United States

â€œTheranosticsâ€ by magnetic nanofluid hyperthermia (MNFH) combined with T&lt;sub&gt;2&lt;/sub&gt;-weighted MR contrast imaging (MRI) using superparamagnetic nanoparticles (SPNPs) has been drawn a huge attraction in nanomedicine [1,2]. However, insufficient AC heating power at the biologically safe range of AC magnetic field (&lt;i&gt;H&lt;sub&gt;AC&lt;/sub&gt;&lt;/i&gt;) (&lt;i&gt;H&lt;sub&gt;appl &lt;/sub&gt;&lt;/i&gt;&amp;lt; 190 Oe, &lt;i&gt;f&lt;sub&gt;appl &lt;/sub&gt;&lt;/i&gt;&amp;lt; 120 kHz) and unsatisfactorily low &lt;i&gt;r&lt;sub&gt;2&lt;/sub&gt;&lt;/i&gt;-relaxivity keep SPNPs challenging for cancer theranostics agent applications [3,4].&lt;br/&gt;In this work, innovatively designed and developed AC and DC magnetic softness enhanced dual-doped (Ni&lt;sub&gt;0.6&lt;/sub&gt;Zn&lt;sub&gt;0.4&lt;/sub&gt;)-Î³Fe&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; (MSEÎ³-IO) SPNPs with significantly enhanced both intrinsic loss power (ILP, ~ 4.0 nH m&lt;sup&gt;2 &lt;/sup&gt;kg&lt;sup&gt;-1&lt;/sup&gt;) at the &lt;i&gt;H&lt;sub&gt;appl&lt;/sub&gt;&lt;/i&gt;Ã—&lt;i&gt;f&lt;sub&gt;appl&lt;/sub&gt;&lt;/i&gt; =1.23Ã—10&lt;sup&gt;9&lt;/sup&gt; A m&lt;sup&gt;-1 &lt;/sup&gt;s&lt;sup&gt;-1&lt;/sup&gt; and &lt;i&gt;r&lt;sub&gt;2&lt;/sub&gt;&lt;/i&gt;-relaxivity (&lt;i&gt;r&lt;sub&gt;2&lt;/sub&gt;&lt;/i&gt; = 660.4 mM&lt;sup&gt;-1 &lt;/sup&gt;s&lt;sup&gt;-1&lt;/sup&gt;) are reported. The significantly enhanced ILP, and &lt;i&gt;r&lt;sub&gt;2&lt;/sub&gt;&lt;/i&gt;-relaxivity of dual-doped MSEÎ³-IO SPNPs were primarily attributed to the distinctly enhanced AC magnetic softness directly related to the&lt;i&gt; H&lt;sub&gt;AC&lt;/sub&gt;&lt;/i&gt; absorption and &lt;i&gt;f&lt;sub&gt;appl&lt;/sub&gt;&lt;/i&gt; resonance efficiency, and the DC magnetic softness dominantly controlled by the occupation and spatial distribution of Ni&lt;sup&gt;2+&lt;/sup&gt; in O&lt;sub&gt;h&lt;/sub&gt; vacancies sites, and Zn&lt;sup&gt;2+&lt;/sup&gt; cations in T&lt;sub&gt;d&lt;/sub&gt; sites of Î³-Fe&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;, respectively. The biocompatibility and bioavailability experimentally evaluated by &lt;i&gt;in-vitro&lt;/i&gt; and &lt;i&gt;in-vivo&lt;/i&gt; studies demonstrated that the dual-doped MSEÎ³-IO SPNPs can be a promising candidate for highly efficient cancer theranostics agent for future nanomedicine.

