Anisotropy of the first critical field and critical current in YBa 2Cu 3O 6.9 single crystals

Abstract

Temperature dependences of the first critical field H c1( T) are studied in YBa 2Cu 3O 6.9 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 √ M 1→0. The temperature behaviours of H c1 have been investigated for both H • c and H ⊤ c . For H ⊤ c , H c1 ( T) exhibits a normal temperature dependence with a low temperature saturation value H ⊤ c c1 (0)=230±20 Oe, in agreement with previously published data. For H • 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 • c c1(0)=950±200 Oe. The origin of the H • c c1( T) anomalies is discussed. The magnetization hysteresis loops M( H) were measured in fields up to 50 kOe at different temperatures. For both H• c and H ⊤ 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/T 0), with parameter values j c1(0)=0.81X10 6 A/cm 2, T 0=40 K and j c2(0)=1.4X10 5 A/cm 2, T 0=27 K for H • c and H ⊤ c , respectively.