Stable magnetic droplet solitons in spin-transfer nanocontacts

Abstract

Magnetic thin films with perpendicular magnetic anisotropy have localized excitations that correspond to reversed, dynamically precessing magnetic moments, which are known as magnetic droplet solitons. Fundamentally, these excitations are associated with an attractive interaction between elementary spin-excitations and have been predicted to occur in perpendicularly magnetized materials in the absence of damping. Although damping suppresses these excitations, it can be compensated by spin-transfer torques when an electrical current flows in nanocontacts to ferromagnetic thin films. Theory predicts the appearance of magnetic droplet solitons in nanocontacts at a threshold current and, recently, experimental signatures of droplet nucleation have been reported. However, to date, these solitons have been observed to be nearly reversible excitations, with only partially reversed magnetization. Here, we show that magnetic droplet solitons exhibit a strong hysteretic response in field and current, proving the existence of bistable states: droplet and non-droplet states. In the droplet soliton state we find that the magnetization in the contact is almost fully reversed. These observations, in addition to their fundamental interest, are important to understanding and controlling droplet motion, nucleation and annihilation.

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.…”

“…Larger fields ultimately destabilize and annihilate the droplet, favoring an alignment of the two layers' magnetization in the direction of the applied field. As we sweep the field down back to zero, the droplet nucleates and annihilates at similar field values as in the sweep up, showing hysteresis in some cases [17,20], which indicates stability of the droplet states (see Appendix B for a sample with pronounced hysteresis).…”

“…The samples we study are nanocontacts (150 nm in nominal diameter) to a magnetic bilayer structure consisting of a free layer (FL) composed of cobalt (Co) and nickel (Ni) with PMA and a polarizing layer (PL) of Ni 80 Fe 20 (permalloy) with in-plane magnetization [20] [see Fig. 1(a)].…”

“…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].…”