Freezing and thawing magnetic droplet solitons

TL;DR

This paper demonstrates the reversible transformation of dynamic magnetic droplets into static nanobubbles by adjusting the magnetic field, revealing new stability properties and potential for microwave signal applications.

Contribution

It introduces a method to freeze and thaw magnetic droplets into stable nanobubbles, showing the role of pinning and magnetic field control in their stability.

Findings

Bubbles are stabilized by pinning and magnetic field adjustments.

The transition between droplets and bubbles is reversible.

Bubbles remain stable for days without external drive.

Abstract

Magnetic droplets are non-topological magnetodynamical solitons displaying a wide range of complex dynamic phenomena with potential for microwave signal generation. Bubbles, on the other hand, are internally static cylindrical magnetic domains, stabilized by external fields and magnetostatic interactions. In its original theory, the droplet was described as an imminently collapsing bubble stabilized by spin transfer torque and, in its zero-frequency limit, as equivalent to a bubble. Without nanoscale lateral confinement, pinning, or an external applied field, such a nanobubble is unstable, and should collapse. Here, we show that we can freeze dynamic droplets into static nanobubbles by decreasing the magnetic field. While the bubble has virtually the same resistance as the droplet, all signs of low-frequency microwave noise disappear. The transition is fully reversible and the bubble can be thawed back into a droplet if the magnetic field is increased under current. Whereas the droplet collapses without a sustaining current, the bubble is highly stable and remains intact for days without external drive. Electrical measurements are complemented by direct observation using scanning transmission x-ray microscopy, which corroborates the analysis and confirms that the bubble is stabilized by pinning.