Annealing에 의한 나노구조 박막의 전기적 특성 연구

고태준(Taejoon Kouh) 한국자기학회 2006 韓國磁氣學會誌 Vol.16 No.1

Electron transport properties of nanostructured Pb thin film, consisting of grains, have been studied. Nanostructured thin films were fabricated on a substrate held at low temperature and their thicknesses were less than 10 ㎚. While temperature of the film increased from 1.3 K to room temperature, the change in normal state sheet resistance has been measured. As the annealing temperature varies, the normal state sheet resistance shows a non-monotonic and irreversible change. Such behavior can be understood with the Pb grain growth due to annealing of the film.

나노구조 박막의 Morphology에 따른 초전도 특성 변화에 관한 연구

고태준(Taejoon Kouh) 한국자기학회 2006 韓國磁氣學會誌 Vol.16 No.1

Transport and tunneling measurements of nanostructured superconducting thin films are presented. To understand the effects of film morphology on their superconducting properties, thermal annealing experiments have been performed. The transition temperature increases with thermal annealing temperature towards the bulk value. Also, thermal annealing results in a shift of transverse phonon mode. These can be understood with changes in film morphology and suggest its importance on the superconducting state properties.

Nanomechanical fluctuations of microcantilever under turbulent mixing

Seunghwan Kim,Taejoon Kouh 한국물리학회 2016 Current Applied Physics Vol.16 No.9

Here, we describe the motion of a microcantilever under water, being stirred by a rotating stir bar. Specifically, the thermomechanical fluctuations of the microcantilever are monitored as a function of the rotation frequency of the stir bar and compared with those under stationary water and in air. While the measured probability density functions both in air and stationary water show well-defined Gaussian distributions in displacement, with increasing rotation frequency of the stir bar, we note the growing tail of the probability density function as well as high level of the noise power spectral density, which can be attributed to the turbulent fluctuation within the water, caused by the motion of the stir bar. The work presented demonstrates the cantilever-based mechanical probing of the fluid motion.

Nanomechanical fluctuations of microcantilever under turbulent mixing

Kim, Seunghwan,Kouh, Taejoon Elsevier 2016 CURRENT APPLIED PHYSICS Vol.16 No.9

<P>Here, we describe the motion of a microcantilever under water, being stirred by a rotating stir bar. Specifically, the thermomechanical fluctuations of the microcantilever are monitored as a function of the rotation frequency of the stir bar and compared with those under stationary water and in air. While the measured probability density functions both in air and stationary water show well-defined Gaussian distributions in displacement, with increasing rotation frequency of the stir bar, we note the growing tail of the probability density function as well as high level of the noise power spectral density, which can be attributed to the turbulent fluctuation within the water, caused by the motion of the stir bar. The work presented demonstrates the cantilever-based mechanical probing of the fluid motion. (C) 2016 Elsevier B.V. All rights reserved.</P>

Relation between Mössbauer spectroscopy and geometrical frustration factors in MCr_1.98⁵⁷Fe_0.02O₄ (M = Co, Zn)

Choi, Kang Ryong,Kouh, Taejoon,Kim, Sam Jin,Kim, Chul Sung WILEY-VCH Verlag 2007 Physica status solidi. PSS. B, Basic solid state p Vol.244 No.12

<P>In order to elucidate the role of Cr ions in MCr<SUB>2</SUB>O<SUB>4</SUB>(M = Co, Zn) exhibiting geometrically frustration and multiferroic property, we have substituted a small amount of Fe ions for Cr sites and investigated the magnetic behavior of Fe ions with Mössbauer spectroscopy. The crystal structure was found to be single-phase cubic spinel with space group of Fd3m. The lattice constants a<SUB>0</SUB> and the internal structural parameter (x) of the oxygen were determined to be 8.340, 8.331 Å and 0.261 and 0.260, respectively. The Mössbauer absorption spectra at 4.2 K show that the well-developed two sextets are superposed with small difference of hyperfine fields(H<SUB>hf</SUB>). The hyperfine fields of CoCr<SUB>1.98</SUB><SUP>57</SUP>Fe<SUB>0.02</SUB>O<SUB>4 </SUB>and ZnCr<SUB>1.98</SUB><SUP>57</SUP>Fe<SUB>0.02</SUB>O<SUB>4</SUB> were determined to be 490 ∼ 480 kOe and 460 ∼ 450 kOe, respectively. Isomer shift values (δ) of the two sextets are found to be 0.33 ∼ 0.35 mm/s relative to those of Fe metal, which are consistent with the high spin Fe<SUP>3+</SUP> charge state. From the results of Mössbauer measurement, it is suggested that Cr<SUP>3+</SUP> ions have two different magnetic sites, and there is a correlation between hyperfine fields and degree of magnetic geometrical frustration. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)</P>

Investigation of cation distribution in single crystalline Fe₃₋_<i>x</i>Mn_<i>x</i>O₄ microspheres based on Mössbauer spectroscopy

Hui Li, Yong,Kouh, Taejoon,Shim, In-Bo,Sung Kim, Chul American Institute of Physics 2012 JOURNAL OF APPLIED PHYSICS - Vol.111 No.7

Investigation of Magnetic Properties of Sr-Doped Ba_3–<i>x</i>Sr_<i>x</i>Co₂Fe₂₄O₄₁ Z-type Hexaferrite by Mössbauer Spectroscopy

Lim, Jung Tae,Kouh, Taejoon,Kim, Chul Sung IEEE 2015 IEEE transactions on magnetics Vol.51 No.11

