arXiv subjects

Bertrand Dupé

Publications and source records attributed to Bertrand Dupé.

At least 19 recordsLinked to original sources

arXiv2026

Cryogenic Voltage Control of Magnetism in Silicon-Integrated \newline SrTiO$_3$/Fe Heterostructures

Cryogenic electronics forms a rapidly emerging research domain for high-performance and power-efficient computing applications. Incorporating nanomagnetic components in cryogenic circuitry adds highly valuable functionality, facilitating downscaling, reducing energy consumption and introducing time-reversal symmetry breaking. Furthermore, low-temperature environments enhance magnetic stability and switching efficiency at nanoscale dimensions, reinforcing the potential of cryogenic nanomagnets. To fully leverage these opportunities, magnetic control schemes require alternative options to current-based writing, which is the main bottleneck regarding power consumption and downscaling. In this regard, voltage-based gating of the magnetic state could drastically enhance operational efficiency and integration density. In this work, we investigate cryogenic Voltage Control of Magnetism (VCM) in epitaxial SrTiO$_3$/Fe thin film heterostructures on a CMOS compatible Si substrate. We demonstrate and quantify voltage-controlled modifications of the magnetic domain structure, consistent with electric field-controlled magnetic anisotropy at the Fe/SrTiO$_3$ interface. These findings provide a viable material system for the development of next-generation magnetic domain-based devices for classical and quantum computing.

cond-mat.mes-hall↗

arXiv2026

Effective Gilbert damping in the stochastic Landau-Lifshitz-Gilbert equation

Quasi particle based (e.g. Boltzmann equation) studies of spin wave transport often assume that their scattering rates follow the simple form $η=αω$, with the Gilbert damping $α$ and frequency $ω$. In this work, we examine the effective damping $α_{eff,T}=η/ω$ observed in atomistic spin dynamics, when temperature and spin wave interactions are introduced for a 1D spin chain. We extract the dynamical correlation functions from spin trajectories propagated using the stochastic Landau-Lifshitz-Gilbert equation, and fit the dynamical structure factor, yielding the dispersion and scattering rates for a wide range of temperatures. The resulting effective damping can be very different from the initially constant Gilbert value. It exhibits a temperature and crystal momentum scaling which we explain based on interactions with the Gilbert bath and spin wave scattering by changes in local magnetic order.

cond-mat.mes-hall↗

arXiv2024

Higher order topological defects in a moiré lattice

Topological defects are ubiquitous, they manifest in a wide variety of systems such as liquid crystals, magnets or superconductors. The recent quest for nonabelian anyons in condensed matter physics stimulates the interest for topological defects since they can be hosted in vortices in quantum magnets or topological superconductors. In addition to these vortex defects, in this study we propose to investigate edge dislocations in 2D magnets as new building blocks for topological physics since they can be described as vortices in the structural phase field. Here we demonstrate the existence of higher order topological dislocations within the higher order moiré pattern of the van der Waals 2D magnet CrCl3 deposited on Au(111). Surprizingly, these higher order dislocations arise from ordinary simple edge dislocations in the atomic lattice of CrCl3. We provide a theoretical framework explaining the higher order dislocations as vortex with a winding Chern number of 2. We expect that these original defects could stabilize some anyons either in a 2D quantum magnet or within a 2D superconductor coupled to it.

cond-mat.mes-hall↗

arXiv2023

Engineering magnetic domain wall energies in multiferroic BiFeO$_3$ via epitaxial strain

Epitaxial strain has emerged as a powerful tool to tune magnetic and ferroelectric properties in functional materials such as in multiferroic perovskite oxides. Here, we use first-principles calculations to explore the evolution of magnetic interactions in the antiferromagnetic multiferroic BiFeO$_3$ (BFO), one of the most promising multiferroics for future technology. The epitaxial strain in BFO(001) oriented film is varied between $\varepsilon_{xx,yy}$ $\in$ $[-2\%, +2\%]$. We find that both strengths of the exchange interaction and Dzyaloshinskii-Moriya interaction (DMI) decrease linearly from compressive to tensile strain whereas the uniaxial magnetocrystalline anisotropy follows a parabolic behavior which lifts the energy degeneracy of the (111) easy plane of bulk BFO. From the trends of the magnetic interactions we can explain the destruction of cycloidal order in compressive strain as observed in experiments due to the increasing anisotropy energy. For tensile strain, we predict that the ground state remains unchanged as a function of strain. By using the domain wall (DW) energy, we envision the region where isolated chiral magnetic texture might occur as function of strain i.e. where the DW and the spin spiral energy are equal. This transition between $-1.5\%$ and $-0.5\%$ of strain should allow topologically stable magnetic states such as antiferromagnetic skyrmions and merons to occur. Hence, our work should trigger experimental and theoretical investigations in this range of strain.

