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DOE OSTI2020

100th Anniversary of the discovery of Ferroelectricity: How did it impact the Current Day Physics

Ferroelectric phenomenon in materials was identified for the first time about 100 years ago in 1920 (paper presented at the American Physical Society (APS) Meeting in Washington, DC, April 1920), at the University of Minnesota, MN, USA; by then a graduate student Joseph Valasek working under the supervision of Prof. W. F. G. Swann. Before this announcement, some prominent physicists had envisaged and even predicted the possibility of an analogous electrical behavior in some materials to that of the well-established hysteresis behavior of magnetic materials. Debye’s work tried to put the concept of Curie temperature in some solids in 1912 but it Schrodinger in the same year who formalized the term ferroelectricity. The names ferroelectrics, ferroelectricity, etc. are fully symbolic of the hysteresis loop-like behavior originally displayed by the magnetic materials. Later on when the physics of ferroelectrics was formalized, most of the terminology, e.g. Curie constant, Curie temperature, etc. that characterized the hysteresis loop or its related characteristics, was inherited from the magnetic materials. Over the years basic ferroelectricity concepts were significantly developed. The observed ferroelectric hysteresis loop between electric polarization vs applied electric field in the then well-known material, Rochelle salt, was presented at the April meeting of the APS, held at, then known as, the National Bureau of Standards or NBS (currently known as the National Institute for Standards and Technology or NIST). Interestingly, even though Valasek submitted the abstract but it was presented by Swann (and Valasek was not present). It happened to be a unique case of the earlier predictions but it did not catch the attention of many researchers working in the field of electronics. Whatever the reason, there was a little attention paid to this newly identified effect in Rochelle salt in 1920s. In late 1920, Valasek’s paper was submitted to Physical Review and that appeared in April 1921. Both ferroelectricity and ferromagnetism are the observational or experimental characteristics of a special class of materials which were not predicted by any basic theoretical approaches to start with at that time.

36 MATERIALS SCIENCE↗

DOE OSTI2025

Modeling kinetic effects of charged vacancies on electromechanical responses of ferroelectrics: Rayleighian approach

Understanding the time-dependent effects of charged vacancies on the electromechanical responses of materials is at the forefront of research for designing materials exhibiting metal-insulator transitions and memristive behavior. A Rayleighian approach is used to develop a model for studying the nonlinear kinetics of the reaction leading to generation of vacancies and electrons via the dissociation of vacancy-electron pairs. Also, diffusion and elastic effects of charged vacancies are considered to model polarization-electric potential and strain-electric potential hysteresis loops. The model captures multiphysics phenomena by introducing couplings among polarization, the electric potential, stress, strain, and concentrations of charged (multivalent) vacancies and electrons (treated as classical negatively charged particles), where the concentrations can vary due to association-dissociation reactions. A derivation of coupled time-dependent equations based on the Rayleighian approach is presented. Three limiting cases of the governing equations are considered, highlighting the effects of (1) nonlinear reaction kinetics on the generation of charged vacancies and electrons, (2) Vegard's law (i.e., the concentration-dependent local strain) on asymmetric strain-electric potential relations, and (3) coupling between a fast component and the slow component of the net polarization on the polarization-electric-field relations. The Rayleighian approach discussed in this work should pave the way for developing a multiscale modeling framework in a thermodynamically consistent manner while capturing multiphysics phenomena in ferroelectric materials. Published by the American Physical Society 2025

Kumar, Rajeev (ORCID:0000000194943488)↗

DOE OSTI2021

Magnetic excitations of the hybrid multiferroic (ND 4 ) 2 FeCl 5 ·D 2 O

We report a comprehensive inelastic neutron scattering study of the hybrid molecule-based multiferroic compound ( ND 4 ) 2 FeCl 5 · D 2 O in the zero-field incommensurate cycloidal phase and the high-field quasicollinear phase. The spontaneous electric polarization changes its direction concurrently with the field-induced magnetic transition, from mostly aligned with the crystallographic a axis to the c axis. To account for such a change in polarization direction, the underlying multiferroic mechanism was proposed to switch from the spin-current model induced via the inverse Dzyaloshinskii-Moriya interaction to the p - d hybridization model. We perform a detailed analysis of the inelastic neutron data of ( ND 4 ) 2 FeCl 5 · D 2 O using linear spin-wave theory to quantify magnetic interaction strengths and investigate the possible impact of different multiferroic mechanisms on the magnetic couplings. Overall our result reveals that the spin dynamics of both multiferroic phases can be well described by a Heisenberg Hamiltonian with easy-plane anisotropy. We do not find notable differences between the optimal model parameters of the two phases. The hierarchy of exchange couplings and the balance among frustrated interactions remain the same between two phases, suggesting that magnetic interactions in ( ND 4 ) 2 FeCl 5 · D 2 O are much more robust than the electric polarization in response to delicate reorganizations of the electronic degrees of freedom in an applied magnetic field.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2022

