United States

Bismuth (Bi) or Bi-based heterostructures has attracted much interest as candidates for spin current generator. Recently, helicity-dependent (HD) photocurrent in Bi or Bi/Cu(or Ag) thin films was reported by using pulse laser-induced terahertz (THz) emission and transport measurement[1, 2]. It is believed that spin current in Bi induced by circularly-polarized laser via photo-spin conversion effect is converted into charge current via spin-charge conversion effect. However, details of photo-spin conversion in Bi have yet to be clarified because the photo-spin and the spin-charge conversion effects are observed simultaneously, thus, one cannot distinguish two effects. Here, we studied THz emission induced by spin-current generation using the photo-spin conversion effect in Bi and the laser-induced demagnetization in an adjacent ferromagnetic Co layer in Bi/Co films to separate two spin-related conversion effects and investigate the photo-spin conversion in Bi.
The samples, glass sub./Bi(tBi)/Co(5)/MgO(2)/Ta(2) (thickness is in nm), were prepared by a DC/RF magnetron sputtering. Laser-induced THz emission in Bi/Co films was investigated by THz time-domain spectroscopy (THz-TDS) with 120-fs Ti: Sapphire laser[3,4]. Figure 1 and Figure 2 show the Bi thickness tBi dependence of THz peak values induced by linearly-polarized laser and HD-THz peak values, respectively. Here, HD-THz is difference between signals obtained with left- and right-circularly polarized laser. We found different tBi dependence as shown in Fig. 1 and Fig. 2, which is possibly due to the difference in spin-relaxation length of two experiments. In the presentation, we will discuss the physics behind the different THz signals obtained in two experiments and quantitative consideration of the photo-spin conversion in Bi. This work is partially supported by KAKENHI (19K15430, 21H05000). K. I. acknowledges Grant-in-Aid for JSPS Fellow (22J22178) and GP-Spin at Tohoku University. S. M. acknowledges CSRN in CSIS at Tohoku University.&lt;br&gt;&lt;br&gt;
**References**&lt;br&gt; [1] Y. Hirai et al. Phys. Rev. Appl. 14, 064015 (2020)
[2] H. Hirose et al. Phys. Rev. B 103, 174437 (2021)
[3] H. Idzuchi, et al., AIP Advances 11, 015321 (2021)
[4] Y. Sasaki et al. Appl. Phys. Lett. 111, 102401 (2017)
&lt;br&gt;&lt;br&gt;
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MMM 2022

Laser induced terahertz wave emission from Bi/Co structure

Bismuth (Bi) or Bi-based heterostructures has attracted much interest as candidates for spin current generator. Recently, helicity-dependent (HD) photocurrent in Bi or Bi/Cu(or Ag) thin films was reported by using pulse laser-induced terahertz (THz) emission and transport measurement[1, 2]. It is believed that spin current in Bi induced by circularly-polarized laser via photo-spin conversion effect is converted into charge current via spin-charge conversion effect. However, details of photo-spin conversion in Bi have yet to be clarified because the photo-spin and the spin-charge conversion effects are observed simultaneously, thus, one cannot distinguish two effects. Here, we studied THz emission induced by spin-current generation using the photo-spin conversion effect in Bi and the laser-induced demagnetization in an adjacent ferromagnetic Co layer in Bi/Co films to separate two spin-related conversion effects and investigate the photo-spin conversion in Bi.
The samples, glass sub./Bi(tBi)/Co(5)/MgO(2)/Ta(2) (thickness is in nm), were prepared by a DC/RF magnetron sputtering. Laser-induced THz emission in Bi/Co films was investigated by THz time-domain spectroscopy (THz-TDS) with 120-fs Ti: Sapphire laser[3,4]. Figure 1 and Figure 2 show the Bi thickness tBi dependence of THz peak values induced by linearly-polarized laser and HD-THz peak values, respectively. Here, HD-THz is difference between signals obtained with left- and right-circularly polarized laser. We found different tBi dependence as shown in Fig. 1 and Fig. 2, which is possibly due to the difference in spin-relaxation length of two experiments. In the presentation, we will discuss the physics behind the different THz signals obtained in two experiments and quantitative consideration of the photo-spin conversion in Bi. This work is partially supported by KAKENHI (19K15430, 21H05000). K. I. acknowledges Grant-in-Aid for JSPS Fellow (22J22178) and GP-Spin at Tohoku University. S. M. acknowledges CSRN in CSIS at Tohoku University. 
**References** [1] Y. Hirai et al. Phys. Rev. Appl. 14, 064015 (2020)
[2] H. Hirose et al. Phys. Rev. B 103, 174437 (2021)
[3] H. Idzuchi, et al., AIP Advances 11, 015321 (2021)
[4] Y. Sasaki et al. Appl. Phys. Lett. 111, 102401 (2017)
 
