• Journal of the Korean Magnetic Resonance Society
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• v.11 no.2
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• pp.64-72
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• 2007

[ $^1H$ ] nuclear magnetic resonance (NMR) experiments have been performed at 30 - 300 K and 7 T to investigate dynamics of hydrogen bond network in the single crystal $(NH_4)_3H(SO_4)_2$. The two proton sites, ammonium proton and hydrogen-bond proton, are identified from the $^1H$ NMR MAS spectrum at 340 K. As temperature decreases, the $^1H$ NMR spectrum shifts to the higher frequency side with a larger linewidth. The spectrum at 65 K shows a distinctive change in line shape toward the ferroelectric transition at 63 K. The measured values of $T_1$ for ammonium and hydrogen-bond protons are similar in the whole range of temperature. $T_1$ of $^1H$ NMR shows a gradual decrease down to 120 K and starts to steeply increase below 100 K. Then $T_1$ shows abrupt decrease below 70 K with a sharp minimum at 63 K, where the ferroelectric transition occurs. This temperature dependence of spectrum and $T_1$ clearly prove that the large change in the dynamics of hydrogen bond network is associated with the ferroelectric phase transition at 63 K.

• Proceedings of the Korean Magnestics Society Conference
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• 2002.12a
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• pp.17-17
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• 2002

The magnetic effects connected to finite-size of the particles become dominant at transition of particles in region several nanometers and less [1]. At reduction of atom numbers in magnetic cluster (tens or hundreds atoms), along the classical features, essential appear the quantum phenomena resulting in spin transformation of magnetic structure, both with reduction of the size and at applying of an external magnetic field. (omitted)

• Journal of The Korean Astronomical Society
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• v.33 no.3
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• pp.159-163
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• 2000

We have investigated one dimensional steady flow model of a typical magnetic flux tube in the solar transition region constrained to observed Differential Emission Measure (DEM) for the average quiet-Sun deduced by Raymond & Doyle (1981) with a flux tube geometry conforming to Doppler shifts of UV lines measured by Chae, Yun & Poland (1998). Because local heating and filling factor in the transition region are not well known, we considered two extreme cases, one characterized by the filling factor= 1 ('filled-up model') and the other set by local heating=0 ('not-heated model'). We examined how much the heating is required for the flux tube by recomputing a model through adjustment of the filling factor in such a way that 'not-heated model' accounts for the observed DEM.

• Journal of Magnetics
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• v.10 no.2
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• pp.63-65
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• 2005

The $^1H$ NMR spin-lattice relaxation in a series of the organic-inorganic hybrid systems $(C_nH_{2n+1}NH_3)_2SnCl_6$ (n = 8, 10, 12, 14) undergoing two successive phase transitions was studied. A discontinuity characteristic of a first order phase transition was observed at the high-temperature conformational transition. Besides, the spin-lattice relaxation rate below the conformational transition temperature was well fitted by four types of molecular motions, from which the chain-length dependence of the activation energies of the molecular groups was obtained.

• Proceedings of the Korean Magnestics Society Conference
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• 2000.09a
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• pp.584-584
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• 2000

Electronic structures of ordered Fe$_3X (X = Al, Si), and their derivative ternary alloys of Fe_2TX (T = 3d transition metal) have been investigated by using the linearized muffin-tin orbital (LMTO) band method. The role of the coupling between substituted transition metal and its neighbors is investigated by calculating the magnetic moments and local density of states (LDOS). It is shown that it is essential to include the coupling beyond nearest neighbors in obtaining the magnetic moment of Fe alloy. The preferential sites of T impurities in Fe_3X are determined from the total energy calculations. The derivative ternary alloys of Fe_2TX have characteristic electronic structures of semi-metal for Fe_2VAI and (nearly) half-metal for Fe_2TAI (T = Cr, Mn) and Fe_2TSi (T = V, Cr, Mn)

