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We report on transport and magnetization studies of the critical current in single crystal ErNi2B2C for applied fields below 4 kG. Below T approximately 2.5 K superconductivity coexists with weak ferromagnetism. We find that the critical currents are strongly enhanced for all field orientations in this ferromagnetic regime, corresponding to a threefold increase of the pinning force of the flux line lattice. We speculate that this increase is due to strong pair breaking by the ferromagnetism.

We have investigated vortex motion below the quasistatic melting transition in a detwinned YBaqCu307 q single crystal by measuring the differential resistance and the voltage noise. With the magnetic field parallel to the copper-oxygen planes and the large, flat surface of the sample, the noise power spectrum permits us to deduce that the surfaces are the dominant source of the noise and that the vortices appear to be fiowing in channels. In certain regimes, the noise power spectrum exhibits unusually sharp and distinct peaks which are periodic in frequency.PACS numbers: 74.60.GeIn a type-II superconductor, in the mixed state, the sample is penetrated by an array of magnetic vortices each of which contains a quantized amount of magnetic Aux. In an applied current, there will be a Lorentz force density f = J X B on the vortices, where 1 is the current density and B is the flux density [1]. If the vortices are at rest, there will be no electric field and the resistance

We have investigated the temperature dependence of the H c flux line lattice structural phase transition from square to hexagonal symmetry, in the tetragonal superconductor LuNi2B2C (Tc = 16.6 K). At temperatures below 10 K the transition onset field, H2(T ), is only weakly temperature dependent. Above 10 K, H2(T ) rises sharply, bending away from the upper critical field. This contradicts theoretical predictions of H2(T ) merging with the upper critical field, and suggests that just below the Hc2(T )-curve the flux line lattice might be hexagonal.Studies of the topology of the magnetic flux line lattice (FLL) in type-II superconductors have a long history. Early neutron scattering experiments on low-κ superconductors such as niobium revealed a multitude of different FLL symmetries and orientations, mainly determined by the symmetry of the atomic crystal structure in the plane perpendicular to the applied field [1]. This is not surprising since deviations from the hexagonal FLL characteristic of an isotropic superconductor, and the locking to the crystalline lattice are driven by the symmetry of the screening current plane and by nonlocal flux line interactions within a range determined by the coherence length ξ 0 , and the electronic mean free path ℓ. Later, similar effects were also observed in the strong type-II superconductor V 3 Si with κ ≈ 17 [2], demonstrating that nonlocal effects can be equally important in high-κ materials.Over the last couple of years, effects of nonlocality have been clearly observed in the borocarbide superconductors. The borocarbides are quaternary intermetallics with stochiometry RNi 2 B 2 C (R = Y, Gd-Lu) and a tetragonal unit cell (I4/mmm) [3]. These materials have attracted attention due to the coexistence of superconductivity (R = Y, Dy-Tm, Lu) and antiferromagnetic ordering (R = Gd-Tm). The borocarbides are strong type-II superconductors with Ginzburg-Landau (GL) parameter, κ = 6-15. The discovery of a square FLL in most of the H c phase diagram [4], which undergoes a smooth transformation into hexagonal symmetry at fields below 1 kOe [5], was the first observation of a purely field driven FLL symmetry transition.Using nonlocal corrections to the London model and incorporating the symmetry of the screening current plane obtained from band structure calculations, one is able to calculate the FLL free energy, and thereby to determine the stable FLL configuration in different fields in the borocarbides [6]. The model succesfully describes the nature of the FLL square to hexagonal symmetry evolution in the borocarbides with H c as the applied field is reduced. Qualitatively, this can be understood as driven by the four fold basal plane anisotropy which makes the vortex current paths "squarish" close to the core. At high densities this leads to a square FLL, whereas at low fields the system appears isotropic resulting in a hexagonal FLL [7]. The onset of the transition occurs as the field decreases, commensing at a critical field H 2 , determined by the range of the nonlocal inter...