Showing 1–22 of 22 results for author: Nahas, Y

Topological textures and emergent altermagnetic signatures in ultrathin BiFeO3

Authors: George Fratian, Maya Ramesh, Xinyan Li, Evangelos Golias, Yousra Nahas, Sebastian Maria Ulrich Schultheis, Julian Skolaut, Marti Checa, Arundhati Ghosal, Jan Priessnitz, F. C. Fobasso Mbognou, Shashank Kumar Ojha, Shiyu Zhou, Alexander Qualls, Kai Litzius, Christoph Klewe, Peter Meisenheimer, Laurent Bellaiche, Libor Šmejkal, Darrell G. Schlom, Yimo Han, Sergei Prokhorenko, Ramamoorthy Ramesh, Paul Stevenson, Angela Wittmann , et al. (1 additional authors not shown)

Abstract: 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… ▽ More 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. △ Less

Submitted 14 January, 2026; originally announced January 2026.

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

Authors: Sajid Husain, Maya Ramesh, Qian Song, Sergei Prokhorenko, Shashank Kumar Ojha, Surya Narayan Panda, Xinyan Li, Yousra Nahas, Yogesh Kumar, Pushpendra Gupta, Tenzin Chang, Alan Ji-in Jung, Rogério de Sousa, James G. Analytis, Lane W. Martin, Zhi Yao, Sang-Wook Cheong, Laurent Bellaiche, Manuel Bibes, Darrell G. Schlom, Ramamoorthy Ramesh

Abstract: 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… ▽ More 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. △ Less

Submitted 7 January, 2026; originally announced January 2026.

Comments: 8 pgaes, 5 figure

Low-energy domain wall racetracks with multiferroic topologies

Authors: Arundhati Ghosal, Alexander Qualls, Yousra Nahas, Shashank Ojha, Peter Meisenheimer, Shiyu Zhou, Maya Ramesh, Sajid Husain, Julia Mundy, Darrell Schlom, Zhi Yao, Sergei Prokhorenko, Laurent Bellaiche, Ramamoorthy Ramesh, Paul Stevenson, Lucas Caretta

Abstract: 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 nanostri… ▽ More 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. △ Less

Submitted 16 July, 2025; originally announced July 2025.

Colossal enhancement of spin transmission through magnon confinement in an antiferromagnet

Authors: Sajid Husain, Maya Ramesh, Xinyan Li, Sergei Prokhorenko, Shashank Kumar Ojha, Aiden Ross, Koushik Das, Boyang Zhao, Hyeon Woo Park, Peter Meisenheimer, Yousra Nahas, Lucas Caretta, Lane W. Martin, Se Kwon Kim, Zhi Yao, Haidan Wen, Sayeef Salahuddin, Long-Qing Chen, Yimo Han, Rogerio de Sousa, Laurent Bellaiche, Manuel Bibes, Darrell G. Schlom, Ramamoorthy Ramesh

Abstract: 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… ▽ More 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. △ Less

Submitted 31 March, 2025; originally announced March 2025.

Comments: 12 pages, 4 figures

Poincaré sphere engineering of dynamical ferroelectric topological solitons

Authors: Lingyuan Gao, Yijie Shen, Sergei Prokhorenko, Yousra Nahas, Laurent Bellaiche

Abstract: 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… ▽ More 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. △ Less

Submitted 19 February, 2025; originally announced February 2025.

Journal ref: Phys. Rev. B 112, L121102 (2025)

Morphogenesis of Spin Cycloids in a Non-collinear Antiferromagnet

Authors: Shashank Kumar Ojha, Pratap Pal, Sergei Prokhorenko, Sajid Husain, Maya Ramesh, Peter Meisenheimer, Darrell G. Schlom, Paul Stevenson, Lucas Caretta, Yousra Nahas, Lane W. Martin, Laurent Bellaiche, Chang-Beom Eom, Ramamoorthy Ramesh

Abstract: 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 morpho… ▽ More 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. △ Less

Submitted 29 October, 2024; originally announced October 2024.

