arXiv subjects

Juba Bouaziz

Publications and source records attributed to Juba Bouaziz.

At least 19 recordsLinked to original sources

arXiv2026

Emergent toroidal induction in a polar Weyl ferromagnet

Spin-orbit coupling (SOC) underpins modern spintronics by enabling the electrical generation of spin torques. Its reciprocal counterpart, in which magnetization dynamics produce electromotive forces through a spin-dependent Berry phase, is known as emergent electromagnetic induction (EEMI). However, this effect has previously been observed only in magnetic textures with spatial gradients, such as domain walls, helices, and skyrmions. Here, we demonstrate that even a spatially uniform ferromagnet can host EEMI through a previously unrecognized Berry-phase mechanism inherent to noncentrosymmetric conductors. In the polar Weyl ferromagnet PrAlGe, an applied alternating current generates spin-orbit torques that drive collective magnetization dynamics. The resulting emergent toroidal moment (T = P \times M), where (P) is the crystal's polar axis and (M) is the net magnetization, acts as a gauge potential whose time derivative (dT/dt) induces a Hall voltage. This contribution appears specifically in the out-of-phase component of the AC Hall response and scales linearly with frequency, providing direct evidence for EEMI. First-principles calculations further reveal that this toroidal vector encodes the collective motion of Weyl nodes in momentum space. These findings establish "emergent toroidal induction" as a new manifestation of spin-orbit entanglement, unifying Berry phase, topology, and spin dynamics while opening a pathway toward intrinsic and energy-efficient spin-charge interconversion.

cond-mat.mtrl-sci↗

arXiv2026

Interpretable machine learning of magnetic transition temperature in Heusler magnets via hierarchical dependence extraction

We employ interpretable machine learning to analyze the material dependence of the magnetic transition temperature $T_c$ in ferromagnetic and ferrimagnetic Heusler compounds. For over 200 candidate materials with the same $F\overline{4}3m$ crystal structure but different chemical formulae and lattice constants, we consider both experimental $T_c$ and those computed via classical Monte Carlo simulations using magnetic interactions derived from ab initio calculations. We use the hierarchical dependence extraction (HDE) procedure [Morée and Arita, Phys. Rev. B 110, 014502 (2024)] to determine how $T_c$ depends on chemical composition and magnetic moments, from leading to higher-order effects, and use these dependencies to construct an explicit expression for $T_c$. Our results show that the HDE framework predicts $T_c$ with accuracy comparable to other machine-learning approaches such as neural network and random forest algorithms while remaining fully interpretable. $T_c$ is primarily governed by the proportions of Fe, Co, and Mn, increasing systematically with their concentration. These findings clarify how chemical composition and magnetic moments influence $T_c$ in collinear Heusler alloys and support the use of the HDE for computationally guided discovery of new functional materials with tailored $T_c$ values.

cond-mat.mtrl-sci↗

arXiv2026

Atomically-sharp magnetic soliton in the square-net lattice EuRhAl$_{4}$Si$_{2}$

Topological spin textures are hallmark manifestations of competing interactions in magnetic matter. Their effective description by nonlinear field theories reflects an energetic frustration that destabilizes uniform order while selecting finite-size, topologically nontrivial configurations as stationary states. Among the most extreme realizations are atomically-sharp domain wall excitations, namely one-dimensional (1D) magnetic solitons, which represent the ultimate scaling limit of magnetic textures. Such solitons may emerge in magnetic systems where effective exchange interactions compete directly with uniaxial magnetic anisotropy. Here we show that the square-net rare earth compound EuRhAl$_{4}$Si$_{2}$ realizes a very susceptible regime where the magnetic anisotropy competes with highly frustrated exchange interactions stabilizing a rare ferrimagnetic $\uparrow\uparrow\downarrow$ state that, under applied magnetic field, supports the formation of atomically-sharp soliton defects. We confirm the bulk response of the 1D magnetic solitons via magnetization and electrical transport measurements. We establish both the zero- and in-field $\uparrow\uparrow\downarrow$ order via neutron diffraction, while magnetic force microscopy visualizes its real-space evolution into a stripe-like array. To elucidate the microscopic origin of the soliton, we relate the Ruderman-Kittel-Kasuya-Yosida (RKKY)-driven exchange interactions and the magnetic anisotropy through density functional theory, and we construct an effective 1D $J_{1}$-$J_{2}$-$K$ model whose atomistic spin dynamics simulations reproduce the observed soliton states as a function of external field. Our results demonstrate that EuRhAl$_{4}$Si$_{2}$ hosts atomically-sharp, field-driven 1D magnetic solitons, providing a new platform for studying 1D topological excitations at the atomic length scale.

