Wydarzenia

Seminarium Międzynarodowego Laboratorium Silnych Pól Magnetycznych i Niskich Temperatur PAN

Critical Currents Anisotropy and Pinning Mechanism in YBa₂Cu_2.97 Mo_0.03O_x Single Crystals

A. Los, B. Dabrowski, K. Rogacki

International Laboratory of High Magnetic Fields and Low Temperatures,Wroclaw, Poland

Due to high critical temperature (T_c = 93 K) and significant critical current densities (jc > 10¬5 A/cm²) the YBCO is one of the most promising superconductors for applications at liquid nitrogen temperatures. However, this material exhibits large critical current anisotropy γ≈10 resulting from different pinning strengths of vortices for field oriented perpendicular or parallel to the superconducting CuO₂ planes. Moreover, the intrinsic pinning centers present in YBCO become less effective at higher temperatures due to increasing superconducting coherence length [1].

Increase of pinning force in high-T_c superconductors has been previously obtained by several approaches; for example, incorporation of the magnetic nanoparticle impurities [2-5] and proton and neutron irradiation [6]. Most of these methods have, however, crucial disadvantages such as decrease of T_c, deterioration of mechanical properties, health hazards, and more the complicated and expensive production of material. Also, not all of these methods are convenient for large-scale production of superconducting tapes and wires. One of the most promising and relatively easy methods to obtain materials with high critical current density is the optimization of chemical composition, which we propose in this work. For this purpose we have investigated the vortex dynamics and critical current densities of pure and substituted Y123 single crystals.

To enhance pinning force at higher temperatures we have partially substituted Mo for Cu in the chain region of the Y123 crystal structure by creating pinning centers with larger dimension, which have been identified as the dimmers of MoO₆. The size of distortion, which is produced by such Mo₂O₁₁ clusters, is close to the superconductor’s coherence length near nitrogen boiling temperature. Moreover, these three-dimensional pinning centers are also effective as weakly interacting, randomly distributed extended point-like defects. We will present the results of critical current anisotropy investigation as derived by the Bean critical state model and the Dew-Hughes pinning mechanism analysis [1]. The results obtained clearly demonstrate that the critical current densities in Y123 can be considerably increased and the anisotropy reduced by novel substitution methods and annealing’s, which create intrinsic nano-sized defects in the right setting of the crystals structure [7].

Acknowledgements: Work at the International Laboratory was supported by the ERA.Net RUS Plus project #146 MAGNES. Work at NIU was supported by the Institute for Nanoscience, Engineering, and Technology – InSET.

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