• Korean Journal of Materials Research
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• v.19 no.11
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• pp.601-606
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• 2009

$BiFeO_3$ (BFO) thin films were prepared on $Pt/TiO_2/Si$ substrate by r.f. magnetron sputtering. The effects of deposition pressure on electrical properties were investigated using measurement of dielectric properties, leakage current and polarization. When BFO targets were prepared, Fe atoms were substituted with Mn 0.05% to increase electrical resistivity of films. (Fe+Mn)/Bi ratio of BFO thin films increases with increasing partial pressure of $O_2$ gas. The deposited films showed the only BFO phase at 10 mTorr, the coexistence of BFO and $Bi_2O_3$ phase at 30-50 mTorr, and the only $Bi_2O_3$ phase at 70 mTorr. The crystallinity of BFO films was reduced due to the higher Bi contents and the decrease of surface mobility of atoms at high temperature. The porosity and surface roughness of films increased with the increase of the deposition pressure. The films deposited at high pressure showed low dielectric constant and high leakage current. The dielectric constant of films deposited at various deposition pressures was 84${\sim}$153 at 1 kHz. The leakage current density of the films deposited at 10${\sim}$70 mTorr was about $7{\times}10.6{\sim}1.5{\times}10.2A/cm^2$ at 100 kV/cm. The leakage current was found to be closely related to the morphology and composition of the BFO films. BFO films showed poor P-E hysteresis loops due to high leakage current.

• Proceedings of the Materials Research Society of Korea Conference
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• 2009.05a
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• pp.33.2-33.2
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• 2009

Dielectric and piezoelectric properties of Lead-free $(1-x)(Bi_{0.5}K_{0.5})TiO_3-xBiFeO_3$ceramics prepared by a conventional solid state reaction method were investigated in the range of x = 0~10 mol%. Piezoelectric coefficient was increased from 31 pC/N at x = 0 mol% to 64 pC/N at x = 6 mol% then decreased with increasing x. Electromechanical coupling factor ($K_p$) was increased up to 0.18 at x = 10 mol%. On the other hand, mechanical quality factor ($Q_m$) was decreased. Grain size was not much changed with various x and a single perovskite with tetragonal symmetry was maintained at all compositions forming a solid solution between $(Bi_{0.5}K_{0.5})TiO_3$ and $BiFeO_3$. Depolarization temperature ($T_d$) was gradually decreased with increasing x from $302^{\circ}C$ at x = 0 to $245^{\circ}C$ at x = 10 mol%.

• Journal of Magnetics
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• v.5 no.1
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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.

• The Korean Journal of Ceramics
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• v.5 no.4
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• pp.341-347
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• 1999

An ambiguity on the correct room temperature structure of $Bi_5Ti_3FeO_{15}$ was resolved using a combined Rietveld refinement of neutron and X-ray diffraction. The structure of this compound has been reported to have a space group of F2mm (adopting 2-fold rotation symmetry along the c-axis) or A21am. However, our diffraction, study reveals that some reflections would violate F-centering and confirm that the belong to $A2_1$am. Out refinement with the space group of $A2_1$am converged at $R_p=6.85%, R_wp=9.23%$ and $\chi^2$=1.66 for an isotropic temperature model with 85 variables. The lattice constants are a=5.4677(1) $\AA$, b=5.4396(1) $\AA$, and c=41.2475(8)$\AA$. In structure, Ti/Fe atoms at the oxygen octahedral sites of the perovskite unit are completely disordered, resulting in that these atoms are transparent in neutron diffraction. The octahedra of the perovskite unit are relatively displaced along the a-axis against the Bi atoms, which contribute as a major component to the spontaneous polarization of $Bi_5Ti_3FeO_{15}$.

• Proceedings of the Korean Vacuum Society Conference
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• 2013.02a
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• pp.260-260
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• 2013

Multiferroic materials have attracted much attention due to their fascinating fundamental physical properties and technological applications in magnetic/ferroelectric data-storage systems, quantum electromagnets, spintronics, and sensor devices. Among single-phase multiferroic materials, $BiFeO_3 $ is a typical multiferroic material with a room temperature magnetoelectric coupling in view of high magnetic-and ferroelectric-ordering temperatures (Neel temperature $T_N$~647 K and Curie temperature $T_C$~1,103 K). Rare-earth ion substitution at the Bi sties is very interesting, which induces suppressed volatility of Bi ion and improved ferroelectric properties. At the same time, Fe-site substitution with magnetic ions is also attracting, and the enhanced ferromagnetism was reported. In this study, $Bi_{1-x}Dy_xFe_{0.95}Co_{0.05}O_3$ (x=0, 0.05 and 0.1) bulk ceramic compounds were prepared by solid-state reaction and rapid sintering. High-purity $Bi_2O_3$, $Dy_2O_3$, $Fe_2O_3$ and $Co_3O_4$ powders with the stoichiometric proportions were mixed, and calcined at $500^{\circ}C$ or 24 h to produce $Bi_{1-x}Dy_xFe_{0.95}Co_{0.05}O_3$. The samples were immediately put into an oven, which was heated up to $800^{\circ}C$ nd sintered in air for 30 min. The crystalline structure of samples was investigated at room temperature by using a Rigaku Miniflex powder diffractometer. The field-dependent magnetization measurements were performed with a vibrating-sample magnetometer. The electric polarization was measured at room temperature by using a standard ferroelectric tester (RT66B, Radiant Technologies).

