  We demonstrate how magnetic droplet soliton pairs, nucleated by two separated
nano-contact (NC) spin torque oscillators, can merge into a single droplet
soliton. A detailed description of the magnetization dynamics of this merger
process is obtained by micromagnetic simulations: A droplet pair with a
steady-state in-phase spin precession is generated through the spin-transfer
torque effect underneath two separate NCs, followed by a gradual expansion of
the droplets volume and the out phase of magnetization on the inner side of the
two droplets, resulting in the droplets merging into a larger droplet. This
merger occurs only when the NC separation is smaller than a critical value. A
transient breathing mode is observed before the merged droplet stabilizes into
a steady precession state. The precession frequency of the merged droplet is
lower than that of the droplet pair, consistent with its larger size. Merged
droplets can again break up into droplet pairs at high enough magnetic field
with a strong hysteretic response.