cond-mat.mtrl-sci↗

arXiv2023

Observation of the sliding phason mode of the incommensurate magnetic texture in Fe/Ir(111)

The nanoscopic magnetic texture forming in a monolayer of iron on the (111) surface of iridium, Fe/Ir(111), is spatially modulated and uniaxially incommensurate with respect to the crystallographic periodicities. As a consequence, a low-energy magnetic excitation is expected that corresponds to the sliding of the texture along the incommensurate direction, i.e., a phason mode, which we explicitly confirm with atomistic spin simulations. Using scanning tunneling microscopy (STM), we succeed to observe this phason mode experimentally. It can be excited by the STM tip, which leads to a random telegraph noise in the tunneling current that we attribute to the presence of two minima in the phason potential due to the presence of disorder in our sample. This provides the prospect of a floating phase in cleaner samples and, potentially, a commensurate-incommensurate transition as a function of external control parameters.

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arXiv2023

Signatures of pressure-enhanced helimagnetic order in van der Waals multiferroic NiI$_2$

The van der Waals (vdW) type-II multiferroic NiI$_2$ has emerged as a candidate for exploring non-collinear magnetism and magnetoelectric effects in the 2D limit. Frustrated intralayer exchange interactions on a triangular lattice result in a helimagnetic ground state, with spin-induced improper ferroelectricity stabilized by the interlayer interactions. Here we investigate the magnetic and structural phase transitions in bulk NiI$_2$, using high-pressure Raman spectroscopy, optical linear dichroism, and x-ray diffraction. We obtain evidence for a significant pressure enhancement of the antiferromagnetic and helimagnetic transition temperatures, at rates of $\sim15.3/14.4$ K/GPa, respectively. These enhancements are attributed to a cooperative effect of pressure-enhanced interlayer and third-nearest-neighbor intralayer exchange. These results reveal a general path for obtaining high-temperature type-II multiferroicity via high pressures in vdW materials.

cond-mat.mtrl-sci↗

arXiv2023

Effects of Pressure on the Electronic and Magnetic Properties of Bulk NiI$_{2}$

Transition metal dihalides have recently garnered interest in the context of two-dimensional van der Waals magnets as their underlying geometrically frustrated triangular lattice leads to interesting competing exchange interactions. In particular, NiI$_{2}$ is a magnetic semiconductor that has been long known for its exotic helimagnetism in the bulk. Recent experiments have shown that the helimagnetic state survives down to the monolayer limit with a layer-dependent magnetic transition temperature that suggests a relevant role of the interlayer coupling. Here, we explore the effects of hydrostatic pressure as a means to enhance this interlayer exchange and ultimately tune the electronic and magnetic response of NiI$_{2}$. We study first the evolution of the structural parameters as a function of external pressure using first-principles calculations combined with x-ray diffraction measurements. We then examine the evolution of the electronic structure and magnetic exchange interactions via first-principles calculations and Monte Carlo simulations. We find that the leading interlayer coupling is an antiferromagnetic second-nearest neighbor interaction that increases monotonically with pressure. The ratio between isotropic third- and first-nearest neighbor intralayer exchanges, which controls the magnetic frustration and determines the magnetic propagation vector $\mathbf{q}$ of the helimagnetic ground state, is also enhanced by pressure. As a consequence, our Monte Carlo simulations show a monotonic increase in the magnetic transition temperature, indicating that pressure is an effective means to tune the magnetic response of NiI$_{2}$.

