Synthesis, structural and magnetic characterization of thin films of the chiral magnets CoZnMn and FeGe

Skyrmions are topologically protected nanometer-sized magnetic vortices that have sparked interest for future spintronics applications aiming at high-density, high-speed and low-power consuming magnetic memory devices. One type of skyrmions appears in non-centrosymmetric materials with spin-orbit coupling. Bulk materials such as FeGe and CoZnMn have been in the focus of recent research as they host skyrmion lattices near or above room temperature. The interest in thin-film materials has immensely increased as they promise better control of skyrmions. The main challenges are the synthesis of skyrmion-hosting materials with high critical temperatures and the compatibility with Si-based thin-film technologies. Currently most of the studied thin films do not exhibit the relevant properties. It has been argued that strain and defects related to the lattice mismatch inhibit the skyrmion phase.Here we present novel approaches to synthesize skyrmion-hosting thin films based on FeGe and CoZnMn: 1) the crystallization of an amorphous thin film on a vdW substrate (commercial graphene), 2) van der Waals (vdW) growth on graphene as a substrate to achieve a strain- and defect-free growth of the overlayer.Sputtered FeGe layers were obtained on graphene on oxidized silicon substrates and annealed via rapid thermal (RTA) and flash lamp annealing (FLA). The goal was to use these annealing techniques to guide the material to the temperature/composition window of the cubic phase. For this, we varied the thin film thickness, Fe:Ge ratio, pre-heating and annealing temperature. The thin film temperature during the FLA was studied through heat transfer simulations. The experiments uncovered the long annealing time via RTA with a capping layer leads to 4 types of morphology: islands, discs, individual crystals and crystals on discs. The sub-second annealing via FLA led to the polycrystalline thin film formation. The obtained materials contain hexagonal and cubic FeGe, Fe-rich phases, pure Ge and Fe oxides analysed by Raman spectroscopy and X-ray diffraction. We provide Raman vibrational modes and the symmetry description from first-principles calculations for FeGex with x=1, 1.5 and 2 to fast and locally characterize thin films and microstructures.Molecular beam epitaxy was used to grow CoZnMn thin films of different stoichiometries on graphene on oxidized Si. It has been found in bulk crystals that the critical temperature can be systematically engineered beyond room temperature by tuning the composition. We report on the structural, compositional and magnetic properties of Co10-xZn10-yZnx+y (0

van der Waals Epitaxy of Co10–xZn10–yMnx+y Thin Films: Chemical Composition Engineering and Magnetic Properties

Ping Che, Simon Robert Escobar Steinvall, Anna Fontcuberta i Morral, Dirk Grundler, Mohammad Hamdi, Jean-Baptiste Leran, Andrea Mucchietto, Rajrupa Paul

Topologically protected magnetic skyrmions have raised interest for future spintronics applications. One of the main challenges is the synthesis of room temperature skyrmion-hosting materials that are compatible with thin-film technology. We present an approach to produce strain-free epitaxial thin films of Co10–xZn10–yMnx+y using molecular beam epitaxy. Bulk Co10–xZn10–yMnx+y is known to host skyrmions at room temperature for specific composition ratios. Our substrate consists of graphene on oxidized silicon. The van der Waals interactions of the grown material with graphene prevents covalent bonding and corresponding strain. We show how defects in the graphene foster nucleation that results in three different kinds of morphologies: islands, columns, and merged films. Susceptibility measurements suggest a phase transition close to room temperature. We detect up to three spin waves resonances suggesting relatively low magnetic damping. This growth technique opens a new route for the integration of complex alloys and skyrmionic device concepts with silicon electronics.

