• Journal of Magnetics
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• v.12 no.4
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• pp.137-140
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• 2007

We have studied the temperature dependent magnetic properties and crystalline phase transitionn in small amount Fe doped nickel chromite. The Crystalline structure of $NiCr_{1.7}Fe_{0.3}O_4$ is spinel cubic (Fd-3m) structure with a lattice constant $a_0=8.317\AA$ at room temperature. The magnetic $N\acute{e}el$ temperature $(T_N)$ of the Fe doped nickel chromite sample is determined to be 250 K. The $M\ddot{o}ssbauer$ spectra exhibit that there are two magnetic phases with the two different sites for the $Cr^{3+}$ ions. The spectrum at 4.2 K is fitted to two magnetic components of the magnetic hyperfine fields $H_{hf}=496$ and 485 kOe. From the spectrum at 295 K, the electric quadrupole splittings are observed with large values of 0.49 and 0.50 mm/s, respectively. The values of the isomer shifts at all temperature ranges show that the Fe ions are ferric states. We are suggested that the dynamic Jahn-Teller distortion and anisotropic magnetic relaxation effects due to the crystalline phase transition.

• 한국초전도학회:학술대회논문집
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• v.9
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• pp.189-195
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• 1999

We have studied the vortex dynamics in YBa$_2Cu_3O_7$ single crystals with columnar defects using micro Hall-probe array. The Hall-probe array technique allowed a simultaneous measurement of the time and spatial dependence of the vortex density so that more detailed information on flux dynamics could be obtained. We found that field profiles inside sample were similar to the Bean's critical state model from the magnetic hysteresis measurement. Normalized relaxation rates were maximum near the center and decreased toward the edge if applied field H$_{app}$ is greater than the penetration field H. But applied magnetic field H$_{app}$ is less than H, relaxation rates were minimum near the center and increased toward edge. We found that glassy exponent ${\mu}$ has the value of ${\sim}$ 1 whose corresponding vortex motion is half-loop excitation. However, single vortex creep, ${\mu}$ ${\sim}$ 1/7, was also found at 30 K and H$_{app}$ ${\cong}$ H'. Calculation of activation energy, U, was possible from direct analysis of the local relaxation data using the basic diffusion equation. From these results, we found that U increases logarithmically with time and U around center was lower than that at the edge.

• Journal of the Korean Magnetics Society
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• v.25 no.6
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• pp.185-188
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• 2015

We have investigated nuclear magnetic resonance of $^{23}Na$ nucleus in $NaMgCl_3$ single crystal in the temperature range 200 K~410 K using FT-NMR spectrometer. The spin-lattice relaxation times $T_1$ of $^{23}Na$ nucleus residing at cubic symmetry in the host crystal was measured as a function of temperature. The $T_1$ of $^{23}Na$ nucleus decreased with increasing temperature. The nuclear spin-lattice relaxation rate $1/T_1$ of $^{23}Na$ in $NaMgCl_3$ single crystal was proportional to the temperature T. This behavior is explained with the characteristic feature of the direct process between the nuclear spins and single phonon, $1/T_1$ being proportional to the absolute temperature. The activation energy calculated was $E_a=4.82J/mol$.

• Journal of Biomedical Engineering Research
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• v.36 no.6
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• pp.241-250
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• 2015

Noninvasive temperature monitoring is feasible with Magnetic Resonance Imaging (MRI) based on temperature sensitive MR parameters such as $T_1$ and $T_2$ relaxation times, Proton Resonance Frequency shift (PRFs), diffusion, exchange process, magnetization transfer contrast, chemical exchange saturation transfer, etc. While the temperature monitoring is very useful to guide the thermal treatment such as RF hyperthermia or thermal ablation, the optimization of the MR thermometry method is essential because the range of temperature measurement depends on the choice of the measurement methods. Useful temperature range depends on the purpose of treatment methods, for example, $42^{\circ}C$ to $45^{\circ}C$ for RF hyperthermia and over $50^{\circ}C$ for thermal ablation. In this paper, MR thermometry methods using $T_1$ and $T_2$ relaxation times and PRFs-based MR thermometry are tried on a 3.0 T MRI system and their results are reported and compared. In addition, the scanning protocol and temperature calculation algorithms from $T_1$ and $T_2$ relaxation times and PRFs are optimized for the different temperature ranges for the purpose of RF hyperthermia and/or thermal ablation.

