Vortex lock-in transition and evidence for transitions among commensurate kinked vortex configurations in single-layered Fe arsenides.

We report an angle-dependent study of the magnetic torque t(ϑ) within the vortex state of single-crystalline LaO0.9F0.1FeAs and SmO0.9F0.1FeAs as a function of both temperature T and magnetic field H. Sharp peaks are observed at a critical angle ϑc at either side of ϑ=90°, where ϑ is the angle between H and the interplanar c axis. ϑc is interpreted as the critical depinning angle where the vortex lattice, pinned and locked by the intrinsic planar structure, unlocks and acquires a component perpendicular to the planes. We observe a series of smaller replica peaks as a function of ϑ and as ϑ is swept away from the planar direction. These suggest commensurability effects between the period of the vortex lattice and the interplanar distance leading to additional kinked vortex configurations. [ABSTRACT FROM AUTHOR]