MMM 2022

AC and DC Magnetic Softness Enhanced Dual doped γ

â€œTheranosticsâ€ by magnetic nanofluid hyperthermia (MNFH) combined with T2-weighted MR contrast imaging (MRI) using superparamagnetic nanoparticles (SPNPs) has been drawn a huge attraction in nanomedicine [1,2]. However, insufficient AC heating power at the biologically safe range of AC magnetic field (HAC) (Happl &lt; 190 Oe, fappl &lt; 120 kHz) and unsatisfactorily low r2-relaxivity keep SPNPs challenging for cancer theranostics agent applications [3,4]. In this work, innovatively designed and developed AC and DC magnetic softness enhanced dual-doped (Ni0.6Zn0.4)-Î³Fe2O3 (MSEÎ³-IO) SPNPs with significantly enhanced both intrinsic loss power (ILP, ~ 4.0 nH m2 kg-1) at the HapplÃ—fappl =1.23Ã—109 A m-1 s-1 and r2-relaxivity (r2 = 660.4 mM-1 s-1) are reported. The significantly enhanced ILP, and r2-relaxivity of dual-doped MSEÎ³-IO SPNPs were primarily attributed to the distinctly enhanced AC magnetic softness directly related to the HAC absorption and fappl resonance efficiency, and the DC magnetic softness dominantly controlled by the occupation and spatial distribution of Ni2+ in Oh vacancies sites, and Zn2+ cations in Td sites of Î³-Fe2O3, respectively. The biocompatibility and bioavailability experimentally evaluated by in-vitro and in-vivo studies demonstrated that the dual-doped MSEÎ³-IO SPNPs can be a promising candidate for highly efficient cancer theranostics agent for future nanomedicine.

technical paper

#### Welcome to the 67th Annual Conference on Magnetism and Magnetic Materials, MMM 2022!

The Conference was held from October 31 to November 4 and offers both in-person and on-demand content.

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Multiferroic materials having good magnetoelectric coupling are of great interest due to their enormous applications in the field of spintronic devices (1). Magnetoelectric (ME) gallium ferrite is an interesting material due to its room temperature (RT) piezoelectricity and near RT ferrimagnetism along with significant ME coupling (10−11 s/m at 4.2 K) (2). The motive of this work is to increase the magnetic transition temperature (TC) of the material above RT so that the material can have strong ME coupling at room temperature and can be implemented for practical applications. Several earlier reports have shown the magnetic transition temperature of Ga2−xFexO3 increases with higher Fe contents (3). Hence, we chose to study the properties of Ga2−xFexO3 (GFO) only for x = 1.2. Y3Fe5O12 (YIG) is another material which is RT ferromagnet with very high resistivity (resistivity ~ 1012 Ω.cm) (4). 
In this work, by forming GFO-YIG composite with only 10% concentration of YIG, phase transition temperature is increased beyond room temperature from ~ 289 K for GFO to ~ 309 K for 0.9 GFO – 0.1 YIG as shown in Fig. 1. The remnant magnetisation is also enhanced from 0.211 emu/g to 2.82 emu/g [inset of Fig. 2] reporting a magnetisation of ~ 8.2 emu/g at 30 kOe [Fig. 2].
References [1] J. F. Scott, Nature Materials, vol. 6, no. 4, pp. 256–257, Mar. 2007.
[2] J. P. Remeika, Journal of Applied Physics, no. 5, 1960.
[3] T. Arima et al., Physical Review B - Condensed Matter and Materials Physics, vol. 70, no. 6, 2004.
[4] W. Swee Yin, J. Hassan, W. Mohd, and D. W. Yusoff, Solid State Science and Technology, vol. 19, pp. 259–267, 2011. 


 Fig. 1 dM/dT versus Temperature plot indicating the phase transition temperature (TC) 




Fig. 2 Magnetic hysteresis loop obtained at room temperature. Inset of (b) shows the zoomed-out magnetic hysteresis loop of 0.9 Ga0.8Fe1.2O3 – 0.1 Y3Fe5O12 composite at a magnetic field range of 4 kOe.

Enhancement of phase transition temperature beyond room temperature by formation of Ga0.8Fe1.2O3 – Y3Fe5O12 composite

