Angular dependence of the irreversibility temperature for Y- and Nd-BaCuO

For c- and a-axis YBaCuO films and a NdBaCuO single crystal the irreversibi lity temperature T-irr(B,Theta) has been determined from the third harmonic of the AC-susceptibility in dependence on held (B) and field orientation ( Theta = angle (B, sample normal n)). The sample geometry and the intrinsic anisotropy give rise to the angular dependence of T-irr. The approximately elliptical lines of constant temperature in a plot B-\\ n versus B-perpendi cular to n, deduced from the experiments, are in agreement with a combinati on of the isotropic Ginzburg-Landau (GL) theory for thin films and the anis otropic GL theory. Anisotropic pinning probably due to twin lamellae in the NdBaCuO single crystal is seen as a bulge in the elliptical contour at The ta = 0 degrees (B \\ c-axis \\ n). (C) 2000 Elsevier Science B.V. All right s reserved.