Vortex-antivortex proliferation from an obstacle in thin film ferromagnets

TL;DR

This paper investigates vortex-antivortex pair nucleation in thin film ferromagnets using micromagnetic simulations, revealing phenomena like Mach cones and wavefronts analogous to fluid dynamics and Bose-Einstein condensates.

Contribution

It demonstrates vortex-antivortex nucleation from obstacles in ferromagnets and explores their interactions and flow regimes using a hydrodynamic analogy.

Findings

Vortex-antivortex pairs nucleate from obstacles in ferromagnets.

Supersonic flow (Mach number > 1) produces Mach cones and wavefronts.

Flow behavior aligns with fluid dynamics and Bose-Einstein condensate observations.

Abstract

Magnetization dynamics in thin film ferromagnets can be studied using a dispersive hydrodynamic formulation. The equations describing the magnetodynamics map to a compressible fluid with broken Galilean invariance parametrized by the longitudinal spin density and a magnetic analog of the fluid velocity that define spin-density waves. A direct consequence of these equations is the determination of a magnetic Mach number. Micromagnetic simulations reveal nucleation of nonlinear structures from an impenetrable object realized by an applied magnetic field spot or a defect. In this work, micromagnetic simulations demonstrate vortex-antivortex pair nucleation from an obstacle. Their interaction establishes either ordered or irregular vortex-antivortex complexes. Furthermore, when the magnetic Mach number exceeds unity (supersonic flow), a Mach cone and periodic wavefronts are observed, which can be well-described by solutions of the steady, linearized equations. These results are reminiscent of theoretical and experimental observations in Bose-Einstein condensates, and further supports the analogy between the magnetodynamics of a thin film ferromagnet and compressible fluids. The nucleation of nonlinear structures and vortex-antivortex complexes using this approach enables the study of their interactions and effects on the stability of spin-density waves.