arXiv subjects

Samir Lounis

Publications and source records attributed to Samir Lounis.

At least 37 records · Page 2Linked to original sources

arXiv2024

Newly discovered magnetic phase: A brief review on Altermagnets

Recently, a new magnetic phase, termed altermagnetism, has caught the attention of the magnetism and spintronics community. This newly discovered magnetic phenomenon differs from traditional ferromagnetism and antiferromagnetic. It generally lacks net magnetization and is characterized by unusual non-relativistic spin-splitting and broken time-reversal symmetry. This leads to novel transport properties such as the anomalous Hall effect, the crystal Nernst effect, and spin-dependent phenomena that cannot be fully explained by traditional magnetic theories. Spin-dependent phenomena such as spin currents, spin-splitter torques, and high-frequency dynamics emerge as key characteristics in altermagnets. This paper reviews the main aspects pertaining to altermagnets by providing an overview of theoretical investigations and experimental realizations. We discuss the most recent developments in altermagnetism, its comparison to other magnetic orders, and future prospects for exploiting its unique properties in next-generation devices.

cond-mat.str-el↗

arXiv2024

Topological magnetism in diluted artificial adatom lattices

The ability to control matter at the atomic scale has revolutionized our understanding of the physical world, opening doors to unprecedented technological advancements. Quantum technology, which harnesses the unique principles of quantum mechanics, enables us to construct and manipulate atomic structures with extraordinary precision. Here, we propose a bottom-up approach to create topological magnetic textures in diluted adatom lattices on the Nb(110) surface. By fine-tuning adatom spacing, previously inaccessible magnetic phases can emerge. Our findings reveal that interactions between magnetic adatoms, mediated by the Nb substrate, foster the formation of unique topological spin textures, such as skyrmions and anti-skyrmions, both ferromagnetic and antiferromagnetic. Since Nb can be superconducting, our findings present a novel platform with valuable insights into the interplay between topological magnetism and superconductivity, paving the way for broader exploration of topological superconductivity in conjunction with spintronics applications.

cond-mat.mes-hall↗

arXiv2024

Kalman filter enhanced Adversarial Bayesian optimization for active sampling in inelastic neutron scattering

Spin waves, or magnons, are fundamental excitations in magnetic materials that provide insights into their dynamic properties and interactions. Magnons are the building blocks of magnonics, which offer promising perspectives for data storage, quantum computing, and communication technologies. These excitations are typically measured through inelastic neutron or x-ray scattering techniques, which involve heavy and time-consuming measurements, data processing, and analysis based on various theoretical models. Here, we introduce a machine learning algorithm that integrates adaptive noise reduction and active learning sampling, which enables the restoration from minimal inelastic neutron scattering point data of spin wave information and the accurate extraction of magnetic parameters, including hidden interactions. Our findings, benchmarked against the magnon spectra of CrSBr, significantly enhance the efficiency and accuracy in addressing complex and noisy experimental measurements. This advancement offers a powerful machine learning tool for research in magnonics and spintronics, which can also be extended to other characterization techniques at large facilities.

cond-mat.mtrl-sci↗

arXiv2024

Ultrafast light-induced magnetization in non-magnetic films: from orbital and spin Hall phenomena to the inverse Faraday effect

The field of orbitronics has emerged with great potential to impact information technology by enabling environmentally friendly electronic devices. The main electronic degree of freedom at play is the orbital angular momentum, which can give rise to a myriad of phenomena such as the orbital Hall effect (OHE), torques and orbital magnetoelectric effects. Here, we explore via realistic time-dependent electronic structure simulations the magnetic response of a non-magnetic material, an ultrathin Pt film, to ultrafast laser pulses of different polarizatons and helicities. We demonstrate the generation of significant orbital and spin magnetizations and identify the underlying mechanisms consisting of the interplay of the OHE, inverse Faraday effect and spin-orbit interaction. Our discoveries advocate for the prospect of encoding magnetic information using light in materials that are not inherently magnetic.

cond-mat.mtrl-sci↗

arXiv2024

3d-oxide molecules to tailor large magnetic anisotropy energies on MgO films

Designing systems with large magnetic anisotropy energy (MAE) is desirable and critical for nanoscale magnetic devices. A recent breakthrough achieved the theoretical limit of the MAE for 3$d$ transition metal atoms by placing a single Co atom on a MgO(100) surface, a result not replicated by standard first-principles simulations. Our study, incorporating Hubbard-$U$ correction and spin-orbit coupling, successfully reproduces and explains the high MAE of a Co adatom on a MgO (001) surface. We go further by exploring ways to enhance MAE in 3d transition metal adatoms through different structural geometries of 3d--O molecules on MgO. One promising structure, with molecules perpendicular to the surface, enhances MAE while reducing substrate interaction, minimizing spin fluctuations, and boosting magnetic stability. Additionally, we demonstrate significant control over MAE by precisely placing 3d--O molecules on the substrate at the atomic level.

