  Magnetic droplet solitons are dynamical magnetic textures that form due to an
attractive interaction between spin waves in thin films with perpendicular
magnetic anisotropy. Spin currents and the spin torques associated with these
currents enable their formation as they provide a means to excite
non-equilibrium spin wave populations and compensate their decay. Recent years
have seen rapid advances in experiments that realize and study magnetic
droplets. Important advances include the first direct x-ray images of droplets,
determination of their threshold and sustaining currents, measurement of their
generation and annihilation time and evidence for drift instabilities, which
can limit their lifetime in spin-transfer nanocontacts. This article reviews
these studies and contrasts these solitons to other types of spin-current
excitations such as spin-wave bullets, and static magnetic textures, including
magnetic vortices and skyrmions. Magnetic droplet solitons can also serve as
current controlled microwave frequency oscillators with potential applications
in neuromorphic chips as nonlinear oscillators with memory.
