Magnetic Field Dependence of the Critical Current of Planar Geometry Josephson Junctions

We report a study on the magnetic field dependence of the critical current of planar geometry Josephson junctions. We have fabricated Josephson junctions by using a focused helium ion beam to irradiate a narrow barrier in the plane of a 25~nm thick Y--Ba--Cu--O film. The London penetration depth $\lambda_L$ is large ($\sim$1~$\mu$m) because of the ultra--thin thickness of the film. As a result, calculations of the Josephson penetration depth $\lambda_J$ are not realistic nor physical. Therefore in this work, we measure $\lambda_J$ experimentally. We tested devices with bridge widths ranging from 4 to 50~$\mu$m, and present measurements of the Fraunhofer quantum diffraction pattern ($I_C(B)$). We observe a crossover from short to long junction behavior, which gives an experimentally measured $\lambda_J$ that ranges between 3~$\mu$m to 5~$\mu$m. The shape of the $I_C(B)$ pattern is strongly affected by the width of the bridge because of self-field effects. As the bridge width increases, Josephson vortices enter the junction and skew the patterns. This work shows that the electronic properties of the planar junctions are very different than those classical ``sandwich'' junctions due to the differences in geometry.