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technical paper

#### Welcome to the 67th Annual Conference on Magnetism and Magnetic Materials, MMM 2022!

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In recent work, the time evolution of the magnetization reversal caused by spin-transfer torque in small perpendicular MRAM cells was investigated experimentally and theoretically [1, 2]. If the magnetization reversal is incoherent, that is, it does not follow a macro-spin model, intermediate states in between magnetization up or down can be found. These states can last up to tens of ns for cell sizes of the order of 50nm, thus lengthening the reversal time.
We investigated time resolved current measurements for our MRAM cells with sizes smaller than 20nm in the expectation that no longer any phenomena of this kind should be observed for very small cells that are below the “single domain limit”. However, it turns out that even MRAM cells smaller than 20nm (see Fig. 1) show typical indications of domain wall pinning, albeit with shorter pinning times. Fig. 2 shows a clear trend that the reversal times – which have been carefully separated from the incubation times - decrease with size. The reversal times are stochastic in nature and for each size a substantial range for the reversal times exists. Even for the smallest cells, we can show that the observed features are not an artifact caused by noise. In the full paper, details of the measurements and interpretations of the data will be given. 
**References** [1] P. Bouquin et al., Phys. Rev Appl, 15, 024037 (2021)
[2] I. Volvach et al., Appl. Phys. Lett. 116, 192408 (2020) 

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Evidence for Domain Wall Reversal in very Small Perpendicular MRAM Cells

Magnetic skyrmions possess intriguing features such as ultra-small size, solitonic nature, and easy mobility with small electrical currents, which make them attractive for low power, high-density memory and logic applications (1). In a skyrmion-based racetrack, information can be encoded either by the presence and absence of skyrmions for Boolean computing or into the analog spatial coordinates of the skyrmions, which then can be translated into the timing information needed for temporal computing (2). However, it is essential to guarantee the positional stability of skyrmions for reliable information extraction because a randomly displaced skyrmion can alter the bit sequence and change the spatial coordinates thereby the encoded analog timings. Using micromagnetic simulations (3) for the minimum energy path (MEP), we compute the energy barriers associated with notches along a racetrack. We vary material parameters, specifically, the strength of the chiral Dzyaloshinskii-Moriya interactions (DMI), the notch geometry, and the thickness of the racetrack to get the optimal barrier height. We find a range of energy barriers up to ~ 45 kBT for a racetrack of 5 nm thickness that can provide a year-long positional lifetime of skyrmions for long-term memory applications while requiring a moderate amount of current (~ 1010 A/m2) to move the skyrmions (4). Moreover, we derive quasi-analytical equations to estimate the energy barrier with material-dependent pre-factor constants. Currently, we are building machine learning-based surrogate models for the computationally-expensive micromagnetics simulations to accelerate the energy barrier prediction for a much broader parameter space. Our results open up possibilities to design practical skyrmion-based racetrack geometries for spintronics applications. 