cond-mat.mtrl-sci↗

arXiv2023

Eigenmodes of magnetic skyrmion lattices

We explore the interplay between topology and eigenmodes by changing the stabilizing mechanism of skyrmion lattices (skX). We focus on two prototypical ultrathin films hosting an hexagonal (Pd/Fe/Ir(111)) and a square (Fe/Ir(111)) skyrmion lattice, which can both be described by an extended Heisenberg Hamiltonian. We first examine whether the Dzyaloshinkskii-Moriya, or the exchange interaction as the leading energy term affects the modes of the hexagonal skX of Pd/Fe/Ir(111). In all cases, we find that the lowest frequency modes correspond to internal degrees of freedom of individual skyrmions, and suggest a classification based on azimuthal and radial numbers $(l,p)$, with up to $l=6$, and $p=2$. We also show that the gyration behavior induced by an in-plane field corresponds to the excitation of $l=1$ deformation modes with varying radial numbers. Second, we examine the square lattice of skyrmions of Fe/Ir(111). Its stabilization mechanism is dominated by the 4-spin interaction. After relaxation, the unit cell does not carry a topological charge, and the eigenmodes do not correspond to internal skyrmion deformations. By reducing the 4-spin interaction, the integer topological charge is recovered, but the charge carriers do not possess internal degrees of freedom, nor are they separated by energy barriers. We conclude that a 4-spin dominated Hamiltonian does not yield skyrmion lattice solutions, and that therefore, a nontrivial topology does not imply the existence of skyrmions.

cond-mat.mes-hall↗

arXiv2022

Spin-current driven Dzyaloshinskii-Moriya interaction in the multiferroic BiFeO3 from first-principles

The electrical control of magnons opens up new ways to transport and process information for logic devices. In magnetoelectrical multiferroics, the Dzyaloshinskii-Moriya (DM) interaction directly allow for such a control and, hence, is of major importance. We determine the origin and the strength of the (converse) spin current DM interaction in the R3c bulk phase of the multiferroic BiFeO3 based on density functional theory. Our data supports only the existence of one DM interaction contribution originating from the spin current model. By exploring then magnon dispersion in the full Brillouin Zone, we show that the exchange is isotropic, but the DM interaction and anisotropy prefer any propagation and any magnetization direction within the full (111) plane. Our work emphasizes the significance of the asymmetric potential induced by the spin current over the structural asymmetry induced by the anionic octahedron in multiferroics such as BiFeO3.

cond-mat.mtrl-sci↗

arXiv2022

Impact of Spin-Orbit Coupling on Quantum Transport in Magnetic Tunnel Junction with an anti-ferromagnetic Capping Layer

Using first-principles calculations, we explore the role of an anti-ferromagnetic heavy-metal, L1$_0$-IrMn, as a capping layer in a perpendicular magnetic tunnel junction (\emph{p}-MTJ). A comparative study is conducted by employing conventional non-magnetic heavy-metals (Ta, W or Mo) capping layers along with an anti-ferromagnetic IrMn in a symmetric-MTJ X/FeCo/MgO/FeCo/X, where X=Ta, W, Mo or IrMn. Firstly, the calculations without including spin-orbit coupling (SOC) are presented where the highest TMR is achieved in IrMn-IrMn MTJ compared to that of Ta-Ta, W-W and Mo-Mo MTJs. The origin of this large TMR is attributed to, both, the large spin-polarization due to reduced lattice-mismatch and the non-identical signatures of both the anti-parallel conduction-channels caused by the anti-ferromagnetic ordering of IrMn that reflects the spins at IrMn/FeCo interface. Moreover, when SOC is switched-on, the increase of TMR is observed in all the MTJs with a particularly giant enhancement in IrMn-IrMn MTJ. This SOC-induced increase in TMR is ascribed to the mixed contribution of $Δ_1$ and $Δ_5$ states and the additional increase in the band levels of the out-of-plane orbitals, \emph{p}$_z$, \emph{d}$_{z}^2$ and \emph{d}$_{xz}$ due to the lifting of degeneracy. Furthermore, it was also observed that the lattice-mismatch-induced strain might create an orbital reconstruction within the capping layer, probably, due to the crystallographic deformation and, in turn, in the adjacent FeCo-electrode. This microscopic mechanism also plays an additional decisive role in enhancing the TMR. Finally, our results indicate that IrMn can offer giant TMR in future spin-orbit toque (SOT) based MRAM devices with a straightforward design strategy.

cond-mat.mes-hall↗

arXiv2022

The role of band matching in the proximity effect between a superconductor and a normal metal