Near‐Room‐Temperature Magnetoelectric Coupling via Spin Crossover in an Iron(II) Complex

Abstract Magnetoelectric coupling is achieved near room temperature in a spin crossover Fe II molecule‐based compound, [Fe(1bpp) 2 ](BF 4 ) 2 . Large atomic displacements resulting from Jahn–Teller distortions induce a change in the molecule dipole moment when switching between high‐spin and low‐spin states leading to a step‐wise change in the electric polarization and dielectric constant. For temperatures in the region of bistability, the changes in magnetic and electrical properties are induced with a remarkably low magnetic field of 3 T. This result represents a successful expansion of magnetoelectric spin crossovers towards ambient conditions. Moreover, the observed 0.3–0.4 mC m −2 changes in the H ‐induced electric polarization suggest that the high strength of the coupling obtained via this route is accessible not just at cryogenic temperatures but also near room temperature, a feature that is especially appealing in the light of practical applications.

Owczarek, Magdalena↗

DOE OSTI2022

Near‐Room‐Temperature Magnetoelectric Coupling via Spin Crossover in an Iron(II) Complex

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2022

Origin of circular dichroism in resonant elastic x-ray scattering from magnetic and polar chiral structures

Circular dichroism in resonant elastic x-ray scattering (CD-REXS) has recently been observed in chiral structures composed of multiferroic materials as well as magnetic moments or electric polarization vectors. In order to comprehensively understand the experimental results of these previous studies, we present here in detail the analytical formulation of CD-REXS for one-dimensional helices composed of magnetic moments and electric polarization vectors, respectively. In particular, by comparing CD-REXS for a proper-screw-shaped Bloch-type helix and cycloid-shaped Néel-type helix, we found that CD-REXS for both magnetic and polar helices can discriminate between both types of helices. We also found that the x-ray polarization factor depending on the scattering geometry is a significant factor in determining the characteristics of CD-REXS for chiral structures. The results obtained from the detailed formulas can be intuitively understood using the concept of mirror reflection. In particular, in this way it can be understood that Bloch- and Néel-type helices correspond to truly chiral and achiral structures, respectively, and why CD-REXS is able to distinguish between these two types of helices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2025

Ferroaxial phonons in chiral and polar Ni⁢Co 2 ⁢Te⁢O 6

Perfect circular dichroism has been observed in Raman scattering by optical phonons in single chiral domain NiCo 2 TeO 6 crystals. The selection rules for optical phonons are determined by the combination of the chiral structure C and electric polarization P along the c axis. These two symmetry operations are equivalent to the ferroaxial order (C·P) = A, so the observed optical phonons are referred to as ferroaxial. For a given Raman scattering geometry, the observed effect may also be described as a perfect nonreciprocal propagation of the ferroaxial optical phonons, whose preferable q-vector direction is determined by the sign of A. Here, the combination of Raman scattering and polarization plane rotation of the transmitted white light allows for identification of the direction of electric polarization P in monodomain chiral crystals. In addition to the ferroaxial selection rules for optical phonons, a weaker ferroaxial dichroism was observed for the magnon peak at 18 cm −1 .

Altermagnets↗

DOE OSTI2020

Strain–Chemical Gradient and Polarization in Metal Halide Perovskites

Metal halide perovskites (MHPs) have attracted broad research interest due to their outstanding optoelectronic performance. This performance has been attributed in part to the presence of polarization in these materials. However, the precise effects of chemical environment and strain condition on the polar states in MHPs have largely been missing. Herein it is revealed for the first time that chemical gradient is directly coupled with strain gradient in CH 3 NH 3 PbI 3 . This strain–chemical gradient induces an electric polarization that can potentially affect charge carrier dynamics. Furthermore, it is unveiled that this electric polarization—unlike ferroelectricity that only exists in noncentrosymmetric materials—can be present in both tetragonal and cubic phases of CH 3 NH 3 PbI 3 . This suggests that the strain–chemical gradient induced polarization is a more convincing explanation of the outstanding photovoltaic properties of MHPs than the hotly debated ferroelectric polarization. Finally, a mechanism of how this polarization impacts photovoltaic action is proposed, which offers insightful advances in the development of MHPs.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Control of magnetic properties of MnBi 2 Te 4 using a van der Waals ferroelectric III 2 -VI 3 film and biaxial strain

