Nonhomogeneous magnetization and superconductivity in superconductor-ferromagnet structures

TL;DR

This paper investigates how nonhomogeneous magnetization affects superconductivity in superconductor-ferromagnet structures, revealing enhanced Josephson current and long-range triplet superconducting correlations that increase conductance below T_c.

Contribution

It introduces new insights into the effects of magnetization inhomogeneity on superconducting properties and the emergence of long-range triplet components in S/F structures.

Findings

Josephson critical current increases with exchange field in antiparallel magnetization.

Long-range triplet component penetrates ferromagnet over a significant length.

Conductance in ferromagnet increases below T_c due to triplet correlations.

Abstract

We study two different superconductor-ferromagnet (S/F) structures. We consider first a Josephson junction which consists of two S/F bilayers separated by an insulating layer. We show that for an antiparallel alignment of the magnetization in the two F layers the Josephson critical current $I_c$ increases with increasing exchange field $h$. The second system we consider is a S/F structure with a local inhomogeneity of the magnetization in the ferromagnet near the S/F interface. Due to the proximity effect not only a singlet but also a triplet component of the superconducting condensate is induced in the ferromagnet. The latter penetrates over the length $\sqrt{D/\epsilon}$ ($D$ is the diffusion coefficient and $\epsilon$ the energy). In the case of temperatures of the order of the Thouless energy this length is comparable to the length of the ferromagnet. This long-range penetration leads to a significant increase of the ferromagnet conductance below the superconducting critical temperature $T_c$. Contrary to the case of the singlet component, the contribution to the conductance due to the odd triplet component is not zero at $T = 0$ and $V = 0$ ($V$ is the voltage) and decays with increasing temperature T in a monotonic way