**References:** 
(1) H. Vakili, J. Xu and W. Zhou, J. Appl. Phys., 130, 070908 (2021) 
(2) H. Vakili, and M. Sakib and S. Ganguly, IEEE J. Explor. Solid-State Comput. Devices Circuits, 1 (2020) 
(3) A. Vansteenkiste, J. Leliaert and M. Dvornik, AIP Adv., 4, 107133 (2014) 
(4) M. Morshed, H. Vakili and A. Ghosh, Phys. Rev. Appl., 17, 064019 (2022) 

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Stabilizing skyrmions deterministically for racetrack memory applications

Spin-Hall oscillators (SHOs) are promising spintronic devices to realize dc controlled GHz frequency signals in nanoscale devices for a variety of applications1,2, including for neuromorphic computing3. However, traditional SHOs have high auto-oscillation threshold currents and those based on magnetic tunnel junctions require a complex etching process. Here, we demonstrate that by combining a metallic ferromagnetic nanowire with low damping and high-quality ferromagnetic insulators, specifically epitaxial lithium aluminum ferrite thin films4, SHO characteristics are dramatically improved. Our magnetic bilayers SHOs are composed of Pt(5nm)/Py(5nm)/(Li0.5Al1.0Fe1.5O4 (LAFO) or Li0.5Al0.5Fe2O4 (LFO)) (x nm) layers with varied thicknesses x (including x=0, i.e. just Pt/Py layers) and two different compositions of insulator. The Pt/Py layers are patterned into 400 nm wide x 400 nm long nanowires between two Au contact pads as shown in Fig. 1a. Ferromagnetic resonance (FMR) measurements of the unpatterned heterostructure and spin-torque ferromagnetic resonance (ST-FMR) of micron-scale wires show that Py and LAFO are strongly ferromagnetically coupled and thus precess coherently. In the nanowires, we observe two prominent auto-oscillation modes in all samples, which we associate with bulk and edge spin-wave modes. Increased oscillator quality factor and maximum emission power (Fig. 1b) as well as lower threshold currents are obtained in samples containing LAFO and LFO. In addition, the maximum emission power and quality factor increases with the LAFO thickness. The performance can be further enhanced by replacing LAFO with LFO which possesses higher saturation magnetization. The improved characteristics are associated with the excitation of larger spin-precession angles and volumes in LAFO and LFO samples. We further find that the presence of LAFO enhances the auto-oscillation amplitude of spin-wave edge modes, consistent with our micromagnetic modeling. 
**References** 1 V.E. Demidov, S. Urazhdin, H. Ulrichs, V. Tiberkevich, A. Slavin, D. Baither, G. Schmitz, and S.O. Demokritov, Nat. Mater. 11, 1028 (2012).
2 Z. Duan, A. Smith, L. Yang, B. Youngblood, J. Lindner, V.E. Demidov, S.O. Demokritov, and I.N. Krivorotov, Nat. Commun. 5, 1 (2014).
3 J. Grollier, D. Querlioz, K. Y. Camsari, K. Everschor-Sitte, S. Fukami, and M. D. Stiles, Nature Electronics 3, 360 (2020)
4 X.Y. Zheng, L.J. Riddiford, J.J. Wisser, S. Emori, and Y. Suzuki, Appl. Phys. Lett. 117, 1 (2020).
 

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Spin Hall oscillators based on ferromagnetic metal/lithium aluminum ferrite bilayers with enhanced output power and quality factor