Topological materials have great potential for ultralow power spin-orbit torque (SOT) spintronic devices thanks to their giant spin Hall effect originated from their topological surface states (TSSs). However, the giant spin Hall angle (θSH > 1) is limited to a few chalcogenide-based topological insulators (TIs) with toxic elements and low melting points, making them challenging for device integration during the silicon Back-End-of-Line (BEOL) process. Here, we focus on a half-Heusler alloy topological semimetal, YPtBi, to overcome this difficulty. We synthesized YPtBi thin films by using co-sputtering method with YPt and Bi targets while changing the growth temperature. Figure 1 (a) shows the X-ray diffraction (XRD) spectra for YPtBi films grown at various temperature. YPtBi(111) peaks were clearly observed up to 600oC. Figure 1 (b) shows the Bi composition measured by X-ray fluorescence (XRF). Bi composition is stable up to 600oC. These results indicate that YPtBi crystal is stable up to 600oC which is high enough for BEOL process. To evaluate the spin Hall effect for YPtBi, we conducted the second harmonic Hall effect measurements in CoPt/YPtBi heterostructures [1]. By controlling the electric conductivity of YPtBi and spin transparency at the CoPt/YPtBi interface, we successfully realized a giant θSH up to 4.1. Figure 2 shows SOT magnetization switching by pulse currents with pulse width of 50 μs to 10 ms under an external magnetic field of 0.5 kOe applied parallel to the current. Thanks to the giant spin Hall effect originated from TSS, small threshold current of about 1×106 A/cm2 was observed, which is one order magnitude smaller than that in heavy metals [2]. Our work opens the door to the next generation spin Hall materials with both giant θSH and high thermal stability [3]. Acknowledgment: this work was supported by Kioxia corporation. The authors thank Tsuyoshi Kondo of Kioxia corporation for fruitful discussion. 
**References** [1] M. Hayashi, J. Kim, M. Yamanouchi, and H. Ohno, Phys. Rev. B, Vol. 89, p.144425 (2014).
[2] B. Jinnai, C. Zhang, A. Kurenkov, M. Bersweiler, H. Sato, S. Fukami, and H. Ohno, Appl. Phys. Lett. Vol. 111, p.102402 (2017).
[3] T. Shirokura, T. Fan, N. H. D. Khang, T. Kondo, and P. N. Hai, Sci. Rep. Vol. 12, p.2426 (2022).
 
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Giant spin Hall effect in a half Heusler alloy topological semimetal with high thermal stability

NiFe2O4 (NFO) [1], is an insulating ferrimagnet with an inverse spinel structure and already has gained a considerable attention of scientific community due to its potential applications in spintronic devices. For NFO thin films, a coherent strain can be introduced by growing them on different substrates: ZnGa2O4 (ZGO) and MgAl2O4 (MAO) with lattice mismatch of 0.06% and 3.1% respectively. This strain greatly affects physical properties such as magnetic damping, where damping for the NFO film grown on ZGO is considerably smaller than the damping of the NFO film grown on MAO film [2]. Magnetic damping should also affect ultrafast processes such as the demagnetization and remagnetization time of the system. Koopmans et al. [3] showed that the demagnetization time is inversely proportional to the effective Gilbert damping.

In this work we explore the underlying mechanisms ultrafast demagnetization processes of strained NFO films by using table-top high harmonic generation (HHG) in the extreme ultraviolet (XUV) regime [4], which enables element-specific tracking of spin dynamics. To understand the HHG asymmetry spectra, we compare the discrete HHG spectra to the reflectivity spectra acquired using a continuous synchrotron-based source. The comparison shows good agreement, thereby facilitating assignment of HHG asymmetry features. Time-resolved HHG scans indicate that the demagnetization amplitude is larger for NFO on the MAO substrate while NFO films on both substrates exhibit the similar demagnetization and remagnetization times. In contrast to damping of spin motion at longer timescales, which is highly sensitive to issues such as defect density, the similarity in the magnetization dynamics in the first few ps following a demagnetization pulse reveals the local nature of the ultrafast dynamics. The results display the utility of M-edge spectroscopy for studying spin dynamics in insulating oxide films, which is highly relevant for spintronics based on ferrimagnetic insulators. 
**References** [1] R. Knut et al., J. Phys.: Condens. Matter 33, 225801(2021)
[2] A. V. Singh et al., Advanced Materials 29, 1701222 (2017).
[3] B. Koopmans et al., Physical Review Letters 85, 844 (2000).
[4] S. Jana et al., Phys. Rev. Research 2, 013180 (2020).
 
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Element specific ultrafast magnetization dynamics of strained NFO films using ultra short XUV pulses

We discuss thermally activated switching between micromagnetic states in nanodisks with perpendicular anisotropy and a large strength of the interfacial Dzyaloshinskii-Moriya interaction, D. For large D, the magnetization on infinite planes acquires a cycloidal configuration in the ground state. However, Isolated singularities that are analogous to disclinations of cholerestic phases [1] are topologically protected and prevent the system from reaching the cycloidal phase (for examples see Fig. 1). In confined geometries, a rich variety of possible configurations with complex magnetization textures that are also energy minima of the energy landscape [2] has been observed. We argue that characterizing these different states as arrangements of disclinations is an effective way for understanding them. We support this statement on results of String Method [3] calculations done using OOMMF to examine the energy landscape of nanodisks (Fig. 2). We found that the transition states correspond to motion of individual merons across the device's edge (first maximum in Fig. 2), and to merging of disclinations inside the structure (2nd, 3rd and 4th maxima in Fig. 2). An additional advantage of emphasizing the importance of these magnetic singularities is the possibility of adopting them for collective coordinate descriptions which can be treated using low-dimensional stochastic methods to study their dynamics in the presence of thermal noise. 

