The recent development of magnetic tomography [1] and laminography [2] techniques has allowed the quantitative characterization of 3D magnetization vector maps in thin films and nanostructures. Thus, vector analysis methods can be used to understand the experimental magnetization configurations and obtain a unified description of the different types of 3D domain walls and magnetic singularities. In this work we have studied by X-ray vector magnetic tomography (XVMT) the 3D magnetization configuration of a 140 nm thick permalloy microstructure [3] and a ferrimagnetic GdCo/NdCo/GdCoâ€™ trilayer [4]. In the Py microstructure, we observe a domain wall with non-trivial 3D configuration decorated by Bloch points such as the one in Fig. 1: composed of a linear tail-to-tail wall and a circulating vortex across the sample thickness. The corresponding emergent field Be (calculated in the right part of Fig. 1) reveals the underlying topological singularity with charge -1, that acts as a sink of Be lines. Changes in domain wall chirality are mediated by topological dipoles and triplets linked by horizontal bundles of emergent field lines. In addition, surface magnetic textures and helical vortices are found at the cross sections of vertical bundles of Be lines [3]. In the GdCo/NdCo/GdCoâ€™ trilayer, the competition between anisotropy, exchange and magnetostatic interactions creates a peculiar stripe domain pattern with an exchange spring wall across the thickness at the top GdCo/NdCo interface [4]. The role of this boundary as a preferred site for nucleation of singularities will be discussed in detail (see e.g. the emergent field dipole in Fig. 2, composed of two Bloch points at different sample depths and opposite topological charges). Work supported by Spanish MICIN and Asturias FICYT.

X ray vector magnetic tomography of Bloch points in magnetic microstructures and multilayers

The spin Hall effect, and its inverse, are important spin-charge conversion mechanisms widely investigated due to their fundamental importance in the development of spintronics devices. Its measurement has been mostly related to electrical detection schemes involving an interface with another magnetic material and thus, a combination of the properties of both materials as well as the interface are measured. The large scattering of reported results from these methods for the same material and temperature call out for a more direct and interface-free approach. Optical detection schemes have been successfully used for the determination of SHE in semiconductor. However, this approach has proven challenging for metallic systems, due to their considerably shorter spin diffusion lengths. Only recently, optical measurements for Pt and W have been reported [1]. Considering that magnetic circular dichroism (XMCD) has become a reference tool for precision measurements of small or diluted magnetic signals, We propose the use of XMCD-PEEM microscopy for direct, interface-free determination of SHE in metals. In particular, we report the observation of spin separation due to SHE in a single layer of Bi-doped Cu (Cu95Bi5), a material in which giant SHE has been already reported [2,3]. We have performed interface free x-ray spectroscopy measurements at the Cu L3,2 absorption edges while applying electrical current to the sample. The sign of spin accumulation depends on the direction of the current and the amplitude of the X-ray magnetic circular dichroism (XMCD) signal scales with the density current and have different sign when measuring in the L2 or L3 absorption edges, as expected for SHE. We have measured an induced magnetic moment of (2.7 ± 0.5) x 10-12μB A-1 cm2 per Cu atom averaged over the probing depth, which is of the same order as for Pt measured by magneto-optics. Our results constitute the proof of concept for the direct, interface free and element-selective measurement of the SHE in a single material by means of X-ray spectromicroscopy and highlight the potential of CuBi for spin-charge conversion applications [4].

![](https://assets.underline.io/uploads/markdown_image/1/image/0f5bce09fb325fa1a3fcfbf281b8cd98.jpg)

(a) Visualization of the spin accumulation in a Cu95Bi5 electrode. (b) XMCD signal.