Comments: 20 pages, 15 figures

Spin-Wave-Assisted Thermal Reversal of Epitaxial Perpendicular Magnetic Nanodots

Authors: S. Rohart, P. Campiglio, V. Repain, Y. Nahas, C. Chacon, Y. Girard, J. Lagoute, A. Thiaville, S. Rousset

Abstract: The magnetic susceptibility of self-organized two-dimensional Co nanodots on Au(111) has been measured as a function of their size in the 2-7~nm diameter range. We show that the activation energy for the thermal reversal displays a power law behavior with the dot volume. Atomic scale simulations based on the Heisenberg hamiltonian show that this behavior is due to a deviation from the macrospin mo… ▽ More The magnetic susceptibility of self-organized two-dimensional Co nanodots on Au(111) has been measured as a function of their size in the 2-7~nm diameter range. We show that the activation energy for the thermal reversal displays a power law behavior with the dot volume. Atomic scale simulations based on the Heisenberg hamiltonian show that this behavior is due to a deviation from the macrospin model for dot size as small as 3~nm in diameter. This discrepancy is attributed to finite temperature effects through the thermal excitation of spin-wave modes inside the particles △ Less

Submitted 7 February, 2024; originally announced February 2024.

Comments: 4 pages, 4 figures

Journal ref: Phys. Rev. Lett. 104, 137202 (2010)

Quantum criticality at cryogenic melting of polar bubble lattices

Authors: W. Luo, A. Akbarzadeh, Y. Nahas, S. Prokhorenko, L. Bellaiche

Abstract: Quantum fluctuations (QFs) caused by zero-point phonon vibrations (ZPPVs) are known to prevent the occurrence of polar phases in bulk incipient ferroelectrics down to 0K1-3. On the other hand, little is known about the effects of QFs on the recently discovered topological patterns in ferroelectric nanostructures4-9. Here, by using an atomistic effective Hamiltonian within classical Monte Carlo (CM… ▽ More Quantum fluctuations (QFs) caused by zero-point phonon vibrations (ZPPVs) are known to prevent the occurrence of polar phases in bulk incipient ferroelectrics down to 0K1-3. On the other hand, little is known about the effects of QFs on the recently discovered topological patterns in ferroelectric nanostructures4-9. Here, by using an atomistic effective Hamiltonian within classical Monte Carlo (CMC) and path integral quantum Monte Carlo (PI-QMC)1,3,10,11, we unveil how QFs affect the topology of several dipolar phases in ultrathin Pb(Zr0.4Ti0.6)O3 (PZT) films. In particular, our PI-QMC simulations show that the ZPPVs do not suppress polar patterns but rather stabilize the labyrinth4, bimeron5 and bubble phases12,13 within a wider range of bias field magnitudes. Moreover, we reveal that quantum fluctuations induce a quantum critical point (QCP) separating a hexagonal bubble lattice from a liquid-like state characterized by spontaneous motion, creation and annihilation of polar bubbles at cryogenic temperatures. Finally, we show that the discovered quantum melting is associated with anomalous physical response, as, e.g., demonstrated by a negative longitudinal piezoelectric coefficient. △ Less

Submitted 25 October, 2023; originally announced October 2023.

Comments: Nature communication, accepted, 21 pages, 4 Figs

Energy storage properties of ferroelectric nanocomposites

Authors: Zhijun Jiang, Zhenlong Zhang, Sergei Prokhorenko, Yousra Nahas, Sergey Prosandeev, Laurent Bellaiche

Abstract: 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 temperatur… ▽ More 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. △ Less

Submitted 23 May, 2023; originally announced May 2023.

Comments: 7 pages, 5 figures

Journal ref: Phys. Rev. B 107, 214105 (2023)

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

Authors: Shuai Yuan, Zuhuang Chen, Sergei Prokhorenko, Yousra Nahas, Laurent Bellaiche, Chenhan Liu, Bin Xu, Lang Chen, Sujit Das, Lane W. Martin

Abstract: 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,… ▽ More 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. △ Less

Submitted 6 May, 2023; originally announced May 2023.

