Interlayer DMI (IL-DMI) has been recently predicted and observed in multilayer thin film heterostructures1-3, coupling two neighboring magnetic films via a non-magnetic interlayer in a chiral fashion. IL-DMI could be utilized to obtain complex chiral magnetic states in one layer, controlled by the magnetic configuration of a neighboring layer, of great interest to spintronic applications. Previous work using magnetometry techniques indicates the existence of non-zero IL-DMI in Synthetic Antiferromagnetic (SAF) bilayers with highly asymmetric properties; one layer close to the spin reorientation transition, and the other fully out-of-plane. In these systems, IL-DMI is manifested as a chiral exchange bias under in-plane magnetic fields. In this contribution, we further investigate these systems via a combination of X-ray magnetic scattering and imaging techniques, revealing the presence of periodic chiral magnetic structures. In particular, X-ray Magnetic Circular Dichroism Photo Electron Emission Microscopy (XMCD-PEEM) measurements evidence the formation of ring-like structures of sizes after applying a field cycling protocol (Fig. 1). In order to understand more in detail the magnetic configuration of these textures, we reconstruct the spatially resolved magnetization vector by combining different relative sample-beam XMCD projections, yielding the presence of 360 degree domain walls separating the outer and inner domains. During the presentation, we will discuss in detail the role that IL-DMI and other magnetic interactions play in the formation and shape of these rings.

Tunable in plane chirality via Interlayer DMI interactions in Synthetic Anti

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

Cylindrical magnetic nanowires are a three-dimensional system in which magnetization may be manipulated by spin-polarized currents in order to controllably nucleate, move and pin magnetic domain walls (DWs). Such control makes them excellent candidates for three-dimensional storage devices such as in the well-known concept of Racetrack memories [1]. Due to their geometry, cylindrical nanowires can host a DW with curling magnetization not subject to the Walker breakdown limitation [2], and thus that can reach velocities over 1 km/s. This kind of DW is known as the Bloch-Point wall (BPW) [3] [4] and it was experimentally reported under static conditions for the first time in 2014 [5]. Recently, velocities above 600 m/s driven by spin-transfer torques have been also reported [6]. In order to achieve the desired control of DW motion, effective pinning sites need to be set along the wire’s axis. Here, we propose a system based on cylindrical Permalloy (Fe20Ni80) nanowires with evenly spaced chemical barriers: Fe-rich segments (Fe80Ni20) of length ranging from 20 to 100 nm and 1 μm separation. We present an overview of their static micromagnetic configuration as well as their response to applied field and nano-second electric current pulses. This is a prerequisite to investigating and understanding the interplay of DWs with chemical modulations. Quantitative understanding is achieved combining magnetic imaging by means of X-ray Magnetic Circular Dichroism (XMCD) coupled to PhotoEmission Electron Microscopy (PEEM), Scanning Transmission X-ray Microscopy (STXM) or X-ray ptychography with &lt;15 nm spatial resolution (Figure 1), and micromagnetic simulations with mumax3 and feeLLGood codes, as well analytical modelling. In addition, Transmission Electron Microscopy (TEM) combined with electron holography was used to extract valuable qualitative and quantitative information on the local magnetic behavior. The Fe-rich barriers have a spontaneous magnetization of 1.4 T whereas the Permalloy segments of 0.8 T. Such magnetization change at the interface generates volume charges and thus a dipolar energy increase. This energy cost can be overcome with a vortex state at the chemical barrier where the magnetization remains axial at the core and curling at the shell. The XMCD images indeed show a curling magnetic state at the chemical barriers (Figure 2). In addition, magnetic images are supported by a quantitative post processing code that simulates the magnetic contrast coming from XMCD-PEEM technique for a specific three dimensional magnetization texture, allowing us to extract the curling angle of the magnetization with respect to the wire’s axis. The strength of curling increases with both the modulation length and wire diameter, with a clear analogy with the vortex state in flat magnetic disks. The strength of curling was assessed by imaging under large axial magnetic field. The chirality of curling is random at rest but can be switched deterministically by means of the Oersted field of nanosecond current pulses, showing again a clear correlation between the strength of curling and the geometry of the modulations.