cond-mat.mes-hall↗

arXiv2024

Current-driven dynamics of antiferromagnetic skyrmions: from skyrmion Hall effects to hybrid inter-skyrmion scattering

Antiferromagnetic (AFM) skyrmions have emerged as a highly promising avenue in the realm of spintronics, particularly for the development of advanced racetrack memory devices. A distinguishing feature of AFM skyrmions is their zero topological charge and hence anticipated zero skyrmion Hall effect (SkHE). Here, we unveil that the latter is surprisingly finite under the influence of spin-transfer torque, depending on the direction of the injected current impinging on intrinsic AFM skyrmions emerging in CrPdFe trilayer on Ir(111) surface. Hinging on first-principles combined with atomistic spin dynamics simulations, we identify the origin of the SkHE and uncover that FM skyrmions in the underlying Fe layer act as effective traps for AFM skyrmions, confining them and reducing their velocity. These findings hold significant promise for spintronic applications, the design of multi-purpose skyrmion-tracks, advancing our understanding of AFM-FM skyrmion interactions and hybrid soliton dynamics in heterostructures.

cond-mat.mtrl-sci↗

arXiv2024

Two-dimensional Shiba lattices as possible platform for crystalline topological superconductivity

Localized or propagating Majorana boundary modes are the key feature of topological superconductors. They are rare in naturally-occurring compounds, but the tailored manipulation of quantum matter offers opportunities for their realization. Specifically, lattices of Yu-Shiba-Rusinov bound states $-$ Shiba lattices $-$ that arise when magnetic adatoms are placed on the surface of a conventional superconductor can be used to create topological bands within the superconducting gap of the substrate. Here, using scanning tunnelling microscopy to create and probe adatom lattices with single atom precision we reveal two signatures consistent with the realization of two types of mirror symmetry protected topological superconductors. The first has edge modes as well as higher-order corner states, and the second has symmetry-protected bulk nodal points. In principle, their topological character and boundary modes should be protected by the spatial symmetries of the adatom lattice. Our results highlight the potential of Shiba lattices as a platform to design the topology and sample geometry of 2D superconductors.

cond-mat.mes-hall↗

arXiv2023

Kagomerization of transition metal monolayers induced by two-dimensional hexagonal boron nitride

The kagome lattice is an exciting solid state physics platform for the emergence of nontrivial quantum states driven by electronic correlations: topological effects, unconventional superconductivity, charge and spin density waves, and unusual magnetic states such as quantum spin liquids. While kagome lattices have been realized in complex multi-atomic bulk compounds, here we demonstrate from first-principles a process that we dub kagomerization, in which we fabricate a two-dimensional kagome lattice in monolayers of transition metals utilizing a hexagonal boron nitride (h-BN) overlayer. Surprisingly, h-BN induces a large rearrangement of the transition metal atoms supported on a fcc(111) heavy-metal surface. This reconstruction is found to be rather generic for this type of heterostructures and has a profound impact on the underlying magnetic properties, ultimately stabilizing various topological magnetic solitons such as skyrmions and bimerons. Our findings call for a reconsideration of h-BN as merely a passive capping layer, showing its potential for not only reconstructing the atomic structure of the underlying material, e.g. through the kagomerization of magnetic films, but also enabling electronic and magnetic phases that are highly sought for the next generation of device technologies.

cond-mat.mtrl-sci↗

arXiv2023

Superconductivity in Nb: from the impact of temperature, Cooper-pairing to dimensionality

The ability to simulate realistically the electronic structure of superconducting materials is important to understand and predict various properties emerging in both the superconducting topological and spintronics realms. We introduce a tight-binding implementation of the Bogoliubov-de Gennes method, parameterized from density functional theory, which we utilize to explore the bulk and thin films of Nb, known to host a significant superconducting gap. The latter is useful for various applications such as the exploration of trivial and topological in-gap states. Here, we focus on the simulations aspects of superconductivity and study the impact of temperature, Cooper-pair coupling and dimensionality on the size of the superconducting order parameter.