Direct X-ray detection of the spin Hall effect in CuBi

Magnetic layers interfaced with materials with strong spin-orbit coupling (SOC) (such as Pt, Ta) manifest several fascinating phenomena when the inversion symmetry is broken1. One direct consequence of SOC is the interplay between charge and spin transport via spin Hall effect (SHE) in heavy metals, which leads to non-equilibrium spin accumulation at the surfaces2. Another mechanism, the Rashba effect, arises when the electrical carriers move in an interfacial electric field and experience the resultant magnetic field that couples with their spins3. Both mechanisms give rise to spin-orbit torques (SOTs) in the magnetic layer, with damping-like and field-like components4. The SOTs are an efficient way to manipulate chiral magnetic textures, such as Néel domain walls (DWs)5,6 and skyrmions, whose chirality is induced by another product of SOC, the Dzyaloshinskii-Moriya interaction (DMI)7. However, up to now, the SOC was mainly studied in 3d-transition-metals (TM) (Co, Ni, Fe)/5d-heavy-metal (Pt). Very recently, interfacial DMI has been observed in TmIG/Pt and TbIG/Pt garnets thin films grown on Gd3Ga5O12 (GGG) substrates, which was attributed to the orbital moment in rare-earth (RE) material8. On the other hand, experiments on [Co/Tb]n multilayers did not show any SOT contribution from the RE9. The underlying origin of SOC and role of RE (specially Gd, which has no atomic orbital momentum) on SOC in ferrimagnets hence remain to be understood and need to be addressed by sophisticated experimental evidence. Here, we demonstrate that the effects of SOC can be obtained without any need of additional nonmagnetic heavy metal in a single layer GdFeCo ferrimagnet (RE-TM alloy). For this study, a Si/SiOx(100nm)//GdFeCo(5nm)/Al(5nm) film was grown by co-evaporation in ultra-high vacuum with a controlled stoichiometry. The ferrimagnet is composed of two sub-lattices of Gd (RE) and FeCo (TM) which are coupled antiferromagnetically. At the magnetic compensation temperature, TM ≈ 275K for this sample, the net magnetization vanishes (MS) and the anisotropy and the coercive fields diverge. To investigate the existence of SOC inside the ferrimagnetic layer, first we quantify the two components of current-induced effective fields, damping-like (HDL) and field-like (HFL) by using second harmonic Hall voltage measurement technique. The temperature dependence of HDL and HFL per current density in GdFeCo layer are shown in Figure 1(a) and 1(b). Both HDL/J and HFL/J diverge at , showing the expected scaling with 1/MS. Another phenomenon induced by SOC is the DMI, which favours chiral magnetic textures. In the single layer GdFeCo ferrimagnet, we observed chiral Néel domain wall structures of width (Δ) = 20 ± 10 nm, using photoemission electron microscopy combined with X-ray magnetic circular dichroism (XMCD-PEEM) as shown in Figure 2(a), which further indicate the presence of internal DMI or SOC in GdFeCo. The DMI is quantified using Brillouin light scattering (BLS) technique in Damon-Eshbach geometry and the DMI constant ( is obtained from the slop of frequency shifts (Δf) of nonreciprocal spin waves vs wave vector (figure 2(c)). Though the DMI amplitude is much smaller in GdFeCo if compared with Pt/Co and Ta/Co systems, the chiral DWs are stabilised due to lower net magnetization of GdFeCo, as the threshold Dc for Néel DW stabilisation is proportional to Ms. The occurrence of these phenomena also requires a broken inversion symmetry. Electron Energy-Loss Spectroscopy (EELS) studies in a scanning transmission electron microscopy (STEM) reveal an inhomogeneity in Gd concentration along the film thickness. This elemental inhomogeneity breaks the spatial inversion symmetry and the combination of broken inversion asymmetry and electronic hybridization of RE-5d and TM-3d electrons favors the emergence of net SOC effects (SOTs, DMI and chiral Néel DWs) inside the GdFeCo layer. These results show that phenomena that can only exist in systems with broken inversion symmetry and strong SOC -- spin-orbit torques, chiral textures, and DMI -- can occur in a ferrimagnetic thin layer without a heavy metal adjacent layer. This should be taken into account when analysing these phenomena even in bi-layers, and may be very useful for applications as it may allow removing the requirement for a heavy-metal layer and improving the efficiency of SOT-driven magnetisation switching.

