Enhanced critical currents of superconducting ErNi2B2C in the ferromagnetically ordered state

Competition between Superconductor - Ferromagnetic stray magnetic fields in YBa2Cu3O7-x films pierced with Co nano-rods

Superconductivity and ferromagnetism are two antagonistic phenomena that combined can lead to a rich phenomenology of interactions, resulting in novel physical properties and unique functionalities. Here we propose an original hybrid system formed by a high-temperature superconducting film, patterned with antidots, and with ferromagnetic nano-rods grown inside them. This particular structure exhibits the synergic influence of superconductor (SC) - ferromagnetic (FM) stray fields, in both the superconducting behaviour of the film and the three-dimensional (3D) magnetic structure of nano-rods. We show that FM stray fields directly influence the critical current density of the superconducting film. Additional functionalities appear due to the interaction of SC stray fields, associated to supercurrent loops, with the non-trivial 3D remanent magnetic structure of FM nano-rods. This work unravels the importance of addressing quantitatively the effect of stray magnetic fields from both, the superconductor and the ferromagnet in hybrid magnetic nano-devices based on high temperature superconductors.

Enhanced critical current density in the pressure-induced magnetic state of the high-temperature superconductor FeSe

We investigate the relation of the critical current density (J_c) and the remarkably increased superconducting transition temperature (T_c) for the FeSe single crystals under pressures up to 2.43 GPa, where the T_c is increased by ~8 K/GPa. The critical current density corresponding to the free flux flow is monotonically enhanced by pressure which is due to the increase in T_c, whereas the depinning critical current density at which the vortex starts to move is more influenced by the pressure-induced magnetic state compared to the increase of T_c. Unlike other high-T_c superconductors, FeSe is not magnetic, but superconducting at ambient pressure. Above a critical pressure where magnetic state is induced and coexists with superconductivity, the depinning J_c abruptly increases even though the increase of the zero-resistivity T_c is negligible, directly indicating that the flux pinning property compared to the T_c enhancement is a more crucial factor for an achievement of a large J_c. In addition, the sharp increase in J_c in the coexisting superconducting phase of FeSe demonstrates that vortices can be effectively trapped by the competing antiferromagnetic order, even though its antagonistic nature against superconductivity is well documented. These results provide new guidance toward technological applications of high-temperature superconductors.

Prediction of polaronlike vortices and a dissociation depinning transition in magnetic superconductors: the example of ErNi2B2C

In borocarbide ErNi2B2C, the phase transition to the commensurate spin density wave at 2.3 K leaves 1/20 part of Ising-like Er spins practically free. Vortices polarize these spins nonuniformly and repolarize them when moving. At a low spin relaxation rate and at low bias currents, vortices carrying magnetic polarization clouds become polaronlike and their velocities are determined by the effective drag coefficient, which is significantly bigger than the Bardeen-Stephen (BS) one. As current increases, at a critical current J(c) vortices release polarization clouds and the velocity as well as the voltage in the I-V characteristics jump to values corresponding to the BS drag coefficient. The nonuniform components of the magnetic field and magnetization drop as velocity increases, resulting in weaker polarization and discontinuous dynamic dissociation depinning transition. As current decreases, on the way back, vortices are retrapped by polarization clouds at the current J(r)<J(c).

Hysteretic vortex pinning in superconductor-ferromagnet nanocomposites

This Letter reports the observation of hysteresis in the vortex pinning in a superconductor-ferromagnetic epitaxial nanocomposite consisting of fcc Gd particles incorporated in a Nb matrix. We show that this hysteretic pinning is associated with magnetic reversal losses in the Gd particles and is fundamentally different in origin to pinning interactions previously observed for ferromagnetic particles or other microstructural features.

Local superconducting density of States of ErNi2B2C

We present local tunneling microscopy and spectroscopy measurements at low temperatures in single crystalline samples of the magnetic superconductor ErNi2B2C . The electronic local density of states shows a striking departure from s-wave BCS theory with a finite value at the Fermi level, which amounts to half of the normal phase density of states.