• Proceedings of the Korean Vacuum Society Conference
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• 2014.02a
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• pp.182-182
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• 2014

Multiferroic materials have attracted much attention due to their own fascinating fundamental physical properties and potential technological applications to magnetic/ferroelectric data storage systems, quantum electromagnets, spintronics, and sensor devices. Among single-phase multiferroic materials, $BiFeO_3$, in particular, has received considerable attention because the enhanced ferromagnetism was found by the Fe-site ion substitution with magnetic ions. The structural, the magnetic and the ferroelectric properties of polycrystalline $BiFe_{1-x}Ni_xO_3$ (x=0, 0.01, 0.02, 0.03 and 0.05), which were prepared by the solid-state reaction and the rapid-sintering method, have been investigated. The x-ray diffraction patterns reveal that all the samples are in single phase and show rhombohedral structure with R3c space group. The magnetic properties are enhanced according to the doping content. The Ni-doped $BiFeO_3$ samples exhibit lossy P-E loop due to the oxygen vacancy. The leakage current density of Ni-doped samples (x=0.01 and 0.02) is increased by four orders of magnitude. On the other hand, the x=0.03 and 0.05 samples show the relative reduction of the leakage current.

• Journal of the Korean Magnetics Society
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• v.13 no.6
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• pp.246-250
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• 2003

The electromagnetic properties of Ni$\_$0.8/Zn$\_$0.2/Fe$_2$O$_4$ having stable characteristics in high frequency range were investigated as functions of Bi$_2$O$_3$, CaO contents. Power loss increased in proportion to the amount of Bi$_2$O$_3$ up to 0.3 wt％ and decreased over 0.3 wt％. Also, permeability increased with Bi$_2$O$_3$ contents. The lowest power loss and highest resonance frequency were obtained to the specimens added Bi$_2$O$_3$ of 0.7wt％ and CaO of 0.3 wt％ due to creation of resistivity layers in the grain boundaries originated by the solid solution of Bi$_2$O$_3$ and CaO.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2008.11a
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• pp.231-232
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• 2008

The coupling between electric, magnetic, and structural order parameters results in the so-called multiferroics, which possess ferroelectricity, ferromagnetism, and/or ferroelasticity. The simultaneous ferroelectricity and ferromagnetism (magnetoelectricity) allow potential applications in information storage, spintronics, and in magnetic or electric field sensors. Perovskite compound $BiFeO_3$ (BFO) is antiferromagnetic below Neel temperature of 647K and ferroelectric with a high Curie temperature of 1043K. It exhibits weak magnetism at room temperature(RT) due to the residual moment from a canted spin structure. It is likely that non-stoichiometry and second-phase formation are the factors which cause leakage in BFO. It has been suggested that oxygen non-stoichiometry leads to valence fluctuations of Fe ions in BFO, resulting in high conductivity. To reduce the large leakage current of BFO, one attempt is fabricating donor doped BFO compounds and thin films. We report here the successful fabrication of the Nd, Ti co-doped $BiFeO_3$ ceramics and thin films by pulsed laser deposition technique.

• Journal of the Korean Magnetics Society
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• v.16 no.5
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• pp.245-248
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• 2006

[ $Tb_{3-x}Bi_xFe_5O_{12}$ ] has been studied by x-ray diffraction (XRD), vibrating sample magnetometer, $M\"{o}ssbauer$ spectroscopy. The crystal structures were found to be a cubic garnet structure with space group Ia3d. The lattice constants increase linearly with increasing bismuth concentration. With increase of bismuth substitution, the $N\'{e}el$ temperature increases but the compensation temperature decreases. We have observed the negative magnetization in Bi-TbIG system which has not been reported in garnet systems. $M\"{o}ssbauer$ spectra were measured at various temperatures from 4.2 K to $N\'{e}el$ temperature. The isomer shifts at room temperature are ${\sim}0.26mm/s$ which is consistent with ferric state.

• Proceedings of the Korean Vacuum Society Conference
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• 2016.02a
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• pp.139.1-139.1
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• 2016

Multiferroic materials are of great interest because of its potential applications in the design of devices combining magnetic, electronic and optical functionalities. Among various multiferroic materials, $BiFeO_3$(BFO) is known to be one of the intensively focused mainly due to the possibility of multiferroism at device working temperature (> $200^{\circ}C$). However, leakage current and weak polarization resulting from oxygen deficiency and crystalline defect should be resolved. Furthermore the magnetic ordering of pure BFO mainly prefers to have antiferromagnetic coupling. Up to now many attempts have been performed to improve the ferromagnetic and the ferroelectric properties of BFO by doping. In this work, we investigated the effects of Ni substitution on the multiferroism of bulk BFO. Four BFO samples (a pure BFO and three Ni-doped BFO's; $BiFe_{0.99}Ni_{0.01}O_3$, $BiFe_{0.98}Ni_{0.02}O_3$ and $BiFe_{0.97}Ni_{0.03}O_3$) were synthesized by the standard solid-state reaction and rapid sintering technique. The XRD results reveal that Ni atoms are substituted into Fe-sites and give rise to phase transition of cubic to rhombohedal. By using vibrating sample magnetometer and standard ferroelectric tester, the multiferroic properties at room temperature were characterized. We found that the magnetic moment of Ni-doped BFO turned out to be maximized for 3% of Ni dopant.