• Proceedings of the International Union of Geodesy And Geophysics Korea Journal of Geophysical Research Conference
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• 2003.05a
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• pp.20-20
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• 2003

In the summers of 1997 and 1998 and in February of 2000 we made 570 measurements of the ambient geomagnetic field 120 cm above the pavement surface of State Route 130, south of Pahoa, the island of Hawaii using a three-component fluxgate magnetometer. We measured at every 15.2 m (50 feet) interval covering a distance of 6, 310 m (20, 704 ft) where both historic and pre-historic highly magnetic basalt flows underlie. We also collected 197 core samples from eight road cuts, 489 specimens of which were subject to AF demagnetizations at 5 - 10 mT level up to a maximum field of 60 mT. We observed significant inclination anomalies ranging from a minimum of $31^{\circ}$ to a maximum $40^{\circ}$ where a uniform inclination value of $36.7^{\circ}$ (International Geomagnetic Reference Field, IGRF) was expected. Since the mean of the observed inclinations is approximately $35^{\circ}$ we assume that the study area is slightly affected by the magnetic terrain effect to a systematically shallower inclinations for being located in the regionally sloping surface of the southern side of the island (Baag, et al., 1995). We observed inclination anomalies showing wider (spacial) wavelength (160 - 600 m) and higher amplitudes in the historic lava flows area than in the northern pre-historic flows. Our observations imply that preexisting inclination anomalies such as those that we observed would have been interpreted as paleosecular variation (PSV). These inclination anomalies can best be attributed to concealed underground highly magnetic dikes, channel type lava flows, on-and-off hydrothermal activities through fissure-like openings, etc. Both the within- and between-site dispersions of natural remanent magnetization (NRM) are largest (up to ${\pm}7^{\circ}$) above the flows of 1955, while the area of pre-historic flows in the northern part of the study area exhibit the smallest dispersion. Nevertheless, mean inclinations of each historic flow of 1955 and 1790 are almost identical to that of the corresponding present field, whereas mean of NRM (after AF demagnetization) inclinations for each of the four pre-historic lava flow units is twelve to thirteen degrees lower than the present field inclination. We observed three cases of very large inclination variations from within a single flow, the best fitting curves of which are linear, second and third order polynomials each from within a single flow, whereas no present field variations are observed. This phenomena can be attributed to the notion that local magnetic anomalies on the surface of an active volcano are not permanent, but are transient. Therefore we believe that local magnetic anomalies of an active volcano may be constantly modified due to on going subsurface injections and circulations of hot material and also due to wide spacial and temporal distribution of highly magnetic basaltic flows that will constantly modify the topography which will in turn modify the local ambient geomagnetic field (Baag, et al., 1995). Our observations bring into question the general reliability of PSV data inferred from volcanic rocks, because on-going various geologic and geophysical activities associated with active volcano would continuously deflect and modify the ambient geomagnetic field.

• Economic and Environmental Geology
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• v.43 no.6
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• pp.625-636
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• 2010

The geophysical characteristics and geological structure of the northeastern part of the Ulleung Basin were investigated from interpretation of geophysical data including gravity, magnetic, bathymetry data, and seismic data. Relative correction was applied to reduce errors between sets of gravity and magnetic data, obtained at different times and by different equipments. The northeastern margin of the Ulleung Basin is characterized by complicated morphology consisting of volcanic islands (Ulleungdo and Dokdo), the Dokdo seamounts, and a deep pathway (Korea Gap) with the maximum depth of -2500 m. Free-air anomalies generally reflect the topography effect. There are high anomalies over the volcanic islands and the Dokdo seamounts. Except local anomalous zones of volcanic edifices, the gradual increasing of the Bouguer anomalies from the Oki Bank toward the Ulleung Basin and the Korea Gap is related to higher mantle level and denser crust in the central of the Ulleung Basin. Complicated magnetic anomalies in the study area occur over volcanic islands and seamounts. The power spectrum analysis of the Bouguer anomalies indicates that the depth to the averaged Moho discontinuity is -16.1 km. The inversion of the Bouguer anomaly shows that the Moho depth under the Korea Gap is about -16~17 km and the Moho depths towards the Oki Bank and the northwestern part of Ulleung Island are gradually deeper. The inversion result suggests that the crust of the Ulleung Basin is thicker than normal oceanic crusts. The result of 20 gravity modeling is in good agreement with the results of the power spectrum analysis and the inversion of the Bouguer anomaly. Except the volcanic edifices, the main pattern of magnetization distribution shows lineation in NE-SW. The inversion results, the 2D gravity modeling, and the magnetization distribution support possible NE-SW spreading of the Ulleung Basin proposed by other papers.