Since the first all-optical switching experiments carried on GdFeCo ferrimagnet [1-2], the all-optical switching represents an ultrafast and easy method to manipulate magnetization without any applied field. Single pulse helicity independent all-optical switching (HI-AOS) has been demonstrated in ferrimagnetic GdFeCo alloy, Gd/FM bilayers where FM is a ferromagnet, and recently in a ferrimagnetic Heusler alloy and Tb/Co multilayer-based electrode [2-5].
In this work, we demonstrate that robust toggle HI-AOS has been observed in ferrimagnetic Tb based multilayers (Fig.1a). The properties of single pulse HI-AOS in Tb-based multilayers mainly differ in two ways compared to GdFeCo alloys. First, the shape of the state diagram of Tb-based multilayers in which the toggle switching can be observed at a pulse duration of 10 ps is completely different from that of GdFeCo alloy [6] (Fig.1d). The threshold switching fluence (Fswitch) is independent of the laser pulse duration of at least up to 10 ps (Fig. 1b). Moreover, the multidomain fluence (Fmulti), above which a multidomain state is observed in the center of the spot, increases slightly for pulse durations below 3 ps and then remains constant for longer pulse durations. This indicates that the mechanisms responsible for the Tb-based multilayers switching differ from those of Gd-based alloys. Second, the observed equilibrium state after the first laser pulse excitation shows a complex structure with rings of opposite magnetization directions when the fluence increases. This ring structure was observed in all Tb-based multilayer films we have studied so far. The number and the size of the rings depend on the sample composition (thickness of the Tb and transition metal layers) as well as on the annealing condition.
Acknowledgments: UFO, COMRAD 
**References** [1] C. D. Stanciu, F. Hansteen, A. V. Kimel, Phys. Rev. Lett. 99, 047601, 2007.
[2] T. A. Ostler, J. Barker, R. F. L. Evans, Nat. Commun. 3, 666, 2012.
[3] M. Beens, M. L. M. Lalieu , A. J. M. Deenen, Phys. Rev. B 100, 220409(R) , 2019.
[4] L. Avilés-Félix, A. Olivier, G. Li, C. S. Davies, Sci. Rep. 10, 5211, 2020.
[5] C. Banerjee, N. Teichert, K. Siewierska, Nat. Commun., 11, 4444, 2020.
[6] J. Wei, B. Zhang, M. Hehn, Phys. Rev. Applied 15, 054065, 2021. 
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Robust toggle switching in Tb based multilayers by a single

Domain walls (DWs) in thin films with perpendicular magnetic anisotropy (PMA) are promising information carriers for the next generation of data storage and logic
operation devices.
[1,2] However, controlling DW motion efficiently remains unsolved. Here, we experimentally demonstrated an enhanced DW velocity in a PMA film using both standing
and travelling surface acoustic waves (SAWs). Two interdigitated transducers (Fig.1a), centre frequency 47.93 MHz (Fig. 1b), were patterned on opposite side of a
Ta(5)/Pt(2.5)/Co(0.5)/Ta(5) film (thicknesses in nm). The film was dc magnetron sputtered onto a lithium niobate substrate. Kerr microscopy was used to measure the DW velocity by
measuring the DW displacement due to a pulsed field. Results showed that DW velocity increased from 7±1 to 34±1 µm/s with an increasing magnetic field from 10.9 to 14.8 Oe without
SAWs (Fig.1c). A significant DW velocity increase can be observed in the presence of the standing SAW (Fig.1c) with applied SAW power from 15.5 to 20.5 dBm. An up to 24-fold DW
velocity increase (870±10 µm/s at 14.8 Oe) can be found in the presence of standing SAWs at 20.5 dBm compared to that without SAW at the same field. A less significant DW increase
(372±6 µm/s at 14.8 Oe) can also be observed with the application of travelling SAWs (Fig.1d). SAWs locally and periodically lower and raise the anisotropy of the thin film due to the
magnetoelastic coupling effect, temporarily assisting the DW to overcome pinning energy barriers during its motion.
[3,4] This process is possibly an accumulative effect, which can explain
the velocity difference between results for the travelling and standing SAW. This study demonstrates the potential for SAWs to efficiently manipulate magnetic DWs. 
**References** [1] Marathe et al. Vacuum, 14, 329, (2017) 
[2] Li et al., J. Appl. Phys, 115, 17E307 (2014) 
[3] Shepley Sci. Rep. 5, 7921 (2015) 
[4] Shuai et al. Appl. Phys. Lett. 120, 252402 (2022) 

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Fig. 1 (a) Experimental set-up; (b) S-parameters of the SAW device; (c) Domain wall velocity against the applied field with standing SAWs (various applied SAW power) and without
SAW. (d) Domain wall velocity against the applied field without SAW and with standing/travelling SAWs (applied SAW power of 20.5 dBm).

