Scanning Tunneling Spectroscopy Study of Bi-Layer La$_{0.7}$Ca$_{0.3}$MnO$_3$/YBa$_2$Cu$_3$O$_{7-\delta}$ Thin Films*

Recent experiments have reported long-range proximity effect in ferromagnet/superconductor (F/S) heterostructures comprised of transition-metal perovskites. To look for direct evidence of this effect, we have performed scanning tunneling spectroscopy (STS) on La$_{0.7}$Ca$_{0.3}$MnO$_3$/YBa$_2$Cu$_3$O$_{7-\delta}$ (LCMO/YBCO) bi-layer thin films. The bi-layer films were epitaxially grown on $<$001$>$ SrTiO$_{3}$ substrates using pulsed laser deposition. STS measurements were made at 4.2 K on bi-layer films with varying LCMO thickness ($\sim$20 to 60 nm) and in a magnetic field applied parallel to the film. The STS data were analyzed for spectral signatures of a pairing gap on the LCMO layer induced by the YBCO layer, to determine the length scale of the F/S proximity effect and the role played by magnetic domain walls.