• 한국재료학회지
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• 제14권8호
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• pp.595-599
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• 2004

We investigated the structural, electrical and magnetic properties of Mn-doped $CuAlO_2$ delafossite ceramics ($CuAl_{1-x}Mn_{x}O_2,\;0\le\;x\;\le0.05$), synthesized by solid-state reaction method in an air atmosphere at a sintering temperature of $1150^{\circ}C$. The solubility limit of Mn ions in delafossite $CuAlO_2$ was found to be as low as about 3 $mol\%$. Positive Hall coefficient and the temperature dependence of conductivity established that non-doped $CuAlO_2$ ceramic is a variable-range hopping p-type semiconductor. It was found that the Mn-doping in $CuAlO_2$ rapidly reduced the hole concentration and conductivity, indicating compensation of free holes. The analysis of the magnetization data provided an evidence that antiferromagnetic superexchange interaction is the dominant mechanism of the exchange coupling between Mn ions in $CuAl_{1-x}Mn_{x}O$ alloy, leading to an almost paramagnetic behavior in this alloy.

• 한국전기전자재료학회논문지
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• 제35권3호
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• pp.297-302
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• 2022

We investigated the origin of magnetic behaviors induced by an asymmetric spin exchange interaction in Fe-site engineered lead iron niobate [Pb(Fe_1/2Nb_1/2)O₃, PFN], which exhibits a room-temperature multiferroicity. The magnitude of spin exchange interaction was regulated by the introduced transition metals with a distinct Bohr magneton, i.e., Cr, Co, and Ni. All compositions were found to have a single-phase perovskite structure keeping their ferroelectric order except for Cr introduction. We discovered that the incorporation of each transition metal imposes a distinct magnetic behavior on the lead iron niobate system; antiferro-, hard ferro-, and soft ferromagnetism for Cr, Co, and Ni, respectively. This indicates that orbital occupancy and interatomic distance play key roles in the determination of magnetic behavior rather than the magnitude of the individual Bohr magneton. Further investigations are planned, such as X-ray absorption spectroscopy, to clarify the origin of magnetic properties in this system.

• 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.

• Journal of Magnetics
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• 제19권1호
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• pp.59-63
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• 2014

Nano-sized nickel substituted cobalt ferrite powders, $Ni_xCo_{1-x}Fe_2O_4$ ($0.0{\leq}x{\leq}1.0$), were fabricated by the sol-gel method, and their crystallographic and magnetic properties were studied. All the ferrite powders showed a single spinel structure, and behaved ferrimagnetically. When the nickel substitution was increased, the lattice constants and the sizes of particles of the ferrite powders decreased. The M$\ddot{o}$ssbauer absorption spectra of $Ni_xCo_{1-x}Fe_2O_4$ ferrite powders could be fitted with two six-line subspectra, which were assigned to a tetrahedral A-site and octahedral B-sites of a typical spinel crystal structure. The increase in values of the magnetic hyperfine fields indicated that the superexchange interaction was stronger, with the increased nickel concentration in $Ni_xCo_{1-x}Fe_2O_4$. This could be explained using the cation distribution, which can be written as, $(Co_{0.28-0.28x}Fe_{0.72+0.28x})[Ni_xCo_{0.72-0.72x}Fe_{1.28-0.28x}]O_4$. The two values of the saturation magnetization and the coercivity decreased, as the rate of nickel substitution was increased. These decreases could be explained using the cation distribution, the magnetic moment, and the magneto crystalline anisotropy constant of the substituted ions.

• 한국재료학회지
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• 제28권7호
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• pp.371-375
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• 2018

The manganese-, nickel-, and aluminum-doped cobalt ferrite powders, $Mn_{0.2}Co_{0.8}Fe_2O_4$, $Ni_{0.2}Co_{0.8}Fe_2O_4$, and $Al_{0.2}CoFe_{1.8}O_4$, are fabricated by the sol-gel method, and the crystallographic and magnetic properties of the powders are studied in comparison with those of $CoFe_2O_4$. All the ferrite powders are nano-sized and have a single spinel structure with the lattice constant increasing in $Mn_{0.2}Co_{0.8}Fe_2O_4$ but decreasing in $Ni_{0.2}Co_{0.8}Fe_2O_4$ and $Al_{0.2}CoFe_{1.8}O_4$. All the $M{\ddot{o}}ssbauer$ spectra are fitted as a superposition of two Zeeman sextets due to the tetrahedral and octahedral sites of the $Fe^{3+}$ ions. The values of the magnetic hyperfine fields of $Ni_{0.2}Co_{0.8}Fe_2O_4$ are somewhat increased in the A and B sites, while those of $Mn_{0.2}Co_{0.8}Fe_2O_4$ and $Al_{0.2}CoFe_{1.8}O_4$ are decreased. The variation of $M{\ddot{o}}ssbauer$ parameters is explained using the cation distribution equation, superexchange interaction and particle size. The hysteresis curves of the ferrite powders reveal a typical soft ferrite pattern. The variation in the values of saturation magnetization and coercivity are explained in terms of the site distributions, particle sizes and the spin magnetic moments of the doped ions.