**References:**

 [1] S. S. P. Parkin et al, Science, vol. 320, no. 5873, p. 190 (2008). [2] R. Hertel, Journal of Physics: Condensed Matter, vol. 28, no. 48, p. 483002 (2016). [3] H. Forster et al., Journal of Applied Physics, vol. 91, no. 10, pp. 6914–6919 (2002). [4] A. Thiaville and Y. Nakatani, ‘Spin dynamics in confined magnetic structures III’, vol. 101 pp. 161–205 (2006). [5] S. Da Col et al., Phys. Rev. B, vol. 89, no. 18, p. 180405 (2014). [6] M. Schöbitz et al., Phys. Rev. Lett., vol. 123, no. 21, p. 217201 (2019).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/0655064c9bacad1f07583fcda00e9f63.jpg)
Figure 1: Reconstructed amplitude ptychographic XAS image at Fe L3 edge of a permalloy cylindrical nanowire of 140 nm of diameter with a Fe-rich chemical barrier (Fe80Ni20) of 100 nm in length (dark region).

![](https://assets.underline.io/uploads/markdown_image/1/image/2e86f6a91ded7f93aaa462b2471792da.jpg)
Figure 2: Reconstructed XMCD ptychographic image of the chemical barrier shown in Fig. 1. The curling magnetization is indicated by the dark and light bipolar contrasts.

 Micromagnetics of Chemical Barriers Inserted within Permalloy Cylindrical Nanowires: Towards the Control of Domain Wall Motion

In the last decades, transition metal oxides are growing in importance in spintronic applications. There are many different magnetic oxides, all of them very stable, with a wide range of properties that could allow broadening the range of spintronic applications [1]. Furthermore, the physical properties of these materials can be tuned by reducing the material to the nanoscale, tuning the composition, and/or incorporating it to heterostructures. However, in many cases, growth defects such as antiphase boundaries can strongly affect their magnetic properties, acting as pinning centers for magnetic domain walls. Considering their potential, it is essential to develop growth methods that allow engineering high quality oxide-based epitaxial nanostructures with customizable functionalities [2], as well as to develop the tools to fully characterize them. Extremely high-quality oxide films can be obtained by depositing metals by molecular beam epitaxy on a hot substrate, while the sample is exposed to an oxidizing agent such as molecular oxygen. We have used this method to fabricate ultrathin spinel islands of up to 100 µm2 area, with nanometer thickness and atomically flat surfaces. By this method many spinel islands (CoFe2O4 [3], Fe3O4 [4] and NiFe2O4 [5]) grow from single crystallographic nuclei and are thus expected to be antiphase boundaries-free. The extremely low defect concentration leads to a robust magnetic order and exceptionally large magnetic domains. Different microscopy techniques can be used to explore the magnetic configuration of these nanoobjects but few provide information about the vector configuration of the magnetic moments. In this work we use PhotoEmission Electron Microscopy with X-Ray Magnetic Circular Dichroic contrast (XMCD-PEEM) in order to obtain 3D information of the magnetic configuration of in-situ grown single crystal nanometer-thick islands, by acquiring images at different photon beam incidence angles. The 3D magnetization maps were used as the initial magnetization configuration for micromagnetic simulations of islands with the same lateral and vertical dimensions as the experimental ones. A careful analysis of the magnetic configuration was done by combining XMCD-PEEM with atomic force microscopy and Low Energy Electron Microscopy (LEEM) maps in order to tackle which effects might be responsible for the differences between measured and simulated experimental domain landscape. We will show how the combination of XMCD-PEEM, LEEM and modelling is a very powerful tool for the study of magnetic configurations of nanometer sized objects, and how the atomic steps in the Ru substrate give rise to efficient pinning sites for the domain walls in the particular case of ultrathin CoFe2O4 ferrites.

