The actual mechanisms occurring at interfaces underlying the Dzyaloshinskii-Moriya interaction (i-DMI) remain largely an open question in nanomagnetism. In this study, we investigate the origin of the i-DMI, aiming at estimating how independent the DMI contributions of the two interfaces of a FM layer are and what their relative weight in the effective DMI amplitude is. The effective DMI is measured by the study of the asymmetric domain wall propagation in the presence of an in-plane magnetic field, in Pt|Co|M stacks. These measurements are completed by Brillouin light scattering (BLS) measurements. We then explore the correlation between the effective i-DMI and the properties of metal M, namely, its atomic number, electronegativity, and work function. A clear linear relationship is found between the i-DMI and the work function difference at the Co|M interface (ΔΦ) [1,2]. We define Δφ = φM - φCo. This result is a strong evidence of the independent DMI contributions of the two interfaces for the chosen Co thickness (1 nm). These findings may guide the optimization of the magnetic properties of future spintronic devices.
This work has been supported by DARPA TEE program grant (MIPR HR-0011831554), ANR grant TOPSKY (ANR-17-CE24-0025), FLAG-ERA SographMEM (ANR-15-GRFL-0005), and the Horizon2020 Framework Program of the European Commission, under FET Proactive Grant Agreement No. 824123 (SKYTOP) and under Marie Sklodowska- FQ Curie Grant Agreement No. 754303 for supporting J.P.G together with the LABEX LANEF in Grenoble (ANR-10-LABX-0051). Univ. Paris-Saclay, CNRS INP, Sesame IMAGeSPIN project (nEX039175) and the LABEX NanoSaclay (ANR10 LABX0035, BLS@PSay and SPiCY projects) are acknowledged for the BLS equipment. 
**References** [1] F. Ajejas et al., Phys. Rev. Mat., June 2022
[2] Y.-K. Park et al., NPG Asia Mater. 10, 995 (2018)
 
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Interfacial potential gradient modulates Dzyaloshinskii Moriya interaction in Pt|Co|Metal multilayers

Spin-orbit interaction in metals and its ability to generate spin-current has been the hallmark of spintronics in the last decade. Beyond its fundamental interest as a source of spin-current, the manipulation of magnetization via transfer of the spin angular momentum has proven to be energy efficient for spin-based memory (e.g. SOT-MRAM) and also neuromorphic devices (e.g. using spin Hall nano-oscillators). Therein, charge to spin conversion is believed to be governed essentially by two main mechanisms, spin Hall effect in the bulk of heavy-metals and Rashba effect at interfaces [1], the latter being often considered negligible in all-metallic interfaces. However, in the case of atomically thin metallic layers, the underlying SOT physical mechanisms at play have not been experimentally fully tackled. 
In this study, we examine the impact of the insertion of a light element interface on the nature of the SOT as well as its efficiency in terms of damping-like (HDL) and field-like (HFL) effective fields in ultrathin ferromagnets. Importantly, we observe unexpectedly large HFL/HDL ratio (∼2.5) upon inserting a 1.4 nm thin Al layer in Pt|Co|Al|Pt as compared to Cu instead of Al as shown in Fig. 1. From our modeling (dotted lines in Fig.1), these experimental results strongly evidence the presence of a large interfacial Rashba effect at Co|Al interface producing giant HFL. The occurrence of such enhanced torques from an interfacial origin is further validated by reducing the contribution from SHE in the bottom Pt layer as well as by demonstrating current-induced magnetization reversal at reduced current. We believe that our results are important from the application point of view as they provide a clear route for reaching ultimate spin-torque efficiency for the associated devices[2]. 
**Acknowledgement:** DARPA TEE (MIPR no. HR0011831554), the Horizon2020 Framework Program of the European Commission under FET-Proactive Grant agreement No. 824123 (SKYTOP) and the the Agence Nationale de la Recherche, France, No. ANR-17-CE24-0025 (TOPSKY). 
**References:** [1] Manchon, A. et al., Rev. Mod. Phys. 91, (2019). 
[2] Krishnia S. et al., arXiv : 2205.08486. 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/d8fd135b8e9157bc0dc4a5f819d2811b.png) 
Co thickness (tCo) dependence of (a) DL-SOT field and (b) FL-like SOT fields in Pt8|Co (tCo)|Al1.4|Pt3 (red ) and Pt8|Co (tCo)|Cu1.4|Pt3 (blue ) samples (c) ζ = HFL/HDL as a function of tCo.

