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Ultrathin asymmetrically sandwiched ferromagnetic films support fast moving chiral magnetic domain walls and skyrmions [1,2]. This paves the way to the realization of prospective racetrack memory concept, the performance of which is determined by the static and dynamic micromagnetic parameters [3]. The necessity of having strong Dzyaloshinskii-Moriya interactions (DMI) and perpendicular magnetic anisotropy requires the utilization of ultrathin magnetic (~1 nm) layers, which compromized structural quality, that substantially enhances the magnetic damping for non-collinear magnetic textures. Here, we present the experimental and theoretical analysis of ultrathin Co films with asymmetric interfaces //CrO x /Co/Pt and estimation of their micromagnetic parameters based on the analysis of the temperature dependence of magnetization as well as imaging of the morphology of magnetic domain walls (DWs) in stripes. Namely, we show that the best fit to the magnetometry data up to room temperature is obtained within a quasi-2D model, accounting for the lowest transversal magnons [4]. The fit provides access to the exchange constant in asymmetric stackes which is found to be about 1 order of magnitude smaller compared to the bulk Co. The experimentally observed tilt of magnetic domain walls in stripes in statics can be explained based on two models: (I) A unidirectional tilt could appear in equilibrium as a result of the competition between the DMI and additional in-plane easy-axis anisotropy, which breaks the symmetry of the magnetic texture and introduce tilts [5]. (II) A static DW tilt could appear due to the spatial variation of magnetic parameters, which introduce pinning centers for moving tilted DWs driven by magnetic field and can fix them at remanence [6]. We found that the second model is in line with the experimental observations and allows to determine self-consistently the DW damping parameter and DMI constant for the particular layer stack. The DW damping is found to be about 0.1 and explained by the enhanced longitudinal relaxation mechanism. The latter is shown to much stronger tan the standard transversal relaxation and can be even stronger than the spin pumping contribution for the case of ultrathin ferromagnetic films [7]. References: [1] N. Nagaosa and Y. Tokura, “Topological properties and dynamics of magnetic skyrmions”, Nat. Nanotechnol. 8, 899 (2013). [2] A. Fert, N. Reyren, and V. Cros, “Magnetic skyrmions: advances in physics and potential applications”, Nat. Rev. Mater. 2, 17031 (2017). [3] C. Garg, S.-H. Yang, T. Phung, A. Pushp and S. S. P. Parkin, “Dramatic influence of curvature of nanowire on chiral domain wall velocity”, Sci. Adv. 3, e1602804 (2017). [4] I. A. Yastremsky, O. M. Volkov, M. Kopte, T. Kosub, S. Stienen, K. Lenz, J. Lindner, J. Fassbender, B. A. Ivanov and D. Makarov, “Thermodynamics and Exchange Sti ff ness of Asymmetrically Sandwiched Ultrathin Ferromagnetic Films with Perpendicular Anisotropy”, Phys. Rev. Appl. 12, 064038 (2019). [5] O. V. Pylypovskyi, V. P. Kravchuk, O. M. Volkov, J. Fassbender, D. D. Sheka and D. Makarov, “Unidirectional tilt of domain walls in equilibrium in biaxial stripes with Dzyaloshinskii–Moriya interaction”, J. Phys. D: Appl. Phys. 53, 395003 (2020). [6] O. M. Volkov, F. Kronast, C. Abert, E. Se. Oliveros Mata, T. Kosub, P. Makushko, D. Erb, O. V. Pylypovskyi, M.-A. Mawass, D. Sheka, S. Zhou, J. Fassbender and D. Makarov, “Domain-Wall Damping in Ultrathin Nanostripes with Dzyaloshinskii-Moriya Interaction”, Phys. Rev. Appl. 15, 034038 (2021). [7] I. A. Yastremsky, J. Fassbender, B. A. Ivanov, and D. Makarov, “Enhanced Longitudinal Relaxation of Magnetic Solitons in Ultrathin Films”, Phys. Rev. Appl. 17, L061002 (2022).