Using the first-principles calculations, we systematically investigated magnetic properties of a MnBi 2 Te 4 (MBT) monolayer in contact with III 2 -VI 3 two-dimensional ferroelectric substrates. As the electric polarization in the ferroelectric films is switched, a semiconductor to half-metal transition occurs in MBT, and its magnetic easy axis in several cases reorients. As an example, the Curie temperature of MBT/In 2 Se 3 can be enhanced to 21.5 or 32 K under the influence of electric polarization and biaxial strain, much higher than that of the pristine MBT monolayer, which is 14.5 K in our calculations. Our results in this work suggest effective ways to tune magnetic properties of MBT and provide useful insights for the further development of these important van der Waals functional materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2021

Asymmetric Character of the Ferroelectric Phase Transition and Charged Domain Walls in a Hybrid Improper Ferroelectric

Abstract In improper ferroelectrics, the spontaneous ordering is typically driven by a structural distortion or a magnetic spin alignment. The induced electric polarization is only a secondary effect. This dependence is a rich source for unusual phenomena and ferroelectric domain configurations for proper, polarization‐driven ferroelectrics. This study focuses on the polar domain structure and the hysteretic behavior at the ferroelectric phase transition in Ca 3 Mn 1.9 Ti 0.1 O 7 as a representative of the recently discovered hybrid improper ferroelectric class of multiferroics. Combining optical second harmonic generation and Raman spectroscopy gives access to the spontaneous structural distortion and the resulting improper electric polarization. This study shows that hybrid improper ferroelectrics contrast proper and improper ferroelectrics in several ways. Most intriguingly, adjacent ferroelectric domains favor head‐to‐head and tail‐to‐tail domain walls over charge‐neutral configurations. Furthermore, the phase transition occurs in an asymmetric fashion. The regime of phase coexistence of the nonpolar and polar phases shows a clear and abrupt upper temperature limit. In contrast, the coexistence toward low temperatures is best described as a fade‐out process, where 100‐nm‐sized islands of the nonpolar phase expand deep into the polar phase.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Evidence for a single-layer van der Waals multiferroic

Here we searched for signatures of type-II multiferroic order in ultrathin van der Waals materials, to determine if such a state, comprising coexisting electrical polarization and spin order with strong magnetoelectric effects, could persist down to a single-atomic layer. We reported the first observation of such a state in triangular lattice material NiI 2 . This system has a layered structure and built-in magnetic frustration that promotes a complex magnetic ground state with proper-screw spin helices. The spin helix pattern breaks inversion symmetry and induces electrical polarization via magnetoelectric coupling. We use circular dichroic Raman measurements to directly probe the magneto-chiral ground state and its electromagnon modes originating from dynamic magnetoelectric coupling. Using birefringence and second-harmonic generation measurements, we detect a highly anisotropic electronic state simultaneously breaking three-fold rotational and inversion symmetry. The evolution of the optical signatures as a function of temperature and layer number surprisingly revealed a multiferroic state that persists down to the ultrathin limit of monolayer NiI 2 . These observations establish NiI 2 and transition metal dihalides as a new platform for studying emergent multiferroic phenomena, chiral magnetic textures and ferroelectricity in the 2D limit.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2020

Imitation of spin density wave order in Cu 3 Nb 2 O 8

Spin density waves, based on modulated local moments, are usually associated with metallic materials, but have recently been reported in insulators which display coupled magnetic and structural order parameters. Here, we discuss one such example, the multiferroic Cu 3 Nb 2 O 8 , which is reported to undergo two magnetic phase transitions, first to an unknown antiferromagnetic phase at $T_N$ ≈ 26.5 K , and then to a helicoidal structure coupled to an electric polarization below $T_2$ ≈ 24 K which breaks the crystallographic inversion symmetry. By analogy with other complex oxides, one might naturally expect this intermediate phase to be a spin density wave phase. We apply spherical polarimetry to confirm the low-temperature magnetic structure, yet only observe a single magnetic phase transition to helicoidal order. We argue that the reported unknown phase actually supports an imitation spin density wave which originates from a decoupling of the components of the magnetic order parameter, as allowed by symmetry and driven by thermal fluctuations. This provides a mechanism for the magnetic, but not nuclear, structure to break inversion symmetry thereby creating an intermediate phase in the proximity of $T_N$ which imitates a spin density wave. As the temperature is reduced, this intermediate structure destabilizes the crystal such that a structural chirality is induced, as reflected by the emergence of the electric polarization, and the imitation spin density wave relaxes into a generic helicoid. This scenario in which critical fluctuations allow the magnetic structure to break inversion symmetry while the crystal structure remains centrosymmetric might be relevant to other complex multiferroics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2024