**References** 
[1] Phys. Rev. B 99, 214408 (2019)
[2] The Philosophical Magazine. 22:178, 739-749
[3] J. Chem. Phys. 126, 164103 (2007)
This research was supported in part by the National Science Centre Poland under OPUS funding Grant No. 2019/33/B/ST5/02013 (GDC, PK) and U.S. National Science Foundation Grant
DMR 1610416 (DLS). 

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Examples of disclinations on the cycloidal phase of a ferromagnet with large DMI. (a) cycloidal phase without singularities. Disclinations occur when the helical axis rotates by a multiple
of π. (b,c,e,f) Examples of disclinations, respectively their values are: (π,2π,-π,-2π). (d) a skyrmion shown as a 2π disclination embedded in a cycloidal background, (g) a meron shown as a
π disclination embedded in a cycloidad background. 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/55824035ff74275789c3c3d8097028e3.png) 
Results of String Method calculations on nanodisks with high DMi for thermally activated switching between two different micromagnetic configurations. The highest maxima occurs
when a meron reaches the in-plane edge. The other maxima are the result of -π disclinations merging briefly into a +2π disclination resulting on re-acommodation of adjacent skyrmionic
domains.

The Role of Disclinations in the Thermal Stability of Magnetization Textures in Confined Helimagnetic Nanostructures

Rare-earth transition metal thin films hold great potential for implementation in devices at the micro and nanoscale [1-3]. While Nd2Fe14B and Pr2Fe14B share almost the same intrinsic magnetic properties (Ms, Ku…), Nd2Fe14B has a limited use for low temperature applications (i.e. aerospace) due to the spin reorientation that occurs at 135 K [4]. This limitation can be overcome using Pr2Fe14B, whose magnetization direction remains along the c-axis until 4.2 K [4]. The understanding and optimization of these systems is of great importance when attempting their integration in novel miniaturized devices [5].
In this study, high coercivity PrFeB thin films of 100 nm have been fabricated by magnetron sputtering. The effect on the substrate temperature (Ts) in the structure and morphology and its resulting impact on the magnetic properties has been evaluated. 
Our research stablishes the optimum Ts to be 600oC where a highly textured growth of Pr2Fe14B is achieved according to the X-Ray Diffraction (XRD) analysis (Fig. 1a). Pr2Fe14B crystalline phase is accompanied by a Pr-rich phase that can be identified in all the XRD patterns. The role that these Pr-rich areas play in the magnetism of these systems has also been thoroughly studied by Energy Dispersive X-Ray Spectroscopy (EDX) (Fig. 1c). Regardless of Ts, all films present strong perpendicular magnetic anisotropy (Fig.2c,d) which is in good accordance with the c-axis oriented preferential growth. Out of plane coercivities up to 14 kOe have been obtained at RT. Atomic and Magnetic Force Microscopy (AFM and MFM, respectively) have also been used to determine influence of Ts on the roughness and magnetic domains (Fig 2a,b). 
**References:** 
[1] X. Liu, T. Okumoto, M. Matsumoto, A. Morisako. J. Appl. Phys. 97, 10K301 (2005). 
[2] T.-S. Chin, J. Magn. Magn. Matter. 209, 75-79 (2000). 
[3] A. Bollero, V. Neu, V. Baltz et al., Nanoscale, 12, 1155-1163 (2020). 
[4] TT. B. Lan, G.C. Hermosa and A.C. Sun, J. Phys. Chem. Solid. 144, 109506 (2020). 
[5] H2020 FET-OPEN project “UWIPOM2”: https://cordis.europa.eu/project/id/857654. 