cond-mat.str-el↗

arXiv2026

Long-range magnetic interactions in Nd$_2$PdSi$_3$ and the formation of skyrmion phases in centrosymmetric metals

We present an extensive X-ray and neutron scattering study of the structure and magnetic excitations of Nd$_2$PdSi$_3$, a sister compound of Gd$_2$PdSi$_3$ which was recently found to host a skyrmion lattice phase despite its centrosymmetric crystal structure. Dispersive magnetic excitations were measured throughout the Brillouin zone and modeled to determine the magnetic interactions between Nd ions. Our analysis reveals that the magnetic interactions in this system extend over large distances and are significantly affected by a crystallographic superstructure formed by ordering of the Pd and Si atoms. The results suggest that the mechanism for the skyrmion phase formation in this family of materials, specifically Gd$_2$PdSi$_3$, is through the long-range RKKY interactions rather than short-range triangular-lattice frustration.

cond-mat.str-el↗

arXiv2025

Calculation of the biquadratic spin interactions based on the spin cluster expansion for \textit{ab initio} tight-binding models

We develop a calculation scheme using \textit{ab initio} tight-binding Hamiltonians to evaluate biquadratic magnetic interactions. This approach relies on the spin cluster expansion combined with the disordered local moment (DLM) method, originally developed within the multiple scattering Korringa-Kohn-Rostoker method. Applying it to a single-orbital Hubbard model with two sublattices, we show that the evaluated DLM biquadratic interactions are in good agreement with those obtained from the strongly correlated limit, demonstrating the wide applicability of the method to various magnetic systems with large local moments. We then apply it to the \textit{ab initio} tight-binding models for elemental magnetic metals; the resulting magnetic interactions align well with previous literature. Finally, we explore its performance in more complex compounds, such as transition metal dichalcogenides with intercalation of 3\textit{d} transition metals and potassium electrosodalite. The obtained results for both compounds show good agreement with experiments. The present approach offers a convenient \textit{ab initio} path for evaluating biquadratic interactions and understanding the electronic mechanisms controlling them.

cond-mat.str-el↗

arXiv2025

Perfectly harmonic spin cycloid and multi-$Q$ textures in the Weyl semimetal GdAlSi

A fundamental question concerns how topological electronic states are influenced by many-body correlations, and magnetic Weyl semimetals represent an important material platform to address this problem. However, the magnetic structures realized in these materials are limited, and in particular, no clear example of an undistorted helimagnetic state has been definitively identified. Here, we report clear evidence of a harmonic helimagnetic cycloid with an incommensurate magnetic propagation vector in the Weyl semimetal GdAlSi via resonant elastic X-ray scattering, including rigorous polarization analysis. This cycloidal structure is consistent with the Dzyaloshinskii-Moriya (DM) interaction prescribed by the polar crystal structure of GdAlSi. Upon applying a magnetic field, the cycloid undergoes a transition to a novel multi-$Q$ state. This field-induced, noncoplanar texture is consistent with our numerical spin model, which incorporates the DM interaction and, crucially, anisotropic exchange. The perfectly harmonic Weyl helimagnet GdAlSi serves as a prototypical platform to study electronic correlation effects in periodically modulated Weyl semimetals.

cond-mat.str-el↗

arXiv2025

Spin Models and Cluster Multipole Method: Application to Kagome Magnets

We present a multi-scale computational approach that combines atomistic spin models with the cluster multipole (CMP) method. The CMP method enables a systematic and accurate generation of complex non-collinear magnetic structures using symmetry-adapted representations. The parameters of the spin model are derived from density functional theory using the magnetic force theorem, with the paramagnetic state as a reference. The energy landscape of CMP-generated structures is inspected at the model Hamiltonian level, and sets of low-energy magnetic structures are identified for each material candidate. The inclusion of relativistic antisymmetric and anisotropic pair interactions lifts partially the degeneracy among these most stable structures. To demonstrate the applicability and predictive capability of the method, we apply it to the non-collinear Mn3X and collinear Fe3X (X = Ga, Ge, and Sn) kagome compounds. The computational efficiency of the method in identifying low-energy structures among multiple CMP configurations highlights its potential for high-throughput screening of complex magnets with unknown magnetic order.

cond-mat.mtrl-sci↗

arXiv2024

Ideal Spin-Orbit-Free Dirac Semimetal and Diverse Topological Transitions in Pr$_8$CoGa$_3$ Family