Magnetic structure and magnetoelectric properties of the spin-flop phase in LiFePO 4

We investigate the magnetic structure and magnetoelectric(ME) effect in the high-field phase of the antiferromagnet LiFePO 4 above the critical field of 31 T. A neutron diffraction study in pulsed magnetic fields reveals the propagation vector to be q = 0 for the high-field magnetic structure. Pulsed-field electric polarization measurements show that, at the critical field, the low-field off-diagonal ME coupling α ab is partially suppressed, and the diagonal element α bb emerges. These results are consistent with a spin-flop transition where the spin direction changes from primarily being along the easy b axis below the transition to being along a above. The persistence of off-diagonal ME tensor elements above the critical field suggests a lowering of the magnetic point-group symmetry and hence a more complex magnetic structure in the high-field phase. In addition, neutron diffraction measurements in low magnetic fields show no observable field-induced spin canting, which indicates a negligible Dzyaloshinskii-Moriya interaction. The observed spin-flop field supports the Hamiltonian recently deduced from inelastic neutron studies and indicates that the system is less frustrated and with a larger single-ion anisotropy than originally thought. Our results demonstrate the effectiveness of combining pulsed-field neutron diffraction and electric polarization measurements to elucidate the magnetic structures and symmetries at the highest attainable field strengths.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2025

Terahertz-field activation of polar skyrons

Unraveling collective modes arising from coupled degrees of freedom is crucial for understanding complex interactions in solids and developing new functionalities. Unique collective behaviors emerge when two degrees of freedom, ordered on distinct length scales, interact. Polar skyrmions, three-dimensional electric polarization textures in ferroelectric superlattices, disrupt the lattice continuity at the nanometer scale with nontrivial topology, leading to previously unexplored collective modes. Here, using terahertz-field excitation and femtosecond x-ray diffraction, we discover subterahertz collective modes, dubbed “skyrons”, which appear as swirling patterns of atomic displacements functioning as atomic-scale gearsets. The key to activating skyrons is the use of the THz field that couples primarily to skyrmion domain walls. Momentum-resolved time-domain measurements of diffuse scattering reveal an avoided crossing in the dispersion relation of skyrons. Atomistic simulations and dynamical phase-field modeling provide microscopic insights into the three-dimensional crystallographic and polarization dynamics. The amplitude and dispersion of skyrons are demonstrated to be controlled by sample temperature and electric-field bias. The discovery of skyrons and their coupling with terahertz fields opens avenues for ultrafast control of topological polar structures.

ferroelectrics↗

DOE OSTI2025

Chemical Compensation Challenges in Processing Antiferroelectric PbZrO 3 Thin Films

The electric field-induced antipolar-to-polar phase transition in antiferroelectric materials is accompanied by large changes in the structural and functional response, making them attractive for many applications ranging from pulsed energy capacitors to high-strain-high (blocking-)force actuators, solid-state heating and cooling systems, and optoelectronic devices. PbZrO 3 , as an end member of the PZT, (x)PbZr 1–x Ti x O 3 , solid solution, has been the subject of many studies. However, processing of perovskite PbZrO 3 is extremely challenging due to phase instabilities induced by Pb loss at the temperatures necessary for perovskite crystallization. Here, we discuss the challenges associated with Pb loss, as well as its compensation through bulk Pb overstoichiometry and interfacial PbO additions, in chemical solution processed, highly oriented PbZrO 3 thin films. For both 042 o - and 001 o -oriented (o-orthorhombic distortion) PbZrO 3 thin films, the crystallization interfaces are the most important contributors to Pb loss and off-stoichiometric outcomes, and no single approach was sufficient to address these challenges. Pb-rich and Pb-deficient nonperovskite phases including ZrO x were only observable through microscopic characterization, and X-ray diffraction alone could not rule out the presence of such secondary phases. “Ideal” macroscopic antiferroelectric polarization-electric field double hysteresis curves were observed despite (at times) the large presence of secondary phases. However, reduced saturation polarization and increased phase transition electric fields were observed concomitantly with the presence of secondary phase(s) and tentatively assigned to the voltage drop across the ZrO x nanocrystals. Based on these results, it is imperative that the presence of secondary phases always be addressed (beyond X-ray diffraction spectra) in order to correctly evaluate the properties of antiferroelectric films.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2024

