Symmetry-broken dissipative exchange flows in thin-film ferromagnets with in-plane anisotropy

TL;DR

This paper investigates how in-plane anisotropy affects dissipative exchange flows in thin-film ferromagnets, revealing symmetry-breaking effects, frequency shifts, and efficiency reductions through analytical and micromagnetic analyses.

Contribution

It introduces a dispersive hydrodynamic model for symmetry-broken exchange flows in anisotropic ferromagnets, extending previous symmetric models and providing insights into material parameter optimization.

Findings

Symmetry-breaking causes harmonic overtones and frequency redshift.

Dissipative exchange flows can be excited above a threshold depending on material and channel length.

Micromagnetic simulations confirm analytical predictions and show driven flows in finite regions.

Abstract

Planar ferromagnetic channels have been shown to theoretically support a long-range ordered and coherently precessing state where the balance between local spin injection at one edge and damping along the channel establishes a dissipative exchange flow, sometimes referred to as a spin superfluid. However, realistic materials exhibit in-plane anisotropy, which breaks the axial symmetry assumed in current theoretical models. Here, we study dissipative exchange flows in a ferromagnet with in-plane anisotropy from a dispersive hydrodynamic perspective. Through the analysis of a boundary value problem for a damped sine-Gordon equation, dissipative exchange flows in a ferromagnetic channel can be excited above a spin current threshold that depends on material parameters and the length of the channel. Symmetry-broken dissipative exchange flows display harmonic overtones that redshift the fundamental precessional frequency and lead to a reduced spin pumping efficiency when compared to their symmetric counterpart. Micromagnetic simulations are used to verify that the analytical results are qualitatively accurate, even in the presence of nonlocal dipole fields. Simulations also confirm that dissipative exchange flows can be driven by spin transfer torque in a finite-sized region. These results delineate the important material parameters that must be optimized for the excitation of dissipative exchange flows in realistic systems.