Anisotropy of spin-polarized transport in a ferromagnet/d-wave superconductor bilayer: Role of the small exchange field

Abstract

The current-voltage characteristic of a ferromagnet/d-wave superconductor (FD) bilayer is calculated as a function of the orientation of the superconductor crystallographic axes relative to the FD boundary by solving the spin-dependent Bogoliubov-de Gennes equations. We found that, regardless of the superconductor orientation, the conductance G(V) always reaches a maximum when the bias voltage matches the exchange field in the ferromagnet, eV = h, providing that the exchange field is larger than the order parameter δ. For small exchange fields, conductance G(V), and zero-bias conductance G(0) in particular, is more sensitive to the rotation of the superconductor crystallographic axis. Zero-bias conductance (ZBC) is isotropic in two limiting cases: when the exchange field in the ferromagnet is strong enough (h ≫ δ), and in the opposite case of normal metal (h = 0) at T = 0. In between these two extreme cases the low-Temperature ZBC is anisotropic, and the anisotropy is most pronounced for h ≈ δ. The maximum of ZBC always occurs when the direction of the gap node is perpendicular to the FD boundary.