Conventional magnetic nanoscale devices are based on planar thin films and straight racetracks hosting magnetic topological solitons. Recent progress in fabrication and characterization methods allows to realise and study of complex-shaped planar and three-dimensional (3D) architectures. In the planar case, boundaries of nanodots lead to the formation of inhomogeneous textures, such as vortices and antivortices. In 3D, the magnetostatic interaction favours a spatially inhomogeneous shape anisotropy, which acts as easy-axis anisotropy along wires or hard axis of anisotropy perpendicular to the film surface. These interactions track the sample geometry and enable curvature-induced symmetry-breaking effects, such as topology-induced magnetization patterning and emergent anisotropic and chiral responses of the Dzyaloshinskii-Moriya interaction (DMI) type [1,2]. Curvature-induced magnetic responses can be classified as being local or nonlocal. In ferromagnets, local effects stem from the exchange interaction and DMI. The curvature-induced DMI originates from exchange: it is linear in curvatures and has the symmetry of the interfacial DMI. Its strength can be comparable with typical values of the intrinsic DMI. This is experimentally confirmed by the stabilization of chiral domain walls (CDW) on the apex of a Permalloy parabola-shaped stripe [3]. The strength of the CDW depinning field gives an estimation for the curvature-induced DMI constant and can be tuned by the geometry. In contrast to curvature itself, also curvature gradients offer a possibility to pin CDW, which was studied with an example of a circular indentation with a conic cross-section profile. This geometry supports circular CDWs described by the forced skyrmion equation, where the effective force acts as the stabilizing factor for large-radius skyrmion and skyrmionium states [4]. The magnetostatic interaction is a source of novel curvature-induced chiral effects, which are essentially nonlocal, in contrast to the conventional DMI [5]. The effect emerges in shells with non-zero mean curvature due to the non-equivalence between the top and bottom surfaces of a geometrically curved shell. It is possible to show that the analysis of nonlocal effects in curvilinear shells can be more intuitive with a split of a conventional volume magnetostatic charge into two terms: (i) tangential charge, governed by the tangent to the sample's surface, and (ii) geometrical charge, given by the normal component of magnetization and the mean curvature. In addition to the shape anisotropy (local effect), four additional nonlocal terms appear, determined by the surface curvature. Three of them are zero for any magnetic texture in shells with the geometry of minimal surfaces. The fourth term becomes zero only for the special symmetries of magnetic textures. The impact of local and nonlocal chiral effects on magnetic textures in curvilinear architectures will be discussed in this presentation.

Asymmetric ultrathin magnetic thin films represent intriguing materil platforms, which support emerging fundamentals effects, such as skyrmion and topological [1] Hall effects and fast motion of chiral magnetic non-collinear textures [2], that underlie prospective memory and logic devices based on spin-orbit torques. Such asymmetric stacks can provide strong perpendicular magnetic anisotropy and Dzyaloshinskii-Moriya interactions (DMI), which is necessary for the sabilization of chiral non-collinear magnetic textures. As the performance of spin-orbitronic devices is determined by the static and dynamic micromagnetic parameters [3], it is crucial to determine all internal micromagnetic parameters for the particular layer combination and sample geometry. In particular, the speed of a domain wall (DW) based racetrack is determined by the DMI constant, $D$, and the DW damping parameter, $\alpha$. The necessity of having strong DMI requires the utilization of ultrathin magnetic (~1 nm) layers, which implies polycrystalinity and compromized structural quality, that substantially enhances the magnetic damping compared to bulk. Accessing this parameters typically requires dynamic experiments, whose interpretations are cumbersome due to the creep regime. Here, we present the experimental and theoretical investigation of tilted DWs in perpendicularly magnetized asymmetric //CrOx/Co/Pt layer stacks with the surface-induced DMI. We will discuss two possible theoretical mechanism for the appearance of titled DWs: (I) A unidirectional tilt could appear in equilibrium as a result of the competition between the DMI and additional in-plane easy-axis anisotropy, which breaks the symmetry of the magnetic texture and introduce tilts [4]. (II) A static DW tilt could appear due to the spatial variation of magnetic parameters, which introduce pinning centers for DWs [5]. A moving DW can be trapped in a tilted state after the external driving field is off. Based on these theoretical approaches, we perform a statistical analysis of the DW tilt angles obtained in staticts after the external magnetic field used for the sample demagnetization was off. We found that the second approach confirms the experimental observations and allows to determine self-consistently the range of DW damping parameters and DMI constants for the particular layer stack. Using two reference fields, which provide two characteristic tilt angles, allow us to retrieve the range of DMI strength $D \geq 0.8$ mJ/m2 and DW damping parameters $\alpha \geq 0.1$. The upper limit for the DMI constant agrees with an independent transport-based measurement giving $D=0.90 \pm 0.13$ mJ/m2, which further refines our estimate of the damping parameter $\alpha=0.13 \pm 0.02$. Thus, the combination of the proposed method with standard metrological techniques opens up opportunities for the quantification of both static and dynamic micromagnetic parameters based on static measurements of the DW morphology. [1] N. Nagaosa and Y. Tokura, “Topological properties and dynamics of magnetic skyrmions”, Nat. Nanotechnol. 8, 899 (2013). [2] A. Fert, N. Reyren, and V. Cros, “Magnetic skyrmions: advances in physics and potential applications”, Nat. Rev. Mater. 2, 17031 (2017). [3] C. Garg, S.-H. Yang, T. Phung, A. Pushp and S. S. P. Parkin, “Dramatic influence of curvature of nanowire on chiral domain wall velocity”, Sci. Adv. 3, e1602804 (2017). [4] O. V. Pylypovskyi, V. P. Kravchuk, O. M. Volkov, J. Fassbender, D. D. Sheka and D. Makarov, “Unidirectional tilt of domain walls in equilibrium in biaxial stripes with Dzyaloshinskii–Moriya interaction”, J. Phys. D: Appl. Phys. 53, 395003 (2020). [5] O. M. Volkov, F. Kronast, C. Abert, E. Se. Oliveros Mata, T. Kosub, P. Makushko, D. Erb, O. V. Pylypovskyi, M.-A. Mawass, D. Sheka, S. Zhou, J. Fassbender and D. Makarov, “Domain-Wall Damping in Ultrathin Nanostripes with Dzyaloshinskii-Moriya Interaction”, Phys. Rev. Appl. 15, 034038 (2021).

