arXiv subjects

Yousra Nahas

Publications and source records attributed to Yousra Nahas.

14 recordsLinked to original sources

arXiv2026

Topological textures and emergent altermagnetic signatures in ultrathin BiFeO3

Magnetoelectric multiferroics, materials with intrinsically coupled electric polarization and magnetic order, promise ultralow-power switching, nonvolatile memory, and energy-efficient signal transduction. Yet practical deployment demands ultrathin films down to the atomic limit, where both orders typically degrade. Maintaining both order parameters at the thinnest scales in complex oxides remains a tremendous challenge, as uncompensated bound charge drives nanoscale depolarization in most ferroelectrics, while off-stoichiometry, reduced anisotropy, and charge transfer can produce magnetic dead layers in ultrathin oxides at substrate interfaces. Here, we realize a multiferroic phase of BiFeO3 that not only sustains both order parameters at room temperature with no dead layer but also exhibits signatures of emergent altermagnetism in the four-unit-cell, ultrathin limit. First-principles calculations, spin symmetry analysis, atomic-resolution imaging, and angle-resolved magnetic imaging reveal that short-circuit electrostatic boundary conditions, together with epitaxial strain, drive a continuous second-order, thickness-driven phase transition that enables the formation of multiferroic topological textures. Moreover, the imposed boundary conditions stabilize a d-wave altermagnetic time-reversal symmetry breaking, with corresponding signatures observed in magnetic circular dichroism. Collectively, these results establish a pathway to stabilize unconventional multiferroicity at device-relevant thicknesses, reframing scaling limits for oxide electronics.

cond-mat.mtrl-sci↗

arXiv2026

Anisotropic magnon transport in an antiferromagnetic trilayer heterostructure: is BiFeO$_3$ an altermagnet?

Magnons provide a route to ultra-fast transport and non-destructive readout of spin-based information transfer. Here, we report magnon transport and its emergent anisotropic nature in BiFeO$_3$ layers confined between ultrathin layers of the antiferromagnet LaFeO$_3$. Due to the confined state, BiFeO$_3$ serves as an efficient magnon transmission channel as well as a magnetoelectric knob by which to control the stack by means of an electric field. We discuss the mechanism of the anisotropic spin transport based on the interaction between the antiferromagnetic order and the electric field. This allows us to manipulate and amplify the spin transport in such a confined geometry. Furthermore, lower crystal symmetric and suppression of the spin cycloid in ultrathin BiFeO$_3$ stabilizes a non-trivial antiferromagnetic state exhibiting symmetry-protected spin-split bands that provide the non-trivial sign inversion of the spin current, which is a characteristic of an altermagnet. This work provides an understanding of the anisotropic spin transport in complex antiferromagnetic heterostructures where ferroelectricity and altermagnetism coexist, paving the way for a new route to realize electric-field control of a novel state of magnetism.

cond-mat.mes-hall↗

arXiv2025

Low-energy domain wall racetracks with multiferroic topologies

Conventional racetrack memories move information by pushing magnetic domain walls or other spin textures with spin-polarized currents, but the accompanying Joule heating inflates their energy budget and can hamper scaling. Here we present a voltage-controlled, magnetoelectric racetrack in which transverse electric fields translate coupled ferroelectric-antiferromagnetic walls along BiFeO3 nanostrips at room temperature. Because no charge traverses the track, the switching dissipates orders of magnitude less energy than the most efficient spin-torque devices with more favourable scaling, making the scheme significantly more attractive at the nanoscale. We further uncover noncollinear topological magnetoelectric textures that emerge at domain walls in BiFeO3, where the nature of these topologies influences their stability upon translation. Among these are polar bi-merons and polar vertices magnetoelectrically coupled with magnetic cycloid disclinations and previously unobserved, topological magnetic cycloid twist topologies. We observe domain wall velocities of at least kilometres per second - matching or surpassing the fastest ferrimagnetic and antiferromagnetic racetracks and approaching the acoustic-phonon limit of BiFeO3 - while preserving these topologies over tens of micrometres. The resulting high velocity, low-energy racetrack delivers nanosecond access times without the thermal overhead of current-driven schemes, charting a path toward dense, ultralow-power racetrack devices which rely on spin texture translation.