Large interfacial Rashba interaction and giant spin orbit torque in atomically thin metallic multilayers

Magnetic topological textures could serve as powerful tools in the development of next-generation spintronics devices, especially in terms of stability and energy consumption. Two-dimensional (2D) magnetic solitons such as chiral domain walls or skyrmions were mostly under focus for the last decade. Beyond these 2D textures, a new interest has risen for more complex quasi-particles that display variations over the thickness, i.e., three-dimensional (3D) objects. Examples include different skyrmion phases [1], bobbers [2] or even hopfions [3]. In this work, we show how by engineering Pt/Co/Al based multilayers with variable Co thickness, we observe the signature of new textures, called skyrmionic cocoons [4] that are only present in a fraction of the magnetic layers. Interestingly, these cocoons can coexist with more standard ‘tubular’ skyrmions going through all the multilayer as evidenced by the existence of two very different contrasts in the magnetic force microscopy (MFM) images recorded at room temperature (Fig. 1a). Moreover, the field dependence of skyrmionic cocoons has also been studied with Fourier transform holography measurements (FTH) with similar observations (Fig. 1b). FTH additionally puts forward various magnetic phase transitions as well as the existence of an attractive interaction between cocoons. The presence of these novel skyrmionic textures has also been investigated by micromagnetic simulations (Fig. 1c). The relaxed states can easily be identified with the different experimental contrasts: the columnar skyrmion tubes (resp. cocoons) corresponds to the strong contrast (resp. weak contrast). Thus, this coexistence of various magnetic textures in multilayers and the discovery of a novel magnetic texture are particularly interesting as they can open new paths for three-dimensional spintronics. 
**References** [1] A.O. Mandru et al. Nature communications 11,1 (2020)
[2] F. Zheng et al. Nature Nanotechnology 13, 451 (2018)
[3] N. Kent et al. Nature communications 12, 1 (2021)
[4] M. Grelier et al. arXiv: 2205.01172 (2022). 
![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/b8a97625e75c0927f841711b49e2982a.png)

Skyrmionic 3D cocoons in magnetic multilayers

Recently, topological insulator(TI)/ferromagnetic(FM) systems, such as Bi2Se3/FM [1,2], Bi1-xSbx/FM [3,4] have attracted much attention with their promising properties for emerging memory technologies, such as magnetic random-access memories(MRAM). Indeed, these systems may drastically reduce the writing current using spin-orbit torque(SOT) switching. In this case SOT is generated by injecting a charge current into the TI topologically protected surface states(SS), then converted into a spin current in the FM layer. The charge current may shunt the TI SS through the metallic FMs which conductivity is often higher and may not contribute efficiently to the SOT[5]. Thus, FMs with lower conductivities are required and may help to better understand the SOT effect. Also, for applications, FMs with high Curie temperature and perpendicular magnetic anisotropy(PMA) are desirable.

In this work, we focused on the molecular beam epitaxy (MBE) growth conditions of Bi1-xSbx(top)/MnxGa1-x(bottom) heterostructure on the semi-insulating GaAs(001) substrate. A high epitaxial quality was confirmed by the high-resolution scanning transmission electron microscopy (STEM) as shown in Fig.1. The Bi0.85Sb0.15 top surface states have been characterized by performing ARPES measurements. Electronic band structure with topological surface states was observed. Mn0.45Ga0.55 exhibit a PMA with a magnetization of 315 kA/m.

