  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.
