Magnetic droplet solitons in orthogonal nano-contact spin torque oscillators

Abstract

We study microwave signal generation as a function of drive current and applied perpendicular magnetic field in nano-contact spin torque oscillators (NC-STOs) based on orthogonal (pseudo) spin valves where the Co fixed layer has a strong easy-plane anisotropy, and the [Co/Ni] free layer has a strong perpendicular magnetic anisotropy. The orthogonal NC-STOs exhibit a dramatic transition from the conventional ferromagnetic resonance-like spin wave mode to a magnetic droplet soliton mode. In particular, the field and current dependence of the droplet soliton near threshold are discussed. Near threshold the droplet soliton undergoes complex dynamics that include mode hopping, as evident in the experimental frequency domain and magnetoresiatance response.

Magnetic droplet solitons in orthogonal spin valves

“…Although experiments and theory agree at high fields where the fixed layer is saturated out-of-plane [44], the low-field behavior of the droplet nucleation current instead scales well with the perpendicular component of the spin polarization [41]. At intermediate fields both these descriptions appear to break down [42].…”

“…While originally developed to achieve zerofield operation, these STNOs were later shown to be able to nucleate and sustain so-called magnetic droplet solitons [40][41][42][43][44], which are the dissipative analog of the magnon drop solitons suggested in the 1970s [22,24]. While the magnetic droplet is of great fundamental interest in itself, it also promises significant advantages in both microwave and memory applications.…”

Effect of Temperature on Magnetic Solitons Induced by Spin-Transfer Torque

“…The polarization degree depends on the magnetization component of the PL normal to the film plane, and the magnetization is proportional to the applied field. Thus, the onset current has an inverse dependence with the applied magnetic field at low fields, as reported in several experimental studies [19,20,23,29]. Chung et al [23] recently used an expression for the onset current as a function of the field that accounts for the effect of a PL that is not aligned with the FL.…”

“…1(b), we plot a measured magnetoresistance curve [17,19,20] at a fixed current of 27 mA ramping the perpendicular applied field from high to low field at a fixed temperature of 150 K. This shows first the creation (at 1 T) and then the annihilation of the droplet state (0.4 T). Figure 1(c) shows a measured droplet stability map in magnetic field and current space [21,23] built from magnetoresistance curves measured at different applied current at a fixed temperature of 150 K. The no droplet state is plotted in gray and corresponds to a lower resistance state, whereas the green area represents the droplet state, a higher resistance state.…”

“…Dissipative magnetic droplet solitons (droplets hereafter) are nonlinear confined wave excitations consisting of partially reversed precessing spins that can be created in films with perpendicular magnetic anisotropy (PMA) through the local suppression of the magnetic damping [16]. Droplets have been experimentally created using the STT effect in nanocontacts to PMA films [17][18][19][20][21][22][23], and droplets are a particular case of STToscillators. Recently reported experiments have shown that the stability of these collective excitations is limited by the appearance of drift instabilities, which were attributed to the disorderlocal variations of the effective magnetic field [21,24].…”