• 한국자기학회지
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• 제9권3호
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• pp.143-148
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• 1999

Spin-Peierls(SP) 전이를 하는 CuGeO3에서 Cu 이온과 Ge 이온을 자성 이온인 Fe 이온으로 일부 치환시킨 Cu1-xGe1-yFex+yO3계를 제조하여 전이온도 전후에서 자기적 특성과 결정학적 특성의 변화를 밝히기 위하여 자기감수율과 Mossbauer 스펙트럼을 측정 분석하였다. 결정구조는 모든 시료가 직방정계(orthorhombic)였고 격자 상수는 Fe의 치환량이 증가할수록 a축과 c축만 약간 감소하였다. 온도를 내리면서 측정한 자기감수율의 값이 12.5K 근처에서 급격히 감소하는 SP 전이를 나타냈는데 이 온도는 Fe의 치환량이 많아질수록 약간씩 낮아졌다. Mossbauer 스펙트럼은 Fe3+ 이온에 의한 두 개의 Zeeman sextet와 한 개의 이중선이 중첩되어 나타났는데 특히 SP 전이 온도(Tsp)에서 Mossbauer변수들이 불연속이었다. 즉, Tsp이하에서 두 번째 Zeeman선의 자기장이 크게 증가했고 이중선의 사중극자 분열값(QS)과 이성질체 이동값(IS)도 증가했는데, 이를 Tsp에서 이중체 형성에 따른 이온들의 위치 이동 계산 결과와 비교하여 초교한 상호작용, 대칭성 및 공유 결합성 등의 변화와 관련시켜 해석하였다.

• Journal of Magnetics
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• 제5권1호
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• pp.16-18
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• 2000

The garnets $Nd_{1-x}Bi_xY_2Fe_5O_{12}$ ($\chi$=0.0, 0.25, 0.5, 0.75 and 1.0) have been studied by x-rays, electron microscopy, ferromagnetic resonance, vibrating sample magnetometer and Mossbauer spectroscopy, Ultra-fine polycrystalline cubic samples have been prepared by a melt-salt routed sol-gel method. The Mossbauer spectra consist of two sets of six-line patterns corresponding to $Fe^{3+}$ at the tetrahedral 24(d) and octahedral 16(a) sites. Magnetic hyperfine fields of $Nd_{0.5}Bi_{0.5}Y_2Fe_5O_{12}$ at 12 K are found to be 548 kOe (octahedral site) and 475 kOe (tetrahedral site), respectively, It is found that Debye temperatures for the tetrahedral and octahedral sites of $Nd_{0.75}Bi_{0.25}Y_2Fe_5O_{12}$ are $\theta_{tet}=436$ K and $\theta_{oct}=285$ K, respectively, The iron ions at both sites are highly covalent ferric. The Nel temperature decreases linearly with Bi concentration, from 630 K fur $\chi$=0.0 to 600 K for $\chi$=1.0, suggesting that the superexchange interaction for the Nd-O-Fe link is stronger than that for the Bi-O-Fe link. As a consequence, the coercivity of $Nd_{1-x}Bi_xY_2Fe_5O_{12}$ drastically decreases and the magnetization remains almost constant as x increases.

• Journal of Magnetics
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• 제21권3호
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• pp.308-314
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• 2016

Cobalt-, zinc-, and nickel-zinc-substituted nano-size manganese ferrite powders, $MnFe_2O_4$, $Mn_{0.8}Co_{0.2}Fe_2O_4$, $Mn_{0.8}Zn_{0.2}Fe_2O_4$ and $Mn_{0.8}Ni_{0.1}Zn_{0.1}Fe_2O_4$, were fabricated using a sol-gel method, and their crystallographic and magnetic properties were subsequently studied. The $MnFe_2O_4$ ferrite powder annealed at temperatures above 523 K exhibited a spinel structure, and the particle size increased as the annealing temperature increased. All ferrites annealed at 773 K showed a single spinel structure, and the lattice constants and particle size decreased with the substitution of Co, Zn, and Ni-Zn. The $M{\ddot{o}}ssbauer$ spectrum of the $MnFe_2O_4$ ferrite powder annealed at 523 K only showed a doublet due to its superparamagnetic phase, and the $M{\ddot{o}}ssbauer$ spectra of the $MnFe_2O_4$, $Mn_{0.8}Co_{0.2}Fe_2O_4$, and $Mn_{0.8}Zn_{0.2}Fe_2O_4$ ferrite powders annealed at 773 K could be fitted as the superposition of two Zeeman sextets due to the tetrahedral and octahedral sites of the $Fe^{3+}$ ions. However, the $M{\ddot{o}}ssbauer$ spectrum of the $Mn_{0.8}Ni_{0.1}Zn_{0.1}Fe_2O_4$ ferrite powder annealed at 773 K consisted of two Zeeman sextets and one quadrupole doublet due to its ferrimagnetic and paramagnetic behavior. The area ratio of the $M{\ddot{o}}ssbauer$ spectra could be used to determine the cation distribution equation, and we also explained the variation in the $M{\ddot{o}}ssbauer$ parameters by using this cation distribution equation, the superexchange interaction and the particle size. Relative to pure $MnFe_2O_4$, the saturation magnetizations and coercivities were larger in $Mn_{0.8}Co_{0.2}Fe_2O_4$ and smaller in $Mn_{0.8}Zn_{0.2}Fe_2O_4$, and $Mn_{0.8}Ni_{0.1}Zn_{0.1}Fe_2O_4$. These variations could be explained using the site distribution equations, particle sizes and magnetic moments of the substituted ions.

