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Register a new userEffective penetration length and interstitial vortex pinning in superconducting films with regular arrays of defects
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In order to compare magnetic and non-magnetic pinning we have nanostructured two superconducting films with regular arrays of pinning centers: Cu (non-magnetic) dots in one case, and Py (magnetic) dots in the other. For low applied magnetic fields, when all the vortices are pinned in the artificial inclusions, magnetic dots prove to be better pinning centers, as has been generally accepted. Unexpectedly, when the magnetic field is increased and interstitial vortices appear, the results are very different: we show how the stray field generated by the magnetic dots can produce an effective reduction of the penetration length. This results in strong consequences in the transport properties, which, depending on the dot separation, can lead to an enhancement or worsening of the transport characteristics. Therefore, the election of the magnetic or non-magnetic character of the pinning sites for an effective reduction of dissipation will depend on the range of the applied magnetic field.
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Nb films containing extended arrays of holes with 45-nm diameter and 100-nm spacing have been fabricated using anodized aluminum oxide (AAO) as substrate. Pronounced matching effects in the magnetization and Little-Parks oscillations of the superconducting critical temperature have been observed in fields up to 9 kOe. Flux pinning in the patterned samples is enhanced by two orders of magnitude as compared to unpatterned reference samples in applied fields exceeding 5 kOe. Matching effects are a dominant contribution to vortex pinning at temperatures as low as 4.2 K due to the extremely small spacing of the holes.
We discuss pinning properties of MgB2 thin films grown by pulsed-laser deposition (PLD) and by electron-beam (EB) evaporation. Two mechanisms are identified that contribute most effectively to the pinning of vortices in randomly oriented films. The EB process produces low defected crystallites with small grain size providing enhanced pinning at grain boundaries without degradation of Tc. The PLD process produces films with structural disorder on a scale less that the coherence length that further improves pinning, but also depresses Tc.
In superconducting thin films, engineered lattice of antidots (holes) act as an array of columnar pinning sites for the vortices and thus lead to vortex matching phenomena at commensurate fields guided by the lattice spacing. The strength and nature of vortex pinning is determined by the geometrical characteristics of the antidot lattice (such as the lattice spacing $a_0$, antidot diameter $d$, lattice symmetry, orientation, etc) along with the characteristic length scales of the superconducting thin films, viz., the coherence length ($xi$) and the penetration depth ($lambda$). There are at least two competing scenarios: (i) multiple vortices sit on each of the antidots at a higher matching period, and, (ii) there is nucleation of vortices at the interstitial sites at higher matching periods. Furthermore it is also possible for the nucleated interstitial vortices to reorder under suitable conditions. We present our experimental results on NbN antidot arrays in the light of the above scenarios.
We study experimentally the critical depinning current Ic versus applied magnetic field B in Nb thin films which contain 2D arrays of circular antidots placed on the nodes of quasiperiodic (QP) fivefold Penrose lattices. Close to the transition temperature Tc we observe matching of the vortex lattice with the QP pinning array, confirming essential features in the Ic(B) patterns as predicted by Misko et al. [Phys. Rev. Lett, vol.95, 177007 (2005)]. We find a significant enhancement in Ic(B) for QP pinning arrays in comparison to Ic in samples with randomly distributed antidots or no antidots.
We use a magnetic force microscope (MFM) to investigate single vortex pinning and penetration depth in NdFeAsO$_{1-x}$F$_x$, one of the highest-$T_c$ iron-based superconductors. In fields up to 20 Gauss, we observe a disordered vortex arrangement, implying that the pinning forces are stronger than the vortex-vortex interactions. We measure the typical force to depin a single vortex, $F_{mathrm{depin}} simeq 4.5$ pN, corresponding to a critical current up to $J_c simeq 7 times 10^5$ A/cm$^2$. Furthermore, our MFM measurements allow the first local and absolute determination of the superconducting in-plane penetration depth in NdFeAsO$_{1-x}$F$_x$, $lambda_{ab}=320 pm 60$ nm, which is larger than previous bulk measurements.
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