Surface Acoustic Waves Effect on Domain Wall Motion

A study of the effect of a nonuniform perpendicular magnetic anisotropy on the magnetic configurations and on the ferromagnetic resonance (FMR) spectra in two systems
was performed by micromagnetic simulations, by applying a short magnetic pulse [1-3].
First system corresponds to magnetic nanorings of different widening with radial distribution of the perpendicular magnetic anisotropy (RPMA) [1]. Both the resonant frequencies and the
number of peaks depend on the lower energy magnetization configuration [4] (figure 1) which in turn is a function of anisotropy gradient. Also, we compare the diagram between nanorings
with and without RPMA showing that the effects of the RPMA are relevant even for the narrowest ring of 10 nm wide [1].
The second system corresponds to nanodisks with radial distribution but with the center of the distributed anisotropy displaced from the center of the nanodisk (figure 2). This break on the
symmetry of the RPMA with respect to the disk center, allows several new magnetic configurations such as skyrmionium-like magnetic states and therefore the resonance spectra of FMR
have several new modes and peaks compared to non-breaking symmetry of RPMA.
The results show that by controlling the RPMA strength K of the anisotropy gradient, and its symmetry, several new resonance modes and magnetic configurations can be obtained for
instance, vortex, meron and knot states in nanorings and skyrmionum in nanodisks.
Authors acknowledge DICYT Grant 042131EM, PAI77190042, Fondecyt 1201491 and Basal AFB 180001. 
**References** 
[1] Saavedra, E. et al. Effect of nonuniform perpendicular anisotropy in ferromagnetic resonance spectra in magnetic nanorings. Sci. Rep. 11, 14230 (2021). 
[2] McMichael, R.D. and Stiles, M.D. Magnetic normal modes of nanoelements. J. Appl. Phys. 97, 10J901 (2005). 
[3] Baker, A. et al. Proposal of a micromagnetic standard problem for ferromagnetic resonance simulations. J. Magn. Magn. Mater. 421, 428 (2017). 
[4] Castro, M.A. et al. New magnetic states in nanorings created by anisotropy gradients. J. Magn. Magn. Mater. 484, 55–60 (2019). 

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Figure 1: a) Minimal energy configurations as a function of the RPMA strength K: I, II, and III correspond to vortex, knot and meron states respectively. b) Imaginary component of the
dynamic susceptibility for R1 = 50 nm c) Spatial distribution of the amplitude for vortex (K =300 kJ/m3), knot (K = 350 kJ/m3) and meron (K = 450 kJ/m3), respectively.

Effect of nonuniform perpendicular anisotropy in ferromagnetic resonance spectra in magnetic nanorings and nanodisks

Corrado C. Capriata, Bengt G. Malm
Division of Electronics and Embedded Systems, KTH Royal Institute of Technology, Stockholm, Sweden
Abstract Body Two-dimensional synchronized arrays of Nanoconstriction Spin-Hall nano-oscillators (NC-SHNO) [1] have several promising features like higher power, better stability,
and applications in neuromorphic computing [2-3]. For all these applications, the synchronization of the oscillator array is crucial. Oscillators are synchronized with the neighboring ones
via dipolar and exchange coupling. In a recent study [4], NC-SHNO arrays were simulated under an injection locked condition and a one-dimensional array was investigated in [5]. Our
previous study [6] demonstrated how local variations of the exchange coupling can induce multiple oscillation modes in single devices and cause variability in the oscillation frequency. In
this study, the aim is to expand the modeling to free-running 2x2 and 4x4 synchronized arrays using the MuMax3 [7] micromagnetic simulator.
Here, we illustrate results from the 2x2 array with 100 nm pitch and 50 nm width (Fig.1 inset). The exchange coupling is reduced to 15 % at a line in between the columns or rows. The
presence of grains is equivalent to the coexistence of multiple lines and it is an expansion towards a real thin film representation. In this case, the exchange is randomly reduced to 10-30 %
and assigned to each grain.
Modifying the exchange coupling inside the array results in a minor shift of the synchronized frequency at low and medium currents while the array is much more disturbed at high
currents, Fig.1. The phase and power analysis, Fig.2, shows that the shape of the oscillating volume is disturbed by the presence of grains. At high current (3 mA), the excited mode
becomes propagating while at lower currents it is localized and well phase synchronized. In general, disturbed oscillators can be synchronized column-wise or diagonally, the extreme case
is only one device of the array delivering output power. Similar results have been found for the 2x2 array with 200 nm pitch and 120 nm width, highlighting that the phase synchronization
is not always guaranteed or can be broken by local variations in the exchange coupling strength in some operating conditions.

