AC- and DC-driven noise and I-V characteristics of magnetic nanostructures

TL;DR

This paper investigates the effects of combined AC and DC voltages on the noise and I-V characteristics of a ferromagnetic nanostructure, revealing frequency-dependent noise and nonlinear current corrections due to nonequilibrium magnetization dynamics.

Contribution

It introduces a theoretical framework for analyzing noise and current behavior in ferromagnetic nanostructures under combined AC and DC drives, including derivation of Langevin dynamics and current corrections.

Findings

Noise exhibits explicit frequency dependence.

Nonequilibrium magnetization dynamics cause linear and nonlinear current corrections.

Current-voltage characteristics are calculated to order 1/S^2.

Abstract

We study a structure consisting of a ferromagnetic (F) layer coupled to two normal metal (N) leads. The system is driven out of equilibrium by the simultaneous application of external dc and ac voltages across the N/F/N structure. Using the Keldysh diagrammatic approach, and modeling the ferromagnet as a classical spin of size S >> 1, we derive the Langevin equation for the magnetization dynamics and calculate the noise correlator. We find that the noise has an explicit frequency dependence in addition to depending on the characteristics of the ac and dc drive. Further, we calculate the current-voltage characteristics of the structure to O(1/S^2) and find that the nonequilibrium dynamics of the ferromagnetic layer gives rise to corrections to the current that are both linear and nonlinear in voltage.