arXiv subjects

Anna Grünebohm

Publications and source records attributed to Anna Grünebohm.

At least 19 recordsLinked to original sources

arXiv2026

Fully Compensated Lines in Ferrimagnets

We generalize the classic Néel diagram and identify another type of ferrimagnetic phase that remains fully magnetically compensated below the Curie temperature, forming a continuous line of compensated points. It exhibits zero net magnetization while retaining non-relativistic, eV-scale reciprocal spin splitting. We further find a persistently enhanced intrinsic switching field over a broad temperature range near the fully compensated phase. Proximity to the compensated line is achieved by minimizing the net local moment while balancing exchange interactions with respect to the number of equivalent atoms in each sublattice. The resulting extended Néel diagram provides practical design rules for engineering fully compensated ferrimagnetic phases via targeted chemical substitution that combines atoms with robust and weak local moments, as demonstrated through density functional theory and Monte Carlo simulations for GdCo$_5$-type ferrimagnets.

cond-mat.mtrl-sci↗

arXiv2026

Competing phases and domain structures of ferroelectric perovskites: the benefit of epitaxial (110) growth

Strain and domain engineering offer powerful routes to control phase and domain stability in ferroelectric thin films. While most studies have focused on (100)-oriented growth, the impact of lower-symmetry orientations remains underexplored. We address this gap in knowledge with first-principles based molecular dynamics simulation for the example of prototypical ferroelectric perovskites under (110) strain. Epitaxial (110) strains may indeed outperform the widely studied (100) orientation, as even modest strain values stabilize a diverse set of metastable nanoscale states with potential high functional tunability. In this regime, the films exhibit multidomain configurations with domain wall normal oriented along the clamped in-plane or the relaxed out-of-plane directions and heterophases in BaTiO$_3$ and KNbO$_3$. Besides, complex superdomain patterns and antiferroelectric-like domains are observed in PbTiO$_3$. These metastable nanoscale configurations may allow for large reversible responses.

cond-mat.mtrl-sci↗

arXiv2026

Ferroelectric switching at edge dislocations in BaTiO$_3$ modelled at the atomic scale

Ferroelectric switching governs the functional properties of ferroelectric perovskites. It is widely accepted that this switching depends on domain nucleation and pinning and that these processes can be controlled by the defect structure. However, an atomistic picture of the influence of one important class of defects - dislocations on ferroelectric switching is missing. This is an important gap in knowledge as dislocations cannot be avoided at interfaces and can also be engineered by plastic deformation at high temperatures. Using atomistic simulations, we show how the cores of $\langle100\rangle$ edge dislocations in BaTiO$_3$ can either act as nucleation centers for ferroelectric switching or pin walls depending on the direction of the applied field. The coupling between electric field and polarization is strongest when the field is applied parallel to the Burgers vector of the dislocation.

cond-mat.mtrl-sci↗

arXiv2026

Synthetic control over marcasite-pyrite polymorph formation in the Fe1-xCoxSe2 series

Transition-metal dichalcogenides of the pyrite-marcasite family are model systems of crystal chemistry. A few of these show polymorphism. The theoretical ground state of CoSe2 is marcasite, but the material is typically synthesized in the pyrite structure. Polymorphism has been observed in nanoparticles and synthetic control of the polymorphs of CoSe2 has not been achieved. We have synthesized material libraries of the Fe1-xCoxSe2 series by combining combinatorial deposition and ex-situ selenization. The approach allows to efficiently explore substitution ranges and crystal structures that form for different synthesis conditions. We find that higher levels of Co content x within the marcasite structure are possible when synthesizing at low temperatures. At a synthesis temperature of only 250° C, we have successfully synthesized marcasite CoSe2 as the majority phase. Density functional theory simulations reveal that the two isomorphs of CoSe2 are extremely close in energy and that the orthorhombic phase is the energetic ground state. Our experimental and theoretical data show that the marcasite structure is the equilibrium phase of Fe1-xCoxSe2 in the entire composition range.

cond-mat.mtrl-sci↗

arXiv2025

Thermal stability of nano-scale ferroelectric domains by molecular dynamics modeling

