Miniature Hall-probe arrays were used to measure the critical current densities for the three main directions of vortex motion in the stoichiometric LiFeAs superconductor. These correspond to vortex lines along the $c$ axis moving parallel to the $ab$ plane, and to vortex lines in the $ab$ plane moving perpendicular to and within the plane, respectively. The measurements were carried out in the low-field regime of strong vortex pinning, in which the critical current anisotropy is solely determined by the coherence length anisotropy parameter ${\varepsilon }_{\xi }$. This allows for the extraction of ${\varepsilon }_{\xi }$ at magnetic fields far below the upper critical field $B_{c2}$. We find that increasing the magnetic field decreases the anisotropy of the coherence length.

• Received 29 August 2011

DOI:https://doi.org/10.1103/PhysRevB.84.180514