  Theoretically, the vortex melting phenomenon occurs at both low and high
magnetic fields at a fixed temperature. While the high field melting has been
extensively investigated in high Tc cuprates, the low field melting phenomena
in the presence of disorder hasn't been well explored. Using bulk magnetization
measurement and high-sensitivity differential magneto-optical imaging
technique, we detect a low-field vortex melting phenomenon in a single crystal
of Ba0.6K0.4Fe2As2. The low field melting is accompanied by a significant
change in local magnetization ~ 3 G, which decreases with increasing applied
field. The observed vortex melting phenomena is traced on a field temperature
phase diagram and which lies very close to theoretically predicted Lindemann
criteria based low field melting line. Our analysis shows a Lindemann number cL
= 0.14 associated with the low field melting. Imaging of low-field vortex
melting features shows the process nucleates via formation of extended
finger-like projections which spreads across the sample with increasing field
or temperature, before entering into an interaction-dominated vortex solid
phase regime. Magnetization scaling analysis shows that the dimensionality of
melting vortex state is close to one. Angular dependence of bulk magnetization
hysteresis loop in our sample shows the presence of extended defects. From our
studies, we propose the sample contains a peculiar geometry of extended defects
arranged in a plane in the sample, with these planes extending through the
sample thickness. In the weak intervortex interaction limit, we argue that
reduced vortex dimensionality due to pinning by these peculiar extended defect
planes strongly enhances thermal fluctuations. It is these extended defects
planes, which we propose are promoting low dimensional vortex melting in the
pnictide system.
