Field induced 3D to 2D crossover of shielding current path in ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{\mathit{x}}$

Magnetization curves of high quality ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{\mathit{x}}$ single crystals in fields slightly misaligned with the ab plane show a peak at small fields (6 Oe at 5 K for a misalignment angle of 6°). This is attributed to flux penetration into the Josephson coupling between the ${\mathrm{CuO}}_2$ layers and a resultant three- to two-dimensional crossover of the shielding current path with the increase of the field. A surface barrier model for the interlayer flux penetration yields the anisotropy ratio γ of ≊700.