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이승화(Seung Wha Lee),이재광(Jae-Gwang Lee),채광표(Kwang Pyo Chae),안성용(Sung Yong An) 한국자기학회 2010 韓國磁氣學會誌 Vol.20 No.5
γ-Fe₂O₃ nanoparticles have been prepared by a sol-gel method. The structural and magnetic properties have been investigated by XRD, VSM and Mossbauer spectroscopy. γ-Fe₂O₃ powder annealed at 150℃ has a spinel structure and superparamagnetical behavior. The estimated size of superparammagnetic γ-Fe₂O₃ nanoparticle is around 7 ㎚. The hyperfine fields at -261℃ for the A and B patterns were found to be 503 and 485 kOe, respectively. The blocking temperature (T<SUB>B</SUB>) of superparammagnetic γ-Fe₂O₃ nanoparticle is about -183℃. The magnetic anisotropy constant of γ-Fe₂O₃ nanoparticle was calculated to be 1.6 × 106 ergs/㎤. γ-Fe₂O₃ nanoparticle annealed at 150℃ can be a candidate for biomedicine applications as magnetic carriers.
Sol-gel 법에 의한 Co - Zn Ferrite 박막의 제조와 자기 특성에 관한 연구
이승화(Seung Wha Lee),양계준(Kea Joon Yang),류연국(Yeon Guk Ryu),안성용(Sung Yong An),김철성(Chul Sung Kim) 한국자기학회 2001 韓國磁氣學會誌 Vol.11 No.4
Co-Zn ferrite thin films grown on thermally oxidized silicon wafers were fabricated by a sol-gel method. Magnetic and structural properties of Co-Zn thin films were investigated by using x-ray diffractometer (XRD), atomic force microscopy (AFM), auger electron spectroscopy (AES) and a vibrating sample magnetometer (VSM). Co-Zn ferrite thin films annealed at 400 ℃ presented have only a single phase spinel structure without any preferred crystallite orientation. Their surface roughness of Co-Zn ferrite thin films was shown as less than 3 ㎚ and the grain size was about 40 ㎚ for annealing temperatures over 600 ℃. A moderate saturation magnetization of Co-Zn ferrite thin films for recording media was obtained in this study and there is no significant difference of their magnetic property with those external fields of parallel and perpendicular to planes of the films. The maximum value of the coercivity was obtained as 1,900 Oe for Co-Zn ferrite thin film annealed at 600 ℃.
이승화(Seung Wha Lee),이재광(Jae-Gwang Lee),채광표(Kwang Pyo Chae),권우현(Woo Hyun Kwon),김철성(Chul Sung Kim) 한국자기학회 2009 韓國磁氣學會誌 Vol.19 No.2
MnFe₂O₄ nanoparticles have been prepared by a sol-gel method. The structural and magnetic properties have been investigated by XRD, SEM, and Mossbauer spectroscopy, VSM. MnFe₂O₄ powder that was annealed at 250 ℃ has spinel structure and behaved superparamagnetically at room temperature. MnFe₂O₄ annealed at 400 and 500 ℃ has a typical spinel structure and is ferrimagnetic in nature. The estimated size of superparammagnetic MnFe₂O₄ nanoparticle is around 17 ㎚. The hyperfine fields of the A and B patterns at 4.2 K were found to be 508 and 475 kOe, respectively. The blocking temperature (T<SUB>B</SUB>) of superparammagnetic MnFe₂O₄ nanoparticle is about 120 K. The magnetic anisotropy constant and relaxation time constant of MnFe₂O₄ nanoparticle were calculated to be 4.9 × 10? ergs/㎤.
