The strengthening of reentrant pinning by collective interactions in the peak effect

Since it was first observed about 40 years ago [1], the peak effect has been the subject of numerous research mainly impelled by the desire to determine its exact mechanisms. Despite these efforts, a consensus on this question has yet to be reached. Experimentally, the peak effect indicates a transition from a depinned vortex phase to a reentrant pinning phase at high magnetic field. To study the effects of intrinsic pinning on the peak effect, we consider Fe$_{x}$Ni$_{1-x}$Zr$_{2}$ superconducting metallic glasses in which the vortex pinning force varies depending on the Fe content and in which a huge peak effect is seen as a function of magnetic field. The results are mapped out as a phase diagram in which it is readily seen that the peak effect becomes broader with decreasing pinning force. Typically, pinning can be understood by increased pinning centers, but here, we show that reentrant pinning is due to the strengthening of interactions (while decreasing pinning strength). Our results demonstrate the strengthening of the peak effect by collective effects.Comment: 4 pages, 4 figure

Superconductivity and short range order in metallic glasses Fex_{x}Ni1−x_{1-x}Zr2_{2}

In amorphous superconductors, superconducting and vortex pinning properties are strongly linked to the absence of long range order. Consequently, superconductivity and vortex phases can be studied to probe the underlying microstructure and order of the material. This is done here from resistance and local magnetization measurements in the superconducting state of Fe$_{x}$Ni$_{1-x}$Zr$_{2}$ metallic glasses with $0\leq x \leq 0.6$. Firstly, we present typical superconducting properties such as the critical temperature and fields and their dependence on Fe content in these alloys. Then, the observations of peculiar clockwise hysteresis loops, wide double-step transitions and large magnetization fluctuations in glasses containing a large amount of Fe are analyzed to reveal a change in short range order with Fe content.Comment: 8 pages, 7 figure

Experimental phase diagram of moving vortices

In the mixed state of type II superconductors, vortices penetrate the sample and form a correlated system due to the screening of supercurrents around them. Interestingly, we can study this correlated system as a function of density and driving force. The density, for instance, is controlled by the magnetic field, B, whereas a current density j acts as a driving force F=jxB on all vortices. The free motion of vortices is inhibited by the presence of an underlying potential, which tends to pin the vortices. Hence, to minimize the pinning strength we studied a superconducting glass in which the depinning current is 10 to 1000 times smaller than in previous studies, which enables us to map out the complete phase diagram in this new regime. The diagram is obtained as a function of B, driving current and temperature and led a remarkable set of new results, which includes a huge peak effect, an additional reentrant depinning phase and a driving force induced pinning phase.Comment: 4 page