To investigate the charge-to-spin current conversion of Bi0.9 Sb0.1(9 nm)/Mn0.45Ga0.55(7 nm) we first performed anomalous Hall effect (AHE) measurements at various temperatures. A large value of ΔRAHE = 0.4Ω was obtained at room temperature. We then perform second harmonic Hall voltage measurements in order to quantify the damping-like HDL(Fig.2.) and field-like SOT HFL by applying an ac current and in-plane external magnetic field. For this relatively large MnGa thickness (7nm), a value of HDL=2.6±1.3mT for a current density of 1010 A/m2 was estimated. Part of future work will focus on controlling the growth of a thinner layer. 
**References** [1] Mellnik, A. et al. Spin-transfer torque generated by a topological insulator. Nature 511, 449–451 (2014)
[2] Wang, Y., Zhu, D., Wu, Y. et al. Room temperature magnetization switching in topological insulator-ferromagnet heterostructures by spin-orbit torques. Nat Commun 8, 1364 (2017).
[3] Fan, T., Khang, N.H.D., Nakano, S. et al. Ultrahigh efficient spin orbit torque magnetization switching in fully sputtered topological insulator and ferromagnet multilayers. Sci Rep 12, 2998 (2022)
[4] Tuo Fan, Nguyen Huynh Duy Khang, Takanori Shirokura, Ho Hoang Huy, and Pham Nam Hai , Low power spin–orbit torque switching in sputtered BiSb topological insulator/perpendicularly magnetized CoPt/MgO multilayers on oxidized Si substrate, Appl. Phys. Lett. 119, 082403 (2021)
[5] Liu, L. et al. Spin-torque switching with the giant spin Hall efect of tantalum. Science 336, 555–558 (2012) 
![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/690c6c54f05b97ed3e575f4921018052.png)

Growth, characterization and spin orbit torque of BiSb topological insulator/MnGa perpendicular ferromagnet systems

Room-temperature skyrmions have been presented as potential candidates for encoding information bits in several types of new spintronic devices [1]. There have been many recent progresses allowing to stabilize sub-100 nm skyrmions at room temperature in crystalline ferromagnetic thick layers or in (amorphous) thin magnetic multilayers (MML) [2]. In most case, at least in extended films or large structures, the application of relatively large perpendicular field (a few 10 mT) is needed which might be unfavorable for most of the anticipated skyrmion based devices. Hence, the stabilization of zero-field isolated skyrmions or skyrmion lattices is an important step towards application. There are recent works demonstrating isolated zero-field skyrmions stabilized due to interlayer exchange in polycrystalline [3], epitaxial [4] or exchange biased [5] trilayers. Here we demonstrate a new approach showing that room-temperature zero-field skyrmion lattices can be stabilized in MML. We develop a system in which perpendicular magnetic anisotropy, chiral order and bias field strength, through a non-magnetic spacer, can be adjusted simultaneously. Typical stacking sequence is schematically presented in Figure 1 (a). The samples have been characterized first by standard magnetometry (Fig. 1b). Relying on the interlayer electronic coupling to an adjacent bias magnetic layer with strong perpendicular magnetic anisotropy (uniform configuration), we demonstrate by Magnetic Force Microscopy that the remnant wormy-stripe “as-grown” domains (Fig. 1d) can be turned into skyrmion lattice phase (Fig. 1c). The skyrmion density and size can be modified by finely tuning the thicknesses of the layers allowing the control of the parameters related in the stabilization of the lattices. ANR TOPSKY (ANR-17-CE24-0025), DARPA TEE program (MIPR#HR0011831554) and EU SKYTOP (H2020 FET Proactive 824123) are acknowledged for financial support. 
**References** [1] A. Fert, N. Reyren, V. Cros, Nat. Rev. Mat. 2, 17031 (2017)
[2] C. Moreau-Luchaire et al., Nat. Nanotechnol. 11, 444 (2016)
[3] G. Chen et al., Appl. Phys. Lett. 106, 242404 (2015)
[4] R. Loconte et al., doi : 10.1021/acs.nanolett.0c00137
[5] K. G. Rana et al., Phys. Rev. Applied 13, 044079 (2020)
 
