Suppression Of Ferroelectricity Research Articles

Improper ferroelectricity in ultrathin hexagonal ferrites films

Suppression of ferroelectricity in ultrathin films of improper ferroelectric hexagonal ferrites or manganites has been attributed to the effect of interfacial clamping; however, the quantitative understanding and related phenomenological model are still lacking. In this work, we report on the paraelectric-to-ferroelectric phase transition of epitaxial h-ScFeO3 films with different thicknesses through in situ reflection high-energy electron diffraction. Based on the interfacial clamping model and the Landau theory, we show that the thickness-dependence of the ferroelectric Curie temperature can be understood in terms of the characteristic length of an interfacial clamping layer and the bulk Curie temperature. Furthermore, we found that the critical thickness of improper ferroelectricity is proportional to the characteristic length of the interfacial clamping layer. These results reveal the essential role of mechanical clamping from interface on the improper ferroelectricity of hexagonal ferrites or manganites and could serve as the guidance to achieve robust improper ferroelectricity in ultrathin films.

Pressure-driven phase transformations and phase segregation in ferrielectric CuInP2S6−In4/3P2S6 self-assembled heterostructures

Layered multi-ferroic materials exhibit a variety of functional properties that can be tuned by varying the temperature and pressure. As-synthesized CuInP$_2$S$_6$ is a layered material that displays ferrielectric behavior at room temperature. When synthesized with Cu deficiencies, CuInP$_2$S$_6$ spontaneously phase segregates to form ferrielectric CuInP$_2$S$_6$ (CIPS) and paraelectric In$_{4/3}$P$_2$S$_6$ (IPS) domains in a two-dimensional self-assembled heterostructure. Here, we study the effect of hydrostatic pressure on the structure of Cu-deficient CuInP$_2$S$_6$ by Raman spectroscopy measurements up to 20 GPa. Detailed analysis of the frequencies, intensities, and linewidths of the Raman peaks reveals four discontinuities in the spectra around 2, 10, 13 and 17 GPa. At ~2 GPa, we observe a structural transition initiated by the diffusion of IPS domains, which culminates in a drastic reduction of the number of peaks around 10 GPa. We attribute this to a possible monoclinic-trigonal phase transition at 10 GPa. At higher pressures (~ 13 GPa), significant increases in peak intensities and sharpening of the Raman peaks suggest a bandgap-lowering and an isostructural electronic transition, with a possible onset of metallization at pressures above 17 GPa. When the pressure is released, the structure again phase-separates into two distinct chemical domains within the same single crystalline framework -- however, these domains are much smaller in size than the as-synthesized material resulting in suppression of ferroelectricity through nanoconfinement. Hydrostatic pressure can thus be used to tune the electronic and ferrielectric properties of Cu-deficient layered CuInP$_2$S$_6$.

Theoretical and Experimental Overview of Structural, Dielectric, Crystallographic, Electronic, Optical, and Physical Tensors of α-DIPAB and Iodine-Doped α-DIPAB Molecular Ferroelectric Crystals

