RESUMO
In superconducting materials a dynamical rearrangement of the vortex lattice occurs by forcing vortices at high velocities, until the system can become unstable. This phenomenon is known as vortex lattice instability, in which a sudden transition drives the superconducting system abruptly to the normal state. We present an experimental study on submicron bridges of NbN and NbTiN ultra-thin films with a thickness of few nanometers. The nanoscale effect on vortex lattice instability is investigated not only by the ultra-thin thickness in wide bridges, but also by changing the direction of the external magnetic field applied parallel and perpendicular to the c-axis epitaxial films. Indeed, measurements are performed for both orientations and show the vortex lattice instability, regardless of the superconducting material. Critical currents I c as well as instability currents I* have been compared. However, only in the parallel configuration an unusual 'flying birds' feature appears in the magnetic field dependence of current switching, as a consequence of the ratio I*/I c that is approaching 1. This amazing tendency becomes relevant for practical applications involving nanostructures, since by scaling down sample thickness and rotating the external field towards the in-plane orientation, the ultra-thin film geometry can mimic the bridge narrowing down to the nanoscale.
RESUMO
Anisotropy effects on flux pinning and flux flow are strongly effective in cuprate as well as iron-based superconductors due to their intrinsically layered crystallographic structure. However Fe(Se,Te) thin films grown on CaF2 substrate result less anisotropic with respect to all the other iron based superconductors. We present the first study on the angular dependence of the flux flow instability, which occurs in the flux flow regime as a current driven transition to the normal state at the instability point (I*, V*) in the current-voltage characteristics. The voltage jumps are systematically investigated as a function of the temperature, the external magnetic field, and the angle between the field and the Fe(Se,Te) film. The scaling procedure based on the anisotropic Ginzburg-Landau approach is successfully applied to the observed angular dependence of the critical voltage V*. Anyway, we find out that Fe(Se,Te) represents the case study of a layered material characterized by a weak anisotropy of its static superconducting properties, but with an increased anisotropy in its vortex dynamics due to the predominant perpendicular component of the external applied magnetic field. Indeed, I* shows less sensitivity to angle variations, thus being promising for high field applications.
RESUMO
The magnetic properties of the triple-layered Sr(4)Ru(3)O(10) have been investigated by means of neutron scattering diffraction. At zero field we find that the magnetic moments are ferromagnetically coupled and oriented along the c-axis with no signatures of either long-range antiferromagnetic order or ferromagnetic components in the ab-plane. The field dependence of the reflection intensity points to a metamagnetic response involving only the planar magnetic moments. The structural refinement indicates a distinct rearrangement of the unit cell as a function of both temperature and in-plane applied field. We show that at the temperature T* ~/= 50 K, below which the metamagnetic behavior is observed, the c-axis lattice parameter exhibits a rapid increase while the in-plane amplitude saturates. A similar upturn of the in-plane lattice parameter after the quench of the c-axis amplitude occurs above a critical magnetic field.