Studies On Flux Pinning Mechanism Of MgB₂

In 2001 the discovery of MgB2 superconductivity excites people in the condensed matter realm and arouses the upsurge of the study in high critical temperature superconductor. MgB2 is a new material with superconductivity, which builds a new way for us to study those high-temperature superconductors (HTSC) with simple composition, will be a rising new star in the electronic material field. The new material is better than HTSC in many ways, such as: little anisotropy, large interfere length and non-weak link phenomenon. All of these advantages make MgB2 have a good applying prospect. But as a new superconductor, it has many unknown character to be studied. In this paper, we mainly studied the mechanism of MgB2 flux pinning. At the base of building theoretical model, we numerically calculated the magnetic moment, current density, pinning force of the MgB2 polycrystalline sample and compared the results with the experimental ones. The major results are as follows.1. Building the theoretical model for study the MgB2 flux pinningThe grain anisotropy of superconductor sample results in the difference of Hc2 in every grain. We used the Monte-Carlo random distribution to determine every grain's orientation and compared the results with the identical orientation ones. After these, we find the differences of the both. Using the Anderson flux pinning theory we studied the magnetization relaxation of MgB2 polycrystalline sample owning to thermal activated flux movement and the effect of relaxing different time. The results show that the larger the magnetic field, the larger the relaxation influence. Surface pinning also influences the magnetization curve of MgB2 polycrystalline sample especially at the lower magnetic field.2. Numerical calculating and comparing the result with experiment onesAt the base of models above, we respectively numerically calculated by using exponent model, Kramer model, and dual-exponent model and compared the results with the experimental ones. We find the dual-exponent modal is better in accordance with experimental results. It maybe has the intrinsic relationship with the two energy gaps of MgB2. We used the dual-exponent model to numerically calculate the magnetization curves, current density curves and pinning force curves from 5K to 35K in accordance with the experimental results.3. Studying the magnetization relaxation of MgB2 polycrystalline sample and grain-boundary fluxpinningWe studied the relationship between the normalized relaxation rate and the magnetic field (or the temperature). The results show us the normalized relaxation rate rises with the rising of the field or the temperature and in a particular value the rising becomes quicker. The grain-boundary flux pinning influences the magnetization curves of MgB2 sample with different grain size. With the grain size smaller, the grain-boundary flux pinning becomes an un-neglected factor.