Abstract
We report on magnetic and magnetoresistance measurements in twocategories of superconducting Nb films grown via magnetron sputtering andMgB2 bulk samples. In thefirst category, films of Tc = 9.25 K were produced by annealing during deposition. In these films, the magneticmeasurements exhibited the so-called ‘second magnetization peak’ (SMP), whichis accompanied by thermomagnetic instabilities (TMI). The characteristic fieldHfj, where the first flux jump occurs, has been studied as a functionof the sweep rate of the magnetic field. Interestingly, in the regimeT<6.4 K, the respectiveline Hfj(T) is constant,Hfj(T<6.4 K) = 40 Oe. A comparisonto TMI observed in MgB2 bulk samples is also performed. Our experimental findings cannot be describedaccurately by current theories on TMI. In the second category, films ofTc = 8.3 Kwere 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(ΔT<20 mK) in the measured magnetoresistance. In contrast to experimental works presented inthe past, the comparison of our magnetic measurements with the magnetoresistance datasuggests that the appearance of surface superconductivity rather than the meltingtransition of vortex matter is the cause of the observed behaviour.
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