NdFe10.7Ti1.3의 결정학적 및 자기적 성질 연구
이승화(Seung Wha Lee),이용종(Young Jong Lee),안성용(Sung Yong An),김철성(Chul Sung Kim),김윤배(Y. B. Kim),김창석(C. S. Kim) 한국자기학회 1996 韓國磁氣學會誌 Vol.6 No.6
NdFe_(10.7)Ti_(1.3) has been studied with X-ray diffraction, Mossbauer spectroscopy and vibrating sample magnetometer(VSM). The alloys were prepared by arc-melting under an argon atmosphere. The NdFe_(10.7)Ti_(1.3) contains some α-Fe, from X-ray and Mossbauer measurements. The NdFe_(10.7)Ti_(1.3) has the ThMn₁₂-type tetragonal structure with α_0=8.607 Å and c_0=4.790 Å. The Curie temperature(Tc) of the NdFe_(10.7)Ti_(1.3) is 590 K from Mossbauer spectroscopy performed at various temperatures ranging from 13 to 800 K. Each spectrum below Tc was fitted with six subspectra of Fe sites in the structure(8i₁, 8i₂, 8j₂, 8j₁, 8f and α-Fe). The area fractions of the subspectra at room temperature are 13.8 %, 15.4 %, 17 %, 16.4 %, 34.1 % and 3.3 %, respectively. Magenetic hyperfine fields for the Fe sites decrease in the order, Hhf(8i)>Hhf(8j)>Hhf(8f). The abrupt changes in the magnetic hyperfine field, isomer shift and magnetic moment observed at about 180 K in NdFe_(10.7)Ti_(1.3) are attributed to spin reorientation.
열처리에 따른 CoFe₂O₄의 Mossbauer 분광학적 연구
이승화(Seung Wha Lee),김철성(Chul Sung Kim) 한국자기학회 1996 韓國磁氣學會誌 Vol.6 No.2
The crystallographic and magnetic properties of the ferrimagnetic CoFe₂O₄ have been studied by X-ray and Mossbauer measurements. The crystal structure is found to be cubic spinel structure with the lattice constant a_0=8.381±0.005 Å and a_0=8.391±0.005 Å for slow-cooled and quenched CoFe₂O₄, respectively. Mossbauer spectra of CoFe₂O₄ have been taken at various temperatures ranging from 13 to 780 K. The isomer shifts indicate that the valence states of the Fe ions for tetrahedral(A) and octahedral(B) sites have ferric character. Debye temperatures for the A and B sites are found to be θ_A=734±5 K and θ_B=248±5 K for slow-cooled and θ_A=531±5 K and θ_B=197±5 K for quenched, respectively. Atomic migration from the A to the B sites starts near 400 K and 350 K for slow-cooled and quenched CoFe₂O₄, respectively, and increases rapidly with increasing temperature to such a degree that about 69 % for slow-cooled and 91 % for quenched of the ferric ions on the A sites have rooved over to the B sites at 700 K.
Nd - Fe - Ti - B 비정질 합금의 자기적 성질 연구
이승화(Seung Wha Lee),안성용(Sung Yong An),김철성(Chul Sung Kim),김윤배(Y. B. Kim),김창석(C. S. Kim) 한국자기학회 1997 韓國磁氣學會誌 Vol.7 No.3
The amorphous state of NdFe_(10.7)TiB_(0.3) and its nanocrystallization have been studied by X-ray diffraction, Mossbauer spectroscopy, and a vibrating sample magnetometer(VSM). NdFe_(10.7)TiB_(0.3) amorphous ribbons were fabricated by a single-roll melt-spinning method. The average hyperfine field Hhf (T) of the amorphous state shows a temperature dependence of [Hhf(T)-Hhf(0)]/Hhf(0)=-0.46(T/T_C)^(3/2) -0.34(T/T_C)^(5/2) for T/T_C < 0.7, indicative of spin wave excitation. The quadrupole splitting just above the Curie temperature T_C is 0.46 ㎜/s, whereas the average quadrupole shift below T_C is zero. The Curie and crystallization temperatures are determined to be T_C=380 K and T_X=490 K, respectively, for a heating rate of 5 K/min. The occupied area of nanocrystalline phase at around 770 K is about 65 %. Above the Curie temperature, VSM data show magnetic moments increase again. The formation of α-Fe is the main reason for the increasing moment as conformed with the Mossbauer measurements.
