spin-resonance analysis of magnetic ordering in Co58Mn20B10Si12 amorphous alloy

Electron-spin-resonance (ESR) spectroscopy was used to study the magnetic structure of the paramagnetic Co58Mn20B10Si12 amorphous alloy, produced by melt spinning. The room-temperature ESR spectrum consisted of two resonance peaks (g=2.00 and g=5.22). The resonance peak at g=5.22 was tentatively attributed to a possible existence of Co2+ ion clusters in the amorphous alloy. It was also shown that the ESR data obtained at different temperatures were consistent with the magnetic measurement, proving that ESR can be a useful tool probing magnetic states in an amorphous metal alloy. Moreover, the ESR spectra from the thermally annealed alloy showed that the peak at g=5.22 is sensitive to the annealing temperature, suggesting that this resonance peak can be used as a signature signal for a magnetic sensor.