Anisotropy and twin boundaries in Ba(Fe_{1-x}Co_x)2As2 single crystals

MARZIALI BERMÚDEZ, MARIANO*; PASQUINI, GABRIELA; BUD'KO, SERGEI L.; CANFIELD, PAUL C.

Ciudad Autónoma de Buenos Aires

Workshop; X Latin American Workshop on Magnetism, Magnetic Materials and their Applications; 2013

In type-II superconductors, the magnetic field penetrates in quanta through singularities called vortices. Their dynamics defines in great manner the properties of the superconductor. Defects such as dislocations and impurities provide a pinning source for the vortices, reducing their mobility and increasing the overall critical current. When spatially correlated defects are present (planes, columns, etc.) the pinning force is dependent on the relative orientation between them and the external magnetic field. Therefore, the prevalence of this kind of defects can be determined by means of a study on the transport properties dependence on the aforementioned orientation.*: expositor. Formato: pósterTwin boundaries are planar defects common in materials with slightly orthorhombic structures. In some of the new pnictide superconductors, and particularly among the family Ba(Fe_{1−x}Co_x)2As2, there exists a structural transition from a tetragonal to an orthorhombic phase, which is completely suppressed above a particular doping concentration x ≈ 6.4%. So far, few studies have been carried out about the angular response of these new compounds in relation to the twin boundaries. Although it is accepted that they increase the critical current, it is not clear whether they act as correlated defects or not. In the present work we performed ac susceptibility measurements in Ba(Fe_{1−x}Co_x)2As2 single crystals with x = 6.2% (orthorhombic, with twin boundaries) and x = 6.7%, 7.2% (tetragonal, without twin boundaries), in the vecinity of the superconducting transition, applying an external DC magnetic field of different magnitudes and orientaions. We obtained similar anisotropy factors for either compound, compatible with those reported by other authors using different thechniques. We found qualitative differences in the angular dependence, revealing that twin boundaries would also behave as correlated defects in these new materials.