Transport and Microscopy Studies in High-T_c Superconductor-Ferromagnet Junctions

We present results on magneto-transport and magneto-microscopy of superconductor-ferromagnet junctions made using layered van der Waals materials which have high transition temperatures. Bi₂Sr₂Ca₁Cu₂O_8+x (BSCCO) is a high-temperature, d-wave superconductor (T_SC ~ 85 K) with a pseudo-gap of 40 meV, even in a monolayer. The superconductivity is two-dimensional (2D), residing in the Cu-O planes. Fe₅GeTe₂ (FGT) is a ferromagnet metal with transition temperature T_FM = 270~330 K. We measure the FM-I-SC tunneling transport of FGT-BSCCO junctions achieved by using the insulating underdoped top layers of BSCCO as the tunnel barrier. We find the expected zero-bias peak in high-barrier junctions with d-wave superconductors. We also explore ways to inject spin polarized electrons into a d-wave 2D superconductor, as an extension of the seminal work by P. M. Tedrow and R. Meservey 1973 PRB. Lastly, we show the ferromagnetic proximity effects of FGT on BSCCO that are characterized by magneto-transport and Kerr microscopy. Our work informs the pursuit for novel quantum excitations, such as Yu-Shiba-Rusinov and Majorana bound states, that can be accessible at high temperatures.