 
**References** 

[1]: V. E. Demidov, et al. "Nanoconstriction-based spin-Hall nano-oscillator", Appl. Phys. Lett. 105, 172410 (2014) 
[2]: A. Smith, et al. “Dimensional crossover in spin Hall oscillators”, Phys. Rev. B 102, 054422 – Published 17 August 2020. 
[3]: M. Zahedinejad, et al. “Two-dimensional mutually synchronized spin Hall nano-oscillator arrays for neuromorphic computing”, Nat. Nanotechnol. 15, 47–52 (2020). 
[4]: A. Houshang, et al. “Phase-Binarized Spin Hall Nano-Oscillator Arrays: Towards Spin Hall Ising Machines”, Phys. Rev. Applied 17, 014003 – Published 3 January 2022. 
[5]: T. Kendziorczyk, at al. “Mutual synchronization of nanoconstriction-based spin Hall nano-oscillators through evanescent and propagating spin waves”, Phys. Rev. B 93, 134413 –
Published 11 April 2016. 
[6]: C. C. M. Capriata, et al. "Impact of Random Grain Structure on Spin-Hall Nano-Oscillator Modal Stability", IEEE Electron Device Letters, vol. 43, no. 2, pp. 312-315, Feb. 2022. 
[7]: J. Leliaert, et al. "Adaptively time stepping the stochastic Landau-Lifshitz-Gilbert equation at nonzero temperature: Implementation and validation in MuMax3", AIP Advances 7,
125010 (2017).
Frequency stability of 2x2 NC-SHNO array under different conditions. 

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Grain Structure Influence on Synchronized Two Dimensional Spin

With the recent progress in pulsed high-brightness photocathodes, Lorentz imaging approaches in ultrafast transmission electron microscopy (UTEM) now offer a pathway for the investigation of magnetization dynamics with nanometer spatial and femtosecond temporal resolution [1,2]. Extending the available frequencies in field-driven ultrafast Lorentz microscopy to the GHz range, we developed an in-situ radiofrequency (RF) excitation sample holder for transmission electron microscopy based on a two-dimensional microwave cavity (Fig. 1a,b). As a first test case, we investigated ferromagnetic resonances in permalloy stripe arrays (Fig. 1f) which support localized magnonic modes identified by the back-reflected RF power (Fig. 1d,g). For ultrafast Lorentz microscopy, the RF excitation is phase-locked to the nanolocalized photoemission of ultrashort electron pulses from a Schottky field emitter, using high harmonics of an amplified laser system as a master clock for their synthetization [3]. Phase-resolved ultrafast Lorentz micrographs are expected to yield high-contrast FMR mode mapping. A simulated micrograph is shown in Fig. 1c. With these developments, we aim to establish ultrafast Lorentz microscopy as a versatile lab-scale imaging tool for femtosecond and picosecond transient states in ultrafast magnetism and magnonics.

Ultrafast Lorentz Microscopy with Magnetic Field Excitation at Microwave Frequencies