cond-mat.mtrl-sci↗

arXiv2025

Colossal enhancement of spin transmission through magnon confinement in an antiferromagnet

Since Felix Bloch's introduction of the concept of spin waves in 1930, magnons (the quanta of spin waves) have been extensively studied in a range of materials for spintronics, particularly for non-volatile logic-in-memory devices. Controlling magnons in conventional antiferromagnets and harnessing them in practical applications, however, remains a challenge. In this letter, we demonstrate highly efficient magnon transport in an LaFeO$_3$/BiFeO$_3$/LaFeO$_3$ all-antiferromagnetic system which can be controlled electrically, making it highly desirable for energy-efficient computation. Leveraging spin-orbit-driven spin-charge transduction, we demonstrate that this material architecture permits magnon confinement in ultrathin antiferromagnets, enhancing the output voltage generated by magnon transport by several orders of magnitude, which provides a pathway to enable magnetoelectric memory and logic functionalities. Additionally, its non-volatility enables ultralow-power logic-in-memory processing, where magnonic devices can be efficiently reconfigured via electrically controlled magnon spin currents within magnetoelectric channels.

cond-mat.mtrl-sci↗

arXiv2025

Poincaré sphere engineering of dynamical ferroelectric topological solitons

Geometric representation lays the basis for understanding and flexible tuning of topological transitions in many physical systems. An example is given by the Poincaré sphere (PS) that provides an intuitive and continuous parameterization of the spin or orbital angular momentum (OAM) light states. Here, we apply this geometric construction to understand and continuously encode dynamical topologies of ferroelectric solitons driven by OAM-tunable light. We show that: (1) PS engineering enables controlled creation of dynamic polar antiskyrmions that are rarely found in ferroelectrics; (2) We link such topological transition to the tuning of the light beam as a ``knob'' from OAM (PS pole) to non-OAM (PS equator) modes; (3) Intermediate OAM-state structured light results in new ferroelectric topologies of temporally hybrid skyrmion-antiskyrmion states. Our study offers new approaches of robust control and flexible tuning of topologies of matter using structured light.

cond-mat.mtrl-sci↗

arXiv2024

Morphogenesis of Spin Cycloids in a Non-collinear Antiferromagnet

Pattern formation in spin systems with continuous-rotational symmetry (CRS) provides a powerful platform to study emergent complex magnetic phases and topological defects in condensed-matter physics. However, its understanding and correlation with unconventional magnetic order along with high-resolution nanoscale imaging is challenging. Here, we employ scanning NV magnetometry to unveil the morphogenesis of spin cycloids at both the local and global scales within a single ferroelectric domain of (111)-oriented BiFeO$_3$ (which is a non-collinear antiferromagnet), resulting in formation of a glassy labyrinthine pattern. We find that the domains of locally oriented cycloids are interconnected by an array of topological defects and exhibit isotropic energy landscape predicted by first-principles calculations. We propose that the CRS of spin-cycloid propagation directions within the (111) drives the formation of the labyrinthine pattern and the associated topological defects such as antiferromagnetic skyrmions. Unexpectedly, reversing the as-grown ferroelectric polarization from [$\bar{1}$$\bar{1}$$\bar{1}$] to [111] induces a magnetic phase transition, destroying the labyrinthine pattern and producing a deterministic non-volatile non cycloidal, uniformly magnetized state. These findings highlight that (111)-oriented BiFeO$_3$ is not only important for studying the fascinating subject of pattern formation but could also be utilized as an ideal platform for integrating novel topological defects in the field of antiferromagnetic spintronics.

