We use nanocontact spin-torque oscillators (STNOs) to explore the applied field and current dependence of magnetic droplets. These dissipative solitons are found in
magnetic layers with perpendicular anisotropy, and they are inherently dynamic and characterized by a core of reversed spins surrounded by a precessing perimeter [1, 2, 3, 4]. The
precession frequency lies between the ferromagnetic and Zeeman resonances, but the droplet is also prone to drift which gives additional dynamics [5, 6]. Orthogonal pseudo-spin-valves
are usually used in experiments to harvest a strong magnetoresistive dynamic signal [2, 3, 5, 6], but here we use an all-perpendicular layout to study the otherwise inaccessible low field
region.
Electrical measurements reveal that the droplet transforms, freezes, into a static bubble at low fields. Figure 1 illustrates the electrical characteristics of the different states. The resistance is
similar for the droplet and the bubble (Fig. 1a), but the difference in noise level is striking (Fig. 1b) and demonstrates the bubble’s static nature. Once formed, the bubble is stable without a
sustaining current. Furthermore, the droplet-to-bubble transition is fully reversible, and the bubble can thaw back to a droplet at sufficient high field and current. The findings are
corroborated by X-ray microscopy, which images the magnetic states during the freezing. Experimental data together with simulations identify pinning as the main mechanism behind the
bubble stability. 
**References** 
[1] Hoefer et al., Theory for a dissipative droplet soliton excited by a spin torque nanocontact Phys. Rev. B 82, 054432 (2010) 
[2] Mohseni et al., Spin Torque–Generated Magnetic Droplet Solitons Science 339, 1295 (2013) 
[3] Chung et al., Spin transfer torque generated magnetic droplet solitons (invited) J. Appl. Phys 115, 172612 (2014) 
[4] Chung et al., Direct Observation of Zhang-Li Torque Expansion of Magnetic Droplet Solitons Phys. Rev. Lett. 120, 217204 (2018) 
[5] Xiao et al., Parametric autoexcitation of magnetic droplet soliton perimeter modes Phys. Rev. B 95, 024106 (2017) 
[6] Chung et al, Magnetic droplet nucleation boundary in orthogonal spin-torque nano-oscillators Nature Com. 7, 11209 (2016) 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/bcccafe9855d49eedd92e0414da5c481.png) 
Figure 1: Phase diagrams based on the resistance and the microwave noise. (a) STNO resistance and (b) integrated (0–0.5 GHz) microwave noise level as a function of field and current. (c)
shows the noise level in (b) overlaid on the resistance in (a) displayed using a gray scale highlighting intermediate resistance levels indicative of droplets/bubbles. The parallel (P) and
antiparallel (AP) states are easily discernible in the MR-map (a) as dark blue and dark red, while both the droplet and the bubble are characterized by intermediate resistance in green–
yellow. The stark difference between the droplet and the bubble is revealed in the noise spectrum (b), where the stability of the bubble is manifested. Note however that the light-blue
flanges in (a) correspond to a different droplet regime not captured in the microwave signal presented in (b).

Freezing and thawing magnetic droplet solitons

X-ray circular magnetic dichroism (XMCD) is a powerful tool for highly-sensitive and element-specific investigations of magnetic materials. This effect can be utilized in Scanning Transmission X-ray Microscopy (STXM) in the soft X-ray range for magnetic imaging at spatial resolutions down to 20 nm. When combined with a pump-probe excitation and single-photon detection scheme, magneto-dynamic imaging can be achieved with a stroboscopic time-resolution of the order of 20 ps. These capabilities have been widely used in the field of magnetism, including vortex core dynamics [1], magnonics in real-space on the scale of tens of nm[2,3,4] and room temperature skyrmionics [5,6] As a photon-based technique, STXM is very tolerant to the sample environments and allows for diverse detection methods. For example, the use of strong magnetic fields can be combined with surface sensitive electron yield imaging, as well as with helium cryostats to provide sample temperatures down to 20K at multi-GHz bandwidths for dynamic excitation. Examples for research utilizing these features will be presented, as well as recent developments such as CCD-Ptychography for super-resolution or using a high- repetition-rate laser as an optical pump source for time-resolved measurements. As an outlook, it will be discussed how STXM can profit from the current developments in high-brilliance x-ray sources.

