A comparison to thermomagnetic instabilities observed in MgB2 bulk samples was performed. The experimental findings could not be described accurately by then current theories of thermomagnetic instabilities. In the second category, films with TC = 8.3K were produced without annealing during deposition. In such films, a peak effect was observed. In high magnetic fields, the peak effect was accompanied by a sharp drop and a narrow hysteretic behavior (ΔT < 20mK) of the measured magnetoresistance. In contrast to previous experimental work, a comparison of the magnetic measurements with magnetoresistance data suggested that the appearance of surface superconductivity rather than the melting transition of vortex matter was the cause of the observed behavior.

From the Second Magnetization Peak to Peak Effect - a Study of Superconducting Properties in Nb Films and MgB2 Bulk Samples. D.Stamopoulos, A.Speliotis, D.Niarchos: Superconductor Science and Technology, 2004, 17, 1261-74