We combine density functional theory and scanning tunneling microscopy to study the proximity effects between a bulk Ru(0001) superconductor and an atomically thin overlayer of Co. We have identified that the Co monolayer can grow in two different stackings: the hcp and a reconstructed \textepsilon-like stacking. We analyze their electronic structure from both experiments and density functional theory. While the magnetic hcp stacking shows a weak proximity effect in combination with Shiba states and with almost no suppression of superconductivity of the substrate, the more complex \textepsilon-like stacking becomes almost fully superconducting and displays an edge state at the island rim. We identify this edge state as a trivial state caused by a local hcp rim around the \textepsilon-core. We explain the weak proximity effect between Ru and the magnetic hcp islands by a low transparency of the interface, while the large chemical unit cell of the non-magnetic \textepsilon-like stacking lifts the momentum conservation at the interface making it transparent and causing a clear proximity effect.

cond-mat.mtrl-sci↗

arXiv2020

Asymmetric skyrmion-antiskyrmion production in ultrathin ferromagnetic films

Ultrathin ferromagnets with frustrated exchange and the Dzyaloshinskii-Moriya interaction can support topological solitons such as skyrmions and antiskyrmions, which are metastable and can be considered particle-antiparticle counterparts. When spin-orbit torques are applied, the motion of an isolated antiskyrmion driven beyond its Walker limit can generate skyrmion-antiskyrmion pairs. Here, we use atomistic spin dynamics simulations to shed light on the scattering processes involved in this pair generation. Under certain conditions a proliferation of these particles and antiparticles can appear with a growth rate and production asymmetry that depend on the strength of the chiral interactions and the dissipative component of the spin-orbit torques. These features are largely determined by scattering processes between antiskyrmions, which can be elastic or result in bound states or annihilation.

cond-mat.mes-hall↗

arXiv2019

Engineering the dynamics of topological spin textures by anisotropic spin-orbit torques

Integrating topologically stabilized magnetic textures such as skyrmions as nanoscale information carriers into future technologies requires the reliable control by electric currents. Here, we uncover that the relevant skyrmion Hall effect, which describes the deflection of moving skyrmions from the current flow direction, acquires important corrections owing to anisotropic spin-orbit torques that alter the dynamics of topological spin structures. Thereby, we propose a viable means for manipulating the current-induced motion of skyrmions and antiskyrmions. Based on these insights, we demonstrate by first-principles calculations and symmetry arguments that the motion of spin textures can be tailored by materials design in magnetic multilayers of Ir/Co/Pt and Au/Co/Pt. Our work advances the understanding of the current-induced dynamics of these magnetic textures, which underlies a plethora of memory and logic applications.

cond-mat.mtrl-sci↗

arXiv2019

Comparison of first-principles methods to extract magnetic parameters in ultra-thin films: Co/Pt(111)

We compare three distinct computational approaches based on first-principles calculations within density functional theory to explore the magnetic exchange and the Dzyaloshinskii-Moriya interactions (DMI) of a Co monolayer on Pt(111), namely (i) the method of infinitesimal rotations of magnetic moments based on the Korringa-Kohn-Rostoker (KKR) Green function method, (ii) the generalized Bloch theorem applied to spiraling magnetic structures and (iii) supercell calculations with non-collinear magnetic moments, the latter two being based on the full-potential linearized augmented plane wave (FLAPW) method. In particular, we show that the magnetic interaction parameters entering micromagnetic models describing the long-wavelength deviations from the ferromagnetic state might be different from those calculated for fast rotating magnetic structures, as they are obtained by using (necessarily rather small) supercell or large spin-spiral wave-vectors. In the micromagnetic limit, which we motivate to use by an analysis of the Fourier components of the domain-wall profile, we obtain consistent results for the spin stiffness and DMI spiralization using methods (i) and (ii). The calculated spin stiffness and Curie temperature determined by subsequent Monte Carlo simulations are considerably higher than estimated from the bulk properties of Co, a consequence of a significantly increased nearest-neighbor exchange interaction in the Co-monolayer (+50%). The calculated results are carefully compared with the literature.

cond-mat.mtrl-sci↗

arXiv2018

Scaling of intrinsic domain wall magneto-resistance with confinement in electromigrated nanocontacts