Ultra-dense domain walls are increasingly important for many devices but their microscopic properties are so far not fully understood. Here we use molecular dynamic simulations to study the domain wall stability in the prototypical ferroelectric BaTiO3 combining core-shell pair potentials and a coarse-grained effective Hamiltonian. We transfer the discussion of the field-driven nucleation and motion of domain walls to thermally induced modifications of the wall without an external driving force. Our simulations show that domain wall dynamics and stability depend crucially on microscopic thermal fluctuations. Enhanced fluctuations at domain walls may result in the formation of critical nuclei for the permanent shift of the domain wall. If two domain walls are close - put in other words, when domains are small - thermal fluctuations can be sufficient to bring domain walls into contact and lead to the annihilation of small domains. This is even true well below the Curie temperature and when domain walls are initially as far apart as 6 unit cells. Such small domains are, thus, not stable and limit the maximum achievable domain wall density in nanoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2025

Point defect design in (Ba,Sr)TiO$_3$ -- an insight on agglomeration

Functional properties of ferroelectrics and their change with time depend crucially on the defect structure. In particular, point defects and bias fields induced by defect dipoles modify the field hysteresis and play an important role in fatigue and aging. However, a full understanding on how order, agglomeration and strength of defect dipoles affect phase stability and functional properties is still lacking. To close these gaps in knowledge, we screen these parameters by \textit{ab\ initio} based molecular dynamics simulations with the effective Hamiltonian method for the prototypical ferroelectric material (Ba,Sr)TiO$_3$. Our findings suggest that the {\it{active surface area}} of the defects, rather than the defect concentration is the decisive factor. For a fixed defect concentration, clustering reduces the {\it{active surface area}} and thus the defect-induced changes of phase stability and field hysteresis. Particularly planar agglomerates of defects appear as promising route for the material design as their impact on the field hysteresis can be controlled by the field direction and as their impact on the phase stability shows a cross-over with the strength of the defect dipoles. For this agglomeration, we show that the recoverable stored energy can outperform the response of pristine (Ba,Sr)TiO$_3$ even in its paraelectric phase due to a pinched double-loop field hysteresis.

cond-mat.mtrl-sci↗

arXiv2025

BaTiO$_3$ -- SrTiO$_3$ composites: a microscopic study on paraelectric cubic inclusions

Composites of ferroelectric and paraelectric perovskites have attracted a lot of attention due to their application potential in energy storage as well as novel computing and memory devices. So far the main focus of research has been on superlattices and ferroelectric particles in a paraelectric matrix, while the impact of paraelectric inclusions on the ferroelectric matrix is surprisingly underrepresented. To close this gap in knowledge we perform molecular dynamics simulations using an $ab\ initio$ derived effective Hamiltonian for BaTiO$_3$--SrTiO$_3$ and reveal the dependency of phase stability and phase transitions on the size and distances of paraelectric inclusions. We discuss how the combination of compressive strain and depolarization fields at the SrTiO$_3$ interfaces induces large local polarization, complex domain structures and coexisting phases as well as diffuse phase transitions and reduced coercive fields.

cond-mat.mtrl-sci↗

arXiv2025

Efficient local atomic cluster expansion for BaTiO$_3$ close to equilibrium

Barium titanate (BTO) is a representative perovskite oxide that undergoes three first-order ferroelectric phase transitions related to exceptional functional properties. In this work, we develop two atomic cluster expansion (ACE) models for BTO to reproduce fundamental properties of bulk as well as defective BTO phases. The two ACE models do not target full transferability but rather aim to examine the influence of implicit and explicit treatment of long-range Coulomb interactions. We demonstrate that both models describe equally well the temperature induced phase transitions as well as polarization switching due to applied electric field. Even though the parametrizations are based on a limited number of configurations that are mostly not far away from the equilibrium, the ACE models are able to capture also properties of important crystal defects, such as oxygen vacancies, stacking faults and domain walls. A systematic comparison shows that the phase transitions as well as the fundamental properties of the investigated defects can be described with similar accuracy with or without explicit treatment of charges and Coulomb interactions allowing for efficient short-range machine learning potentials.

cond-mat.mtrl-sci↗

arXiv2025

Control of ferroelectric domain wall dynamics by point defects: Insights from ab initio based simulations