Self-consistent ab initio calculations of the structural, crystallographic, electronic, optical, and physical properties as well as spontaneous polarization of P21 (α)-ferroelectric phase of diisopropylammonium bromide (α-DIPAB) molecular crystal are carried out. Examination of the total density of states as obtained using the generalized gradient approximation and hybrid exchange–correlation functional (HSE06) methods yields an optical bandgap of ∼ 5 eV and 6 eV, respectively. Furthermore, by using the Berry phase approach, we found that polar DIPAB exhibits a spontaneous polarization of 22.64 μC/cm2, indicating that this remarkable ferroelectric molecule could be used as an alternative to replace several known ferroelectrics for piezoelectric and optoelectronic applications such as capacitors, sensors, and data storage as well as microelectromechanical system (MEMS) devices. Using the vdW + DF2 approximation, we identify the majority of the vibrational modes in the Raman spectra and analyze the modes generated by the bromine (Br) deficiency. We find that the Br deficiency strongly affects the electric and elastic properties of α-DIPAB. Furthermore, the elastic, dielectric, and piezoelectric tensors of α-DIPAB are calculated and interpreted. α-DIPAB is found to exhibit a static dielectric tensor of ∼ 2.5, i.e., slightly smaller than that of typical perovskite-based ferroelectrics. In addition, the clamped-ion piezoelectric tensor is calculated. We found that the piezoelectric coefficient e21 corresponding to applying uniaxial strain in the direction perpendicular to the N–C bonds is smaller than the e22 principle coefficient. Moreover, the components e15 = 0.220 C/m2 and e35 = −0.2032 C/m2 correspond to applying shear strain η31, indicating a reasonable piezoelectric response of this polar crystal and making it a cheap attractive candidate for piezoelectric applications. The components of the elastic moduli tensor are calculated and interpreted. The α-DIPAB exhibits Young’s modulus of up to 50 GPa along specific crystalline directions despite its relatively low hardness. We attribute such a large value to the presence of a dense H–Br bond network between the Br and DIPAB molecule. The Poisson ratio is also strongly anisotropic, with values ranging from 0.5 to 0.2. The DIPAB systems are brittle based on the ratio between the bulk and shear elastic constants, suggesting that this material has potential for use in flexible electronic applications. A real-part dielectric anomaly at T = 140 K induced by iodine doping of α-DIPAB exhibits switchable behavior. This abnormality can be attributed to suppression of ferroelectricity induced by trapping of vortices at dislocation defects generated by iodine. Another dielectric anomaly found in the vicinity of the Curie temperature of α-DIPAB can be explained in terms of a structural phase-induced ferroelectric-to-paraelectric transition.

Effect of the oxygen vacancy on the ferroelectricity of 90° domain wall structure in PbTiO3: A density functional theory study

The effect of oxygen vacancy on the ferroelectricity of the 90° domain wall structure in PbTiO3 has been investigated by ab initio (first-principles) density functional theory calculations. Based on the position of oxygen vacancies with respect to the 90° domain wall, the oxygen vacancies are classified into two types, i.e., vacancy on the PbO plane (O1-vac) and vacancy on the TiO plane (O2- and O3-vac). Oxygen vacancies in the region adjacent to the 90° domain wall give the minimum formation energies, which indicates that the oxygen vacancies tend to accumulate in the area near the 90° domain wall. The minimum vacancy energies for all oxygen vacancies occur in the region adjacent to the 90° domain wall. The pinning energies of the oxygen vacancies to the 90° domain wall are determined to be 0.330 eV, 0.130 eV, and 0.228 eV for the O1-, O2-, and O3-vacancies, respectively. Such a pinning effect of oxygen vacancies results in a shift of the central position of the domain wall in the crystal lattice, and also, the width of the domain wall is enlarged. This result is found to be the most pronounced for O1-vac on the PbO plane. Finally, the suppression of ferroelectricity caused by the domain wall is strengthened by the effect of the pinned oxygen vacancies.

Ferroelectric Polarization Switching of Hafnium Zirconium Oxide in a Ferroelectric/Dielectric Stack

The ferroelectric polarization switching in ferroelectric hafnium zirconium oxide (Hf0.5Zr0.5O2, HZO) in the HZO/Al2O3 ferroelectric/dielectric stack is investigated systematically by capacitance-voltage and polarization-voltage measurements. The thickness of dielectric layer is found to have a determinant impact on the ferroelectric polarization switching of ferroelectric HZO. A suppression of ferroelectricity is observed with thick dielectric layer. In the gate stacks with thin dielectric layers, a full polarization switching of the ferroelectric layer is found possible by the proposed leakage-current-assist mechanism through the ultrathin dielectric layer. Theoretical simulation results agree well with experimental data. This work clarifies some of the critical parts of the long-standing confusions and debating related to negative capacitance field-effect transistors (NC-FETs) concepts and experiments.

