Order-disorder transition of vortex matter in Mg0.95B2

Third-harmonic susceptibility has been employed to probe the order-disorder transition of vortex matter of a good-quality sample of MgB2 with random disorder. In the major part of the temperature interval, the measured boundary line can be adjusted by a power-law function, in accordance with theoretical models for the melting of the ordered phase of a 3D vortex-glass. For larger temperatures, however, thermally-induced depinning anticipates disorder, which is also predictable for the vortex matter behavior of a clean system with small amounts of randomly distributed disorder, as is the case of the Mg0.95B2 sample studied here.