Understanding the electrical manipulation of the antiferromagnetic order is a crucial aspect to enable the design of antiferromagnetic devices working at THz frequencies. Focusing on collinear insulating antiferromagnetic NiO Pt thin films as a materials platform, we identify the crystallographic orientation of the domains that can be switched by currents and quantify the N el vector direction changes. We demonstrate electrical switching between different T domains by current pulses, finding that the N el vector orientation in these domains is along [ 5 5 19], different compared to the bulk amp; 10216;112 amp; 10217; directions. The final state of the in plane component of the N el vector nIP after switching by current pulses j along the [1 10] directions is nIP amp; 8741;j. By comparing the observed N el vector orientation and the strain in the thin films, assuming that this variation arises solely from magnetoelastic effects, we quantify the order of magnitude of the magnetoelastic coupling coefficient as b0 2b1 3 107J m3. This information is key for the understanding of current induced switching in antiferromagnets and for the design and use of such devices as active elements in spintronic devices

Asymmetrically sandwiched thin magnetic layers with perpendicular anisotropy and Dzyaloshinskii-Moriya interaction (DMI) is the prospective material science platform for spin-orbitronic technologies that rely on the motion of chiral magnetic textures, like skyrmions or chiral domain walls (DWs). The dynamic performance of a DW-based racetracks is defined by the strength of DMI and the DW damping. The determination of the latter parameter is typically done based on technically challenging DW motion experiments. Here, we propose a method to access both parameters, DMI constant and DW damping, yet in static experiments by monitoring the tilt of magnetic DWs in nanostripes. We experimentally demonstrate that in perpendicularly magnetized //CrO x /Co/Pt stacks, DWs can be trapped on edge roughness in a metastable tilted state as a result of the DW dynamics driven by external magnetic field. The measured tilt can be correlated to the DMI strength and DW damping in a self-consistent way in the frame of a theoretical formalism based on the collective coordinate approach.

