Chul-Sung Kim

Crystallization and Mössbauer studies of melt-spun NdFe10.7TiB0.3Nδ alloys

Magnetic properties of melt-spun NdFe10.7TiB0.3Nδ ribbons have been investigated as functions of quenching rate and nitriding period. NdFe10.7TiB0.3 were prepared with substrate velocity vs⩽18u2009m/s and were nitrogenated at 500u2009°C for 15 min. The NdFe10.7TiB0.3Nδ retains the ThMn12-type tetragonal structure with lattice constants a0=8.640u2009A and c0=4.811u2009A, but with an increase in the unit cell volume. The NdFe10.7TiB0.3Nδ was confirmed to have uniaxial anisotropy by x-ray diffraction. Mossbauer spectra were taken at various temperatures ranging from 13 to 855 K. The Curie and Debye temperatures are determined to be Tc=833u2009K and Θ=390 K, respectively. Each spectrum below Tc was fitted with six subspectra of Fe sites (8i1, 8i2, 8j2, 8j1, 8f, and α-Fe). The area fraction of the subspectra at 13 K are 10.2%, 8.2%, 16.5%, 17.5%, 44.3%, and 3.3%, respectively. The magnetic hyperfine fields for the Fe sites decrease in the order, Hhf(8i)>Hhf(8j)>Hhf(8f).

Pressure-sensing based on photothermally coupled operation of micromechanical beam resonator

Here, we demonstrate the pressure-sensing scheme based on the photothermal effect in the miniaturized beam resonator in the moderate pressure range. Since the resonance frequency of the small beam resonator under thermal stress can be easily modulated by the convective cooling of the gas molecules, the pressure change has been monitored by tracking the frequency shift under constant optical power. Our experimental measurements as well as the analytical model show that the described technique ensures the fast response to the external pressure variation with high responsivity as well as much sought-after scalability, desirable for many technological applications.

Effects of La/sup 3+/ substitution on the magnetic properties of double perovskites A/sub 2/FeMoO/sub 6/ (A=Ca and Ba)

We have studied effects of the partial substitution of La/sup 3+/ for A/sup 2+/ on the magnetic properties of double perovskites A/sub 2/FeMoO/sub 6/ (A=Ca and Ba). Polycrystalline A/sub 2-x/La/sub x/FeMoO/sub 6/ (0/spl les/x/spl les/0.3) samples have been prepared by the conventional solid-state reaction in a stream of 5% H/sub 2//Ar gas. The X-ray data indicate that A=Ca is monoclinic with the space group P2/sub 1//n and A=Ba is cubic with the space group Fm3m. The substitution of La/sup 3+/ for A/sup 2+/ results in a cell volume increase for A=Ca and a cell volume reduction for A=Ba. Saturation magnetization, which is 3.5 /spl mu//sub B//f.u. for undoped A=Ca (x=0) and 3.9 /spl mu//sub B//f.u. for undoped A=Ba (x=0), decreases with increasing x. This decrease of magnetization arises from the reduction of moment associated with the electron doping and the disorder at the Fe and Mo sites. The partial substitution of La/sup 3+/ for Ba/sup 2+/ considerably enhances the Curie temperature T/sub c/ from 316 K for x=0 to 334 K for x=0.2. However, the T/sub c/ in Ca/sub 2-x/La/sub x/FeMoO/sub 6/ is not enhanced significantly with increasing x.

Dependence of Frequency and Magnetic field on Self Heating Effect of NiFe2O4 Nanoparticles for Hyperthermia

The aim of this investigation is to study the properties of superparamagnetic (SPM) ferrite nanoparticles with a specially designed RF-induced LC-circuit system for hyperthermia in both isolated solid state nanoparticle powders and cell culture media. The effect of nanoparticle quantity, as well as the magnetic field strength and operating frequency, on the maximum temperature rise for hyperthermia applications was studied. Besides the physical and magnetic properties of the nanoparticles, the physical origin of temperature rise and specific absorption rate will be considered to explore the possibility for real clinical treatments.