cond-mat.supr-con↗

arXiv2023

Magnetic exchange interactions at the proximity of a superconductor

Interfacing magnetism with superconductivity gives rise to a wonderful playground for intertwining key degrees of freedom: Cooper pairs, spin, charge, and spin-orbit interaction, from which emerge a wealth of exciting phenomena, fundamental in the nascent field of superconducting spinorbitronics and topological quantum technologies. Magnetic exchange interactions (MEI), being isotropic or chiral such as the Dzyaloshinskii-Moriya interactions (DMI), are vital in establishing the magnetic behavior at these interfaces as well as in dictating not only complex transport phenomena, but also the manifestation of topologically trivial or non-trivial objects as skyrmions, spirals, Yu-Shiba-Rusinov states and Majorana modes. Here, we propose a methodology enabling the extraction of the tensor of MEI from electronic structure simulations accounting for superconductivity. We apply our scheme to the case of a Mn layer deposited on Nb(110) surface and explore proximity-induced impact on the MEI. Tuning the superconducting order parameter, we unveil potential change of the magnetic order accompanied with chirality switching. Owing to its simple formulation, our methodology can be readily implemented in state-of-the-art frameworks capable of tackling superconductivity and magnetism. Our findings opens intriguing exploration paths, where chirality and magnetism can be engineered depending on the conducting nature of magneto-superconducting interfaces. We thus foresee implications in the simulations and prediction of topological superconducting bits as well as in cryogenic superconducting hybrid devices involving magnetic units.

cond-mat.supr-con↗

arXiv2023

Universal patterns of skyrmion magnetizations unveiled by defect implantation

Skyrmions are spin-swirling textures hosting wonderful properties with potential implications in information technology. Such magnetic particles carry a magnetization, whose amplitude is crucial to establish them as robust magnetic bits, while their topological nature gives rise to a plethora of exquisite features such as topological protection, the skyrmion and topological Hall effects as well as the topological orbital moment. These effects are all induced by an emergent magnetic field directly proportional to the three-spin scalar chirality, $χ= (\mathbf{S}_i\times\mathbf{S}_j)\cdot \mathbf{S}_k$, and shaped by the peculiar spatial dependence of the magnetization. Here, we demonstrate the existence of novel chiral magnetizations emerging from the interplay of spin-orbit interaction and either $χ$ or the two-spin vector chirality $\boldsymbolκ = \mathbf{S}_i\times\mathbf{S}_j$. By scrutinizing correlations among the spin, orbital (trivial and chiral) magnetizations, we unveil from ab-initio universal patterns, quantify the rich set of magnetizations carried by single skyrmions generated in PdFe bilayer on Ir(111) surface and demonstrate the ability to engineer their magnitude via controlled implantation of impurities. We anticipate that our findings can guide the design of disruptive storage devices based on skyrmionic bits by encoding the desired magnetization with strategic seeding of defects.

cond-mat.mtrl-sci↗

arXiv2023

Electrical engineering of topological magnetism in two-dimensional heterobilayers

The emergence of topological magnetism in two-dimensional (2D) van der Waals (vdW) magnetic materials promoted 2D heterostructures as key building-blocks of devices for information technology based on topological concepts. Here, we demonstrate the all-electric switching of the topological nature of individual magnetic objects emerging in 2D vdW heterobillayers. We show from the first principles that an external electric field modifies the vdW gap between CrTe $_2$ and (Rh, Ti)Te$_2$ layers and alters the underlying magnetic interactions. This enables switching between ferromagnetic skyrmions and meron pairs in the CrTe$_2$/RhTe$_2$ heterobilayer while it enhances the stability of frustrated antiferromagnetic merons in the CrTe$_2$/TiTe$_2$ heterobilayer. We envision that the electrical engineering of distinct topological magnetic solitons in a single device could pave the way for novel energy-efficient mechanisms to store and transmit information with applications in spintronics.

cond-mat.mtrl-sci↗

arXiv2023

Spin-resolved spectroscopic evidence for spinarons in Co adatoms

Single cobalt atoms on the (111) surfaces of noble metals were for a long time considered prototypical systems for the Kondo effect in scanning tunneling microscopy experiments. Yet, recent first-principle calculations suggest that the experimentally observed spectroscopic zero-bias anomaly (ZBA) should be interpreted in terms of excitations of the Co atom's spin and the formation of a novel quasiparticle, the spinaron, a magnetic polaron resulting from the interaction of spin excitations with conduction electrons, rather than in terms of a Kondo resonance. Here we present state-of-the-art spin-averaged and spin-polarized scanning tunneling spectroscopy measurements on Co atoms on the Cu(111) surface in magnetic fields of up to 12 T, that allow us to discriminate between the different theoretical models and to invalidate the prevailing Kondo-based interpretation of the ZBA. Employing extended ab-initio calculations, we instead provide strong evidence for multiple spinaronic states in the system. Our work opens a new avenue of research to explore the characteristics and consequences of these intriguing hybrid many-body states as well as their design in man-made nanostructures.