**References:**

[1] S. Gariglio, et al. Rep. Prog. 82, 012501, (2019) [2] S. Farokhipoor et al. Nature, 515, 379, (2015). [3] L. Martín-García et al. Adv. Mater. 27, 5955, (2015) [4] S. Ruiz-Gómez, et al. Nanoscale 10, 5566 (2018). [5] A. Mandziak et al. Scientific reports 8, 1, (2018).

**Images:**

 Domain wall pinning in epitaxial spinel ferrites grown on Ru

Permanent magnets are receiving an increasing economical impact due to their role in the Low Carbon Green Transition being employed for energy generation (wind generators), energy storage (flywheels) and electric transport. However, the most performing magnets, composed by rare earth (RE) elements, are in the head of the list of the Critical Raw Materials due to the recent uncertainty in supply and the increase of the costs of the RE. This last question has pushed the research of alternatives to RE magnets. Between the different solutions, the improvement of the performances of the hard ferrites is receiving an increasing interest [1-4]. In fact, ferrite magnets constitute by far the most widely used permanent magnet by weight [2,3] and any improvement of the performances is expected to have an important impact [4]. The most promising approach to improve the performances of ferrite magnets is the development of nanocomposites formed by magnetically coupled high magnetization and hard magnetic nanophases [4]. We present a research on a novel class of hybrid magnet composed by micrometric nanowires (NWs) and micrometric ferrites that exhibit improved hard magnetic properties. We have investigated the properties of composites containing micrometric FeCo nanowires and micrometric SrFe12O19 powders [5] to produce bonded magnets. FeCo NWs were prepared by template-assisted electrochemical deposition using nanoporous templates and polycarbonate nanoporous membranes to obtain NWs with different diameters (30, 50, and 100 nm). FeCo NWs−strontium ferrite mixture composites in the powder form were fabricated by mixing each phase. The evolution of the magnetic properties of the mixtures considering the different diameter of the NWs and the soft-hard ratio will be shown. The coercive field of the composites decreases as the soft contain is larger, while remanence magnetization remains almost constant but also increases depending on NWs size. The magnetic coupling between the two phases has been investigated using remanence measurements. Even the soft nature of the NWs, the hard and soft phases are magnetically coupled up to a 20%wt of the soft phase in the case of the NWs of 50 nm of diameter. A 20% increase in remanence together with a weak decrease in coercivity, as consequence of the magnetodipolar interaction between ferrite particles and FeCo NWs, is observed. In fact, the oriented bonded composites magnets exhibit a larger energy product up to 40% larger than the corresponding ferrite ones. The magnetic properties of these magnets will be discussed considering the magnetic configuration of the Nanowires. This research was supported by EU- H2020 AMPHIBIAN Project (H2020-NMBP-2016-720853) and by the JECS Trust.

**References:**

[1] EU-H2020, AMPHIBIAN Project. Website : amphibianproject.eu. [2] R. C. Pullar, Progress in Materials Science 57, 1191 (2012). [3] S H Mahmood and I Abu-Aljarayesh Hexaferrite Permanent Magnetic Materials. Materials Research Forum LLC, (2016) [4] C. de Julián Fernández et al., J. Applied Phys. D: Applied Physics, (2020) in press

**Images:**

 Metal Nanowire−Strontium Ferrite Composites for Free Rare-Earth Magnets

LOA: Thin Films and Patterned Structures

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Presentations

Tunable in plane chirality via Interlayer DMI interactions in Synthetic Anti

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

Micromagnetics of Chemical Barriers Inserted within Permalloy Cylindrical Nanowires: Towards the Control of Domain Wall Motion

Domain wall pinning in epitaxial spinel ferrites grown on Ru

Metal Nanowire−Strontium Ferrite Composites for Free Rare-Earth Magnets

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