**Acknowledgements:** 
Authors acknowledge financial support from EU through the H2020 FET Open UWIPOM2 project (Ref. 857654). J. S-M. acknowledges financial support from Comunidad de Madrid (PEJD-2019-PRE/IND-17045). 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/955034a15cdb0a0f619d8946ed3387f8.png) 
Fig. 1. a) XRD pattern of PrFeB thin films grown at different Ts, b) Scanning electron microscopy (SEM) image of a PrFeB film grown at 600oC, c) EDX mapping of a PrFeB film with a Ts = 600oC showing the distribution of Pr and Fe (c.1) Prdistribution (c.2) Fe distribution (c.3). The scale bar is 800 nm in all cases. 
![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/10cb6499fc26bfe27947f0693ed28999.png) 
Fig. 2. a) AFM and b) MFM image of a PrFeB film grown at 600oC, c) Out of plane and d) In Plane room temperature hysteresis loops with a maximum applied field of 20 kOe for PrFeB thin films grown at different Ts.

High coercive PrFeB Films with Strong Perpendicular Magnetic Anisotropy

Skyrmionsâ€”topologically protected vortex-like spin structuresâ€”have been proposed as the new information carriers in racetrack memory devices [1]. In order to realise such devices a small size, high speed of propagation, and minimal deflection angle are required. Modelling has shown that synthetic antiferromagnets (SAFs) present the ideal materials system to realise these aims [2]. However, their magnetic compensation makes observation of skyrmions difficult and indeed this was only recently achieved [3]. There is thus significantly more to understand about the behaviour of synthetic antiferromagnets and the skyrmions therein. Here we present a comprehensive magnetic force microscopy (MFM) and XMCD-PEEM study of a SAF multilayer composed of 20 magnetic layers alternating between CoB and CoFeB each coupled antiferromagnetically with a Ru spacer layer to the one above and below. PEEM results show that in the SAF phase the sample is spontaneously single-domain, and when exposed to a complicated field protocol, large defect-pinned domains can be stabilized. As shown in the hysteresis loop presented in Fig. 1 the SAF undergoes a phase transition between the compensated antiferromagnetic state and its field-polarised ferromagnetic state as a field is applied. MFM shows that in our samples this is as a result of defect-driven bubble nucleation where FM ordered regions nucleate and then expand to cover the entire film. Once the FM regions exceed a critical size they collapse into a skyrmion/stripe domain pattern and so retain a net zero magnetisation. At a narrow range of fields, e.g. as shown in Fig. 2 we observe a phase coexistence between the compensated AF state and the net-zero magnetisation FM state. As the magnetic field is increased we go on to observe isolated skyrmions at fields below saturation as expected in a FM system. These results give perspective on the ferromagnetic nature of skyrmions observed in systems at fields just below saturation and can help to inform the observation of true antiferromagnetic skyrmions.

Phase Coexistence and Transitions between Anti and Ferromagnetic States in a Synthetic Antiferromagnet

Research interest in rare-earth lean SmFe12-based compounds has been revived due to their excellent intrinsic hard magnetic properties making them potential for new permanent magnet materials that are not dependent on scarce elements such as Dy [1,2]. In order to realize these materials for practical applications, the main challenge is development of anisotropic SmFe12-based sintered magnet with a large coercivity and high remanent magnetization. 
We carried out machine-learning on a dataset extracted from literature on Sm(Fe,X)12-based alloys to understand the most influential parameters for coercivity. It was found addition of V to the alloy composition is the most influential to coercivity [3]. The prediction from machine learning was experimentally validated by developing SmFe11Ti and SmFe10TiV melt-spun ribbons where coercivity increased from 0.5 T to 1.1 T upon addition of V. Detailed microstructure characterizations revealed enhanced coercivity is originated from formation of Sm-rich intergranular phase enveloping SmFe12-based grains. The formation of Sm-rich intergranular phase acts as pinning cites against magnetic domain wall propagation resulting in an enhanced coercivity in the V-containing magnet. Motivated by this study, we have developed anisotropic Sm(Fe,Ti,V)12-based sintered magnet via conventional powder processing including the development of jet-milled powders, preparation of green compact, and liquid sintering process [4,5]. The sintered magnets showed coercivity of μ0Hc=0.6-1.0 T and remanent magnetization of μ0Mr = 0.6-0.8 T depending on Ti content (Fig. 1). Detailed multi-scale microstructure characterizations showed the origin of realizing a coercivity of above 0.6 T is the formation of Sm-rich intergranular phase [4,5]. However, {101} twins were also observed in the microstructure of the magnets as shown in Fig. 2 (a). The origin of the twins is induced stress in the powders during the jet-milling process. MOKE observations showed that the magnetization reversal can start from twinned grains which can be detrimental for coercivity (Fig. 2). However, micromagnetic simulations showed that the existence of intergranular phase isolating individual grains can overcome the detrimental effect of twins to coercivity by preventing the propagation of reversed domains to the neighboring grains and hence resulting in a large coercivity. Based on our detailed microstructure investigations and micromagnetic simulations, we will discuss how an optimum microstructure can be realized in the anisotropic SmFe12-based magnets which results in a large μ0Hc and μ0Mr. 
**References:** [1] K. Ohashi, Y. Tawara, R. Osugi, M. Shimao, J. Appl. Phys. 64 (1988) 5714-5716. 
[2] P. Tozman, H. Sepehri-Amin, Y.K. Takahashi, S. Hirosawa, K. Hono, Acta Mater. 153 (2018) 354. 
[3] Xin Tang, J Li, AK Srinithi, H Sepehri-Amin, T Ohkubo, K Hono, Scripta Mater. 200 (2021) 113925. 
[4] J.S. Zhang, Xin Tang, H. Sepehri-Amin, A.K. Srinithi, T. Ohkubo, K. Hono, Acta Mater. 217 (2021) 117161. 
[5] J.S. Zhang, Xin Tang, A. Bolyachkin, A.K. Srinithi, T. Ohkubo, H. Sepehri-Amin, K. Hono, Submitted. 

