Abstract
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. This perspective discusses 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.
Highlights
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
Magnetic droplet solitons are a localized region of highly excited spin waves that form in thin films with perpendicular magnetic anisotropy
These objects while intriguing—and potentially useful in information processing—were not possible to realize and study experimentally. This is because they were predicted to occur in materials without magnetic damping and to require a non-equilibrium spin-wave population
Summary
Magnetic droplet solitons are a localized region of highly excited spin waves that form in thin films with perpendicular magnetic anisotropy Until recently, these objects while intriguing—and potentially useful in information processing—were not possible to realize and study experimentally. There are a number of important fundamental characteristics of such droplets They are generated at a threshold current that is determined by the magnetic damping and magnetic field. Their spin-precession frequency is less than the lowest propagating spin-wave modes in the film, the ferromagnetic resonance frequency. An STNO containing a droplet is a current controlled oscillator with oscillation frequencies in the GHz range These are nonlinear oscillators in that droplet characteristics are a nonlinear function of parameters, such as the current and magnetic field. Their particle-like nature, which includes the possibility of droplet interaction and self modulation, together with hysteresis in their response, makes these oscillators of interest in neuromorphic computing, such as for implementing reservoir computing.
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