• 한국초전도ㆍ저온공학회논문지
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• 제17권2호
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• pp.1-17
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• 2015

$MgB_2$ thin films with superior superconducting properties are very promising for superconducting magnets, electronic devices and coated conductor electric power applications. A clear understanding of flux pinning mechanism in $MgB_2$ films could be a big aid in improving the performance of $MgB_2$ by the enhancement of $J_c$. The fabrication advancement and the understanding of flux pinning mechanism of $MgB_2$ thin and thick films fabricated by using hybrid physical-chemical vapor deposition (HPCVD) are reviewed. The distinct kind of $MgB_2$ films, such as single-crystal like $MgB_2$ thin films, $MgB_2$ epitaxial columnar thick films, and a-axis-oriented $MgB_2$ films are included for flux pinning mechanism investigation. Various attempts made by researchers to improve further the flux pinning property and $J_c$ performance by means of doping in $MgB_2$ thin films by using HPCVD are also summarized.

• Progress in Superconductivity
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• 제13권2호
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• pp.122-126
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• 2011

Superconducting $MgB_2$ thin films were fabricated by using a novel aerosol deposition technique wherein the pre-reacted powder is directly transformed into respective thin film. The formed thin films were characterized by X-ray diffraction technique and FE-SEM to understand its structure and morphology and the superconducting behavior has been characterized with the four probe resistivity measurement. The as-deposited thin films were formed into the frustrated amorphous structure, which were relaxed on the further heat treatment at $900^{\circ}C$ for 3 hrs. The relaxed amorphous $MgB_2$ thin films showed a comparatively high superconducting onset at about 38 K.

• Progress in Superconductivity
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• 제11권1호
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• pp.8-12
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• 2009

We investigated the effects of the irradiation of light-ions on the superconducting $MgB_2$ thin films fabricated by using HPCVD. Deuterium and helium ions were irradiated on $MgB_2$ thin films by various doses, from $1{\times}10^{10}cm^{-2}\;to\;8{\times}10^{15}cm^{-2}$. During these experiments some reasonable results and unpredictable results have been obtained. The reasonable results are that the peak of the reduced maximum pinning force shifts by increasing the pinning sites in $MgB_2$ films and the slightly change of critical current density of films. We obtained some unusual results, which are the increasing of the transition temperature and the change of residual resistance ratio. Among the data of deuterium and helium ion irradiation experiments, the results of helium ion irradiation have most notable points so we will discuss mainly about helium irradiation experiments.

• Progress in Superconductivity
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• 제3권1호
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• pp.45-48
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• 2001

We have fabricated $MgB_2$thin films on (1 1 02)$ A1_2$$O_3$substrates by using a two-step method. Amorphous B thin films were deposited by a pulsed laser deposition technique and sintered in Mg vapor at various temperatures from 800 to $950^{\circ}C$. Superconducting properties of the thin films were investigated by temperature dependences of magnetization and critical current density. Structural studies were carried out by an x-ray diffraction and a scanning electron microscope. The films fabricated at $900^{\circ}C$ showed the highest transition temperature of 39 K and critical current density of ~$10^{7}$ A/$\textrm{cm}^2$ at 15 K.

• 한국초전도ㆍ저온공학회논문지
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• 제21권3호
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• pp.13-17
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• 2019

We investigate the effect of thermal annealing on $MgB_2$ thin films with thicknesses of 400 and 800 nm, irradiated by 350 keV C-ions with a dose of $1{\times}10^{15}atoms/cm^2$. Irradiation by low-energy C-ions produces atomic lattice displacement in $MgB_2$ thin films, improving magnetic field performance of critical current density ($J_c$) while reducing the superconducting transition temperature ($T_c$). Interestingly, the lattice displacement and the $T_c$ are gradually restored to the original values with increasing thermal annealing temperature. In addition, the magnetic field dependence of $J_c$ also returns to that of the pristine state together with the restoration of $T_c$. Because $J_c$(H) is sensitive to the type and density of the disorder, i.e. vortex pinning, the recovery of $J_c$(H) in irradiated $MgB_2$ thin films by thermal annealing indicates that low-energy C-ion irradiation on $MgB_2$ thin films primarily causes lattice displacement. These results provide new insights into the application of low-energy irradiation in strategically engineering critical properties of superconductors.

• Progress in Superconductivity
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• 제14권1호
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• pp.30-33
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• 2012

We have grown $MgB_2$ superconducting thin films on the SiC buffer layers by means of hybrid physical-chemical vapor deposition (HPCVD) technique. Prior to that, SiC was first deposited on $Al_2O_3$ substrates at various temperatures from room temperature to $600^{\circ}C$ by using the pulsed laser deposition (PLD) method in a vacuum atmosphere of ${\sim}10^{-6}$ Torr pressure. All samples showed a high transition temperature of ~40 K. The grain boundaries of $MgB_2$ samples with SiC layer are greater in amount, compare to that of the pure $MgB_2$ samples. $MgB_2$ with SiC buffer layer samples show interesting change in the critical current density ($J_c$) values. Generally, at both 5 K and 20 K measurements, at lower magnetic field, all $MgB_2$ films deposited on SiC buffer layers have low $J_c$ values, but when they reach higher magnetic fields of nearly 3.5 Tesla, $J_c$ values are enhanced. $MgB_2$ film with SiC grown at $600^{\circ}C$ has the highest $J_c$ enhancement at higher magnetic fields, while all SiC buffer layer samples exhibit higher $J_c$ values than that of the pure $MgB_2$ films. A change in the grain boundary morphologies of $MgB_2$ films due to SiC buffer layer seems to be responsible for $J_c$ enhancements at high magnetic fields.