2021

Spin-valley optoelectronics based on two-dimensional materials

Dmitrii Unuchek

Two-dimensional (2D) materials, in particular graphene and transition metal dichalcogenides (TMDC), have attracted great scientific interest over the last decade, revealing exceptional mechanical, electrical and optical properties. Owing to their layered nature, subnanometer-thick single-layer forms of these crystals can be chemically grown or mechanically isolated, representing an ultimately scaled-down platform for further miniaturization of electronic devices. Indeed, the large family of 2D materials contains hundreds of members, including metals, semiconductors, insulators, and others, covering the whole spectrum of potential applications. Availability of all necessary building blocks for the construction of all-2D solid-state devices makes this platform ideal for fabrication of ultrathin, transparent and flexible devices. Extensively investigated graphene has demonstrated excellent electrical properties. However, the absence of a bandgap is an obstacle for its interaction with light and therefore limits applications in optics. In this aspect, atomically thin TMDC semiconductors appear to be an alternative, more promising platform, as they possess a direct band gap in the visible range in the monolayer form. Despite being only three atoms thick, these semiconductors demonstrate exceptionally large light absorption, efficient light emission, and strong light-matter interaction, attracting justified interest from the optics community. We will focus on their potential applications for optoelectronics in Chapter 4. Even more, broken inversion symmetry and strong spin-orbit coupling in single-layer TMDC crystals reveal a new quantum number, the so-called valley index. Indeed, this unique degree of freedom of charge carriers is known for some materials since the 1970s. However, in the case of 2D semiconductors, spin-valley locking mechanism and valley contrasting optical selection rules open new ways for addressing, manipulation and sensing of this pseudospin, opening the whole new field of valleytronics. Due to the large splitting of the valence band, spin and valley degree of freedom become locked in these atomically thin materials. We employ this unique feature in Chapter 5 for indirect injection of spins into graphene by pumping valley polarized carriers in an adjacent TMDC monolayer. Being gapless, graphene does not allow direct optical injection of spins. Another striking feature of 2D materials is their unique ability to be stacked in vertical heterostructures with strong electrical coupling between layers. The weak van der Waals force, which keeps components together, provides freedom in the assembly process, so that a lattice mismatch or a twist angle becomes unimportant in the first approximation. This provides a novel platform for harvesting complementary properties of the constituent materials, allowing the engineering of new artificial meta-materials as we demonstrate in Chapter 6. Furthermore, synergistic effects in van der Waals heterostructures enable completely new properties and phenomena, which do not exist in the single materials, with twist angle being an important knob for tuning these effects. We observe and exploit such novel phenomena arising in heterostructures of 2D semiconductors in the last chapter of this thesis. On the way to the realization of practical optoelectronic and valleytronic applications, this thesis studies fundamental aspects of rich spin-valley physics of atomically thin semiconductors

EPFL2019

Atomic layer deposition of Ni and Ni80Fe20 for tubular spin-wave nanocavities

Maria Carmen Giordano

Magnetic thin films and magnetic nanostructures have become essential components of modern technological applications. Modern branches of magnetisms focus on spin-charge coupling (spintronics) and the collective excitation of spin waves in magnetically ordered materials (magnonics). In particular, spin waves represent a promising charge-free medium to encode and transmit information in the GHz frequency regime, relevant for microwaves technologies. The ever-increasing demand for compact and portable electronic and telecommunication devices leads to the need for further miniaturization and high integration density of their functional components. These aspects have prompted nanomagnetism to venture the third dimension. This development is also motivated by novel physical phenomena and functionalities predicted to arise from three-dimensional (3D) complex magnetic configurations. In order to advance the research in 3D spintronics and 3D magnonics, it will be essential to have control over the fabrication of 3D nanoarchitectures and to access the new properties of individual nanostructures emerging from new complex 3D spin-textures. These two challenges are addressed in this thesis. Ferromagnetic nanotubes (NTs) represent the ideal 3D system for the study of shape dependent static and dynamic magnetic properties. Their magnetic configurations, as well as the spin wave confinement and propagation inside them, can be engineered by changing their geometrical parameters. First, in order to fabricate 3D magnetic device architectures, we explored atomic layer deposition (ALD). The thickness and quality of the thin films deposited with ALD are irrespective of the geometry of the surface on which they are deposited. This characteristic makes it ideal to fabricate 3D nanomagnets by magnetically coating 3D nanotemplates, e.g. nanowires (NWs) in the case of NTs. In this thesis we propose ALD processes for the conformal coating by means of two ferromagnetic materials conventionally used in planar spintronics and magnonics systems: Ni and permalloy (Py, Ni80Fe20), the second material expected to have a lower spin wave damping. By identifying correlations between ALD process parameters and thin film properties we optimized the materials in terms of conformality, electrical resistivity, anisotropic magnetoresistance (AMR) effect, spin wave damping and, in the case of permalloy, also stoichiometry. The optimized materials were then transferred onto GaAs nanowires templates, obtaining ferromagnetic NTs with hexagonal cross sections. We achieved Ni NTs with the lowest resistivity ever reported in similar ALD-grown Ni systems and a large AMR effect. We observed the lowest spin waves damping for permalloy NTs, exhibithing a value comparable with permalloy thin films achieved with conventional planar deposition techniques. Second, we investigated magnetic states and spin waves confinement in nanotubes experimentally and via micromagnetic simulations. We combined magnetoresistance measurements with Brillouin light scattering spectroscopy (BLS) and micromagnetic simulations to study Ni and Py NTs. BLS measurements, performed while irradiating NTs with microwaves, show that these nanotubes form spin-wave nanocavities which impose discrete wave vectors and confine GHz microwave signals on the nanoscale. Our experimental results show that the confinement can be engineered by changing the NT's geometrical parameters and magnetic states.