New examples of ferroelectric nematic materials showing evidence for the antiferroelectric smectic-Z phase

Abstract We present a new ferroelectric nematic material, 4-((4′-((trans)-5-ethyloxan-2-yl)-2′,3,5,6′-tetrafluoro-[1,1′-biphenyl]-4-yl)difluoromethoxy)-2,6-difluorobenzonitrile (AUUQU-2-N) and its higher homologues, the molecular structures of which include fluorinated building blocks, an oxane ring, and a terminal cyano group, all contributing to a large molecular dipole moment of about 12.5 D. We observed that AUUQU-2-N has three distinct liquid crystal phases, two of which were found to be polar phases with a spontaneous electric polarization P s of up to 6 µC cm –2 . The highest temperature phase is a common enantiotropic nematic (N) exhibiting only field-induced polarization. The lowest-temperature, monotropic phase proved to be a new example of the ferroelectric nematic phase (N F ), evidenced by a single-peak polarization reversal current response, a giant imaginary dielectric permittivity on the order of 10 3 , and the absence of any smectic layer X-ray diffraction peaks. The ordinary nematic phase N and the ferroelectric nematic phase N F are separated by an antiferroelectric liquid crystal phase which has low permittivity and a polarization reversal current exhibiting a characteristic double-peak response. In the polarizing light microscope, this antiferroelectric phase shows characteristic zig-zag defects, evidence of a layered structure. These observations suggest that this is another example of the recently discovered smectic Z A (SmZ A ) phase, having smectic layers with the molecular director parallel to the layer planes. The diffraction peaks from the smectic layering have not been observed to date but detailed 2D X-ray studies indicate the presence of additional short-range structures including smectic C-type correlations in all three phases—N, SmZ A and N F —which may shed new light on the understanding of polar and antipolar order in these phases.

36 MATERIALS SCIENCE↗

DOE OSTI2024

Enhancements of electric field and afterglow of non-equilibrium plasma by Pb(ZrxTi1−x)O3 ferroelectric electrode

Abstract Manipulating surface charge, electric field, and plasma afterglow in a non-equilibrium plasma is critical to control plasma-surface interaction for plasma catalysis and manufacturing. Here, we show enhancements of surface charge, electric field during breakdown, and afterglow by ferroelectric barrier discharge. The results show that the ferroelectrics manifest spontaneous electric polarization to increase the surface charge by two orders of magnitude compared to discharge with an alumina barrier. Time-resolved in-situ electric field measurements reveal that the fast polarization of ferroelectrics enhances the electric field during the breakdown in streamer discharge and doubles the electric field compared to the dielectric barrier discharge. Moreover, due to the existence of surface charge, the ferroelectric electrode extends the afterglow time and makes discharge sustained longer when alternating the external electric field polarity. The present results show that ferroelectric barrier discharge offers a promising technique to tune plasma properties for efficient plasma catalysis and electrified manufacturing.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2020

Unconventional crystal-field splitting in noncentrosymmetric BaTiO 3 thin films

Understanding the crystal-field splitting and orbital polarization in noncentrosymmetric systems such as ferroelectric materials is fundamentally important. In this work, taking BaTiO 3 as a representative material, we investigate titanium crystal-field splitting and orbital polarization in noncentrosymmetric TiO 6 octahedra with resonant x-ray linear dichroism at the Ti L 2,3 edge. The high-quality BaTiO 3 thin films were deposited on DyScO 3 (110) single crystal substrates in a layer-by-layer way by pulsed laser deposition. The reflection high-energy electron diffraction and element specific x-ray absorption spectroscopy were performed to characterize the structural and electronic properties of the films. In sharp contrast to conventional crystal-field splitting and orbital configuration (d xz /d yz < d xy < d 3z 2 -r 2 < d x 2 -y 2 or d xy < d xz /d yz < d x 2 -y 2 < d 3z 2 -r 2 ) expected from compressive or tensile epitaxial strain, respectively, it is revealed that d xz , d yz , and d xy orbitals are nearly degenerate, whereas d 3z 2 -r 2 and d x 2 -y 2 orbitals are split with an energy gap ~100 meV in the epitaxial BaTiO 3 films. We find that the unexpected degenerate orbitals d xz /d yz /d xy result from the competition between the orbital splitting induced by epitaxial strain and that induced by polar distortions of BaTiO 3 films. Our results provide a route to manipulate orbital degree of freedom by switching electric polarization in ferroelectric materials.

36 MATERIALS SCIENCE↗