Topological semimetals, known for their intriguing properties arising from band degeneracies, have garnered significant attention. However, the discovery of a material realization and the detailed characterization of spinless Dirac semimetals have not yet been accomplished. Here, we propose from first-principles calculations that the $RE_8\mathrm{Co}X_3$ group ($RE$ = rare earth elements, $X$ = Al, Ga, or In) contains ideal spinless Dirac semimetals whose Fermi surfaces are fourfold degenerate band-crossing points (without including spin degeneracy). Despite the lack of space inversion symmetry in these materials, Dirac points are formed on the rotation-symmetry axis due to accidental degeneracies of two bands corresponding to different 2-dimensional irreducible representations of $C_{6v}$ group. We also investigate, through first-principles calculations and effective model analysis, various phase transitions caused by lattice distortion or elemental substitutions from the Dirac semimetal phase to distinct topological semimetallic phases such as nonmagnetic linked-nodal-line and Weyl semimetals (characterized by the second Stiefel-Whitney class) and ferromagnetic Weyl semimetals.

cond-mat.mtrl-sci↗

arXiv2024

Spin Dynamics of the Centrosymmetric Skyrmion Material GdRu2Si2

Magnetic skyrmion crystals are traditionally associated with non-centrosymmetric crystal structures; however, it has been demonstrated that skyrmion crystals can be stabilized by competing interactions in centrosymmetric crystals. To understand and optimize the physical responses associated with topologically-nontrivial skyrmion textures, it is important to quantify their magnetic interactions by comparing theoretical predictions with spectroscopic data. Here, we present neutron diffraction and spectroscopy data on the centrosymmetric skyrmion material GdRu$_2$Si$_2$, and show that the key spectroscopic features can be explained by the magnetic interactions calculated using density-functional theory calculations. We further show that the recently-proposed 2-$\mathbf{q}$ "topological spin stripe" structure yields better agreement with our data than a 1-$\mathbf{q}$ helical structure, and identify how the magnetic structure evolves with temperature.

cond-mat.str-el↗

arXiv2023

Origin of Incommensurate Magnetic Order in Rare-Earth Magnetic Weyl Semimetals

We investigate rare-earth magnetic Weyl semimetals through first-principles simulations, analyzing the connection between incommensurate magnetic order and the presence of Weyl nodes in the electronic band structure. Focusing on PrAlSi, NdAlSi, and SmAlSi, we demonstrate that the reported helical ordering does not originate from the nesting of topological features at the Fermi Surface or the Dzyaloshinskii-Moriya interaction. Instead, the helical order arises from frustrated isotropic short-range superexchange between the 4f moments facilitated by pd-hybridization with the main group elements. Employing a spin Hamiltonian with isotropic exchange and single-ion anisotropy we replicate the experimentally observed helical modulation.

cond-mat.mtrl-sci↗

arXiv2023

Investigation of magnetic properties of $4f$-adatoms on graphene

Rare-earth (RE) atoms on top of 2D materials represent an interesting platform with the prospect of tailoring the magnetic anisotropy for practical applications. Here, we investigate the ground state and magnetic properties of selected $4f$-atoms deposited on a graphene substrate in the framework of the DFT+$U$ approach. The inherent strong spin-orbit interaction in conjunction with crystal field effects acting on the localized $4f$-shells results in a substantial magnetic anisotropy energy (tens of meVs), whose angular dependence is dictated by the $C_{6v}$ symmetry of the graphene substrate. We obtain the crystal field parameters and investigate spin-flip events via quantum tunneling of magnetization in the view of achieving a protected quantum-spin behavior. Remarkably, the large spin and orbital moments of the open $4f$-shells (Dy, Ho and Tm) generate a strong magneto-elastic coupling which provides more flexibility to control the magnetic state via the application of external strain.

cond-mat.mes-hall↗

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

Crucial role of Fe in determining the hard magnetic properties of Nd$_2$Fe$_{14}$B

Nd$_2$Fe$_{14}$B's unsurpassed, hard magnetic properties for a wide range of temperatures result from a combination of a large volume magnetization from Fe and a strong single-ion anisotropy from Nd. Here, using finite temperature first-principles calculations, we focus on the other crucial roles played by the Fe atoms in maintaining the magnetic order on the Nd sublattices, and hence the large magnetic anisotropy, and directly generating significant uniaxial anisotropy at high temperatures. We identify effective spins for atomistic modelling from the material's interacting electrons and {quantify pairwise and higher order, non-pairwise magnetic interactions among them. We find the Nd spins couple most strongly to spins on sites belonging to two specific Fe sublattices, 8$j_1$, 8$j_2$. Moreover the Fe 8$j_1$ sublattice also provides the electronic origin of the unusual, nonmonotonic temperature dependence of the anisotropy of Y$_2$Fe$_{14}$B.} Our work provides atomic-level resolution of the properties of this fascinating magnetic material.

cond-mat.mtrl-sci↗

arXiv2022

Fermi-surface origin of skyrmion lattices in centrosymmetric rare-earth intermetallics