• Progress in Superconductivity and Cryogenics
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• v.22 no.4
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• pp.62-66
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• 2020

We fabricated superconducting quantum interference devices (SQUIDs) based on Nb Josephson junctions, and characterized the key parameters of the SQUIDs. The SQUIDs are double relaxation oscillation SQUIDs (DROSs) having larger flux-to-voltage transfer coefficient than the standard DC-SQUIDs. SQUID sensors were fabricated by using Nb junction technology consisted of a DC magnetron sputtering and a conventional photolithography process. In multichannel SQUID systems for whole-head magnetoencephalography measurement with a helmet-type SQUID array, we need about 336 SQUID sensors for each system. In this paper, we fabricated a few hundred SQUID sensors, measured the critical current, flux modulation voltage and decided if each tested SQUID can be used for the multichannel systems. As the criterion for the acceptance of the sensors, we chose the critical current and amplitude of the modulation voltage to be 8 ㎂ and 80 ㎶, respectively. The average critical current of the SQUIDs was 10.58 ㎂. The typical flux noise of the SQUIDs with input coil shorted was 2 μΦ0/√Hz at white region.

• Journal of Magnetics
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• v.17 no.4
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• pp.233-241
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• 2012

In this work, Monte Carlo simulation was used to investigate the magnetization properties of thin ferromagnetic films under a perturbation from a supplied heat pulse on one surface of the films. The finite difference method was used to extract the local temperature of each layer of the films as a function of time for various heat source power and heating period. Then, with the variation of the films temperature, Metropolis method was used to update the magnetic moment in magnetic grain, under the Ising framework and using the FePt parameters. With the extracted magnetization profiles, the relationship between magnetization relaxation in accordance with relevant heat parameters and films thickness was reported and discussed, with a purpose to form a database for future use.

• The Bulletin of The Korean Astronomical Society
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• v.42 no.2
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• pp.81.2-81.2
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• 2017

Polarized mid-infrared emission from polycyclic aromatic hydrocarbons (PAHs) can provide a crucial test of basic physics of alignment of nanoparticles and opens a potential new window into studying magnetic fields. In this talk, I will present a new model of polarized PAH emission that takes into account the effect of PAH alignment with the magnetic field due to resonance paramagnetic relaxation. I will then present our predictions for the polarization level of the strong PAH emission features from the interstellar medium. I will present the first detection of polarization of PAH emission at 11.3micron which is consistent with our theoretical prediction. Finally, I will discuss important implications of this work for tracing magnetic fields via mid-IR PAH features and for constraining the polarization of anomalous microwave emission that is useful for the quest of CMB B-modes.

• Journal of Astronomy and Space Sciences
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• v.29 no.2
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• pp.145-149
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• 2012

I describe a series of processes, including hierarchical fragmentation, gravitational scattering, Kozai cycles within triple systems, tidal friction and magnetic braking, that I believe are responsible for producing the modest but significant fraction of stars that are observed as contact binaries. I also discuss further processes, namely heat transport, mass transport, nuclear evolution, thermal relaxation oscillations, and further magnetic braking with tidal friction, that influence the evolution during contact. The endpoint, for contact, is that the two components merge into a single star, as recently was observed in the remarkable system V1309 Sco. The single star probably throws off some mass and rotates rapidly at first, and then slows by magnetic braking to become a rather inconspicuous but normal dwarf or subgiant. If however the contact binary was part of a triple system originally-as I suggested above was rather likely-then the result could be a $widish$ binary with apparently non-coeval components. There are several such known.

• Earthquakes and Structures
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• v.9 no.3
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• pp.577-601
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• 2015

The thermomagnetic effect on plane wave propagation at the liquid-solid interface with nonclassical thermoelasticity is investigated. It is assumed that liquid-solid half-space is under initial stress. Numerical computations are performed for the developed amplitude ratios of P, SV and thermal waves under Cattaneo-Lord-Shulman theory, Green-Lindsay theory and classical thermoelasticity. The system of developed equations is solved by the application of the MATLAB software at different angles of incidence for Green and Lindsay model. The effect of initial stress and magnetic field in the lower half-space are discussed and comparison is made in LS, GL and CT models of thermoelasticity. In the absence of magnetic field, the obtained results are in agreement with the same results obtained by the relevant authors. This study would be useful for magneto-thermoelastic acoustic device field.

• Proceedings of the Korean Magnestics Society Conference
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• 2000.09a
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• pp.121-146
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• 2000

A novel system of magneto-optical microscope magnetometer (MOMM), capable of simultaneous local problems of magnetic properties as well as real-time magnetic domain evolution imaging of ferromagnetic thin films with 400-nm spatial resolution, New findings in domain reveral dynamics of Co-based multilayers: The reversal ratio of V/R is a governing physical parameter. The activation volumes of wall-motion and nucleation processes are generally unequal. Submicron-scale local coercivity variation determines domain reversal dynamics. A thermally activated relaxation process during domain reversal is existed on the submicron-scale in realistic films. Local variation of magnetic properties should be considered for a realistic simulation. The fantastic capabilities of the MOMM can open many possibilities to broaden and deepen our understanding of domain reversal phenomena in ferromagnetic thin films.