• Progress in Superconductivity
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• 제5권2호
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• pp.105-108
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• 2004

We have studied effects of in-situ annealing on the fabrication of superconducting MgB$_2$ thin films prepared by rf magnetron co-sputtering. The Films were deposited on A1$_2$O$_3$ (1102) substrates at room temperature by using Mg and B targets. To trap remnant $O_2$ gas in the chamber, we used 20 mtorr Af sputter-gas balanced with 5 mol % of H$_2$ gas. To enhance adhesion to the substrate a thin layer of B was deposited prior to the codeposition of Mg and B. After completion of the film deposition, an additional Mg layer was deposited on top to compensate for Mg loss during the subsequent in-situ annealing. We have investigated the effects of two most important annealing parameters that are the Mg-to-B composition ratio and the annealing temperature. The range of the Mg-to-B composition ratio was from 0.42 to 0.85, and that of the annealing temperature was 500 $^{\circ}C$∼750 $^{\circ}C$. The Best result was obtained for the composition ratio of about 10% Mg excess from the stoichiometry and the annealing temperature of 700 $^{\circ}C$. Based on these results, we obtained films with T$_{c}$ : 36.5 K by further refining the fabrication process.s.

• 한국초전도ㆍ저온공학회논문지
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• 제15권2호
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• pp.12-15
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• 2013

The pure and carbon (C)-doped $MgB_2$ thin films were fabricated on $Al_2O_3$ (0001) substrates at a temperature of $650^{\circ}C$ by using hot-filament-assisted hybrid physical-chemical vapor deposition technique. The $T_c$ value for pure $MgB_2$ film is 38.5 K, while it is between 30 and 35 K for carbon-doped $MgB_2$ films. Expansion in c-axis lattice parameter was observed with increase in carbon doping concentration which is in contrast to carbon-doped $MgB_2$ single crystals. Significant enhancement in the critical current density was obtained for C-doped $MgB_2$ films as compared to the undoped $MgB_2$ film. This enhancement is most probably due to the incorporation of C into $MgB_2$ and the high density of grain boundaries, both help in the pinning of vortices and result in improved superconducting performance.

• Progress in Superconductivity
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• 제4권2호
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• pp.153-156
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• 2003

We have studied fabrication of $MgB_2$ thin film on $SrTiO_3$ (001) and r-cut $A1_2$$O_3$ substrates by rf magnetron sputtering method using and $ MgB_2$ single target and two targets of Mg and B, respectively. Based on P -T phase diagram of $MgB_2$ and vapor pressure curves of Mg and B, a three-step process was employed. B layer was deposited at the bottom to enhance the film adhesion to the substrate. Secondly, co-sputtering of Mg and B was done. Finally, Mg was sputtered on top to compensate fur the loss of Mg during annealing. Subsequently, $MgB_2$ films were in-situ annealed in various conditions. The sample fabricated using the three-step process showed $T_{c}$ of 24 K and formation of superconducting $MgB_2$ phase was confirmed by XRD spectra. In case of co-sputtering deposition, $T_{c}$ depended on annealing time and argon pressure. However, those made by single-target sputtering showed non-superconducting behavior or low transition temperature, at best.est.

• 한국초전도ㆍ저온공학회논문지
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• 제24권4호
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• pp.1-5
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• 2022

We investigate the effect of surface disorder on the lower critical field (H_c1) of MgB₂ thin films with a thickness of 850 nm, where the disorder on the surface region is produced by the irradiation of 140 keV Co ions with the dose of 1 × 10¹⁴ ions/cm². The thickness of the damaged region by the irradiation is around 143 nm, corresponding to ~17% of the whole thickness of the film, thereby forming the disordered MgB₂ overlayer on the pure MgB₂ layer. The magnetic field dependence of magnetization, M(H), for the pristine MgB₂ thin film and the film with overlayer is measured at various temperatures, and H_c1 is determined from the difference (△M) between the Meissner line and magnetization signal with the criterion of △M = 10^-3 emu. Intriguingly, the film with the disordered overlayer shows a remarkably large H_c1(0) = 108 Oe compared to the H_c1(0) = 84 Oe of pristine film, indicating that the disordered MgB₂ overlayer on the pure MgB₂ layer serves to prevent the penetration of vortices into the sample. These results provide new ideas for improving the superheating field to design high-performance superconducting radio-frequency cavities.