cond-mat.mtrl-sci↗

arXiv2023

Energy storage properties of ferroelectric nanocomposites

An atomistic effective Hamiltonian technique is used to investigate the finite-temperature energy storage properties of a ferroelectric nanocomposite consisting of an array of BaTiO$_{3}$ nanowires embedded in a SrTiO$_{3}$ matrix, for electric field applied along the long axis of the nanowires. We find that the energy density \textit{versus} temperature curve adopts a nonlinear, mostly temperature-independent response when the system exhibits phases possessing an out-of-plane polarization and vortices while the energy density more linearly increases with temperature when the nanocomposite either only possesses vortices (and thus no spontaneous polarization) or is in a paraelectric and paratoroidic phase for its equilibrium state. Ultrahigh energy density up to $\simeq$140 J/cm$^{3}$ and an ideal 100% efficiency are also predicted in this nanocomposite. A phenomenological model, involving a coupling between polarization and toroidal moment, is further proposed to interpret these energy density results.

cond-mat.mtrl-sci↗

arXiv2023

Hexagonal close-packed polar-skyrmion lattice in ultrathin ferroelectric PbTiO3 films

Polar skyrmions are topologically stable, swirling polarization textures with particle-like characteristics, which hold promise for next-generation, nanoscale logic and memory. While understanding of how to create ordered polar skyrmion lattice structures and how such structure respond to applied electric fields, temperature, and film thickness remains elusive. Here, using phase-field simulations, the evolution of polar topology and the emergence of a phase transition to a hexagonal close-packed skyrmion lattice is explored through the construction of a temperature-electric field phase diagram for ultrathin ferroelectric PbTiO3 films. The hexagonal-lattice skyrmion crystal can be stabilized under application of an external, out-of-plane electric field which carefully adjusts the delicate interplay of elastic, electrostatic, and gradient energies. In addition, the lattice constants of the polar skyrmion crystals are found to increase with film thickness, consistent with expectation from Kittel law. Our studies pave the way for the development of novel ordered condensed matter phases assembled from topological polar textures and related emergent properties in nanoscale ferroelectrics.

cond-mat.mtrl-sci↗

arXiv2023

Dynamics of polar vortex crystallization

Vortex crystals are commonly observed in ultra-thin ferroelectrics. However, a clear physical picture of origin of this topological state is currently lacking. Here, we show that vortex crystallization in ultra-thin Pb(Zr0.4,Ti0.6)O3 films stems from the softening of a phonon mode and can be thus described as a SU(2) symmetry-breaking transition. This result sheds light on the topology of the polar vortex patterns and bridges polar vortices with smectic phases, spin spirals, and other modulated states. Finally, we predict an ac-field driven resonant switching of the vortex tube orientation which could enable new low-power electronic technologies.

cond-mat.mtrl-sci↗

arXiv2023

Dynamical control of topology in ferroelectric skyrmions via twisted light

Twisted light carries a non-zero orbital angular momentum, that can be transferred from light to electrons and particles ranging from nanometers to micrometers. Up to now, the interplay between twisted light with dipolar systems has scarcely been explored, though the latter bear abundant forms of topologies such as skyrmions and embrace strong light-matter coupling. Here, using first-principles-based simulations, we show that twisted light can excite and drive dynamical polar skyrmions and transfer its nonzero winding number to ferroelectric ultrathin films. The skyrmion is successively created and annihilated alternately at the two interfaces, and experiences a periodic transition from a markedly "Bloch" to "Neel" character, accompanied with the emergence of a "Bloch point" topological defect with vanishing polarization. The dynamical evolution of skyrmions is connected to a constant jump of topological number between "0" and "1" over time. These intriguing phenomena are found to have an electrostatic origin. Our study thus demonstrates that, and explains why, this unique light-matter interaction can be very powerful in creating and manipulating topological solitons in functional materials.

