We report on magnetic measurements as a function of field, temperature and time (relaxation) in superconducting Nb films of critical temperature T-c = 9.25 K. The magnetic measurements as a function of field exhibited a "second magnetization peak" (SMP) which in general is accompanied by thermomagnetic instabilities (TMIs). The lines H-smp(T) where the SMP occurs and the H-fj(T) where the first flux jump in the virgin magnetization curves is observed, end at a characteristic point (T-0, H-0) approximate to (7.2K, 80 Oe). Relaxation measurements showed that for T < T-0 approximate to 7.2 K the activation energy U-0 and the normalized relaxation rate S exhibit non-monotonic behavior as a function either of temperature or field. The extrema observed in U-0 and S coincide with the points H-on(T) or H-smp(T) of the SMP. In the regime T > T-0 approximate to 7.2 K both U-0 and S present a conventional monotonic behavior. These results indicate that the SMP behavior observed in Our Nb films is promoted by the anomalous relaxation of the magnetization. (C) 2004 Elsevier B.V. All rights reserved.

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 T-c = 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 H-fj, 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 H-fj(T) is constant, H-fj(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 T-c = 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 (DeltaT < 20 rnK) in the measured niagnetoresistance. 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.

We have studied the vortex matter phase diagram in two carbon doped MgB2-xCx (x=0.04 and 0.1) single crystals. We have found that, as in pristine MgB2, the peak effect is also present in carbon doped crystals. The regime where the peak effect is developed is more extended in comparison to the undoped crystal. The enhancement of the peak effect regime, combined with the higher H-c2(c)(0), makes doped MgB2 more favorable for practical applications. The absence of significant bulk pinning below the line H-on(T), which denotes the onset points of the peak effect, implies that the Bragg glass phase is present for H

We report on combined resistance and magnetic measurements in a hybrid structure (HS) of randomly distributed anisotropic CoPt magnetic nanoparticles (MN) embedded in a 160 nm Nb thick film. Our resistance measurements exhibited a sharp increase at the magnetically determined bulk upper-critical fields H-c2(T). Above these points the resistance curves are rounded, attaining the normal state value at much higher fields identified as the surface superconductivity fields H-c3(T). When plotted in reduced temperature units, the characteristic field lines H-c3(T) of the HS and of a pure Nb film, prepared at exactly the same conditions, coincide for H<10 kOe, while for fields H>10 kOe they strongly segregate. Interestingly, the characteristic value H=10 kOe is equal to the saturation field H-sat(MN) of the MN. The behavior mentioned above is observed only for the case where the field is normal to the film's surface, while it is absent when the field is parallel to the film. Our experimental results suggest that the observed enhancement of surface superconductivity field H-c3(T) is possibly due to the not uniform local reduction of the external magnetic field by the dipolar fields of the MN.

By using isothermal magnetization measurements in polycrystalline MgB2 samples, we estimate the H-c2(c) in the interval [0, T-c]. By combining these measurements to the estimated H-c2(ab) from the onset of the diamagnetic transition in isofield and isothermal magnetic measurements, an estimation of the anisotropy parameter can be achieved. The H-c2(c) values coming from high quality polycrystalline samples agrees nicely to those obtained on single crystals. Our results show a temperature variation of the gamma(T) = H-c2(ab) / H-c2(c) with gamma(T-c) approximate to 3.