Dong Hyeok Choi

Hyperfine interaction analysis on the multiferroic properties of Fe doped MnWO4 using Mössbauer spectroscopy

We have investigated the hyperfine interaction and multiferroic properties of Fe doped Mn0.98Fe0.02WO4 polycrystalline powders using Mossbauer spectroscopy. The crystal structures are determined to be monoclinic, space group P2∕c, with the lattice constants a=4.827A, b=5.756A, c=4.994A, and β=88.86°. The Mossbauer spectra below Neel temperature (15K) show the eight-absorbed spectrum lines. It suggests that the electric quadrupole interaction is as large as the magnetic dipole interaction. From the Mossbauer spectra of 4.2K, we have obtained hyperfine field (Hhf) to be 72.1kOe, quadrupole splitting (ΔEQ) to be 2.0mm∕s, and the asymmetry parameter (η) of an electric field gradient (EFG) to be 0. However, η at 9K is 0.30. Also, the observed EFG is asymmetric in the noncollinear spin spiral configuration ranging from 9to13K, indicating that EFG shows the indirect evidence of a spontaneous polarization.

Microstructure of the Oriented Hexagonal HoMnO₃ Thin Films by PLD

We have fabricated (0001) oriented hexagonal HoMnO₃thin films with thickness of 300 nm using Pulsed Laser Deposition (PLD) technique on Pt(111)/Ti/SiO₂/Si substrates. The XRD θ -2θ pattern shows only (0002), (0004), and (0008) reflection of a hexagonal phase, and the full width at half maximum (FWHM) of (0004) peak is under 1.6˚. The chemical state of Mn from XPS spectra of the films reveals the presence of Mn 3+ only. The temperature dependence of dielectric constant shows a weak anomaly at magnetic Neel temperature (T N ), which is about 70 K.

SYNTHESIS OF Bi-Zn-Al-B-Si-O NANO-GLASS FOR MULTILAYER CHIP INDUCTORS

The ferrite added with Bi-Zn-Al-B-Si-O nano-glass was sintered at 840-900°C, 2 h and the initial permeability, quality factor, density and saturation magnetization of the samples were measured. The initial permeability of the nano-glass added sample was about 62.6 at 0.1 wt%, and showed a sharp increase from 0.4 to 0.5 wt%. Then, it reached a maximum value (211.2) at 1.0 wt% addition. Above 1.0 wt% addition, the initial permeability decreased again with a further increase in the nano-glass content. The quality factor and saturation magnetization of 0.5 wt% nano-glass added NiZnCu ferrites sintered at 880 °C was about 143 and 386 emu/cc, respectively. It was shown that the Bi-Zn-Al-B-Si-O nano-glass systems were useful as a sintering aid in the fabrication of MLCIs.

Magnetic Properties of Barium Ferrite Thin Films Grown Under Oxygen Pressure by Pulsed Laser Deposition

This work presents the magnetic properties of M-type hexagonal barium ferrite (BaM) thin films deposited on Pt(111) substrate by pulsed laser deposition (PLD). This study also investigates the crystalline structure and c-axis orientation of magnetization of the BaM thin films by controlling various oxygen pressures.