다양한 외부 조건들이 MgB2 박막에서의 자기 소용돌이 사태에 미치는 영향

Abstract

In early 2001, the superconductivity was discovered in magnesium diboride (MgB2). This compound was paid much attention because it had transition temperature as high as 40 K although it has very simple structure. This magnesium diboride also has a relatively high upper critical field up to 50 T and a very high critical current density (Jc ~ 107 A/cm2). Due to the high Jc, Hc and Tc, MgB2 attracts much attention for the industrial application. But ironically, such high critical current density makes an unexpected phenomenon called by the vortex avalanche which reduce its Jc and generate unexpected noise in low temperature (T < 15 K) and low magnetic field (H < 2000 Oe) region. This may be caused by its high Jc, which in turn produces high Joule heating coupled with an induced electric field. A stable high Jc under the various external conditions is essential for practical applications of superconducting thin films. Therefore, the removal of the dendritic avalanche is important to recover the high critical current density. In this thesis, I studied the vortex avalanche while changing the various external parameters for four different categories and investigated its origin. First subject was about the demagnetization effect. While I changed the width of MgB2 thin film, the appearance of a vortex avalanche showed also different behaviors. As the sample’s width becomes smaller, the avalanche behavior is also reduced and disappears below threshold width. And I also investigated the microscopic features of the vortex avalanche having the dendritic shape in various widths of samples. These results showed that the vortex avalanche strongly depends on the demagnetization effect.Secondly, I considered the gold coating effect on the film to eliminate the vortex avalanche. It is known that a metal coating on a superconducting film can suppress such instability while dissipating the thermal energy to metal layer. I also found that the eddy current shielding can suppress the vortex avalanche. For this study, I checked the eddy current effect by designing the special geometry and found that it can be also another reason to suppress the vortex avalanche.Next, I studied the vortex avalanche for the MgB2 thin films prepared by different methods. One was produced by using pulsed laser deposition (PLD) system and another MgB2 thin film was synthesized by using the hybrid physical chemical vapor deposition (HPCVD) method in which the avalanche behavior did not appear. To find the origin of such difference, I compared the two cases and confirmed that their microstructure which can induce the vortex avalanche differs to each other.Lastly, I studied the effect of the sweeping rate of an applied magnetic field in vortex avalanche. As the sweeping rate goes to fast, the stronger vortex avalanche behavior was appeared and showed saw-tooth like patterns of the flux jumps observed in a very slow sweeping rate. Using these results, I could obtain the phase diagram which distinguishes the stable and the vortex avalanche phase in magnetic field and temperature (H-T) axis.