Signature of a crossed Andreev reflection effect (CARE) in the magnetic response of YBCO junctions with the itinerant ferromagnet SrRuO_3

Abstract

Magnetic properties of SFS and SF ramp-type junctions with $YBa_2Cu_3O_{7-δ}$ (YBCO) electrodes (S), and the itinerant ferromagnet $SrRuO_3$ (SRO - F), were investigated. We looked for a crossed Andreev reflection effect (CARE) in which an electron from one magnetic domain in F is Andreev reflected as a hole into an adjacent, oppositely polarized, domain while a pair is transmitted into S. CARE is possible in SRO since the width of its domain walls is of the order of the YBCO coherence length (2-3 nm). Our junctions behave as typical magnetic tunneling junctions, as the conductance spectra were always asymmetric, and a few showed bound state peaks at finite bias that shifted with field according to the classical Tedrow and Meservey theory. In many of our SFS junctions with a barrier thickness of 10-20 nm, a prominent zero bias conductance peak (ZBCP) has been observed. This peak was found to decrease \textit{linearly} with magnetic field, as expected for Andreev and CARE scattering. In contrast, in SF junctions, the observed ZBCP was found to decrease versus field almost \textit{exponentially}, in agreement with the Anderson-Appelbaum theory of scattering by magnetic states in F. Thus, transport in our SFS and SF junctions depends strongly on the size of the F layer. We also found that in both cases, the ZBCP height at zero field decreased with increasing magnetic order of the domains in F, in agreement with the CARE mechanism.