<P>The polycrystalline Ba3-xSrxCo2Fe24O41 (x = 0.0, 0.5, 1.0, and 1.5) samples were synthesized by the solid-state reaction method. The crystalline structure of the samples was found to be a hexagonal structure with a space group of P6(3)/mmc. The lattice constants a(0), c(0), and V-u of the samples decrease with increasing Sr concentration. From the field-dependent hysteresis curves under 10 kOe at 295 K, the values of M-s increase while the values of H-c decrease. The zero-field-cooled magnetization curves under 100 O-e between 4.2 and 300 K show the spin transition, and the value of T-s decreases from 230 K for x = 0.0 to 135 K for x = 1.5 with increasing Sr concentration due to the reduction of planar anisotropy with the difference in ionic radius between Ba2+ and Sr2+ ions. From the isomer shift value (delta) obtained from Mossbauer spectra, the charge states of all samples are determined to be Fe3+ high spin state. With increasing Sr concentration, the reduction of hyperfine field < H-hf > suggests the change in hyperfine interaction between Fe ions because of the larger ionic radius of Ba2+ ion than that of Sr2+ ion, leading to decreasing M-s.</P>

Hyperthermia Properties of Fe₃O₄ Nanoparticle Synthesized by Hot-injection Polyol Process

Seong Noh Lee(이성노),Taejoon Kouh(고태준),In-Bo Shim(심인보) 한국자기학회 2014 韓國磁氣學會誌 Vol.24 No.2

The Fe₃O₄ nanoparticle was synthesized by the hot-injection method while varying the injection time of the precursor solution. The crystal structure was determined to be cubic inverse spinel with space group of Fd-3m based on X-ray diffraction (XRD) measurements and the morphology of the prepared Fe₃O₄ nanoparticle was studied with a high-resolution transmission electron microscope (HR-TEM). When the precursor solution was injected for 0.5 min, the size of the Fe₃O₄ nanoparticle was 7.63 nm, while the size of the obtained particle was 21.27 nm with the injection time of 60 min. The magnetic properties of the prepared Fe₃O₄ nanoparticle were investigated by both vibrating sample magnetometer (VSM) and <SUB>57</SUB>Co Mossbauer spectroscopy at various temperatures. From the hyperthermia measurement, we observed that the temperature of the Fe₃O₄ nanoparticle powder reached around 120 ℃ under 250 Oe at 50 kHz, when the injection time of the precursor solution was 60 min.

The effect of proton irradiation on magnetic properties of lithium ferrites

Hyun, Sung Wook,Kouh, Taejoon,Kim, Sam Jin,Kim, Chul Sung American Institute of Physics 2009 JOURNAL OF APPLIED PHYSICS - Vol.105 No.7

<P>The effect of proton irradiation on magnetic properties of lithium ferrites has been investigated with x-ray diffraction (XRD), magnetization, and Mossbauer spectroscopy measurements. Li0.5Fe2.5O4 powders have been fabricated by the sol-gel method. Following the annealing at 700 degrees C, these samples have been proton irradiated with 1, 5, and 10 pC/mu m(2). The analysis of XRD patterns by Rietveld refinement method shows that these samples have ordered cubic spinel structures with space group of P4(3)32. We have observed that the corresponding lattice constant a(0) linearly increases from 8.3301 to 8.3314 +/- 0.0001 angstrom with increasing proton irradiation. Compared to nonirradiated sample, which has the saturation magnetization (M-s) of 66.4 emu/g and oxygen occupancy of 3.9980 at room temperature, the values of magnetization and oxygen occupancy at room temperature are 66.0, 62.6, and 60.8 emu/g and 3.9840, 3.9452, and 3.9272, respectively, for 1, 5, and 10 pC/mu m(2) irradiated powders. Also, the coercivity (H-c) decreases from 175.6 to 154.0 Oe with increasing proton irradiation. The Mossbauer spectra taken at room temperature show that the values of isomer shift (delta) for the tetrahedral (A) and octahedral (B) sites are consistent with the Fe3+ valence state. The results suggest that the proton irradiation induces the oxygen vacancy defects, which in turn leads to the changes in magnetic properties. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3070611]</P>

Investigation of Microscopic Crystal Field in Co-Doped Lithium-Iron Phosphate

Seung Je Moon,Taejoon Kouh,Choong Sub Lee,Chul Sung Kim IEEE 2009 IEEE transactions on magnetics Vol.45 No.6

<P>We present the study on the structural and the magnetic properties of LiFePO<SUB>4</SUB> and LiFe<SUB>0.8</SUB>Co<SUB>0.2</SUB>PO<SUB>4</SUB>. The crystal structure was determined to be orthorhombic with space group <I>P</I>nma by Rietveld refinement method. We have observed the increase in the lattice constants with Co<SUP>2+</SUP> ion substituted. The Mossbauer spectra of olivine LiFePO<SUB>4</SUB> and LiFe<SUB>0.8</SUB>Co<SUB>0.2</SUB>PO<SUB>4</SUB> have shown distorted line broadening below <I>T</I> <SUB>N</SUB> . In order to explain the observed line broadening, we have analyzed the Mossbauer spectra by considering the full Hamiltonian for the <SUP>57</SUP>Fe nucleus, and both the magnetic dipole and the electric quadrupole interactions. Also, we have observed the anomalous change in the electric quadrupole splitting near the <I>T</I> <SUB>N</SUB>.</P>