Comments: 4 Figures

Motion and teleportation of polar bubbles in ultra-thin ferroelectrics

Authors: S. Prokhorenko, Y. Nahas, Q. Zhang, V. Govinden, N. Valanoor, L. Bellaiche

Abstract: Polar bubble domains are complex topological defects akin to magnetic skyrmions that can spontaneously form in ferroelectric thin films and superlattices. They can be deterministically written and deleted and exhibit a set of properties, such as sub-10 nm radius and room-temperature stability, that are highly attractive for dense data storage and reconfigurable nano-electronics technologies. Howev… ▽ More Polar bubble domains are complex topological defects akin to magnetic skyrmions that can spontaneously form in ferroelectric thin films and superlattices. They can be deterministically written and deleted and exhibit a set of properties, such as sub-10 nm radius and room-temperature stability, that are highly attractive for dense data storage and reconfigurable nano-electronics technologies. However, possibilities of controlled motion of electric bubble skyrmions, a critical technology requirement currently remains missing. Here we present atomistic simulations that demonstrate how external electric-field perturbations can induce two types of motion of bubble skyrmions in low-dimensional tetragonal PbZr$_{0.4}$Ti$_{0.6}$O$_3$ systems under residual depolarizing field. Specifically, we show that, depending on the spatial profile and magnitude of the external field, bubble skyrmions can exhibit either a continuous motion driven by the external electric field gradient or a discontinuous, teleportation-like, skyrmion domain transfer. These findings provide the first analysis of dynamics and controlled motion of polar skyrmions that are essential for functionalization of these particle-like domain structures. △ Less

Submitted 24 March, 2023; originally announced March 2023.

Dynamics of polar vortex crystallization

Authors: Suyash Rijal, Yousra Nahas, Sergei Prokhorenko, Laurent Bellaiche

Abstract: 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 pola… ▽ More 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. △ Less

Submitted 14 February, 2023; originally announced February 2023.

Comments: 3 figures

Dynamical control of topology in ferroelectric skyrmions via twisted light

Authors: Lingyuan Gao, Sergei Prokhorenko, Yousra Nahas, Laurent Bellaiche

Abstract: 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-ba… ▽ More 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. △ Less

Submitted 2 February, 2023; originally announced February 2023.

Journal ref: Phys. Rev. Lett. 132, 026902 (2024)

Ferroelectric Solitons Crafted in Epitaxial Bismuth Ferrite Superlattices

Authors: V. Govinden, P. R. Tong, X. Guo, Q. Zhang, S. Mantri, S. Prokhorenko, Y. Nahas, Y. Wu, L. Bellaiche, H. Tian, Z. Hong, D. Sando, V. Nagarajan

Abstract: In ferroelectrics, complex interactions among various degrees of freedom enable the condensation of topologically protected polarization textures. Known as ferroelectric solitons, these particle-like structures represent a new class of materials with promise for beyond CMOS technologies due to their ultrafine size and sensitivity to external stimuli. Such polarization textures have scarcely been r… ▽ More In ferroelectrics, complex interactions among various degrees of freedom enable the condensation of topologically protected polarization textures. Known as ferroelectric solitons, these particle-like structures represent a new class of materials with promise for beyond CMOS technologies due to their ultrafine size and sensitivity to external stimuli. Such polarization textures have scarcely been reported in multiferroics. Here, we report a range of soliton topologies in bismuth ferrite strontium titanate superlattices. High-resolution piezoresponse force microscopy and Cs-corrected high-angle annular dark-field scanning transmission electron microscopy reveal a zoo of topologies, and polarization displacement mapping of planar specimens reveals center-convergent and divergent topological defects as small as 3 nm. Phase field simulations verify that some of these topologies can be classed as bimerons, with a topological charge of plus and minus one, and first-principles-based effective Hamiltonian computations show that the co-existence of such structures can lead to non-integer topological charges, a first observation in a BiFeO3-based system. Our results open new opportunities in multiferroic topotronics. △ Less

Submitted 19 September, 2022; originally announced September 2022.

Stability of ferroelectric bubble domains

Authors: Vivasha Govinden, Suyash Rijal, Qi Zhang, Yousra Nahas, Laurent Bellaiche, Nagarajan Valanoor, Sergei Prokhorenko

Abstract: 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 combinatio… ▽ More 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. △ Less

Submitted 20 May, 2022; originally announced May 2022.