cond-mat.mes-hall↗

arXiv2022

Stability of ferroelectric bubble domains

Nanoscale ferroelectric topologies such as vortices, anti-vortices, bubble patterns etc. are stabilized in thin films by a delicate balance of both mechanical and electrical boundary conditions. A systematic understanding of the phase stability of bubble domains, particularly when the above factors act simultaneously, remains elusive. Here we present first-principle-based simulations in combination with scanning probe microscopy of ultrathin epitaxial (001) PbZr0.4Ti0.6O3 heterostructures to address this gap. The simulations predict that as-grown labyrinthine domains will transform to bubbles under combinations of reduced film thickness, increased mechanical pressure and/or improved electrical screening. These topological transitions are explained by a common fundamental mechanism. Namely, we argue that, independently of the nature of the driving force, the evolution of the domain morphology allows the system to conserve its original residual depolarization field. Thereby, the latter remains pinned to a value determined by an external or built-in electric bias. To verify our predictions, we then exploit tomographic atomic force microscopy to achieve the concurrent effect of reducing film thickness and increased mechanical stimulus. The results provide a systematic understanding of phase stability and demonstrate controlled manipulation of nanoscale ferroelectric bubble domains.

cond-mat.mtrl-sci↗

arXiv2021

Electrocaloric effects in multiferroics

An atomistic effective Hamiltonian is used to compute electrocaloric (EC) effects in rare-earth substituted BiFeO$_{3}$ multiferroics. A phenomenological model is then developed to interpret these computations, with this model indicating that the EC coefficient is the sum of two terms, that involve electric quantities (polarization, dielectric response), the antiferromagnetic order parameter, and the coupling between polarization and antiferromagnetic order. The first one depends on the polarization and dielectric susceptibility, has the analytical form previously demonstrated for ferroelectrics, and is thus enhanced at the ferroelectric Curie temperature. The second one explicitly involves the dielectric response, the magnetic order parameter and a specific magnetoelectric coupling, and generates a peak of the EC response at the Néel temperature. These atomistic results and phenomenological model may be put in use to optimize EC coefficients.

cond-mat.mtrl-sci↗

arXiv2020

Berezinskii-Kosterlitz-Thouless Phase in Two-dimensional Ferroelectrics

The celebrated Berezinskii-Kosterlitz-Thouless (BKT) phase transition refers to a topological transition characterized, e.g., by the dissociation of vortex-antivortex pairs in two-dimensional (2D) systems. Such unusual phase has been reported in various types of materials, but never in the new class of systems made by one-unit-cell-thick (1UC) ferroelectrics (also coined as 2D ferroelectrics). Here, the use of a first-principles-based effective Hamiltonian method leads to the discovery of many fingerprints of a BKT phase existing in-between the ferroelectric and paraelectric states of 1UC tin tellurium being fully relaxed. Moreover, epitaxial strain is found to have dramatic consequences on the temperature range of such BKT phase for the 1UC SnTe. Consequently, our predictions extend the playground of BKT theory to a novel class of functional materials, and demonstrate that strain is an effective tool to alter BKT characteristics there.

cond-mat.mtrl-sci↗

arXiv2020

Possible Kitaev Quantum Spin Liquid State in 2D Materials with S=3/2

Quantum spin liquids (QSLs) form an extremely unusual magnetic state in which the spins are highly correlated and fluctuate coherently down to the lowest temperatures, but without symmetry breaking and without the formation of any static long-range-ordered magnetism. Such intriguing phenomena are not only of great fundamental relevance in themselves, but also hold the promise for quantum computing and quantum information. Among different types of QSLs, the exactly solvable Kitaev model is attracting much attention, with most proposed candidate materials, e.g., RuCl$_3$ and Na$_2$IrO$_3$, having an effective $S$=1/2 spin value. Here, via extensive first-principle-based simulations, we report the investigation of the Kitaev physics and possible Kitaev QSL state in epitaxially strained Cr-based monolayers, such as CrSiTe$_3$, that rather possess a $S$=3/2 spin value. Our study thus extends the playground of Kitaev physics and QSLs to 3$d$ transition metal compounds.

cond-mat.mtrl-sci↗