Mossbauer 분광학을 이용한 Ni0.5Co0.5Fe₂O₄의 Atomic migration 연구
이승화(Seung Wha Lee),홍성렬(Sung Real Hong),박승일(Seung leI Park),김철성(Chul Sung Kim) 한국자기학회 1995 韓國磁氣學會誌 Vol.5 No.1
The crystallographic and magnetic properties of the ferrimagnetic Ni_(0.5)Co_(0.5)Fe₂O₄ have been studied by X-ray and Mossbauer measurements. The Crystal structure is found to be spinel structure with the lattice constant a_0 = 8.346 ± 0.005 Å. Mossbauer spectra of Ni_(0.5)Co_(0.5)Fe₂O₄ have been taken at various temperatures rauging from 13 to 780 K. The isomer shifts indicate that the valence states of the Fe ions for tetrahedral(A) and octahedral (B) sites have ferric character. Debye temperatures for the A and B sites are found to be 441± 5 K and 321± 5 K, respectively. Atomic migration from the A to the B sites starts near 500 K and increases rapidly with increasing temperature to such a degree that 51 % of the ferric ions at the A sites have moved over to the B sites by 700 K.
Multiferroic Bi₂/₃La₁/₃FeO₃ 나노입자의 Mossbauer 연구
이승화(Seung Wha Lee) 한국자기학회 2006 韓國磁氣學會誌 Vol.16 No.1
La substituted perovskite BiFeO₃ have been prepared by a sol-gel method. Magnetic and structural properties of the powders were characterized with Mossbauer spectroscopy, XRD, SEM, and TG-DTA. The crystal structure is found to be a rhombohedrally distorted perovskite structure with the lattice constant a=3.985 Å and α=89.5°. Bi₂/₃La₁/₃FeO₃ powders that were annealed at and above 600 ℃ have a single-phase perovskite structure. However, powders annealed at 900 ℃ have a typical perovskite structure with small amount of Bi₂O₃ phase. The Neel temperature of Bi₂/₃La₁/₃FeO₃ is found to be 680±3 K. The isomer shift value at room temperature is found to be 0.27 ㎜/s relative to the Fe metal, which is consistent with high-spin Fe³+ charge states. Debye temperature for Bi₂/₃La₁/₃FeO₃ is found to be 305±5 K. The average hyperfine field Hhf(T) of the Bi₂/₃La₁/₃FeO₃ shows a temperature dependence of [Hhf(T)-Hhf(0)]/Hhf(0)=-0.42(T/TN)³/²-0.13(T/TN)^(5/2) for T/TN<0.7, indicative of spin-wave excitation.
나노입자 Zn_(0.5)Ni_(0.5)Fe₂O₄의 초상자성 성질 연구
이승화(Seung Wha Lee) 한국자기학회 2006 韓國磁氣學會誌 Vol.16 No.1
Zn_(0.5)Ni_(0.5)Fe₂O₄ nanoparticles have been prepared by a sol-gel method. The structural and magnetic properties have been investigated by XRD, SEM, and Mossbauer spectroscopy, VSM. Zn_(0.5)Ni_(0.5)Fe₂O₄ powder that was annealed at 300 ℃ has spinel structure and behaved superparamagnetically at room temperature. The estimated size of superparammagnetic Zn_(0.5)Ni_(0.5)Fe₂O₄ nanoparticle is around 7 ㎚. The hyperfine fields of the A and B patterns at 4.2 K were found to be 510 and 475 kOe, respectively. The blocking temperature (TB) of superparammagnetic Zn_(0.5)Ni_(0.5)Fe₂O₄ nanoparticle is about 90 K. The magnetic anisotropy constant and relaxation time constant of Zn_(0.5)Ni_(0.5)Fe₂O₄ nanoparticle were calculated to be 1.6×10^6 ergs/㎤.