• Journal of Biomedical Engineering Research
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• v.28 no.3
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• pp.317-324
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• 2007

The application of diffusion tensor imaging (DTI) and fiber tractography to Wallerian degeneration (WD) is important because this technique is a very potent tools for quantitatively evaluating fiber tracts in vivo brain. We analyzed a case and control using tracts of interest (TOI) analysis to quantify WD. We scanned a case of old infarction and an age-matched healthy volunteer. T1 magnetization prepared rapid acquisition gradient echo (MPRAGE), fluid attenuated inversion recovery (FLAIR) and 12-direction diffusion tensor imaging (DTI) were obtained and analyzed using TOI analysis. The value of mean diffusity ($D_{av}$) and fracional anisotrophy (FA) were analyzed statistically by MWU test. A p-value of less than 0.05 was considered to indicate statistical significance. A comparison of the global fiber diffusion characteristics shows WD of both the corpus callosum and the ipsilateral superior longitudinal fasciculus. The corpus callosum in particular showed trans-hemispherical degeneration. Local fiber characteristics along the geodesic paths show WD in the corpus callosum, ipsilateral superior longitudinal fasciculus, ipsilateral corticospinal tract, and ipsilateral corticothalamic tract. We have demonstrated changes in $D_{av}$ and FA values and a clear correspondence with the WD in various tracts. TOI analysis successfully revealed radial WD in white matter tracts from a region of encephalomalacia and primary gliosis, although they were only slightly affected.

• Journal of the Korean Magnetics Society
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• v.1 no.2
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• pp.1-8
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• 1991

The electronic and magnetic structure of Fe overlayers on W(110) is determined by means of the all-electron local spin density full potential linearized augmented plane wave (FLAPW) method with a single slab approach. Charge and spin densities, magnetic moments, contact hyperfine fields, and layer projected density of states (LDOS) are presented. For bilayer Fe coverage, we find magnetic moments to be 2.90 and 2.30 ${\mu}_B$ for the surface and subsurface Fe layers, respectively, corresponding to a 18% enhancement of the total magnetization compared with the calculated bulk value (2.22${\mu}_B$);For monolayer coverage the moment is 2.56 ${\mu}_B$ which is enhanced by 16% compared to bulk. Unusual changes in the magnetic hyperfine interaction are found in going from a monolayer to a bilayer coverage. Comparison of the results to the theoretical ones of the clean Fe(110) to discuss the hybridization and the negative pressure effects. We discuss our results by comparing them to experimental results.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.9 no.5
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• pp.516-520
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• 1999

Mechanical alloying (MA) by planetary type ball mill of pure iron powders was carried out under the ammonia gas atmosphere. The powders of metastable iron nitrides was synthesized up to the nitrogen content of 23 at% N. The observed phases are identified as the super-saturated bcc solid solution for the nitrogen concentration below 14.5 at% N and the non-equilibrium hcp phase stable at high temperature for 20.8 at.% N. Magnetization of Fe-N powders gradually decreases with increasing the N concentration on contrast to the enhancement reported for the bct iron nitrides. Neutron diffraction experiments also provide detailed information concerning the local atomic structure surrounding the nitrogen atoms. The coordination number of Fe atom around a nitrogen atom for the iron nitride containing 9.5 at% N turns out to be 3.9 atoms. This suggests that a nitrogen atom is situated at a center of the tetrahedron formed by iron atoms.

• Proceedings of the Korean Magnestics Society Conference
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• 2011.12a
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• pp.7-8
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• 2011

Although yttrium iron garnet (YIG) has provided a great vehicle for the study of spin waves in the past, associated difficulties in film deposition and device fabrication using YIG had limited the applicability of spin waves to practical devices. However, microfabrication techniques have made it possible to characterize both the resonant as well as the travelling characteristics of spin waves in permalloy (Py). A variety of methods have been used for measuring spin waves, including Brillouin light scattering (BLS), magneto-optic Kerr effect (MOKE), vector network analyzer ferromagnetic resonance (VNA-FMR), and pulse inductive microwave magnetometry (PIMM). PIMM is one of the most preferred methodologies of measuring travelling spin waves. In this method, an electrical impulse is applied at one of two coplanar waveguides patterned on top of oxide-insulated Py, producing a local disturbance in the magnetization of the Py. The resulting disturbance travels down the Py in the form of waves, and is inductively picked up by the other coplanar waveguide. We investigate the effect of the pulse width of excitation pulses on the generated spin wave packets using both experimental results and micromagnetic simulations. We show that spin wave packets generated from electrical pulses are a superposition of two separate spin wave packets, one generated from the rising edge and the other from the falling edge, which interfere either constructively or destructively with one another, depending upon the magnitude and direction of the field bias conditions. A method of spin wave amplitude modulation is also presented by the linear superposition of spin waves. We use interfering spin waves resulting from two closely spaced voltage impulses for the modulation of the magnitude of the resultant spin wave packets.

