Flux Pinning Mechanism in Single-Crystalline MgB2 Thin Films

We investigate the flux pinning mechanism for single-crystalline MgB2 thin films showing a peculiar flux pinning force behavior. The vortex-vortex distance is considered in this study through an analysis of the critical current density (J(c)) and the flux pinning force density (F-p) over a wide range of temperatures from 5 to 35 K. Two unusual kinds of peaks, a near zero-field peak like a hump and another peak with a broad shape, are observed in the field dependences of the F-p. The second peak with a broad shape begins to be depressed at temperatures above 25 K while the first sharp peak is only detected at temperatures above 30 K. In the single-crystalline MgB2 thin films weak collective pinning is dominant at all temperatures, and the pinning mechanism shows a crossover from delta T-c-pinning to delta l-pinning with increasing magnetic field. We describe these unique features by considering the relation between the inter-vortex distances and the vortex-vortex interaction energy.