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Development of rare earth lean SmFe12

The transfer and control of angular momentum is a key aspect for spintronic applications. When a thin nickel film is subjected to ultrashort laser pulses, it can lose its magnetic order almost completely within merely femtosecond times. This phenomenon, which can also be observed in many other materials, offers opportunities for rapid information processing or ultrafast spintronics at frequencies approaching those of light. Consequently, ultrafast demagnetization is central to modern material research, but a crucial question has remained elusive: If a material loses its magnetization within only femtoseconds, where is the missing angular momentum on such short time scales?

Here we use molecular dynamics simulations to investigate the role of phonons during ultrafast demagnetization in nickel. For this purpose, we transfer angular momentum corresponding to the observed amount of demagnetization into the lattice and calculate the resulting changes in the diffraction pattern. Our results are in line with ultrafast electron diffrac- tion measurements which show an almost instantaneous, long- lasting, non-equilibrium popu-
lation of anisotropic high-frequency phonons that appear as quickly as the magnetic order is lost. The anisotropy plane is perpendicular to the direction of the initial magnetization and the atomic oscillation amplitude is 2 pm. Theory and experiment indicate a rotational lattice motion on atomic dimensions after the excitation with the laser pulse that takes up the missing angular momentum [1] before the onset of a macroscopic Einstein-de Haas rotation [2].

Acknowledgements This research was supported by the German Research Foundation (DFG) via SFB 1432. 
**References** [1] S. R. Tauchert et al. Nature 602, 73–77 (2022).
[2] C. Dornes et al. Nature 565, 209-212 (2019)
 
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Polarized phonons carry the missing angular momentum in femtosecond demagnetization

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

Spin-orbitronics and single pulse alloptical switching (AOS) of magnetization are two major successes of the fast advancing field of nanomagnetism in recent years, with high potential for enabling novel fast and energy-efficient memory and logic platforms. Fast current induced domain wall motion and single shot AOS have been individually demonstrated in different ferrimagnetic alloys. However, the stringent requirement for their composition control makes them challenging material for wafer scale production. Here, we demonstrate simultaneously fast current-induced domain wall motion and energy efficient AOS in a synthetic ferrimagnetic system based on [Co/Gd]2 multilayers. We firstly show AOS is present in its full composition range. We find current driven domain wall velocities over 2 km/s at room temperature conditions is achieved by compensating the total angular momentum through layer thickness tuning. Furthermore, analytical modelling of the current-induced domain wall motion reveals that Joule heating needs to be treated transiently to properly describe the current-induced domain wall motion for our sub-ns current pulses. Our studies establish Co/Gd based synthetic ferrimagnets to be a unique material platform for domain wall devices with access to ultrafast single pulse AOS [1]. 
**References** [1] Pingzhi Li, Thomas J Kools, Bert Koopmans and Reinoud Lavrijsen. Ultrafast racetrack based on compensated Co/Gd-based synthetic ferrimagnet with all-optical switching. arXiv preprint arXiv:2204.11595, 2022.

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