• Economic and Environmental Geology
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• v.19 no.3
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• pp.231-237
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• 1986

Samples of porphyries, andesites, decites and sandstones were collected from 14 sites in order to study paleomagnetism and to determine K-Ar age in Guryongpo area. K-Ar age dating indicates that porphyries and volcanic rocks formed 41.7 and 22.7~19.4Ma, respectively. The mean direction of remanent magnetization for each site was generally well grouped after alternating field demagnetization. Both normal and reversed directions are present. The mean magnetic direction of the porphyries of Late Eocene and that of the volcanic rocks of Late Oligocene to Early Miocene have similar, easterly declinations. Overall magnetic direction is $Dm=43.8^{\circ}$, $Im=53.5^{\circ}$, ${\alpha}95=12.2^{\circ}$. the reliability of the observed paleomagnetic directions is ascertained by the presence of normal and reversed polarities. This is also conformed by the similarity of the declinations of the normal and reversed polarities. Observed easterly declinations in this area are attributed to local clockwise rotation of the land mass by approximately 40~50 degrees since early Miocene.

• Journal of the Korean Magnetics Society
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• v.8 no.6
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• pp.335-340
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• 1998

Colossal magnetoresistance $La_{0.67}Ca_{0.33}Mn_{0.99}^{57}Fe_{0.01}O_3$ material has been produced by a metal-salt routed sol-gel process method. Magnetic properties of $La_{0.67}Ca_{0.33}Mn_{0.99}^{57}Fe_{0.01}O_3$ have been studied with x-ray diffraction, Rutherford back-scattering spectroscopy(RBS), vibrating sample magnetometer, and Mossbauer spectroscopy. Crystalline $La_{0.67}Ca_{0.33}Mn_{0.99}^{57}Fe_{0.01}O_3$ was perovskite cubic structure with a lattice parameter $a_0=3.868$\AA$$. And there was no appreciable change in the value of the lattice parameter when a small amount (x=0.01) of iron was added. However, Mossbauer and VSM data indicate the Curie temperature of the $La_{0.67}Ca_{0.33}Mn_{0.99}^{57}Fe_{0.01}O_3$ decreased from 282 to 270 k and also the saturation magnetization from 84 to 81 emu/g at 77 K. Mossbauer spectra of $La_{0.67}Ca_{0.33}Mn_{0.99}^{57}Fe_{0.01}O_3$ have been taken at various temperatures ranging form 4.2 K to room temperature. Analysis of $^{57}Fe$ Mossbauer data in terms of the local configurations of Mn atoms has permitted the influence of the magnetic hyperfine interactions to be monitored. The isomer shifts show that the charge state of all Fe ions are ferric. The magnetoresistance of $La_{0.67}Ca_{0.33}Mn_{0.99}^{57}Fe_{0.01}O_3$ was about 33 % at semiconductor-metal transition temperature $T_{SC-M}=250K$.

• Journal of the Korean Magnetics Society
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• v.26 no.1
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• pp.1-6
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• 2016

Pd has the highest magnetic susceptibility among single element metals and often shows ferromagnetism under some special environments. In this paper, we report magnetism of 5- and 9-monolayers (ML) calculated by using full-potential linearized augmented plane wave method. Exchange-correlation interaction is taken into account in local density approximation (LDA) and generalized gradient approximation (GGA) and calculational results in LDA and GGA are compared with each other. It is found that calculations by LDA are more reliable compared to those by GGA because LDA prediction of paramagnetism of bulk Pd is consistent with experiments, whereas GGA predicts wrongly ferromagnetim of bulk Pd. Calculational results in LDA on a 5-ML Pd(111) thin film shows a ferromagnetic ground state unlike a paramagnetic ground state of bulk Pd. The center Pd layer of the 5-ML Pd(111) thin film has the largest magnetic moment ($0.273{\mu}_B$) among the layers and |m| = 1 orbital states play a dominant role in stabilizing the ferromagnetism of the 5-ML Pd(111) thin film. A 9-ML Pd(111) thin film in a ferromagnetic state has almost the same total energy as in a paramagnetic state. Since the magnetization of the 9-ML Pd(111) thin film is stable, the ferromagnetic state may be meta-stable.