![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/c7358254fecfd168f1a707d1ac7e39b6.png)

Zero field room

In nanomagnetism and spintronics, magnetic multilayers continue to thrive as pivotal structures to engineer complex non-trivial spin textures thanks to the tunability of their composition and their properties. Two-dimensional magnetic textures such as skyrmions or chiral domain walls were mostly under focus for the last decade, but recently interest has surged for more complex objects which display an inhomogeneous behavior over the vertical dimension [1,2]. Interesting examples include bobbers which could become remarkable assets for the development of logic devices [3] or the recently observed hopfions [4] or even different skyrmions phases [5]. Our own efforts to engineer and explore three-dimensional textures in Pt/Co based multilayers allows us to observe the signature of new textures, which we name skyrmionic cocoons. At low magnetic field, they resemble tubular skyrmions but upon an increase of the out-of-plane field, they shrink and disappear in the outer layers (Fig 1.b,c), becoming elongated ellipsoids. To observe such textures, we grew multilayers displaying variable thicknesses of a ferromagnetic material, thereby introducing a non-uniform effective anisotropy. By carefully tuning the thickness of each magnetic layer, it was possible to observe in a single sample two distinct types of objects, as shown by the strong difference in the Magnetic Force Microscopy (MFM) contrasts (Fig 1.a). With the support of micromagnetic simulations (Fig 1.b), we identify them as two vertically coupled cocoons (large contrast) and single cocoons only present in half of the layers (weak contrast). The existence and field-dependent behavior of such textures could represent interesting possibilities for potential applications.

![](https://assets.underline.io/uploads/markdown_image/1/image/e2c9cf21f889d9f4fd7c83a4aa3ac691.jpg)

Fig 1. a) MFM measurement at 265 mT on a multilayer with variable thickness of Co. b) Corresponding micromagnetic simulations at different fields displayed with isosurfaces, mz=0 (white) and mz=-0.8 (red), with plane cuts identified by the coloured lines. The yellow dotted ellipses evidence the two kinds of objects correlated with the MFM image: coupled cocoons (left) and single cocoons (right). c) Simulated vertical cuts showing the evolution of a cocoon at different out-of-plane magnetic fields.

Non-Collinear Three-Dimensional Textures In Magnetic Multilayers: Hatching of Skyrmionic Cocoons

Charge-spin interconversion using large Spin Orbit (SO) material has been intensively studies thanks to potential spintronic application in information and communication technology. It has been demonstrated that the interface between heavy metal and both metallic and insulating ferromagnetic layers could lead to spin current absorption and manipulation of the magnetization driven by electrical pulse[1][2]. More recently Rashba surface states and Topological insulator (TI) have been reported like a good candidate for the spin to charge interconversion[3]. Nevertheless, current shunting by metallic ferromagnet at the interface with TI makes hard to address unambiguously the charge to spin conversion from surface states. As a supplementary effect, the role of proximity effect and exchange interaction in between ferromagnetic material and TI needs to be clarified. To comprehend this issue, we first grow a thin film of magnetic insulator Tm3Fe5O12(TmIG) by off-axis sputtering and demonstrated the high quality of the film in the thickness range of 5 to 15 nm (Figure. 1(a)). We achieved to demonstrate, as a reference experiment, spin current absorption at the interface in between Pt and TmIG by 2nd harmonic experiment to extract torque and magnetization reversal driven by pulse current experiment. The high quality of the film makes possible to growth by MBE Bi2Se3 TI on top of it as illustrated in Figure 1(b). The TmIG films exhibit perpendicular magnetic anisotropy (Figure 1 (c)) with nearly bulk magnetization. We will address a comparative study of the interfacial effect when using SHE material (Pt) or TI (Bi2Se3) with ferromagnetic insulator TmIG. This work has been supported by the Defense Advanced Research ProjectsAgency (DARPA) Topological Excitations in Electronics (TEE) program, through Grant MIPR No. HR0011831554.

