Generation and annihilation time of magnetic droplet solitons

Abstract

Magnetic droplet solitons were first predicted to occur in materials with uniaxial magnetic anisotropy due to a long-range attractive interaction between elementary magnetic excitations, magnons. A non-equilibrium magnon population provided by a spin-polarized current in nanocontacts enables their creation and there is now clear experimental evidence for their formation, including direct images obtained with scanning x-ray transmission microscopy. Interest in magnetic droplets is associated with their unique magnetic dynamics that can lead to new types of high frequency nanometer scale oscillators of interest for information processing, including in neuromorphic computing. However, there are no direct measurements of the time required to nucleate droplet solitons or their lifetime–experiments to date only probe their steady-state characteristics, their response to dc spin-currents. Here we determine the timescales for droplet annihilation and generation using current pulses. Annihilation occurs in a few nanoseconds while generation can take several nanoseconds to a microsecond depending on the pulse amplitude. Micromagnetic simulations show that there is an incubation time for droplet generation that depends sensitively on the initial magnetic state of the nanocontact. An understanding of these processes is essential to utilizing the unique characteristics of magnetic droplet solitons oscillators, including their high frequency, tunable and hysteretic response.

Multiple magnetic droplet soliton modes

“…There has been clear evidence for droplet soliton formation in ferromagnetic materials with perpendicular magnetic anisotropy [8][9][10][11][12][13][14][15][16][17], including direct images taken using x-ray microscopy [4,18]. Electrical measurements of voltage across the nanocontact, both ac and dc, provide an understanding of droplet modes due to the giant magnetoresistance effect.…”

Tuning the dynamics of magnetic droplet solitons using dipolar interactions

“…This current density possesses a clear threshold which is higher compared to the auto-oscillation mode (Slonczewski solution) due to the strong nonlinearity of the droplet [19,21]. Moreover, such a nonlinear dynamic requires a short time to develop in the system, named as the nucleation time tn [27,37]. Figure 2(a) presents the nucleation time of the droplet tn with respect to the applied current to the NC, while the thickness of the free layer varies.…”

“…This essentially means that droplets are dissipative solitons since they only sustain if the damping of the system is compensated by the STT (gain). The strong nonlinearity of the droplets given by their very large angle of precession leads to various intriguing nonlinear and instability dynamics [23][24][25][26][27][28][29][30][31]. This also leads to a strong output power compared to similar devices, suggesting the benefits of droplet-STNOs in microwave data processing devices, magnonic and spintronic applications.…”

“…4(c), which demonstrate that the that the ratio of the NC diameter to the droplet effective diameter approached to unity in higher thicknesses. The presence of the Oersted field of the current can lead to several instability processes of the droplet, such as drift resonances, inertial dynamics as well as the excitation of the perimeter modes [25][26][27][28][29][30].…”

Observation of magnetic droplets in magnetic tunnel junctions

“…This was mainly due to the large precession angle of the magnetic droplet mode [7,18,19]. However, all experimental work on magnetic droplets has, until now, focused on spin-valve (SV) structures [18,19,[21][22][23] and spin-hall nano-oscillators (SHNOs) [24,25]. The very low magnetoresistance (MR) (approximately 1%) in SVs and SHNOs limits power emission and any further use in STNO-based applications.…”