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 $\tau(\theta)$ within the vortex state of single-crystalline LaO$_{0.9}$F$_{0.1}$FeAs and SmO$_{0.9}$F$_{0.1}$FeAs as a function of both temperature $T$ and magnetic field $H$. Sharp peaks are observed at a critical angle $\theta_c$ at either side of $\theta=90^{\circ}$, where $\theta$ is the angle between $H$ and the inter-planar \emph{c}-axis. $\theta_c$ 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 $\theta$ and as $\theta$ is swept away from the planar direction. These suggest commensurability effects between the period of the vortex lattice and the inter-planar distance leading to additional kinked vortex configurations.
Comment: 5 pages, 5 figures