United States

In this talk, we first report the roadmap and development of advanced perpendicular magnetic tunnel junctions based on low-damping magnetic materials with crystalline anisotropy that can be scaled down to sub-3-nm diameter with 10 years’ retention time [1]. Then, we will present the perpendicular magnetic tunnel junctions (p-MTJs) switched utilizing bipolar electric fields. Traditional voltage-controlled magnetic anisotropy only linearly lowers the energy barrier of ferromagnetic layer via electric field effect and efficiently switches p-MTJs only with a unipolar behavior. Here we propose and demonstrate a bipolar electric field effect switching of 100-nm p-MTJs through voltage-controlled exchange coupling (VCEC) [2]. The switching current density, ~1.1×105 A/cm2, is one order of magnitude lower than that of the best-reported spin-transfer torque devices. Theoretical results suggest that electric field induces a ferromagnetic-antiferromagnetic exchange coupling transition and generates a field-like interlayer exchange coupling torque, which cause the bidirectional magnetization switching. We will further report our recent results to switch the p-MTJs using the interplay of SOT and VCEC, which lowers the VCEC switching current density to ~5×103 A/cm2[3]. These results could eliminate the major obstacle in the development of spin memory devices beyond their embedded applications. This technology can be used directly for the future spin-orbit-torque (SOT) MRAM [3] and spin logic devices [4].&lt;br&gt;
**References**&lt;br&gt; [1] D. Zhang, et al, Phys. Rev. Applied, (2018) vol. 9, 044028&lt;br&gt;
[2] D. Zhang, et al, Nano Letters (2022), doi.org/10.1021/acs.nanolett.1c03395&lt;br&gt;
[3] B. Zink, et al, Advanced Electronic Materials, (2022), DOI: 10.1002/aelm.202200382&lt;br&gt;
[4] M. Mankalale, et al, IEEE Journal on Exploratory Solid-State Computational Devices and Circuits, 3 (2017) 27-36; 10.1109/JXCDC.2017.2690629&lt;br&gt;

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MMM 2022

Perpendicular Magnetic Tunnel Junctions with Crystalline Anisotropy and Energy efficient Switching of Synthetic Antiferromagnet pMTJs by voltage

In this talk, we first report the roadmap and development of advanced perpendicular magnetic tunnel junctions based on low-damping magnetic materials with crystalline anisotropy that can be scaled down to sub-3-nm diameter with 10 years’ retention time [1]. Then, we will present the perpendicular magnetic tunnel junctions (p-MTJs) switched utilizing bipolar electric fields. Traditional voltage-controlled magnetic anisotropy only linearly lowers the energy barrier of ferromagnetic layer via electric field effect and efficiently switches p-MTJs only with a unipolar behavior. Here we propose and demonstrate a bipolar electric field effect switching of 100-nm p-MTJs through voltage-controlled exchange coupling (VCEC) [2]. The switching current density, ~1.1×105 A/cm2, is one order of magnitude lower than that of the best-reported spin-transfer torque devices. Theoretical results suggest that electric field induces a ferromagnetic-antiferromagnetic exchange coupling transition and generates a field-like interlayer exchange coupling torque, which cause the bidirectional magnetization switching. We will further report our recent results to switch the p-MTJs using the interplay of SOT and VCEC, which lowers the VCEC switching current density to ~5×103 A/cm2[3]. These results could eliminate the major obstacle in the development of spin memory devices beyond their embedded applications. This technology can be used directly for the future spin-orbit-torque (SOT) MRAM [3] and spin logic devices [4]. 
**References** [1] D. Zhang, et al, Phys. Rev. Applied, (2018) vol. 9, 044028 
[2] D. Zhang, et al, Nano Letters (2022), doi.org/10.1021/acs.nanolett.1c03395 
[3] B. Zink, et al, Advanced Electronic Materials, (2022), DOI: 10.1002/aelm.202200382 
[4] M. Mankalale, et al, IEEE Journal on Exploratory Solid-State Computational Devices and Circuits, 3 (2017) 27-36; 10.1109/JXCDC.2017.2690629 

