• Bulletin of the Korean Chemical Society
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• 제29권6호
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• pp.1233-1242
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• 2008

The hydration effect on the intrinsic magnetism of natural salmon double-strand DNA was explored using electron magnetic resonance (EMR) spectroscopy and superconducting quantum interference device (SQUID) magnetic measurements. We learned from this study that the magnetic properties of DNA are roughly classified into two distinct groups depending on their water content: One group is of higher water content in the range of 2.6-24 water molecules per nucleotide (wpn), where all the EMR parameters and SQUID susceptibilities are dominated by spin species experiencing quasi one-dimensional diffusive motion and are independent of the water content. The other group is of lower water content in the range of 1.4-0.5 wpn. In this group, the magnetic properties are most probably dominated by cyclotron motion of spin species along the helical π -way, which is possible when the momentum scattering time (${\tau}_k$) is long enough not only to satisfy the cyclotron resonance condition (${\omega}_c{\tau}_k$ > 1) but also to induce a constructive interference between the neighboring double helices. The same effect is reflected in the S-shaped magnetization-magnetic field strength (M-H) curves superimposed with the linear background obtained by SQUID measurements, which leads to larger susceptibilities at 1000 G when compared with the values at 10,000 G. In particular, we propose that the spin-orbital coupling and Faraday's mutual inductive effect can be utilized to interpret the dimensional crossover of spin motions from quasi 1D in the hydrate state to 3D in the dry state of dsDNA.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2012년도 제43회 하계 정기 학술대회 초록집
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• pp.218-218
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• 2012

Recently, hexagonal manganites have attracted much attention because of the coexistence of ferroelectricity and antiferromagnetic (AFM) order. The crystal structure of hexagonal manganites consists of $MnO_5$ polyhedra in which $Mn^{3+}$ ion is surrounded by three oxygen atoms in plane and two apical oxygen ions. The Mn ions within Mn-O plane form a triangular lattice and couple the spins through the AFM superexchange interaction. Due to incomplete AFM coupling between neighboring Mn ions in the triangular lattice, the system forms a geometrically-frustrated magnetic state. Among hexagonal manganites, $YMnO_3$, in particular, is the best known experimentally since the f states are empty. In addition, for applications, $YMnO_3$ thin films have been known as promising candidates for non-volatile ferroelectric random access memories. However, $YMnO_3$ has low magnetic order temperature (~70 K) and A-type AFM structure, which hinders its applications. We have synthesized $YMn1_{-x}Cr_xO_3$ (x = 0, 0.05 and 0.1) samples by the conventional solid-state reaction. The powders of stoichiometric proportions were mixed, and calcined at $900^{\circ}C$ for $YMn1_{-x}Cr_xO_3$ for 24 h. The obtained powders were ground, and pressed into 5-mm-thick disks of 1/2-inch diameter. The disks were directly put into the oven, and heated up to $1,300^{\circ}C$ and sintered in air for 24 h. The phase of samples was checked at room temperature by powder x-ray diffraction using a Rigaku Miniflex diffractometer with Cu $K{\alpha}$ radiation. All the magnetization measurements were carried out with a superconducting quantum-interference-device magnetometer. Our experiments point out that the Cr-doped samples show the characteristics of a spin-glass state at low temperatures.

• 한국분말재료학회지
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• 제9권5호
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• pp.359-364
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• 2002

We report the structure, thermal and magnetic properties of a non-equilibrium $Al_{0.6}(Fe_{50}Cu_{50})_{0.4}$ alloy powder produced by rod milling and chemical leaching. An X-ray diffractometry(XRD), a transmission electron microscope(TEM), a differential scanning calorimeter(DSC), a vibrating sample magnetometer(VSM), and superconducting quantum interference device(SQUID) were utilized to characterize the as-milled and leaching specimens. The crystallite size reached a value of about 8.82 nm. In the DSC experiment, the peak temperatures and crystallization temperatures decreased with increasing milling time. The activation energy of crystallization is 200.5 kJ/mole for as-milled alloy powder. The intensities of the XRD peaks of as-milled powders associated with the bcc type $Al_{0.5}Fe_{0.5}$ structure formative at $350^{\circ}C$ sharply increase with increasing annealing temperature. Above $400^{\circ}C$, peaks alloted to $Al_{0.5}Fe_{0.5}$ and $Al_{5}Fe_{2}$ are observed. After annealing at $600^{\circ}C$ for 1h, the leached Ll specimen transformed into bcc $\alpha$-Fe and fcc Cu phases, accompanied by a change in the structural and magnetic properties. The saturation magnetization decreased with increasing milling time, and a value of about 8.42 emu/g was reached at 500 h of milling. The coercivity reached a maximum value of about 142.7 Oe after 500 h of milling. The magnetization of leached specimens as function of fields were higher at 5 K, and increased more sharply at 5 K than at 100 K.

