Critical current of a Josephson junction containing a conical magnet

Abstract

We calculate the critical current of a superconductor/ferromagnetic/superconductor (S/FM/S) Josephson junction in which the FM layer has a weakened conical magnetic structure composed of an in-plane rotating antiferromagnetic phase and an out-of-plane ferromagnetic component. In view of the realistic electronic properties and magnetic structures that can be formed when conical magnets such as Ho are grown with a polycrystalline structure in thin-film form by methods such as direct current sputtering and evaporation, we have modeled this situation in the dirty limit with a large magnetic coherence length (${\ensuremath{\xi}}_{f}$). This means that the electron mean free path is much smaller than the normalized spiral length $\ensuremath{\lambda}/2\ensuremath{\pi}$ which in turn is much smaller than ${\ensuremath{\xi}}_{f}$ (with $\ensuremath{\lambda}$ as the length a complete spiral makes along the growth direction of the FM). In this physically reasonable limit we have employed the linearized Usadel equations: we find that the triplet correlations are short ranged and manifested in the critical current as a rapid oscillation on the scale of $\ensuremath{\lambda}/2\ensuremath{\pi}$. These rapid oscillations in the critical current are superimposed on a slower oscillation which is related to the singlet correlations. Both oscillations decay on the scale of ${\ensuremath{\xi}}_{f}$. We derive an analytical solution and also describe a computational method for obtaining the critical current as a function of the conical magnetic layer thickness.