The control of ferroelectric domain walls and their dynamics on the nanoscale becomes increasingly important for advanced nanoelectronics and novel computing schemes. One common approach to tackle this challenge is the pinning of walls by point defects. The fundamental understanding on how different defects influence the wall dynamics is, however, incomplete. In particular, the important class of defect dipoles in acceptor-doped ferroelectrics is currently underrepresented in theoretical work. In this study, we combine molecular dynamics simulations based on an \textit{ab\ initio}-derived effective Hamiltonian and methods from materials informatics, and analyze the impact of these defects on the motion of 180$^{\circ}$ domain walls in tetragonal BaTiO$_3$. We show how these defects can act as local pinning centers and restoring forces on the domain structure. Furthermore, we reveal how walls can flow around sparse defects by nucleation and growth of dipole clusters, and how pinning, roughening and bending of walls depend on the defect distribution. Surprisingly, the interaction between acceptor dopants and walls is short-ranged. We show that the limiting factor for the nucleation processes underlying wall motion is the defect-free area in front of the wall.

cond-mat.mtrl-sci↗

arXiv2024

Electric field direction dependence of the electrocaloric effect in BaTiO3

Single-crystalline ferroelectric (FE) perovskites show a large electrocaloric effect at electric field-induced phase transitions, promising for solid-state cooling technologies. However, paraelectric-FE transition temperatures are often too high for practical applications, and lower transitions are underrepresented in literature. Particularly, the role of thermal hysteresis and electric field direction on the caloric response is critical, especially for polycrystalline materials, but not yet fully understood. Using ab initio-based coarse-grained molecular dynamics simulations, we show how transition temperatures depend on the direction of the applied field. Also, we reveal that the choice of electric field direction can reduce thermal hysteresis and can adjust the temperature ranges where large and reversible caloric responses occur. Furthermore, we propose a phenomenological descriptor for the qualitative changes in transition temperature with field direction. This descriptor is valid for both BaTiO3 and PbTiO3, even though both materials show different microscopic electric field coupling. Finally, we identify favorable temperature and texturing conditions for large and reversible caloric responses in polycrystals.

cond-mat.mtrl-sci↗

arXiv2023

Ab initio study of transition paths between (meta)stable phases of Nb and Ta-substituted Nb

Although Niobium is a well characterized material it still shows some anomalies that are not yet understood. Therefore we revisit its metastable phases using density functional theory. First, we systematically compare energies and ground state volumes of chosen crystal structures and discuss possible transition paths to the bcc ground state structure and the energy landscape for tetragonal distortions. Furthermore, we discuss their stability by means of their phonon spectra and vibronic free energies. Second we analyze the impact of tantalum impurities on phase stability. Surprisingly we find new aspects of the energy landscape of the material which have been overlooked so far: A new local energy minimum on the bcc to omega transition path, a flat energy landscape with respect to uniaxial strain along [111] and a considerable stabilization of the sigma phase by Ta substitution.

cond-mat.mtrl-sci↗

arXiv2023

Microscopic insights on field induced switching and domain wall motion in orthorhombic ferroelectrics

Surprisingly little is known about the microscopic processes that govern ferroelectric switching in orthorhombic ferroelectrics. To study microscopic switching processes we combine ab initio-based molecular dynamics simulations and data science on the prototypical material BaTiO$_3$. We reveal two different field regimes: For moderate field strengths, the switching is dominated by domain wall motion while a fast bulk-like switching can be induced for large fields. Switching in both field regimes follows a multi-step process via polarization directions perpendicular to the applied field. In the former case, the moving wall is of Bloch character and hosts dipole vortices due to nucleation, growth, and crossing of two dimensional 90$^{\circ}$ domains. In the second case, the local polarization shows a continuous correlated rotation via a an intermediate tetragonal multidomain state.

cond-mat.mtrl-sci↗

arXiv2022

Pinning of domain walls by strontium layer in BaTiO3 perovskite: an atomic-scale study

We use atomistic simulations to study the interactions between two-dimensional domain walls and Sr inclusions in the prototypical ferroelectric BaTiO$_3$. Based on nudged elastic band calculations we predict that the energy barrier for domain wall movement increases in the vicinity of small planar Sr inclusions which may act as pinning centers. We link this observation to the local increase in polarization by larger oxygen off-centering and validate our predictions by molecular dynamics simulations of field-driven domain walls at finite temperatures.

cond-mat.mtrl-sci↗

arXiv2022

Domain Wall Acceleration by Ultrafast Field Application: An Ab Initio-Based Molecular Dynamics Study