cond-mat.str-el↗

arXiv2023

Intrinsic antiferromagnetic multimeronic Néel spin-textures in ultrathin films

The realization of topological antiferromagnetic (AFM) solitons in real materials is a major goal towards their use in information technology. While they bear various advantages with respect to their ferromagnetic cousins, their observation is scarce. Utilizing first-principles simulations, here we predict new chiral particles in the realm of AFM topological magnetism, frustrated multimeronic spin-textures hosted by a Néel magnetic state, arising in single Mn layers directly grown on Ir(111) surface or interfaced with Pd-based films. These topological structures are intrinsic, i.e. they form in a single AFM material, can carry distinct topological charges and can combine in various multimeronic sequences with enhanced stability against external magnetic fields. We envision the frustrated Néel AFM multimerons as exciting highly-sought after AFM solitons having the potential to be utilized in novel spintronic devices hinging on non-synthetic AFM quantum materials.

cond-mat.mtrl-sci↗

arXiv2023

CrTe$_2$ as a two-dimensional material for topological magnetism in complex heterobilayers

The discovery of two-dimensional (2D) van der Waals magnetic materials and their heterostructures provided an exciting platform for emerging phenomena with intriguing implications in information technology. Here, based on a multiscale modelling approach that combines first-principles calculations and a Heisenberg model, we demonstrate that interfacing a CrTe$_2$ layer with various Te-based layers enables the control of the magnetic exchange and Dzyaloshinskii-Moriya interactions as well as the magnetic anisotropy energy of the whole heterobilayer, and thereby the emergence of topological magnetic phases such as skyrmions and antiferromagnetic Neel merons. The latter are novel particles in the world of topological magnetism since they arise in a frustrated Neel magnetic environment and manifest as multiples of intertwined hexamer-textures. Our findings pave a promising road for proximity-induced engineering of both ferromagnetic and long-sought antiferromagnetic chiral objects in the very same 2D material, which is appealing for new information technology devices employing quantum materials.

cond-mat.mtrl-sci↗

arXiv2023

A spin model for intrinsic antiferromagnetic skyrmions on a triangular lattice

Skyrmions are prospected as the potential future of data storage due to their topologically protected spin structures. However, traditional ferromagnetic (FM) skyrmions experience deflection when driven with an electric current, hindering their usage in spintronics. Antiferromagnetic (AFM) skyrmions, consisting of two FM solitons coupled antiferromagnetically, are predicted to have a zero Magnus force, making them promising candidates for spintronic racetrack memories. Currently, they have been stabilized in synthetic AFM structures, i.e. multilayers hosting FM skyrmions, which couple antiferromagnetically through a non-magnetic spacer, while recent first-principles simulations predict their emergence in an intrinsic form, within an row-wise AFM single monolayer of Cr deposited on PdFe bilayer grown on Ir(111) surfaces. The latter material forms a triangular lattice, where single and interlinked AFM skyrmions can be stabilized. Here, we explore the minimal Heisenberg model enabling the occurrence of such AFM solitons and the underlying phase diagrams by accounting for the interplay between the Dzyaloshinskii-Moriya and Heisenberg exchange interactions, as well as the magnetic anisotropy and impact of magnetic field. By providing the fundamental basis to identify and understand the behavior of intrinsic AFM skyrmions, we anticipate our model to become a powerful tool for exploring and designing new topological magnetic materials to conceptualize devices for AFM spintronics.

cond-mat.mtrl-sci↗

arXiv2023

Spin excitation of Co atoms at monatomic Cu chains

The zero-bias anomaly in conductance spectra of single Co atoms on Cu(111) observed at $\approx$ 4 K, which has been interpreted as being due to a Kondo resonance, is strongly modified when the Co atoms are attached to monatomic Cu chains. Scanning tunneling spectra measured at 340 mK in magnetic fields exhibit all characteristics of spin-flip excitations. Their dependence on the magnetic field reveals a magnetic anisotropy and suggests a non-collinear spin state. This indicates that spin-orbit coupling (SOC), which has so far been neglected in theoretical studies of Co/Cu(111), has to be taken into account. According to our density functional theory and multi-orbital quantum Monte Carlo calculations SOC suppresses the Kondo effect for all studied geometries.

cond-mat.mes-hall↗

arXiv2022

Microscopic theory of spin-relaxation of a single Fe adatom coupled to substrate vibrations

Understanding the spin-relaxation mechanism of single adatoms is an essential step towards creating atomic magnetic memory bits or even qubits. Here we present an essentially parameter-free theory by combining \textit{ab-initio} electronic and vibrational properties with the many-body nature of atomic states. Our calculations account for the millisecond spin lifetime measured recently on Fe adatoms on MgO/Ag(100) and reproduce the dependence on the number of decoupling layers and the external magnetic field. We show how the atomic interaction with the environment should be tuned in order to enhance the magnetic stability, and propose a clear fingerprint for experimentally detecting a localized spin-phonon excitation.

cond-mat.mes-hall↗