In this work we study the evolution of intrinsic domain wall magnetoresistance (DWMR) with domain wall confinement. Clean permalloy notched half-ring nanocontacts are fabricated using a special ultra-high vacuum electromigration procedure to tailor the size of the wire in-situ and through the resulting domain wall confinement we tailor the domain wall width from a few tens of nm down to a few nm. Through measurements of the dependence of the resistance with respect to the applied field direction we extract the contribution of a single domain wall to the MR of the device, as a function of the domain wall width in the confining potential at the notch. In this size range, an intrinsic positive MR is found, which dominates over anisotropic MR, as confirmed by comparison to micromagnetic simulations. Moreover, the MR is found to scale monotonically with the size of the domain wall, $δ_{DW}$, as 1/$δ_{DW}^b$, with $b=2.31\pm 0.39 $. The experimental result is supported by quantum-mechanical transport simulations based on ab-initio density functional theory calculations.

cond-mat.mes-hall↗

arXiv2018

Magnetic Interactions in BiFeO$_3$: a First-Principles Study

First-principles calculations, in combination with the four-state energy mapping method, are performed to extract the magnetic interaction parameters of multiferroic BiFeO$_3$. Such parameters include the symmetric exchange (SE) couplings and the Dzyaloshinskii-Moriya (DM) interactions up to second nearest neighbors, as well as the single ion anisotropy (SIA). All magnetic parameters are obtained not only for the $R3c$ structural ground state, but also for the $R3m$ and $R\bar{3}c$ phases in order to determine the effects of ferroelectricity and antiferrodistortion distortions, respectively, on these magnetic parameters. In particular, two different second-nearest neighbor couplings are identified and their origins are discussed in details. Moreover, Monte-Carlo (MC) simulations using a magnetic Hamiltonian incorporating these first-principles-derived interaction parameters are further performed. They result (i) not only in the accurate prediction of the spin-canted G-type antiferromagnetic structure and of the known magnetic cycloid propagating along a $<$1$\bar{1}$0$>$ direction, as well as their unusual characteristics (such as a weak magnetization and spin-density-waves, respectively); (ii) but also in the finding of another cycloidal state of low-energy and that awaits to be experimentally confirmed. Turning on and off the different magnetic interaction parameters in the MC simulations also reveal the precise role of each of them on magnetism.

cond-mat.mtrl-sci↗

arXiv2018

Electrical switching of perpendicular magnetization in L10 FePt single layer

Electrical manipulation of magnetization is essential for integration of magnetic functionalities such as magnetic memories and magnetic logic devices into electronic circuits. The current induced spin-orbit torque (SOT) in heavy metal/ferromagnet (HM/FM) bilayers via the spin Hall effect in the HM and/or the Rashba effect at the interfaces provides an efficient way to switch the magnetization. In the meantime, current induced SOT has also been used to switch the in-plane magnetization in single layers such as ferromagnetic semiconductor (Ga,Mn)As and antiferromagnetic metal CuMnAs with globally or locally broken inversion symmetry. Here we demonstrate the current induced perpendicular magnetization switching in L10 FePt single layer. The current induced spin-orbit effective fields in L10 FePt increase with the chemical ordering parameter (S). In 20 nm FePt films with high S, we observe a large charge-to-spin conversion efficiency and a switching current density as low as 7.0E6 A/cm2. We anticipate our findings may stimulate the exploration of the spin-orbit torques in bulk perpendicular magnetic anisotropic materials and the application of high-efficient perpendicular magnetization switching in single FM layer.

cond-mat.mtrl-sci↗

arXiv2018

Stability and magnetic properties of Fe double-layers on Ir (111)

We investigate the interplay between the structural reconstruction and the magnetic properties of Fe doublelayers on Ir (111)-substrate using first-principles calculations based on density functional theory and mapping of the total energies on an atomistic spin model. We show that, if a second Fe monolayer is deposited on Fe/Ir (111), the stacking may change from hexagonal close-packed to bcc (110)-like accompanied by a reduction of symmetry from trigonal to centered rectangular. Although the bcc-like surface has a lower coordination, we find that this is the structural ground state. This reconstruction has a major impact on the magnetic structure. We investigate in detail the changes in the magnetic exchange interaction, the magnetocrystalline anisotropy, and the Dzyaloshinskii Moriya interaction depending on the stacking sequence of the Fe double-layer. Based on our findings, we suggest a new technique to engineer Dzyaloshinskii Moriya interactions in multilayer systems employing symmetry considerations. The resulting anisotropic Dzyaloshinskii-Moriya interactions may stabilize higher-order skyrmions or antiskyrmions.

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