Visualization of vortex chains in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+y}$ by magneto-optical imaging

We visualized vortex chains in the crossing-lattices state in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+y}$ by using magneto-optical imaging technique. From the spacing of vortex chains, we could determine the anisotropy parameter quantitatively, from $\gamma =800\mathrm{}\pm 80$ for a slightly underdoped sample to $\gamma =490\mathrm{}\pm 50$ for slightly overdoped samples. Vortex chains can be rotated by changing the direction of in-plane field in the ${\mathrm{CuO}}_2$ plane. Hysteretic motion and the forking of vortex chains are discussed in relation to the indirect pinning of Josephson vortices through pinning of pancake vortices.