Suppression of Superconductivity in Bi₂Sr₂CaCu₂O_8+x due to Proximity Effect from Fe₅GeTe₂

We present experimental results on superconductor-ferromagnet junctions made using layered van der Waals materials which have high transition temperatures. Bi₂Sr₂Ca₁Cu₂O_8+x (BSCCO) is a high-temperature type-II superconductor (T_SC ~ 85 to 90 K) with a pseudo-gap of 40 meV, even in a monolayer, and a large critical field. The two-dimensional superconductivity of BSCCO resides in its Cu-O planes. Fe₅GeTe₂ (FGT) is a ferromagnet metal with transition temperature (T_FM = 270~330 K) and hosts complex spin textures. By stacking bulk flakes (50 to 70 nm thickness) of FGT on top of BSCCO in the cross-bar geometry and fabricating devices with several contacts, we can measure the electrical properties of the capped FGT/BSCCO and uncapped BSCCO regions in the same device. As a control, we introduce a thin layer (<3 nm) of hexagonal boron nitride (h-BN) to act as a diffusion barrier and increase the separation between FGT and BSCCO. In this talk, we discuss the transport results from these devices, cross-sectional TEM and EELS analysis, micro-XPS depth profiles, and theoretical modeling to support our findings.