**References:**

1. Soumyanarayanan, A. et al. Nature 539, 509–517 (2016). 2. Stamm, C. et al. Phys. Rev. Lett. 119, 087203 (2017). 3. Sánchez, J. C. R. et al. Nat. Commun. 4, 1–7 (2013). 4. Garello, K. et al. Nat. Nanotechnol. 8, 587–593 (2013). 5. Emori, S. et al. Nat. Mater. 12, 611–616 (2013). 6. Ryu, K.-S. et al. Nat. Nanotechnol. 8, 527–533 (2013). 7. Thiaville, A. et al. Epl 100, (2012). 8. Caretta, L. et al. Nat. Commun. 11, 1–9 (2020). 9. Yu, J. et al. Nat. Mater. 18, 29–34 (2019).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/906aed22f1aa90a30aad88a7fc2c9d8a.jpg)
Figure 1. (a) HDL and (b) HFL effective fields per current density as a function of temperature. Purple points correspond to an up saturated film state and khaki to down one. The diagrams represent the direction and magnitude of Gd (black) and FeCo (red) magnetic moments. The dotted lines are guides to the eyes.

![](https://assets.underline.io/uploads/markdown_image/1/image/4b4afae03c3181e42876985bb777aa9a.jpg)
Figure 2. (a) Multidomain XMCD-PEEM images at the Gd M4,5 edge (1178.7 eV) at T = 230 K. The green arrow shows the direction of the X-ray beam at a grazing angle of 16°. The dark (bright) contrast corresponds to a down (up) magnetic domain. (b) Intensity profiles (black lines) averaged over the yellow regions in (a). The thick yellow line is a fit to the experimental data using the theoretical profile. The presence of a peak or a dip in the line scan is an evidence of chiral Néel DWs. (c) The DMI induced frequency shifts (𝛥f) of nonreciprocal spin waves vs wave vector magnitudes |ksw| for GdFeCo thin film

 Direct Observation of Spin-Orbit Torques, Dzyaloshinskii-Moriya Interaction and Chiral Spin Textures in Single Layer Ferrimagnets

Thanks to its extreme surface sensitivity and element specificity, X-ray photoemission electron microscopy permits imaging magnetic nanostructures with sub-nanometer thickness and even fully compensated synthetic antiferromagnets. In addition, XPEEM is sensitive to both in plane and out of plane magnetization components, so it can unambiguously determine the character and chirality of domain walls. Several skyrmionic systems have been measured at the ALBA Synchrotron Light Source XPEEM, profiting the functional sample environments that allow applying small magnetic fields and/or current pulses in order to study skyrmion nucleation and propagation and the effect, for example, of disorder or of targeted modulations of the magnetic anisotropy. I will give an overview of the strenghts and limits of the technique, the experiments carried out in DMI systems over the past few years, and an outlook for the near future including instrumental upgrades in development.

**References:**

Olivier Boulle et al., Room temperature chiral magnetic skyrmion in ultrathin magnetic nanostructures, Nature Nanotechnology 11, 449 (2016) MIchael Foerster et al., Custom sample environments at the ALBA XPEEM, Ultramicroscopy 171, 63 (2016) Roméo Juge et al., Magnetic skyrmions in confined geometries: Effect of the magnetic field and the disorder, Journal of Magnetism and Magnetic Materials 455, 3 (2018) Lucia Aballe et al., Pulse picking in synchrotron-based XPEEM, Ultramicroscopy 202, 10 (2019) Roméo Juge et al., Current-Driven Skyrmion Dynamics and Drive-Dependent Skyrmion Hall Effect in an Ultrathin Film, Physical Review Applied, 12, 044007 (2019) Anna Mandziak et al., Combining high temperature sample preparation and in-situ magnetic fields in XPEEM, Ultramicroscopy 214, 113010 (2020) Sachin Krishnia et al., Making spin-orbit coupling visible in single layer ferrimagnets: direct observation of spin-orbit torques and chiral spin textures, submitted Teng Xu et al., Ferrimagnetic Néel skyrmions stabilized on top of noncollinear antiferromagnetic Mn3Sn, submitted Kumari Gaurav Rana et al., Room temperature skyrmions in an exchange biased antiferromagnet, submitted

**Images:**

 A close look at skyrmions in ultrathin films and synthetic antiferromagnets.

ROB: Recent Advances In Magnetic Imaging

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Presentations

X ray vector magnetic tomography of Bloch points in magnetic microstructures and multilayers

Direct X-ray detection of the spin Hall effect in CuBi

Direct Observation of Spin-Orbit Torques, Dzyaloshinskii-Moriya Interaction and Chiral Spin Textures in Single Layer Ferrimagnets

A close look at skyrmions in ultrathin films and synthetic antiferromagnets.

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