date:MV LOMONOSOV STATE UNIV,DEPT PHYS,HIGH TC SUPERCONDUCT LAB,MOSCOW 117234,USSR
Temperature dependences of the first critical field H(c1)(T) are studied in YBa2Cu3O6.9 single crystal by the onset of the trapped moment. The advantage of this method for H(c1) measurements has been demonstrated in the case of type II superconductors with large critical currents j(c). The trapped magnetic moment M(t), calculated in the framework of the Bean model with j(c)(H) = constant, extended by taking into account reversible magnetization, is proportional to (H(a)-H(c1))2. Then H(c1) can be determined by using the extrapolation square-root M(t) --> 0. The temperature behaviours of H(c1) have been investigated for both H parallel-to c and H perpendicular-to c. For H perpendicular-to c, H(c1)(T) exhibits a normal temperature dependence with a low temperature saturation value H(c1) perpendicular-to c(0) = 230 +/- 20 Oe, in agreement with previously published data. For H parallel-to c, H(c1)(T) demonstrates an almost linear T behaviour without the usual low temperature saturation but with indications of an anomalous onset of a positive curvature below 35 K. The low temperature extrapolated value is H(c1) parallel-to c(0) = 950 +/- 200 Oe. The origin of the H(c1) parallel-to c(T) anomalies is discussed. The magnetization hysteresis loops M(H) were measured in fields up to 50 kOe at different temperatures. For both H parallel-to c and H perpendicular-to c the magnetization, renormalized to M(max) at H = H(max), shows a universal field dependence. The temperature behaviour of critical currents j(c)(T) is exponential, j(c)(T) = j(c)(0) exp(-T/T0), with parameter values j(c1)(0) = 0.81 X 10(6) A/cm2, T0 = 40 K and j(c2)(0) = 1.4 X 10(5) A/cm2, T0 = 27 K for H parallel-to c and H perpendicular-to c, respectively.