Effect of vortex-vortex interactions on the critical current density of single-crystalline MgB₂ thin films

We have investigated the influence of inter-vortex interactions on the critical current density (J_c) and flux pinning properties of single-crystalline MgB₂ thin films grown using a hybrid physical-chemical vapor deposition system. Magnetic field dependences of J_c were measured over a 5-35 K range of temperatures for fields applied parallel and perpendicular to the ab-plane of the film's surface. Two regions along the J_c(H) curve, the low-field plateau region and the high-field slope region, which are respectively known as the single-vortex and collective pinning regimes, are clearly distinguishable. A ratio a₀/λ between the inter-vortex spacing a₀ and the penetration depth λ was calculated at each temperature, to investigate the effect of vortex-vortex interactions on the J_c(H). We found that the a₀/λ dependences of the normalized J_c(H,T) data tend to fall onto one curve irrespective of the temperature, if only we use a certain average value of λ. Furthermore, the flux pinning mechanism shows a crossover from δT_c-pinning to δl-pinning with increasing magnetic fields, indicating the coexistence of different pinning mechanisms in MgB₂.