The levitation force between a superconductor and a permanent magnet has been investigated for the development of superconducting magnetic bearings (SMBs). Depending on the proposed application, the SMBs can be arranged with two kinds of symmetries: rotational or linear. The SMBs present passive operation, low level of noise and no friction, but they need a cooling system for their operation. Nowadays the cooling problem may be easily solved by the use of a commercial cryocooler. The levitation force of SMBs is directly related to the quality of the superconductor material (which depends on its critical current density) and the permanent magnet arrangement. Also, research about the YBa 2 Cu 3 O x (Y123) bulk materials has shown that artificial holes enhance the superconducting properties, in particular the magnetic trapped field. In this context, this work proposes the investigation of the levitation force of a bulk Y123 sample with multiple holes and the comparison of its performances with those of conventional plain Y123 superconductors.

We have performed small-angle neutron scattering (SANS) of the flux line lattice (FLL) in a F e doped N bSe2 sample which presents a large peak effect in the critical current. The scattered intensity and the width of the Bragg peaks of the equilibrium FLL indicate an ordered structure in the peak effect zone. The history dependence in the FLL structure has been studied using field cooled and zero field cooled procedures, and each state shows the same intensity of Bragg scattering and good orientational order. These results strongly suggest that the peak effect is unrelated to a bulk disordering transition, and confirm the role of a heterogeneous distribution of screening current.The peak effect in the critical current of a type II superconductor is a long standing problem in the field of superconductivity. Since its observation [1, 2], no clear consensus has emerged as to its origin. Most models are based on the Pippard idea [3] who attributed the peak effect to the loss of rigidity of the flux lines lattice (FLL), which falls close to B c2 more rapidly than the pinning strength due to inhomogeneities. Larkin and Ovchinikov (LO) have treated the complex problem of the elastic FLL in the presence of random pinning centers [4]. The main issues are the statistical calculations of a volume V c over which the lattice remains correlated and the corresponding pinning force per unit volume F p ∝ V −1/2 c . In addition to the intense theoretical interest, there is the possible link between the FLL bulk disorder and the critical current. Other interpretations of the critical current deal with dominant pinning of flux lines by the surface [5,6], without significant role of FLL bulk order. Recently, the explanation of the peak effect as a genuine phase transition has been addressed both theoretically and experimentally, and the peak effect is now considered as a classical example of an order-disorder transition in an elastic system [7]. A superconductor with a peak effect is remarkable by its anomalous transport properties. They have been tackled by quite thorough series of measurements in N bSe 2 , with a large focus on the metastable properties [8]. Two coexisting macroscopic FLL states with different pinning strengths were observed [9] and the kinetics between these two states can explain most of the anomalous transport properties. Paltiel et al have also shown that the critical current in the peak effect zone is heterogeneous and more important close to the edge of the samples [10]. However, the technique of Hall probe does not allow concluding on the structure of the FLL. The genuine nature of FLL states with the two different critical currents remains unknown, even if one is usually assumed to be much more disordered following LO approach [10]. However, this latter assumption needs to be experimentally controlled using a structural probe: indeed, a correlation between the critical current and the FLL bulk order can not be a priori assumed, as shown by different Small Angle Neutron Scattering (SANS) experiments [11,12,1...

A systematic study of irreversible magnetization was performed in bulk Niobium after different surface treatments. Starting with smooth surfaces and abrading them, a strong increase of the critical current is observed up an apparent limiting value. An impressive change of the critical current is also observed in the surface superconductivity (SSC) state, reaching values of the same order of magnitude as in the mixed state. We explain also the observation of strong SSC for magnetic field perpendicular to larges facets in terms of nucleation of SC along bumps of a corrugated surface.