Dependence of thermal activation energy on thickness and magnetic field in (Pr/sub 0.6/Y/sub 0.4/)Ba/sub 2/Cu/sub 3/O/sub 7//YBa/sub 2/Cu/sub 3/O/sub 7//(Pr/sub 0.6/Y/sub 0.4/)Ba /sub 2/Cu/sub 3/O/sub 7/ trilayers

We have studied the activation energy of thermally activated vortex motion using (Pr/sub 0.6/Y/sub 0.4/)Ba/sub 2/Cu/sub 3/O/sub 7//YBa/sub 2/Cu/sub 3/O/sub 7//(Pr/sub 0.6/Y/sub 0.4/)Ba /sub 2/Cu/sub 3/O/sub 7/ trilayer structures. The dependence of the activation energy on the thickness of the YBa/sub 2/Cu/sub 3/O/sub 7/ layer and magnetic field was studied for YBa/sub 2/Cu/sub 3/O/sub 7/ layer thicknesses or 1.2/spl sim/120 nm and fields up to 7 T. We obtain a longitudinal vortex correlation length of 15/spl sim/20 mm in YBa/sub 2/Cu/sub 3/O/sub 7/. When the YBa/sub 2/Cu/sub 3/O/sub 7/ layer is a few unit-cells thick, the activation energy shows a logarithmic field dependence. In the thickness range between 10 to 30 nm, the dependence of the activation energy on magnetic field shows a crossover behavior from logarithmic dependence at low fields to one of power law, U/sub o//spl sim/H/sup -/spl alpha// (with /spl alpha//spl sim/0.5), at high fields. This crossover field does not depend on the thickness of the YBa/sub 2/Cu/sub 3/O/sub 7/ layer.<>