Seminar experimentelle Physik der kondensierten Materie

Resonant inleastic X-ray scattering (RIXS) especially in the soft X-ray region has seen a tremendous increase in applicability and scientific insight over the recent years. This was largely enabled by progress in instrumentation and theoretical description. Now the time is ripe to apply RIXS to pressing problems and develop the technique further making full use of the capabilities of novel X-ray sources. In my talk, I will address three main themes from my research: 1. Time-resolved RIXS at free-electron lasers applied to relevant dynamic processes in chemistry (on surfaces, in liquids and in solid catalysts) 2. RIXS with micrometer spatial resolution to resolve domain dynamics in complex materials and on devices in-operando 3. Non-linear spectroscopies in the soft X-ray range to enhance information content and signal levels I will show and discuss experimental results from all research themes and point to future development directions.

From 2D skyrmions to 3D cocoons : nucleation, motion and electrical detection of non collinear topogical spin textures Vincent Cros Unité Mixte de Physique, CNRS, Thales, Université Paris-Saclay, 91767, Palaiseau, France. In the last decade, magnetic multilayers have proven to be essential structures for creating and investigating complex, topologically non-trivial spin textures through the ability to tune their composition and properties [1-2]. Two-dimensional magnetic textures such as skyrmions (or chiral domain walls) were mostly under focus. First, I will share some of our recent results showing the skyrmion nucleation can be precisely controlled using injection of current pulses through artificial notches and show how the spin-orbit torques, responsible for the skyrmion motion, can be optimized in multilayers. More specifically, I will explain how, in atomically thin Co, the SOTs amplitudes, both for damping and field-like symmetries, varies significantly when a light element, such as Al is deposited on top of Co, surpassing the values existing in literature [3]. Then I will describe how the presence and the displacement of skyrmions can be precisely followed through a simple electrical detection. By relying on our ability to perform fully-electrical manipulation and detection of magnetic skyrmions in multilayers, I will present some recent device developments for performing a basic unconventional computation operation in hardware. Beyond the 2D skyrmions, a strong interest has emerged for more complex magnetic objects which display a non-homogeneous behavior over the vertical dimension, giving them a 3D character e.g. magnetic bobbers [4] or the recently observed hopfions [5]. In the second part of my talk, I will present our recent results on 3D spin textures, called skyrmionic cocoons [6], that have a typical ellipsoidal shape and that can be stabilized in aperiodic magnetic multilayers with a variable thickness for the ferromagnetic elements. Interestingly, these skyrmionics cocoons can coexist with more standard tubular skyrmions going through all the multilayer as evidenced by the existence of two very different contrasts in room temperature magnetic force microscopy. They can also be electrically detected using magneto-transport measurements, an interesting feature for potential applications. The presence of these novel skyrmionic textures as well as the understanding of their layer resolved chiral and topological properties have been investigated by micromagnetic simulations. Finally, I will describe how the use of x-ray holography and x-ray laminography gives a precise insight into the 3D distribution of the magnetization which demonstrate the 3D nature of skyrmionic cocoons. Financial supports from FLAG-ERA SographMEM (ANR-15-GRFL-0005), from ANR MEDYNA (ANR-20-CE42-0012), from “Investissements d’Avenir" program SPiCY (ANR-10-LABX-0035), from France 2030 government grant (ANR-22-PEPR-Electronique-EMCOM and ANR-23-PEPR-Spin) and the EU Horizon2020 Programme under FET-Proactive Grant agreement No. 824123 (SKYTOP) are acknowledged. [1] A. Fert, N. Reyren and V. Cros, Nat. Rev. Materials 2, 17031 (2017) [2] K. Everschor-Sitte et al, J. Appl. Phys. 124, 240901 (2018) [3] S. Krishnia, VC et al, arXiv:2205.08486 (2022) [4] F. Zheng et al. Nat. Nanotech., 13, 451 (2018) [5] N. Kent et al. Nat. Comm. 12, 1 (2021) [6] M. Grelier, VC et al. Nature Comm, 13, 6843 (2022)

Magnetic skyrmions are nanoscale, chiral topological solitons which exhibit a wide variety of interesting dynamical phenomena that have solicited much interest for fundamental reasons and technological applications alike. In this talk, I will discuss some recent experimental and theoretical results on two aspects of skyrmion dynamics in ferromagnetic thin film systems. The first involves the resonant dynamics in multilayered films of [Pt/FeCoB/AlOx]20, which are found to host dense robust skyrmion lattices at room temperature with a relatively low Gilbert damping of ∼0.02 [1]. Broadband ferromagnetic resonance measurements, combined with micromagnetic simulations, reveal distinct resonant modes detected in the skyrmion lattice phase. These are found to involve localised excitations, along with skyrmion core precession emitting spin waves into uniform background with wavelengths in the 50–80 nm range. The second aspect involves thermal diffusion of skyrmions in frustrated systems under spin-orbit torques, where the helicity dynamics leads to an anomalous drift that strongly depends on the strength of the Dzyaloshinskii-Moriya interaction. Such drift processes suggest the importance of helicity coupling to spin-orbit torques and may have bearing on dipole-stabilized bubbles for which drive-dependent skyrmion Hall angles and low drift velocities have been reported. [1] T. Srivastava et al, arXiv:2111.11797 [cond-mat.mes-hall].

Spintronics, based on the freedoms of charge and spin of the electron, is a key technology in the 21st century. Magnetic random access memory (MRAM), which uses giant magnetoresistance (GMR), has several advantages compared with electronics. Although conventional magnets composed of transition metals are normally used, in our study, we use molecule-based nano-magnets and single-molecule magnets (SMMs) to overcome “Moore`s Limitation”. SMMs are also available for quantum computer. I will talk about the molecular spin qubits for quantum computer ([1]Crystal Engineering Method, [2]g-Tensor Engineering Method, [3]Orbital Engineering Method, and [4]Molecular Technology Method) as well as high-density memory devices such as single-molecule memory device, SMMs encapsulated into SWCNT, and metallic conducting SMMs with negative magnetoresistances.