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

TL;DR

This study investigates the magnetic response of YBCO/SrRuO3 junctions, providing evidence for crossed Andreev reflection effects that depend on magnetic domain structure and layer thickness.

Contribution

We demonstrate the presence of crossed Andreev reflection (CARE) in YBCO/SrRuO3 junctions, linking it to domain wall width and magnetic order, which is a novel observation in such junctions.

Findings

Zero bias conductance peak decreases linearly with magnetic field in SFS junctions.

ZBCP decreases exponentially with magnetic field in SF junctions.

ZBCP height at zero field decreases with increasing magnetic order.

Abstract

Magnetic properties of SFS and SF ramp-type junctions with $YBa_2Cu_3O_{7-\delta}$ (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.