Magnetic field and orientation dependence of vortex motion in YBa₂Cu₃O₇/PrBa₂Cu₃O ₇ multilayers

We have studied the vortex motion in YBa₂Cu₃O₇/PrBa₂Cu₃O ₇ (YBCO/PBCO) multilayers by measuring the magnetic field H and orientation dependence of the average activation energy Ū. The Ū of YBCO(24 Å)/PBCO(96 Å) multilayer system shows a logarithmic H dependence in the field region of 0.1-4 T. The H direction dependence of Ū shows that all Ū versus H sin φ of experimental points fall on the same line, Ū versus H (φ = 90°), in the whole applied field and orientation regions. Here, φ is the angle between H and the film plane under the condition of H ⊥ the transport current. These results suggest two dimensional vortex motion in the YBCO(24 Å)/PBCO(96 Å) multilayers. The deviations from the logH dependence at low fields (H < 0.1 T) and high fields (H > 4 T) are observed. The deviation at low fields is discussed on the basis of the interaction between vortices. The H dependence of Ū seems to follow a power-law behaviour in the high field. This suggests a possibility of coupling between the YBCO layers.