Plasmonic surface lattice resonances, visible in periodic arrays of nanodisks, arise from radiative coupling between the localized surface plasmon resonance[1-3] of individual particles and
diffracted orders[4]. Here, we study the impact of these optical modes on the magneto-optical properties[5,6] and energy absorption efficiency[7] of [Co/Gd/Pt]N nanodisks by measuring
the response of different arrays to optical excitations as function of the light wavelength, the disk diameter, and the array period. We demonstrate that surface lattice resonances allow AllOptical single pulse switching[8,9] of the nanodisks using much less energy than for the thin film, with an energy absorption enhanced by more than 400 %. Besides, these optical modes
increase the magneto-optical Faraday effect by more than 2000 %. The influence of the disk diameter and array period on the amplitude, width and position of the resonances is in
qualitative agreement with theoretical calculations and opens the way to design magnetic metasurfaces for all-optical magnetic recording. 
**References** 
[1] Liu, T.-M., et al. Nanoscale Confinement of All-Optical Magnetic switching in TbFeCo – Competition with Nanoscale Heterogeneity. Nanoletters, 15:6862 – 6868, 2015. 
[2] Savoini, M., et al. Highly efficient all-optical switching of magnetization in GdFeCo microstructures by interference-enhanced absorption of light. Physical Review B, 86:140404, 2012. 
[3] Cheng, F., et al. All-Optical Manipulation of Magnetization in Ferromagnetic Thin Films Enhanced by Plasmonic Resonances. Nanoletters. 20:6437 – 6443, 2020. 
[4] Kataja, M., et al. Surface lattice resonances and magneto-optical response in magnetic nanoparticle arrays. Nature Communications, 6:1 – 8, 2015. 
[5] Freire-Fernández, F., et al. Surface-plasmon-polariton driven narrow-linewidth magneto-optics in Ni nanodisk arrays. Nanophotonics, 9:113 – 121, 2019. 
[6] Freire-Fernández, F., et al. Magnetoplasmonic properties of perpendicularly magnetized [Co/Pt]N nanodots. Physical Review B, 101:054416, 2020. 
[7] Kataja, M., et al. Plasmon-induced demagnetization and magnetic switching in nickel nanoparticle arrays. Applied Physics Letters, 112:072406, 2018. 
[8] Radu, I., et al. Transient ferromagnetic-like state mediating ultrafast reversal of antiferromagnetically coupled spins. Nature, 472:205 – 208, 2011. 
[9] Lalieu, M. L. M., et al. Deterministic all-optical switching of synthetic ferrimagnets using single femtoseccond laser pulses. Physical Review B, 96:220411, 2017. 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/ce745f3e9cc680537e51e8ee5aa93ef2.png) 
Fig. 1. Magnetic [Co/Gd/Pt]N metasurface.

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/018ffca0228f5b63139eb1c00164c594.png) 
Fig. 2. a) Ratio of the All-Optical switching threshold fluence measured on the film and metasurfaces with P = 500 nm and different array diameters. b) Average Faraday readout-sensitivity
among 6 samples, defined as the ratio of the Faraday angle of [Co/Gd/Pt]N metasurfaces on resonance and in the continuous [Co/Gd/Pt]N films at the same wavelength, scaled with the
packing density.

Energy efficient single pulse switching of magnetic nanodisks using surface lattice resonances

One of the most promising magnetic systems studied in the last years are the 3D nanostructures with curved geometry where novel and interesting magnetization textures and dynamics are involved [1-2]. One of these nanostructures, cylindrical nanowires, present multiple topologically non-trivial magnetization structures such as Bloch-point domain walls, faster and more stable as compared to those in planar nanowires, vortices, and skyrmion tubes [2-3]. 
This talk presents different ways to manipulate the magnetization configuration and the domain wall dynamics in cylindrical nanowires. The control and stabilization of domains and domain walls have been achieved in multi-segmented structures [4] by alternating the magnetic anisotropy or by imprinting notches along the nanostructures length. The domain wall dynamics, under the influence of magnetic field or current pulses, was investigated by XMCD-PEEM and micromagnetic simulations. The results indicate that large current densities induce domain wall nucleation while smaller currents move domain walls preferably against the current direction but also along it. The pinning of domain walls has been observed not only at geometrical constrictions but also induced by the Oersted field which creates chiral domain walls. 
**References:** [1] R. Streubel et al, J. Phys. D: Appl. Phys. 2012, 49, 363001. 
[2] A. Fernández-Pacheco et al., Nat Commun 2017, 8, 15756. 
[3] J.A. Fernandez-Roldan et al., Nanoscale 2018, 10, 5923. 
[4] C. Bran et al., ACS Nano 2020, 14, 12819–12827.

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