Electrocaloric effects in multiferroics

Authors: Zhijun Jiang, Bin Xu, Sergey Prosandeev, Yousra Nahas, Sergei Prokhorenko, Jorge Íñiguez, L. Bellaiche

Abstract: 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… ▽ More 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. △ Less

Submitted 26 February, 2021; originally announced February 2021.

Comments: 6 pages, 3 figures

Berezinskii-Kosterlitz-Thouless Phase in Two-dimensional Ferroelectrics

Authors: Changsong Xu, Yousra Nahas, Sergei Prokhorenko, Hongjun Xiang, L. Bellaiche

Abstract: 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). Her… ▽ More 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. △ Less

Submitted 1 June, 2020; originally announced June 2020.

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

Authors: Changsong Xu, Junsheng Feng, Mitsuaki Kawamura, Youhei Yamaji, Yousra Nahas, Sergei Prokhorenko, Yang Qi, Hongjun Xiang, L. Bellaiche

Abstract: 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… ▽ More 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. △ Less

Submitted 27 February, 2020; originally announced February 2020.

Giant electrocaloric response in the prototypical Pb(Mg,Nb)O$_{3}$ relaxor ferroelectric from atomistic simulations

Authors: Zhijun Jiang, Y. Nahas, S. Prokhorenko, S. Prosandeev, D. Wang, Jorge Íñiguez, L. Bellaiche

Abstract: An atomistic effective Hamiltonian is used to investigate electrocaloric (EC) effects of Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_{3}$ (PMN) relaxor ferroelectrics in its ergodic regime, and subject to electric fields applied along the pseudocubic [111] direction. Such Hamiltonian qualitatively reproduces (i) the electric field-versus-temperature phase diagram, including the existence of a critical point where… ▽ More An atomistic effective Hamiltonian is used to investigate electrocaloric (EC) effects of Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_{3}$ (PMN) relaxor ferroelectrics in its ergodic regime, and subject to electric fields applied along the pseudocubic [111] direction. Such Hamiltonian qualitatively reproduces (i) the electric field-versus-temperature phase diagram, including the existence of a critical point where first-order and second-order transitions meet each other; and (ii) a giant EC response near such critical point. It also reveals that such giant response around this critical point is microscopically induced by field-induced percolation of polar nanoregions. Moreover, it is also found that, for any temperature above the critical point, the EC coefficient-versus-electric field curve adopts a maximum (and thus larger electrocaloric response too), that can be well described by the general Landau-like model proposed in [Jiang et al, Phys. Rev. B 96, 014114 (2017)] and that is further correlated with specific microscopic features related to dipoles lying along different rhombohedral directions. Furthermore, for temperatures being at least 40 K higher than the critical temperature, the (electric field, temperature) line associated with this maximal EC coefficient is below both the Widom line and the line representing percolation of polar nanoregions. △ Less

Submitted 5 February, 2018; originally announced February 2018.

Comments: 6 pages, 5 figures

Journal ref: Phys. Rev. B 97, 104110 (2018)

Electrocaloric effects in the lead-free Ba(Zr,Ti)O$_{3}$ relaxor ferroelectric from atomistic simulations

Authors: Zhijun Jiang, Sergei Prokhorenko, Sergey Prosandeev, Y. Nahas, D. Wang, J. Íñiguez, E. Defay, L. Bellaiche

Abstract: Atomistic effective Hamiltonian simulations are used to investigate electrocaloric (EC) effects in the lead-free Ba(Zr$_{0.5}$Ti$_{0.5}$)O$_{3}$ (BZT) relaxor ferroelectric. We find that the EC coefficient varies non-monotonically with the field at any temperature, presenting a maximum that can be traced back to the behavior of BZT's polar nanoregions. We also introduce a simple Landau-based model… ▽ More Atomistic effective Hamiltonian simulations are used to investigate electrocaloric (EC) effects in the lead-free Ba(Zr$_{0.5}$Ti$_{0.5}$)O$_{3}$ (BZT) relaxor ferroelectric. We find that the EC coefficient varies non-monotonically with the field at any temperature, presenting a maximum that can be traced back to the behavior of BZT's polar nanoregions. We also introduce a simple Landau-based model that reproduces the EC behavior of BZT as a function of field and temperature, and which is directly applicable to other compounds. Finally, we confirm that, for low temperatures (i.e., in non-ergodic conditions), the usual indirect approach to measure the EC response provides an estimate that differs quantitatively from a direct evaluation of the field-induced temperature change. △ Less

Submitted 27 June, 2017; originally announced June 2017.