• Progress in Superconductivity
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• 제10권2호
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• pp.139-143
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• 2009

We have fabricated a low-noise 61-channel axial-type first-order gradiometer system for measuring fetal magnetocardiography(MCG) signals. Superconducting quantum interference device(SQUID) sensor was based on double relaxation oscillation SQUID(DROS) for detecting biomagnetic signal, such as MCG, magnetoencphalogram(MEG) and fetal-MCG. The SQUID sensor detected axial component of fetal MCG signal. The pickup coil of SQUID sensor was wound with 120 ${\mu}m$ NbTi wire on bobbin(20 mm diameter) and was a first-order gradiometer to reject the environment noise. The sensors have low white noise of 3 $fT/Hz^{1/2}$ at 100 Hz on average. The fetal MCG was measured from $24{\sim}36$ weeks fetus in a magnetically shielded room(MSR) with shielding factor of 35 dB at 0.1 Hz and 80 dB at 100 Hz(comparatively mild shielding). The MCG signal contained maternal and fetal MCG. Fetal MCG could be distinguished relatively easily from maternal MCG by using independent component analysis(ICA) filter. In addition, we could observe T peak as well as QRS wave, respectively. It will be useful in detecting fetal cardiac diseases.

• 한국자기학회지
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• 제13권2호
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• pp.59-63
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• 2003

고분자 재료인 이온교환수지 박막 안에서의 이온교환반응과 전기화학적 환원반응을 이용하여 코발트 나노 입자를 제조하였다. 코발트 나노 입자의 구조와 자기특성을 투과전자현미경과 초전도양자간섭기를 이용하여 고찰하였다. 투과전자현미경 결과로부터 고분자 박막(MF-4SK) 1 gram에 코발트가 $7.8{\times}10^{19}$ atoms 포함된 시편에서 코발트가 나노 크기로 입자를 형성하고 있음을 확인하였으며, 자기측정 결과로부터 코발트 나노 입자가 blocking temperature($T_{B}$) 이상에서 초상자성을 나타내는 것을 확인하였다. 온도에 따른 자화 측정 곡선으로부터 500 Oe 자기장 하에서 $T_{B}$가 대략 185 K인 결과를 얻었으며, 300 K에서의 자화곡선(M-H곡선) 결과를 이용하여 Langevin function fit하여 계산한 코발트 입자의 평균 반경은 4.0 nm로, 투과전자현미경으로 관찰한 크기와 일치하는 것을 확인하였다. 이 결과는 고분자 박막 내에서 코발트 나노 입자가 자성 단상(single domain) 구조를 이루고 있음을 보여주는 것으로, 강자성 나노 입자들의 초상자성 거동을 고찰하였다.

• 한국자기학회지
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• 제17권2호
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• pp.71-75
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• 2007

계면활성제를 첨가한 음향화학적 침전법으로 두 가지 크기의 아연페라이트 나노입자를 합성하였다. 열분석장치(TGA/DSC)를 이용하여 열적 특성 및 결정화 온도를 조사하였고, 결정구조 확인을 위하여 X선 회절실험을 실시하였다. 제조된 입자는 $240^{\circ}C$ 부근에서 결정생성이 시작되었으며, 결정구조는 전형적인 스피넬 구조를 나타내었다. Scherrer식에 의해 측정된 입자의 크기는 11.2nm와 13.4nm이었다. SQUID를 통하여 자기적 성질을 분석한 결과, 작은 크기의 입자에 대한 방해온도 $T_B$(Blocking temperature)가 큰 입자의 경우보다 더 높게 나타났다.

• Progress in Superconductivity
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• 제5권2호
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• pp.98-104
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• 2004

We analyzed a noise-sensitivity profile of a specific SQUID sensor system for the localization of brain activity. The location of a neuromagnetic current source is estimated from the recording of spatially distributed SQUID sensors. According to the specific arrangement of the sensors, each site in the source space has different sensitivity, that is, the difference in the lead field vectors. Conversely, channel noises on each sensor will give a different amount of the estimation error to each of the source sites. e.g., a distant source site from the sensor system has a small lead-field vector in magnitude and low sensitivity. However, when we solve the inverse problem from the recorded sensor data, we use the inverse of the lead-field vector that is rather large, which results in an overestimated noise power on the site. Especially, the spatial sensitivity profile of a gradiometer system measuring tangential fields is much more complex than a radial magnetometer system. This is one of the causes to make the solutions of inverse problems unstable on intervening of the sensor noise. In this study, in order to improve the localization accuracy, we calculated the noise-sensitivity profile of our 40-channel planar SQUID gradiometer system, and applied it as a normalization weight factor to the source localization using synthetic aperture magnetometry.

