Magnetic droplet nucleation boundary in orthogonal spin-torque nano-oscillators

Abstract

Static and dynamic magnetic solitons play a critical role in applied nanomagnetism. Magnetic droplets, a type of non-topological dissipative soliton, can be nucleated and sustained in nanocontact spin-torque oscillators with perpendicular magnetic anisotropy free layers. Here, we perform a detailed experimental determination of the full droplet nucleation boundary in the current–field plane for a wide range of nanocontact sizes and demonstrate its excellent agreement with an analytical expression originating from a stability analysis. Our results reconcile recent contradicting reports of the field dependence of the droplet nucleation. Furthermore, our analytical model both highlights the relation between the fixed layer material and the droplet nucleation current magnitude, and provides an accurate method to experimentally determine the spin transfer torque asymmetry of each device.

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.…”

“…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. Their approximation for the onset current as a function of the applied field summarizes the previous discussion; its derivation is based on the Slonczewski critical current condition [30] in a material with PMA and consists of a term proportional to the applied field that dominates at large fields and another that is inversely proportional to the same field that dominates at lower field values,…”

“…Representative high-frequency noise spectra versus dc current of our samples at room temperature are shown in Appendix A. A low-frequency signal with a characteristic time scale of hundreds of megahertz has been recently measured [21,23] in droplet states and has been associated with center-of-mass droplet motion, such as the droplet drifting outside the nanocontact region.…”

“…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].…”