Crystallographic and magnetic properties of NdFe10.7TiM0.3(M=B, Ti)

NdFe10.7TiM0.3(M=B, Ti) has been studied with x‐ray diffraction, Mossbauer spectroscopy, and a vibrating sample magnetometer. The alloys were prepared by arc‐melting under an argon atmosphere. The NdFe10.7TiB0.3 exhibits a pure single phase, whereas the NdFe10.7Ti1.3 contains some α‐Fe, from x‐ray and Mossbauer measurements. The NdFe10.7TiB0.3 has the ThMn12‐type tetragonal structure with a0=8.587 A and c0=4.788 A. The Curie temperature (TC) is 570 K from Mossbauer spectroscopy performed at various temperatures ranging from 13 to 770 K. Each spectrum below TC was fitted with five subspectra of Fe sites in the structure (8i1, 8i2, 8j1, 8j2, and 8fu2009). The area fraction of the subspectra at room temperature are 16.4%, 8.2%, 14.8%, 21.3%, and 39.3%, respectively. Magnetic hyperfine fields for the Fe sites decrease on the order of Hhf(8i)≳Hhf(8j)≳Hhf(8fu2009). The average hyperfine field Hhf(T) of the NdFe10.7TiB0.3 shows a temperature dependence of [Hhf(T)−Hhf(O)]/Hhf(O)=−0.39(T/TC)3/2 & −0.17(T/TC)5/2 for T/TC<0...

Effect of the Formation of an Initial Oxide Layer on the Fabrication of the Porous Aluminium Oxide

We have investigated the effect of the formation of an initial oxide layer on the fabrication of the porous aluminium oxide. The porous aluminium oxide was fabricated by two-step anodization process with a electropolished aluminium foil. Before the first anodization step, the initial oxide layer with thickness of 10 nm was formed under the applied voltage of 1 V and later the anodization was continued under 40 V using oxalic acid solution. With the formation of the initial oxide layer, the anodization process was stable and the anodization current was constant throughout the process. In case of the absence of the initial oxide layer, the anodization was very unstable and the continuous increase in the anodization current was observed. This indicates the formation of the initial oxide layer on the aluminium surface prevents the burning of the surface due to the nonuniform distribution of the applied electric field, and allows the stable anodization process required for the porous aluminium oxide.

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.

The Effect of Cation Ion Ordering in In- Cr- Sulphur Spinel

This paper investigates the magnetic properties of the FeCr₂S₄ and FeIn₂S₄ with special emphasis on cation ordering related to the quadrupole interactions. In order to study microscopic interaction mechanism, the Mossbauer spectra of sulphur spinel have been studied. The Neel temperatures were found to be 175 and 15 K for the FeCr₂S₄ and FeIn₂S₄, respectively, by Mossbauer spectroscopy. It can be understood as the strength of inter-sublattice exchange interaction Fe²⁺(A)-S^2--Cr³⁺(B) is stronger than that of the intra-sublattice exchange interaction Fe²⁺(B)-S^2--Fe²⁺(B).

In-field Mɬssbauer spectra for LaxSr1-xFeO3-y (x=1/3, 1/2) above their charge disproportionation temperature

The charge disproportionation (CD) was first observed by Mossbauer spectroscopy in the sample La1/3Sr2/3FeO3, in which the Fe ions could take non-integral charge states between Fe3+ and Fe5+ depending on the composition. The results of external field Mossbauer spectroscopy for LaxSr1-xFeO3-y (x=1/3,1/2) concerning how the external magnetic field affects the CD state above their transition temperature were discussed. Chemical analysis showed that oxygen non-stoichiometry y were 0.04 and 0.025 for x=1/3 and 1/2, respectively. External magnetic field up to 6 T was applied both parallel and perpendicular to the direction of gamma-ray propagation using a split magnet superconductor cryostat. As the external magnetic field is parallel to the gamma-ray direction, the spectral lines 2 and 5 are suppressed (the lines 3 and 4 are overwhelmed by the strong central singlet). The magnetic moments are aligned perpendicular to the gamma-ray direction.

Synthesis and Magnetic Properties of LiFeO2 Powders by a Sol-Gel Method

Ordered LiFeO2 powders were prepared by the sol-gel method. The crystal structures were measured by X-ray diffraction (XRD). Thermal analysis such as thermogravimetry analysis (TGA) and differential thermal analysis (DTA) were performed on the dried powder obtained from the LiFeO2 precursor solution. The particle morphology of the samples was observed using a scanning electron microscope (SEM). Magnetic properties were measured using a vibrating sample magnetometer (VSM). Mossbauer spectra of 57Fe at room temperature for LiFeO2 powders are shown paramagnetic behavior as demonstrated by the single quadrupole doublet with zero hyperfine fields.