Direct observation of magnetic droplet solitons in all-perpendicular spin torque nano-oscillators

TL;DR

This study demonstrates the direct observation and nucleation of magnetic droplet solitons in all-perpendicular spin torque nano-oscillators under modest fields, using advanced microscopy techniques to reveal their size and structure.

Contribution

It provides the first direct imaging and electrical characterization of magnetic droplets in all-perpendicular NC-STNOs, expanding understanding of their formation and properties.

Findings

Magnetic droplets can be nucleated in all-perpendicular NC-STNOs with modest fields.

Droplet diameter is approximately 150 nm, nearly twice the nano-contact size.

The droplet perimeter width is about 70 nm, with a fully reversed core observed.

Abstract

Magnetic droplets are non-topological dynamical solitons that can be nucleated and sustained in nano-contact based spin torque nano-oscillators (NC-STNOs) with perpendicular anisotropy free layers. While originally predicted in all-perpendicular NC-STNOs, all experimental demonstrations have so far relied on orthogonal devices with an in-plane polarizing layer that requires a strong magnetic field for droplet nucleation. Here, we instead show the nucleation and sustained operation of magnetic droplets in all-perpendicular NC-STNOs in modest perpendicular fields and over a wide range of nano-contact size. The droplet is observed electrically as an intermediate resistance state accompanied by broadband low-frequency microwave noise. Using canted fields, which introduce a non-zero relative angle between the free and fixed layer, the actual droplet precession frequency can also be determined. Finally, the droplet size, its perimeter width, and its fully reversed core are directly observed underneath a 80 nm diameter nano-contact using scanning transmission x-ray microscopy on both the Ni and Co edges. The droplet diameter is 150 nm, i.e. almost twice the nominal size of the nano-contact, and the droplet has a perimeter width of about 70 nm.