• 한국초전도학회:학술대회논문집
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• 한국초전도학회 2000년도 High Temperature Superconductivity Vol.X
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• pp.2-6
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• 2000

We have developed an extremely sensitive radio frequency amplifier based on the dc superconducting quantum interference device (dc SQUID). Unlike a conventional semiconductor amplifier, a SQUID can be cooled to ultra-low temperatures (100 mK or less) and thus potentially achieve a much lower noise temperature. In a conventional SQUID amplifier, where the integrated input coil is operated as a lumped element, parasitic capacitance between the coil and the SQUID washer limits the frequency up to which a substantial gain can be achieved to a few hundred MHz. This problem can be circumvented by operating the input coil of the SQUID as a microstrip resonator: instead of connecting the input signal open. Such amplifiers have gains of 15 dB or more at frequencies up to 3 GHz. If required, the resonant frequency of the microstrip can be tuned by means of a varactor diode connected across the otherwise open end of the resonator. The noise temperature of microstrip SQUID amplifiers was measured to be between $0.5\;K\;{\pm}\;0.3\;K$ at a frequency of 80 MHz and $1.5\;K\;{\pm}\;1.2\;K$ at 1.7 GHz, when the SQUID was cooled to 4.2 K. An even lower noise temperature can be achieved by cooling the SQUID to about 0.4 K. In this case, a noise temperature of $100\;mK\;{\pm}\;20\;mK$ was achieved at 90 MHz, and of about $120\;{\pm}\;100\;mK$ at 440 MHz.

• Progress in Superconductivity
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• 제2권1호
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• pp.1-5
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• 2000

We have developed an extremely sensitive radio frequency amplifier based on the dc superconducting quantum interference device (dc SQUID). Unlike a conventional semiconductor amplifier, a SQUID can be cooled to ultra-low temperatures (100 mK or less) and thus potentially achieve a much lower noise temperature. In a conventional SQUID amplifier, where the integrated input coil is operated as a lumped element, parasitic capacitance between the coil and the SQUID washer limits the frequency up to which a substantial gain can be achieved to a few hundred MHz. This problem can be circumvented. by operating the input coil of the SQUID as a microstrip resonator: instead of connecting the input signal between the two ends of the coil, it is connected between the SQUID washer and one end of the coil; the other end is left open. Such amplifiers have gains of 15 dB or more at frequencies up to 3 GHz. If required, the resonant frequency of the microstrip can be tuned by means of a varactor diode connected across the otherwise open end of the resonator. The noise temperature of microstrip SQUID amplifiers was measured to be between 0.5 K $\pm$ 0.3 K at a frequency of 80 MHz and 1.5 K $\pm$: 1.2 K at 1.7 GHz, when the SQUID was cooled to 4.2 K. An even lower noise temperature can be achieved by cooling the SQUID to about 0.4 K. In this case, a noise temperature of 100 mK $\pm$ 20 mK was achieved at 90 MHz, and of about 120 $\pm$ 100 mK at 440 MHz.

• Progress in Superconductivity
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• 제11권2호
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• pp.152-157
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• 2010

We have fabricated the low noise liquid helium(LHe) dewar with a different shape of thermal shield to apply the 64-channel SQUID(Superconducting Quantum Interference Device) gradiometer. The first shape of thermal shield was made of an aluminum plate with a wide width of 100 mm slit and the other shape was modified with a narrow width of 20 mm slit. The two types of dewars were estimated by comparing the thermal noise and the signal-to-noise ratio(SNR) of magnetocardiography(MCG) using the $1^{st}$ order SQUID gradiometer system cooled each dewar. The white noise was different as a point of the dewar. The noise was increased as close as the edge of dewar, and also increased at the thermal shield with the more wide width slit. The white noise of the dewar with thermal shield of 100 mm slit was 6.5 fT/$Hz^{1/2}$ at the center of dewar and 25 fT/$Hz^{1/2}$ at the edge, and the white noise of the other one was 3.5 - 7 fT/$Hz^{1/2}$. We measured the MCG using 64-channel SQUID gradiometer cooled at each LHe dewar and compared the SNR of MCG signal. The SNR was improved of 10 times at the LHe dewar with a modified thermal shield.