We show from first-principles that barrel-shaped structures within the Fermi surface of the centrosymmetric intermetallic compounds GdRu$_2$Si$_2$ and Gd$_2$PdSi$_3$ give rise to Fermi surface nesting, which determines the strength and sign of quasi-two-dimensional Ruderman-Kittel-Kasuya-Yosida pairwise exchange interactions between the Gd moments. This is the principal mechanism leading to their helical single-$q$ spin-spiral ground states, providing transition temperatures and magnetic periods in good agreement with experiment. Using atomistic spin-dynamic simulations, we draw a direct line between the subtleties of the three-dimensional Fermi surface topology and the stabilization of a square skyrmion lattice in GdRu$_2$Si$_2$ at applied magnetic fields as observed in experiment.

cond-mat.mtrl-sci↗

arXiv2021

Transverse Transport in Two-Dimensional Relativistic Systems with Non-Trivial Spin Textures

Using multiple scattering theory, we show that the generally accepted expression of transverse resistivity in magnetic systems that host skyrmions, given by the linear superposition of the ordinary (OHE), the anomalous (AHE) and the topological Hall effect (THE), is incomplete and must be amended by an additional term, the "non-collinear" Hall effect (NHE). Its angular form is determined by the magnetic texture, the spin-orbit field of the electrons, and the underlying crystal structure, allowing to disentangle the NHE from the various other Hall contributions. Its magnitude is proportional to the spin-orbit interaction strength. The NHE is an essential term required for decoding two- and three-dimensional spin textures from transport experiments.

cond-mat.mes-hall↗

arXiv2021

A new view on the origin of zero-bias anomalies of Co atoms atop noble metal surfaces

Many-body phenomena are paramount in physics. In condensed matter, their hallmark is considerable on a wide range of material characteristics spanning electronic, magnetic, thermodynamic and transport properties. They potentially imprint non-trivial signatures in spectroscopic measurements, such as those assigned to Kondo, excitonic and polaronic features, whose emergence depends on the involved degrees of freedom. Here, we address systematically zero-bias anomalies detected by scanning tunneling spectroscopy on Co atoms deposited on Cu, Ag and Au(111) substrates, which remarkably are almost identical to those obtained from first-principles. These features originate from gaped spin-excitations induced by a finite magnetic anisotropy energy, in contrast to the usual widespread interpretation relating them to Kondo resonances. Resting on relativistic time-dependent density functional and many-body perturbation theories, we furthermore unveil a new many-body feature, the spinaron, resulting from the interaction of electrons and spin-excitations localizing electronic states in a well defined energy.

cond-mat.mes-hall↗

arXiv2020

Spin-orbit torques and their associated effective fields from gigahertz to terahertz

Terahertz spintronics offers the prospect of devices which are both faster and more energy-efficient. A promising route to achieve this goal is to exploit current-induced spin-orbit torques. However, the high-frequency properties of these quantities remain unexplored both experimentally and theoretically, within a realistic material-specific approach. Here we investigate the dynamical transverse components of the torques and uncover contributions longitudinal to the magnetic moment capable of changing its magnitude. We show that, while the torques can be drastically altered in the dynamical regime, the effective magnetic fields that accompany them present a frequency-independent behaviour, ranging from the static limit up to the terahertz domain - including the ferromagnetic resonance of the system. The outcomes of this work point to new ways to control magnetic units in next-generation spintronic devices.

cond-mat.mes-hall↗

arXiv2020

Zero-point quantum swing of magnetic couples

Quantum fluctuations are ubiquitous in physics. Ranging from conventional examples like the harmonic oscillator to intricate theories on the origin of the universe, they alter virtually all aspects of matter -- including superconductivity, phase transitions and nanoscale processes. As a rule of thumb, the smaller the object, the larger their impact. This poses a serious challenge to modern nanotechnology, which aims total control via atom-by-atom engineered devices. In magnetic nanostructures, high stability of the magnetic signal is crucial when targeting realistic applications in information technology, e.g. miniaturized bits. Here, we demonstrate that zero-point spin-fluctuations are paramount in determining the fundamental magnetic exchange interactions that dictate the nature and stability of the magnetic state. Hinging on the fluctuation-dissipation theorem, we establish that quantum fluctuations correctly account for the large overestimation of the interactions as obtained from conventional static first-principles frameworks, filling in a crucial gap between theory and experiment [1,2]. Our analysis further reveals that zero-point spin-fluctuations tend to promote the non-collinearity and stability of chiral magnetic textures such as skyrmions -- a counter-intuitive quantum effect that inspires practical guidelines for designing disruptive nanodevices.

cond-mat.mtrl-sci↗