  We report on magnetic and magnetoresistance measurements in two categories of
superconducting Nb films grown via magnetron sputtering and MgB2 bulk samples.
In the first category, films of Tc = 9.25 K were produced by annealing during
deposition. In these films, the magnetic measurements exhibited the so-called
second magnetization peak (SMP), which is accompanied by thermomagnetic
instabilities (TMI). The characteristic field Hfj, where the first flux jump
occurs, has been studied as a function of the sweep rate of the magnetic field.
Interestingly, in the regime T < 6.4 K, the respective line Hfj(T) is constant,
Hfj(T < 6.4 K) = 40 Oe. A comparison to TMI observed in MgB2 bulk samples is
also performed. Our experimental findings cannot be described accurately by
current theories on TMI. In the second category, films of Tc = 8.3 K were
produced without annealing during deposition. In such films, we observed a peak
effect (PE). In high magnetic fields the PE is accompanied by a sharp drop and
a narrow hysteretic behaviour in the measured magnetoresistance. In contrast to
experimental works presented in the past, the comparison of our magnetic
measurements with the magnetoresistance data suggests that the appearance of
surface superconductivity rather than the melting transition of vortex matter
is the cause of the observed behaviour.