**References:**

[1] Van Waeyenberge et al., Nature 444, 461 (2006). [2] S Wintz et al., Nat. Nanotech. 11, 948 (2016). [3] V. Sluka et al., Nat. Nanotech. 14, 328 (2019). [4] N. Träger et al., Phys. Status Solidi RRL , 14: 2000373 (2020). [5] S Woo et al., Nat. Mater. 15, 501 (2016). [6] K Litzius et al., Nat. Phys. 13, 170 (2017). [7] S. Mayr et al. (unpublished) (2020).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/ab4ee7588fc632d843781219a2c096de.jpg)

Schematics and time-resolved STXM image (normalized snapshot) revealing the coherent flow of magnons in a ferrimagnetic insulator [7]

Advanced Scanning Transmission X-Ray Microscopy for the Investigation Magnetic Phenomena

Spin waves are collective excitations of a spin system, where the magnetic moments precess with a spatially periodic phase shift that determines the wavelength [1]. From a technological point of view, the use of spin waves as signal carriers in future spintronic logic and memory devices has the advantages of a potentially lower power consumption and improved miniaturization prospects compared to the present charge-based CMOS technology [2]. For achieving nanoscale integration, topological spin textures such as magnetic vortex cores can be driven by alternating magnetic fields to excite coherent spin waves with short, sub-micrometer wavelengths, as shown recently for both synthetic ferrimagnetic systems and single ferromagnetic layers [3, 4]. In this contribution, we will demonstrate the versatility of this excitation mechanism by showing that the application of a static magnetic bias field makes the vortex core a manipulable spin-wave source that can be continuously translated in space without altering the resulting spin-wave dispersion relation [5]. We employ time-resolved scanning transmission x-ray microscopy (TR-STXM) to directly image vortex-core spin textures as well as the emission and propagation of spin waves from them. By applying a static in-plane magnetic bias field, we can continuously displace the vortex core from its equilibrium position in the center of a structured element. For a synthetic ferrimagnet (Co/Ru/Ni81Fe19), it is possible to shift the core relatively close to the edge while maintaining a radial, isotropic spin-wave emission pattern (see Figure 1). The wavelengths of the excited spin waves are not noticeably changed by the applied magnetic bias field, but remain tunable by the driving frequency in the same way as without bias field. For a single Ni81Fe19 layer with a thickness of the order of 100 nm, we observe a similar behavior for relatively small magnetic bias fields but when the vortex core approaches the edge of the disk, it also expands (see Figure 2). This expansion can be seen as a transformation from a zero-dimensional point source to a one-dimensional curved object where its shape and extent is controlled by the magnitude of the applied magnetic bias field. Such an expansion leads to a directional emission of the spin waves that propagate away from the core in mainly two opposite directions rather than radially as observed for non-expanded cores. For certain combinations of magnetic bias fields and driving frequencies, focusing effects appear. Such effects provide promising means for a controlled and directional propagation of spin waves without the need for additional patterning or waveguides.

**References:**

[1] A.G. Gurevich and G.A. Melkov, Magnetization Oscillations and Waves. Vol. 1, CRC Press (1996) [2] A.V. Chumak et al., Nat. Phys. Vol.11, p.453-461 (2015) [3] S. Wintz et al. Nat. Nanotechnol. Vol.11, p.948-953 (2016) [4] G. Dieterle et al., Phys. Rev. Lett. Vol.122, p.117202 (2019) [5] S. Mayr et al., Nano Lett. accepted (2021)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/1ab356e578b624eb1b59b9aeeb5b4f0a.jpg)
Fig. 1: TR-STXM snapshots of a synthetic ferrimagnet, taken with out-of-plane magnetic sensitivity at the Fe L3 edge at a magnetic bias field of 5 mT. (a) Absolute absorption image showing the black vortex core shifted from the center of the structure (green circle). (b) Normalized contrast displaying the radial spin-wave emission from the core for a 1 GHz alternating field excitation.

![](https://assets.underline.io/uploads/markdown_image/1/image/2d254de94235b1183f441f680c4fed5a.jpg)
Fig. 2: TR-STXM images with out-of-plane sensitivity displaying the response of the vortex in a Ni81Fe19 layer to a magnetic bias field of 22.5 mT. (a) Absolute absorption image showing the expansion of the vortex core (encircled in green). (b) Normalized image displaying only the temporal changes with respect to the average magnetization, showing the spin waves emitted from the expanded vortex core in two directions for an excitation frequency of 8.6 GHz.

 Spin-wave emission from vortex cores under static magnetic bias fields

GOE: Magnetization Dynamics V

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Freezing and thawing magnetic droplet solitons

Advanced Scanning Transmission X-Ray Microscopy for the Investigation Magnetic Phenomena

Spin-wave emission from vortex cores under static magnetic bias fields

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