Single vortex manipulation in superconducting NdFeAsO$_{1-x}$F$_x$

Vortex pinning challenges have severely hampered attempts to incorporate cuprate high $T_c$ superconductors into technology. Understanding and improving the pinning of quantized magnetic vortices in high $T_c$ superconductors remains an important challenge. We use a homebuilt low temperature magnetic force microscope to image and manipulate individual vortices in single crystal NdFeAsO$_{1-x}$F$_x$ ($T_c$=50K). By exerting a large force on a single vortex we can deliberately depin it from its original position and permanently move it to a different, predetermined position. We can also drag the top of a single vortex for a short distance without permanently depinning its full length. By dragging individual vortices along different directions, we observe a 4-fold anisotropy of the dragging distance, with the easy-drag direction along the Fe-Fe axis. Our results shed light on the questions of anisotropy and pinning mechanisms in iron pnictides.