• Journal of the Korean Magnetic Resonance Society
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• v.22 no.4
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• pp.101-106
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• 2018

Magnetite is the oldest magnet material known to mankind. It is getting attention again from solid state physics researchers now a days because it is one of the most strongly correlated electron systems. Spin, charge, and orbital orders are interplaying with lattice and involved in the Verwey transition where magnetization, conductivity, and structure changes suddenly. The peculiar ordering states above and below the transition temperature mainly originate from the coexistence of $Fe^{2+}$ and $Fe^{3+}$ ions in the B site of the inverse spinel structure. In particular, the state of the charge and orbital order was the oldest and most intriguing problem. NMR has made significant contribution to the investigation of this question. A. Abragam stated that there is no doubt that NMR is a very powerful tool for the study of ferromagnetic and antiferromagnetic materials. In this mini-review, a short history of NMR investigation of magnetite is presented, providing a support to Abragam's claim.

• 한국초전도학회:학술대회논문집
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• v.9
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• pp.222-222
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• 1999

From extensive measurements of the resistance and the dynamic resistance as functions of magnetic field and temperature, we find that the transport in the insulating state beyond the superconductor-insulator (S-I) transition is dominated by bosons(Cooper pairs and/or vortices) and cannot be described by the theory of the fermionic insulating phase. The maximum of the magnetoresistance at B = B$_m$ and the following negative slope in R(B) with increasing field can be explained by the crossover from the "Bose-glass" to the "Fermi-glass" phase as suggested by Paalanen, Hebard, and Ruel. The zero bias peak in dv/dl for biases below the characteristic voltage V$_c$ (or current $I_c$), gives a clue for the assumption of the "dirty boson" model which states that the insulating state above the critical magnetic field is the phase where Cooper pairs are localized due to the Coulomb blockade with a nonvanishing order parameter. The shift to a lower value of the critical magnetic field by overlaying thin Au layer, which is known as a strong spin-orbit scatterer, also supports the bosonic nature of the S-I transition.

• Journal of the Korean Magnetics Society
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• v.19 no.2
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• pp.43-46
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• 2009

The magnetic and structural properties of the (8, 0) BN nanotubes with transition metals (TM) of Fe, Co, or Ni substitution for B or N were investigated using a first-principles calculation. It was found that TM substitution makes the cross section being distorted and the bond length TM-B or TM-N being longer than that of the original B-N one. The magnetic moment is larger for the TM substitution for B than one for N, and it is mainly due to the 3d electrons of TM atoms.

• Korean Journal of Materials Research
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• v.16 no.12
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• pp.747-753
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• 2006

(BaSr)$TiO_3$ (BST) thick films were prepared by tape casting method, using $BaTiO_3$ and $SrTiO_3$ powder slurry and their dielectric properties were investigated. With an additive, $Li_2CO_3$, the sintering temperature was lowered by $200^{\circ}C$. Sintering density was 5.7 g/$cm^3$ and the BST thick films exhibited a maximum dielectric constant, tunability at temperatures near phase transition point. Whilst their characteristics were deteriorated above the phase transition temperature, they were unchanged below the phase transition temperature, which is presumedly due to the acceleration of $90^{\circ}$ domain formation, its contribution to the relaxation of internal stress and the increase in sintering according to the replacement of Li.

• Journal of Magnetics
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• v.19 no.4
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• pp.345-348
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• 2014

Magnetorheological suspensions (MRSs) are smart materials that have the potential to revolutionize several industrial sectors because of their special rheological behaviors. In this paper, MRS, based on carbonyl iron (CI) microparticles that were dispersed in silicone oil with oleic acid, were prepared. We showed that the electroconductibility of MRS was significantly influenced by the intensity of the external magnetic field that was applied. The resistance value can vary from infinite to below $300{\Omega}$ after applying an external magnetic field. The results indicated that this MRS had the property of magnetic-field-tuned insulator to conductor transition. This system has potential applications in controllable MRS electrical devices.