Vortex pinning in layered organic superconductors: κ-(BEDT-TTF)2Cu[N(CN)2]Br

Magnetization studies on organic single-crystal superconductors of κ-(BEDT-TTF)2Cu[N(CN)2]Br with the field H parallel to the b axis (perpendicular to the conducting plane) show anomalous field and temperature dependence of vortex pinning in the mixed state. At high temperatures, the magnetization M decays with increasing field with a power-law dependence. The normalized relaxation rate S=d(lnM)/d(lnt) decreases monotonically with H. At low temperatures (T<7 K), a change of sign in the curvature of M(H) is observed, accompanied by a corresponding change in S as a function of H. Measurement of the irreversibility field Hrev shows a universal power-law dependence of Hrev on (1-T/Tc) in the temperature range investigated. We suggest that the magnetic anomaly observed is due to a dimensional crossover in the nature of vortex pinning. copyright 1996 American Institute of Physics