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technical paper

#### Welcome to the 67th Annual Conference on Magnetism and Magnetic Materials, MMM 2022!

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LaMn3+O3 is an AFM insulator with TN = 140 K but transforms into a FM insulator with Co2+ or Ni2+ substituted at the Mn-site in La2CoMnO6 and La2NiMnO6 (1), accompanied by a change of Mn valence from 3+ to 4+. However, LaMn0.96Mo0.04O3 is a metallic ferromagnet at low temperatures (2). The FM Curie temperature TC decreased only by 7 K upon 6% Mo6+: d0 substitution in La0.67Sr0.33Mn1-xMoxO3 (3). It was stated that Mo6+ doping induced Mn2+ (d5) which interacted with Mn3+(d4) through Zener’s double exchange giving rise to FM and metallic behavior. But, studies on Mo-doped manganites are yet rare. Here, we report the physical characterization of LaMn0.94Mo0.06O3 prepared by solid-state route. Fig.1(a) shows the temperature dependence of magnetization (M) and inverse susceptibility (Χ-1) measured under H = 1kOe. Fig. 1(b) shows the dc resistivity (ρ) and thermopower (S). Tc obtained from the inflection point of dM/dT is 237 K whereas the Χ-1 deviates from the Curie-Weiss behavior below 260 K. This is possibly due to Griffiths phase, i.e. nano-sizes FM clusters preformed above TC in the PM matrix. The ρ shows an IM transition with a peak at 235 K. However, S shows a peak around 258 K, which is about 20 K above the TC, suggesting that S is sensitive to nucleation of FM nano domains or short-range FM correlation. Fig. 1(c) shows ESR signal as a derivative microwave absorption as a function of dc magnetic field at selected temperatures near FM transition. The spectrum at 300 K is typical of ESR. While the spectrum at 300 K can be fitted to a single Lorentzian curve, the spectrum at 250 K reveals another low-intensity peak at a lower field (~1900 Oe) in addition to the ESR signal around 3200 Oe. The intensity of the ESR-signal decreases dramatically at 245 K but the intensity of the low-field peak increases. At 230 K, we do not see the ESR signal. These results indicate that the low field peak is due to FMR, but this feature start appearing above TC confirms that FM metallic nano domains are preformed above TC in the PM state and increase in size with lowering temperature and percolate leading to long-range FM and metallicity. 

**References:** (1) G. Blasse, Ferromagnetic interactions in non-metallic perovskites, J. Phys. Chem. 26, 1969 (1965);S. Vasala and M. Karppinen, A2B’B’’O6 perovskites: A review, Prog. Soild State Chem. 42, 1 (2015). 

(2) W. J. Lu et al., Induced ferromagnetism in Mo-substituted LaMnO3, Phys. Rev. B, 73, 174425 (2006). 

(3) L. Chen et al., Critical behavior of Mo-doping La0.67Sr0.33M1-xMoxO3 perovskite, Physica B, 404,1879 (2009). 

R. M. thanks the Ministry of Education, Singapore for supporting this work (Grant no. R144-000-442-114)


(a) T-dependence of M and X-1; (b) T-dependence of ρ and S; 1(c) ESR at selected T

Magnetism and Electron Spin Resonance (ESR) in Mo doped LaMnO3

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

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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Laser induced terahertz wave emission from Bi/Co structure

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

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

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.

Nucleation and Motion of Domain Walls in Cylindrical Nanowires

Magnetic skyrmions are compact chiral spin textures that exhibit a rich variety of topological phenomena and hold potential for the development of high-density memory devices and novel computing schemes driven by spin currents. In this invited talk, we will demonstrate the room-temperature interfacial stabilization and current-driven control of skyrmion bubbles in the ferrimagnetic insulator Tm3Fe5O12 coupled to Pt, showing the current-induced motion of individual skyrmion bubbles (1). The ferrimagnetic order of the crystal together with the interplay of spin–orbit torques and pinning determine the skyrmion dynamics in Tm3Fe5O12 and result in a strong skyrmion Hall effect characterized by a negative deflection angle and hopping motion. Further, we will show that the velocity and depinning threshold of the skyrmion bubbles can be modified by exchange coupling Tm3Fe5O12 to an in-plane magnetized Y3Fe5O12 layer, which distorts the spin texture of the skyrmions and leads to directional-dependent rectification of their dynamics. This effect, which is equivalent to a magnetic ratchet, is exploited to control the skyrmion flow in a racetrack-like device. 

**References:** 
(1) S. Vélez et al., Nature Nanotechnology (2022). https://doi.org/10.1038/s41565-022-01144-x 

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