• Journal of Magnetics
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• 제21권1호
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• pp.40-45
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• 2016

Nickel substituted nano-sized ferrite powders, $Co_{1-x}Ni_xFe_2O_4$, $Mn_{1-x}Ni_xFe_2O_4$ and $Mn_{1-2x}Zn_xNi_xFe_2O_4$ ($0.0{\leq}x{\leq}0.2$), were fabricated using a sol-gel method, and their crystallographic and magnetic properties were subsequently compared. The lattice constants decreased as quantity of nickel substitution increased, while the particle size decreased in $Co_{1-x}Ni_xFe_2O_4$ ferrite but increased for the $Mn_{1-x}Ni_xFe_2O_4$ and $Mn_{1-2x}Zn_xNi_xFe_2O_4$ ferrites. For the $Co_{1-x}Ni_xFe_2O_4$ and $Mn_{1-x}Ni_xFe_2O_4$ ($0.0{\leq}x{\leq}0.2$) ferrite powders, the $M{\ddot{o}}ssbauer$ spectra could be fitted as the superposition of two Zeeman sextets due to the tetrahedral and octahedral sites of the $Fe^{3+}$ ions. However, the $M{\ddot{o}}ssbauer$ spectrum of $Mn_{0.8}Zn_{0.1}Ni_{0.1}Fe_2O_4$ consisted of two Zeeman sextets and one single quadrupole doublet due to the ferrimagnetic and paramagnetic behavior. The area ratio of the $M{\ddot{o}}ssbauer$ spectra could be used to determine the cation distribution equation, and we also explain the variation in the $M{\ddot{o}}ssbauer$ parameters by using this cation distribution equation, the superexchange interaction and the particle size. The saturation magnetization decreased in the $Co_{1-x}Ni_xFe_2O_4$ and $Mn_{1-2x}Zn_xNi_xFe_2O_4$ ferrites but increased in the $Mn_{1-x}Ni_xFe_2O_4$ ferrite with nickel substitution. The coercivity decreased in the $Co_{1-x}Ni_xFe_2O_4$ and $Mn_{1-2x}Zn_xNi_xFe_2O_4$ ferrites but increased in the $Mn_{1-x}Ni_xFe_2O_4$ ferrite with nickel substitution. These variations could thus be explained by using the site distribution equations, particle sizes and spin magnetic moments of the substituted ions.

• Bulletin of the Korean Chemical Society
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• 제17권9호
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• pp.794-798
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• 1996

Solid solutions of the nonstoichiometric Dy1-xSrxCoO3-y system with the compositions of x=0.00, 0.25, 0.50, 0.75, and 1.00 have been synthesized by the solid state reaction at 1000 ℃ under atmospheric air pressure. The crystallographic structures of the solid solutions are analyzed by the powder X-ray diffraction patterns at room temperature. The analyses assign the compositions of x=0.00 and 0.25 to the orthorhombic system with space group of Pbnm/D2h16, the compositions of x=0.50 and 0.75 to the tetragonal system like a typical SrCoO2.86, and the composition of x=l.00 or SrCoO2.50 to the brownmillerite type system with space group of I**a. The reduced lattice volumes increase with x value due to the larger radius of Sr2+ ion than that of Dy3+ ion. The mole ratio of Co4+ ion to total Co ion with mixed valence state between Co3+ and Co4+ ions at B sites or τ value has been determined by an iodometric titration. All the samples except for the DyCoO3 compound show the mixed valnce state and thus the composition of x=0.50 has the maximum τ value in the system. The oxygen vacancies increasing with x value are randomly distributed over the crystal lattice except for the composition of x=l.00 which have the ordering of the oxygen vacancies. The nonstoichiometric chemical formulas of the Dy1-xSrxCo3+1-τCo4+τO3-(x-τ)/2 system are formulated from the x, τ, and y values. The electrical conductivity in the temperature range of 100 to 900 K increases with τ value linearly because of positive holes of the Co4+ ions in π* band as a conducting carrier. The activation energy of the x=0.50 as Ea=0.17 eV is minimum among other compouds. Broad and high order transition due to the overlap between σ* and π* bands broadened by the thermal activation is observed near 1000 K and shows a low temperature-semiconducting behavior. Magnetic properties following the Currie-Weiss law show the low to high spin transition in the cobaltate perovskite. Especially, the composition of x=0.75 presents weak ferromagnetic behavior due to the Co3+-O2--Co4+ indirect superexchange interaction.