Comments: 9 pages, 4 figures

Journal ref: Phys. Rev. B 96, 014114 (2017)

Magnetoresistance through a single molecule

Authors: Stefan Schmaus, Alexei Bagrets, Yasmine Nahas, Toyo K. Yamada, Annika Bork, Martin Bowen, Eric Beaurepaire, Ferdinand Evers, Wulf Wulfhekel

Abstract: The use of single molecules to design electronic devices is an extremely challenging and fundamentally different approach to further downsizing electronic circuits. Two-terminal molecular devices such as diodes were first predicted [1] and, more recently, measured experimentally [2]. The addition of a gate then enabled the study of molecular transistors [3-5]. In general terms, in order to increas… ▽ More The use of single molecules to design electronic devices is an extremely challenging and fundamentally different approach to further downsizing electronic circuits. Two-terminal molecular devices such as diodes were first predicted [1] and, more recently, measured experimentally [2]. The addition of a gate then enabled the study of molecular transistors [3-5]. In general terms, in order to increase data processing capabilities, one may not only consider the electron's charge but also its spin [6,7]. This concept has been pioneered in giant magnetoresistance (GMR) junctions that consist of thin metallic films [8,9]. Spin transport across molecules, i.e. Molecular Spintronics remains, however, a challenging endeavor. As an important first step in this field, we have performed an experimental and theoretical study on spin transport across a molecular GMR junction consisting of two ferromagnetic electrodes bridged by a single hydrogen phthalocyanine (H2Pc) molecule. We observe that even though H2Pc in itself is nonmagnetic, incorporating it into a molecular junction can enhance the magnetoresistance by one order of magnitude to 52%. △ Less

Submitted 13 February, 2011; originally announced February 2011.

Comments: To appear in Nature Nanotechnology. Present version is the first submission to Nature Nanotechnology, from May 18th, 2010

Elastic and magnetic effects on the infrared phonon spectra of MnF2

Authors: R. Schleck, Y. Nahas, R. P. S. M. Lobo, J. Varignon, M. B. Lepetit, C. S. Nelson, R. L. Moreira

Abstract: We measured the temperature dependent infrared reflectivity spectra of MnF2 between 4 K and room temperature. We show that the phonon spectrum undergoes a strong renormalization at TN. The ab-initio calculation we performed on this compound accurately predict the magnitude and the direction of the phonon parameters changes across the antiferromagnetic transition, showing that they are mainly induc… ▽ More We measured the temperature dependent infrared reflectivity spectra of MnF2 between 4 K and room temperature. We show that the phonon spectrum undergoes a strong renormalization at TN. The ab-initio calculation we performed on this compound accurately predict the magnitude and the direction of the phonon parameters changes across the antiferromagnetic transition, showing that they are mainly induced by the magnetic order. In this material, we found that the dielectric constant is mostly from phonon origin. The large change in the lattice parameters with temperature seen by X-ray diffraction as well as the A2u phonon softening below TN indicate that magnetic order induced distortions in MnF2 are compatible with the ferroelectric instabilities observed in TiO2, FeF2 and other rutile-type fluorides. This study also shows the anomalous temperature evolution of the lower energy Eu mode in the paramagnetic phase, which can be compared to that of the B1g one seen by Raman spectroscopy in many isostructural materials. This was interpreted as being a precursor of a phase transition from rutile to CaCl2 structure which was observed under pressure in ZnF2. △ Less

Submitted 13 July, 2010; v1 submitted 16 October, 2009; originally announced October 2009.

Comments: 8 pages, 8 figures, updated version accepted in PRB

Journal ref: Phys. Rev. B 82, 054412 (2010)