ANGULAR AND TEMPERATURE DEPENDENCES OF THE 1ST CRITICAL-FIELD HC1 AND OF THE VORTICES DIRECTION IN Y1BA2CU3O7

We report investigations of the first critical field Hc1(T,θ) and the magnetization M(H,T,θ) of Y1Ba2Cu3O7 single crystals as a fonction of angle θ between the effective field and the c-axis and as a fonction of temperature. We discuss the uncertainties on the measured Hc1 induced by the size effects of the crystals and by spurious demagnetizing fields due to irregularities in the shape of the sample. When H is oriented away from the symmetry directions, we observe that the vectors M, B and H are disaligned for H ≫ Hc1. © 1990.