Curvilinear magnetism is the emerging field in micromagnetism which studies influences of external geometry and its topology on magnetic vector fields [1]. Much attention was paid to fundamental theoretical investigations of curvature-induced effects for local [2,3] and non-local magnetic interactions [4], which results in the prediction of various magnetochiral effects [2,5], topologically-induced magnetic patterns [5,6], stabilization of individual skyrmions [7,8] and skyrmion lattices [9] on curvilinear defects. Recently, we provided the very first experimental confirmation and quantitative assessment of the existence of the curvature-induced chiral interaction of exchange origin in a conventional soft ferromagnetic material [10]. In its turn, the interplay between the intrinsic and exchange-induced Dzyaloshinskii-Moriya interaction (DMI) paves the way to a mesoscale DMI [3], whose symmetry and strength depends both on the geometrical and material parameters of the magnetic system. Extending this concept we proposed a novel approach towards artificial magnetoelectric materials with helimagnetic nanohelices embedded in a piezoelectric matrix [11], where electric field could control magnetic states through the utilization of curvature-induced effects. [1] R. Streubel et. al., J. Phys. D: Appl. Phys. 49,363001 (2016). [2] Y. Gaididei et al., Phys. Rev. Lett. 112, 257203 (2014). [3] O. Volkov et al., Sci. Rep. 8, 866 (2018). [4] D. D. Sheka et al., Commun. Phys. 3, 128 (2020). [5] V. P. Kravchuk et al., Phys. Rev. B 85, 144433 (2012). [6] O. V. Pylypovskyi et al., Phys. Rev. Lett. 114, 197204 (2015). [7] V. P. Kravchuk et al., Phys. Rev. B 94, 144402 (2016). [8] O. V. Pylypovskyi et al., Physical Review Applied 10, 064057 (2018). [9] V. P. Kravchuk et al., Phys. Rev. Lett. 120, 067201 (2018). [10] O. M. Volkov et al., Phys. Rev. Lett. 123, 077201 (2019). [11] O. M. Volkov et al., J. Phys. D: Appl. Phys. 52, 345001 (2019).

The optical control of magnetism offers an attractive possibility to manipulate small magnetic domains for prospective memory devices on ultrashort time scales. Here, we report on the local deterministic transformation of the magnetic domain pattern from stripes to bubbles in out-of-plane magnetized Co/Pt multilayers controlled only by the helicity of ultrashort laser pulses. Relying on the experimentally determined average size of stripe domains and the magnetic layer thickness, we calculate the temperature and characteristic fields at which the stripe-bubble transformation occurs. Furthermore, we demonstrate that in the narrow range of the laser power, the helicity induces a drag on domain walls.

Dzyaloshinskii-Moriya interaction (DMI) is a key ingredient which allows to obtain chiral non-collinear magnetic textures, e.g. chiral domain walls and skyrmions. The conventional spin-orbit induced DMI emerges in gyrotropic crystals or at the interfaces. Therefore, tailoring of DMI is done by optimizing materials. A viable alternative to the material screening approach relies on the use of geometrically broken symmetries of conventional materials, where local geometrical curvatures generate effective exchange-induced DMI. Here, we provide the very first experimental confirmation of the existence of the curvature-induced DMI in a Permalloy parabolic nanostripe. By analyzing the evolution of transversal domain wall (DW) [1] under the influence of external field we correlate the depinning field of the DW with the curvature-induced DMI field. We put forth a framework to analyze this field and assess the strength of the effective DMI. [1] O. Volkov et. al, Physica Status Solidi – Rapid Research Letters, 1800309 (2018).

The chemically ordered B2 Fe50Rh50 alloy is antiferromagnetic. By inducing chemical disorder its structure can be changed to the ferromagnetic A2 structure. Following the laser writing method published here [1] we used a pulsed laser to induce ferromagnetism locally in Fe50Rh50 thin films of 10, 20, and 30nm thickness. XMCD measurements on the laser-treated region revealed the formation of an annulus of FM contrast and a non-FM center. Transmission electron microscopy (TEM) on a section through the annulus found the FM region to be A2 and the enclosed non-contrast region of the fcc A1 structure. The surrounding untreated region remained in the B2 structure.

International audience; We investigate the effect of electric current pulse injection on domain walls in La 0.7 Sr 0.3 MnO 3 (LSMO) half-ring nanostructures by high resolution x-ray magnetic microscopy at room temperature. Due to the easily accessible Curie temperature of LSMO, we can employ reasonable current densities to induce the Joule heating necessary to observe effects such as hopping of the domain walls between different pinning sites and nucleation/annihilation events. Such effects are the dominant features close to the Curie temperature, while spin torque is found to play a small role close to room temperature. We are also able to observe thermally activated domain wall transformations and we find that, for the analyzed geometries, the vortex domain wall configuration is energetically favored, in agreement with micromagnetic simulations.

Présentation orale; International audience