Freezing and thawing magnetic droplet solitons

Abstract

Magnetic droplets are non-topological magnetodynamical solitons displaying a wide range of complex dynamic phenomena with potential for microwave signal generation. Bubbles, on the other hand, are internally static cylindrical magnetic domains, stabilized by external fields and magnetostatic interactions. In its original theory, the droplet was described as an imminently collapsing bubble stabilized by spin transfer torque and, in its zero-frequency limit, as equivalent to a bubble. Without nanoscale lateral confinement, pinning, or an external applied field, such a nanobubble is unstable, and should collapse. Here, we show that we can freeze dynamic droplets into static nanobubbles by decreasing the magnetic field. While the bubble has virtually the same resistance as the droplet, all signs of low-frequency microwave noise disappear. The transition is fully reversible and the bubble can be thawed back into a droplet if the magnetic field is increased under current. Whereas the droplet collapses without a sustaining current, the bubble is highly stable and remains intact for days without external drive. Electrical measurements are complemented by direct observation using scanning transmission x-ray microscopy, which corroborates the analysis and confirms that the bubble is stabilized by pinning.

Field-Free High-Frequency Exchange-Spring Spin-Torque Nano-Oscillators

“…It is noteworthy, that the slope of the resistance vs field has now changed sign compared to the t-AP state, consistent with the reversed magnetization of the droplet. In contrast to the droplet forming in the high-field P state (Figure a), where the all-perpendicular symmetry of the state cancels out the droplet microwave signal, ,, here, the microwave signal from the droplet is directly observed, again confirming the low-field tilted state of the Co layer with an in-plane component of its magnetization. , Finally, the droplet again comes with low-frequency microwave noise, as it moves around in the NC region.…”

“…At stronger negative current ( I dc = −10 mA; red curve), the P state is replaced by an intermediate resistance state (the t-AP state remains unaffected), indicative of a partially reversed core in the free layer due to the STT, consistent with the nucleation of a free layer magnetic droplet soliton (“droplet”, from hereon) observed in all-PMA STNOs . This explanation is directly corroborated by the intense low-frequency microwave noise appearing at high fields which is strong evidence of an unstable droplet. , …”

“…10 This explanation is directly corroborated by the intense low-frequency microwave noise appearing at high fields which is strong evidence of an unstable droplet. 10,34 However, in contrast to the all-PMA STNOs, another highfrequency signal shows up with a linear magnetic field dependence. This new auto-oscillating signal is very close to the fitted exchange-spring FMR frequency from Figure 1d and exhibits an identical field dependence.…”

“…Magnetic anisotropies of those ferromagnetic layers are key in determining the operation performances in STNOs. For instance, in STNOs with in-plane magnetic anisotropic (IMA) ferromagnetic layers, the microwave line width and power can be improved dramatically by synchronizing the propagating spin waves. , Nevertheless, a high magnetic field is generally required in these in-plane STNOs to produce a noncollinear magnetic configuration, for the sake of maximizing the current-induced spin torques and detecting the auto-oscillating signals. , Using materials with perpendicular magnetic anisotropy (PMA) as the free layer and/or the reference layer − may be a solution to this issue, where novel dynamic mode (magnetic droplet) can even be generated at a low field regime. , Specifically, STNOs with two IMA free layers and two PMA reference layers were studied theoretically, showing larger precession amplitude and high-frequency field-free operation, promising for developing high-power high-frequency STNOs …”