Optimizing ferroelectrics for contemporary high-frequency applications asks for the fundamental understanding of ferroelectric switching and domain wall (DW) motion in ultrafast field pulses while the microscopic understanding of the latter is so far incomplete. To close this gap in knowledge, ab initio-based molecular dynamics simulations are utilized to analyze the dynamics of 180$^\grad# DWs in the prototypical ferroelectric material BaTiO 3 . How ultrafast field application initially excites the dipoles in the system and how they relax to their steady state via transient negative capacitance are discussed. Excitingly, a giant boost of the DW velocity related to the nonequilibrium switching of local dipoles acting as nucleation centers for the wall movement is found. This boost may allow to tune the local ferroelectric switching rate by the shape of an applied field pulse.

cond-mat.mtrl-sci↗

arXiv2020

Impact of domains on the orthorhombic-tetragonal transition of BaTiO$_3$: an ab initio study

We investigate the multi-domain structures in the tetragonal and orthorhombic phases of BaTiO$_3$ and the impact of the presence of domain walls on the intermediary phase transition. We focus on the change in the transition temperatures resulting from various types of domain walls and their coupling with an external electric field. We employ molecular dynamics simulations of an ab initio effective Hamiltonian in this study. After confirming that this model is applicable to multi-domain configurations, we show that the phase transition temperatures strongly depend on the presence of domains walls. Notably we show that elastic 90$^{\circ}$ walls can strongly reduce thermal hysteresis. Further analysis shows that the change in transition temperatures can be attributed to two main factors - long-range monoclinic distortions induced by walls within domains and domain wall widths. We also show that the coupling with the field further facilitates the reduction of thermal hysteresis for orthorhombic 90$^{\circ}$ walls making this configuration attractive for future applications.

cond-mat.mtrl-sci↗

arXiv2020

Multi-step stochastic mechanism of polarization reversal in rhombohedral ferroelectrics

A stochastic model for the field-driven polarization reversal in rhombohedral ferroelectrics is developed, providing a description of their temporal electromechanical response. Application of the model to simultaneous measurements of polarization and strain kinetics in a rhombohedral Pb(Zr,Ti)O3 ceramic over a wide time window allows identification of preferable switching paths, fractions of individual switching processes, and their activation fields. Complementary, the phenomenological Landau-Ginzburg-Devenshire theory is used to analyze the impact of external field and stress on switching barriers showing that residual mechanical stress may promote the fast switching.

cond-mat.mtrl-sci↗

arXiv2018

Tailoring the electrocaloric effect by internal bias fields and field protocols

In acceptor doped ferroelectrics and in ferroelectric films and nanocomposites, defect dipoles, strain gradients, and the electric boundary conditions at interfaces and surfaces often impose internal bias fields. In this work we delicately study the impact of internal bias fields on the electrocaloric effect (ECE), utilizing an analytical model and \emph{ab initio}-based molecular dynamics simulations. We reveal the complex dependency of the ECE on field protocol and relative strength of internal and external fields. The internal fields may even reverse the sign of the response (inverse or negative ECE). We explore the transition between conventional and inverse ECE and discuss reversible and irreversible contributions to the field-induced specific entropy change. Most importantly, we predict design routes to optimize the cooling and heating response for small external fields by the combination of internal field strengths and the field loading protocol.

cond-mat.mtrl-sci↗

arXiv2017

The impact of hysteresis on the electrocaloric effect at first-order phase transitions

We study the impact of thermal hysteresis at the first-order structural/ferroelectric phase transitions on the electrocaloric response in bulk BaTiO$_3$ by performing molecular dynamics simulations for a first-principles-based effective Hamiltonian. We demonstrate that the electrocaloric response can conceptually be separated in two contributions: a transitional part, stemming from the discontinuous jump in entropy at the first order phase transition, and a configurational part, due to the continuous change of polarization and entropy within each phase. This latter part increases with the strength of the applied field, but for small fields it is very small. In contrast, we find a large temperature change of $\sim 1$ K resulting from the transition entropy, which is essentially independent of the field strength. However, due to the coexistence region close to the first order phase transition, this large electrocaloric response depends on the thermal history of the sample and is generally not reversible. We show that this irreversibility can be overcome by using larger fields.

cond-mat.mtrl-sci↗