![](https://assets.underline.io/uploads/markdown_image/1/image/d2384c7e071cf59f2e36c9d864aa2f2a.jpg)

Figure 1 (a) X-Ray (θ-2θ) diffraction patterns recorded on TmIG(5-15nm) thin films grown on GGG substrate. (b) RHEED patterns recorded before (on TmIG surface) and after Bi2Se3 growth. (c) Magnetization hysteresis recorded in out-of-plane field on TmIG(15nm) sample.

Towards the development of Tm3Fe5O12 magnetic insulator and Bi2Se3 topological insulator for charge-spin interconversion

Microwave response of magnetic textures in ferromagnets offers rich physics and has been intensively investigated over the past decades for their potential application in magnonics. Eigenexcitations of films featuring bubble domains [1,2] and stripes [3] have been accounted for, broadly consisting of acoustic, optical and domain wall oscillation modes. Recently, topologically non-trivial chiral magnetic structures called skyrmions [4] have gathered a lot of interest owing to their robust, tiny and particle-like nature. They exhibit unique spectral signatures which have been observed mainly in bulk systems below room temperature [5,6]. The experimental observation and interpretation of their resonant dynamics in thin films remain elusive owing to high damping coefficients. Here we report room temperature investigation of resonant skyrmion dynamics in [Pt/CoFeB/AlOx]×20 multilayers. We optimize our sample by tuning the different layer thicknesses to host magnetic skyrmions at room temperature, along with a fairly low damping parameter α= 0.02. We use magnetic force microscopy to observe the static magnetic domain configuration consisting of randomly distributed skyrmions and dense skyrmion lattice as a function of an out-of-plane magnetic field. Broadband ferromagnetic resonance measurements are performed to study the dynamic response with an in-plane excitation field. Besides the Kittel mode (KM) above saturation, three distinct modes, a low frequency mode (LFM), an intermediate frequency mode (IFM) and a high frequency mode (HFM), appear with unique frequency-field dispersions in the non-saturated state (see Fig). Micromagnetic simulations reveal the characteristics of these modes, where the LFM features a mode localized in the skyrmion edges, while for the IFM skyrmion edge dynamics couples with the uniform background. In the HFM, the precession of skyrmion cores emits omnidirectional spin waves into uniform background, producing interference patterns in the lattice. Our results thus open up new avenues for skyrmion-based magnonic devices.

![](https://assets.underline.io/uploads/markdown_image/1/image/20e701a47ef97bc8c284fee76af80155.jpg)

Fig : Frequency-field dispersion map measured by FMR over a frequency range of 0.1 GHz to 20 GHz with an out-of-plane magnetic field swept from -500 mT to 500 mT.

Localized and propagating spin-wave modes in thin film multilayers hosting skyrmions

