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 τ(θ) within the vortex state of single-crystalline LaO_0.9F _0.1FeAs and SmO_0.9F_0.1FeAs as a function of both temperature T and magnetic field H. Sharp peaks are observed at a critical angle θ_c at either side of θ=90 ^â̂̃, where θ is the angle between H and the interplanar c axis. θ_c 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.