THE ANGULAR-DEPENDENCE OF THE IRREVERSIBILITY FIELD OF LA2-XSRXCUO4-DELTA SINGLE-CRYSTALS

The experimental study of La2-xSrxCuO4-delta superconducting single crystals is reported. The magnetization measurements were carried out at T=4.2 K in magnetic fields tilted by an arbitrary angle theta to the crystal c axis. The irreversibility field H(irr), defined by extrapolation of pinning force curves, shows a strong dependence on theta. At 30-degrees < theta < 85-degrees, H(irr) scales as (cos theta)-2/3. At theta close to 90-degrees, the curve crosses over to a steeply decreasing dependence. It is proposed that in the field inclined with respect to the ab plane pinning is due to collective interaction of kinked vortices with c-axis-aligned crystalline defects. When vortices are parallel to the ab plane no kinks are present and pinning is weaker. Simple scaling considerations based on this assumption allow to explain the observed angular dependence and the crossover of the irreversibility field.