Generation of spin-current and its transport across interfaces, as well as its interaction with magnetic moments, are the central themes of spintronic community to enable fast and efficient magnetization dynamics for data storage devices. There are two main mechanisms to produce the spin-current in non-magnetic materials: (a) spin Hall effect (SHE) in heavy metals such as Pt or Ta and (b) Rashba effect (RE) at the interfaces1. Irrespective of the origin, the spin-current exerts torques on adjacent ferromagnetic layer with two components: damping-like (DL) & field-like (FL)2. However, disentangling properly SHE from RE remains highly challenging. In this experimental study we identify the mechanisms of spin-transport in ultrathin magnetic multilayer whose thicknesses range from less to above the characteristic spin- decoherence length. To this aim, we quantify the different SOT contributions in series of samples Pt8|Cox|Al1.4|Pt3 and P8|Cox|Cu1.4|Pt3 (as a reference ) with x varying from 0.4 to 1.4 nm. We extract, in each case, a spin decoherence length to be around 1.1 nm. Surprisingly, SOT investigation in the Pt8|Cox|Al 1.4|Pt 3 reveals a dominating FL torque (HFL/HDL&gt;1), which is a signature of a strong Rashba interface in Al-based systems. As a comparison, we find that the Rashba interaction is small in Pt8|Cox|Cu1.4|Pt3 samples. Moreover, we determine the SOTs characteristics in Pt(x)/Co(0.9)/Al(5) by varying the Pt thickness (tPt). The bulk spin Hall angle of Pt(θSHE) and ratio of HFL and HDL as a function of tPt are shown in Figure 1(a) and (b). The increase in HFL/HDL in the range of Pt thickness where the bulk SHE of Pt started decreasing, strongly favors our argument of the presence of Rashba interface3, exhibiting thus the specific role of Al insertion layers to enhance spin-orbit effects. Financial support from the Agence Nationale de la Recherche, France, No. ANR-17-CE24-0025 (TOPSKY), the Horizon2020 Framework Programme of the European Commission under FET-Proactive Grant agreement No. 824123 (SKYTOP), FLAGERA SOGRaphMEM and the DARPA TEE programme through grant MIPR no. HR0011831554 are acknowledged

![](https://assets.underline.io/uploads/markdown_image/1/image/ae330d77631623209411e1c48a048ff1.jpg)

Figure 1(a) θSHE vs tPt in Ta(5)/Pt(x)/Co(0.9)/Al(5). (b) Ratio of HFL/HDL as a function of tPt.

Rashba Spin-Orbit Torque in Interface Engineered Ultrathin Metallic Multilayers

EOE: Spin-Orbitronics I

technical paper

#### Welcome to the 67th Annual Conference on Magnetism and Magnetic Materials, MMM 2022!

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FOB: Spin Hall and Spin-Orbit Torque I

EOD: Skyrmions II

EOF: Spin-Orbitronics II

**Please click on the poster to see the abstract or leave your comment to the author.** 
**Session Chair:** Yue Li, *Argonne National Laboratory*

EPB: Skyrmions and Spin-Orbitronics

poster

**Session chair:** Aisha Aqeel, University of Munich



GOG - Fundamental Interactions in Skyrmions and Novel Phases

The 15th Joint MMM-INTERMAG Conference (2022 Joint) is sponsored jointly by AIP Publishing and the IEEE Magnetics Society. Members of the international scientific and engineering communities interested in recent developments in fundamental and applied magnetism are invited to attend and contribute to the technical sessions. The technical program will include invited and contributed papers in oral and poster sessions, invited symposia, a plenary session, and an evening session, with about 1500 presentations overall. This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

The 2022 Joint Conference will offer both in-person and prerecorded on-demand content.

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2022 Joint MMM-INTERMAG

**Session chair:** Chuanpu Liu, Colorado State University



GOI - Magnetization Control

**Session chair:** Stanislas Rohart, Université Paris-Saclay Faculte des Sciences d'Orsay



HOD - DMI, Skyrmions and other Topological Objects

**Session chair:** Jorge Puebla, RIKEN



Nicolas Reyren

9

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Presentations

Interfacial potential gradient modulates Dzyaloshinskii Moriya interaction in Pt|Co|Metal multilayers

Large interfacial Rashba interaction and giant spin orbit torque in atomically thin metallic multilayers

Skyrmionic 3D cocoons in magnetic multilayers

Growth, characterization and spin orbit torque of BiSb topological insulator/MnGa perpendicular ferromagnet systems

Zero field room

Non-Collinear Three-Dimensional Textures In Magnetic Multilayers: Hatching of Skyrmionic Cocoons

Towards the development of Tm3Fe5O12 magnetic insulator and Bi2Se3 topological insulator for charge-spin interconversion

Localized and propagating spin-wave modes in thin film multilayers hosting skyrmions

Rashba Spin-Orbit Torque in Interface Engineered Ultrathin Metallic Multilayers

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