Highly tunable magnetic spirals and electric polarization in Gd0.5Dy0.5MnO3

Recent progress in the field of multiferroics led to the discovery of many new materials in which ferroelectricity is induced by cycloidal spiral orders. The direction of the electric polarization is typically constrained by spin anisotropies and magnetic field. Here, we report that the mixed rare-earth manganite, Gd$_{0.5}$Dy$_{0.5}$MnO$_3$, exhibits a spontaneous electric polarization along a general direction in the crystallographic ac-plane, which is suppressed below 10 K but re-emerges in an applied magnetic field. Neutron diffraction measurements show that the polarization direction results from a large tilt of the spiral plane with respect to the crystallographic axes and that the suppression of ferroelectricity is caused by the transformation of a cycloidal spiral into a helical one, a unique property of this rare-earth manganite. The freedom in the orientation of the spiral plane allows for a fine magnetic control of ferroelectricity, i.e. a rotation as well as a strong enhancement of the polarization depending on the magnetic field direction. We show that this unusual behavior originates from the coupling between the transition metal and rare-earth magnetic subsystems.

Neutron and high-pressure X-ray diffraction study of hydrogen-bonded ferroelectric rubidium hydrogen sulfate.

The pressure- and temperature-dependent phase transitions in the ferroelectric material rubidium hydrogen sulfate (RbHSO4) are investigated by a combination of neutron Laue diffraction and high-pressure X-ray diffraction. The observation of disordered O-atom positions in the hydrogen sulfate anions is in agreement with previous spectroscopic measurements in the literature. Contrary to the mechanism observed in other hydrogen-bonded ferroelectric materials, H-atom positions are well defined and ordered in the paraelectric phase. Under applied pressure RbHSO4 undergoes a ferroelectric transition before transforming to a third, high-pressure phase. The symmetry of this phase is revised to the centrosymmetric space group P21/c, resulting in the suppression of ferroelectricity at high pressure.

Pressure effect on magnetism and multiferroicity inMn2GeO4

The effect of high pressure exceeding 6 GPa on magnetism and multiferroicity was investigated for the olivine ${\text{Mn}}_{2}$${\text{GeO}}_{4}$ that shows successive magnetic transitions at ambient pressure and a ferroelectric ground state driven by spin-spiral order. We measured heat capacity, dielectric constant, and electric polarization at various pressures using a diamond anvil cell. The pressure evolution of the magnetic structures was also investigated by powder neutron diffraction measurements using a Paris-Edinburgh press. We found that all of the magnetic transition temperatures are enhanced monotonically by applying pressure. Furthermore, the spin-driven ferroelectricity persists up to about 6 GPa but suddenly vanishes by 6.3 GPa. A consistent description of all data is achieved if an incommensurate-commensurate phase transition occurs with pressurization and causes the suppression of ferroelectricity. We discuss the origin of the observed pressure effects on the magnetism and multiferroicity in ${\text{Mn}}_{2}$${\text{GeO}}_{4}$ in terms of a pressure-induced change in the superexchange magnetic interactions.

Origin of giant spin-lattice coupling and the suppression of ferroelectricity in EuTiO3from first principles

We elucidate the microscopic mechanism that causes a suppression of ferroelectricity and an enhancement of octahedral rotations in EuTiO3 from first principles. We find that the hybridization of the rare-earth Eu 4f states with the B-site Ti cation drives the system away from ferroelectricity. We also show that the magnetic order dependence of this hybridization is the dominant source of spin-phonon coupling in this material. Our results underline the importance of rare-earth f electrons on the lattice dynamics and stability of these transition metal oxides.

Evolution of multifunctional behavior in site specific cation substituted Na0.5Bi0.45Gd0.05Ti0.95Mn0.05O3 ceramics

A novel multifunctional behavior was observed by site specific cation substitution in NBT ceramics prepared by conventional solid state reaction technique. X-ray diffraction studies revealed very weak orthorhombic phase co-existence in co-substituted NBT system along with the primary R3c phase. Raman spectroscopy of NBT-GM ceramics reveals a substantial change in all phonon modes in terms of suppression and frequency shift, indicates that the substitution affects the nature of cation displacement, octahedral tilt and further the Bi-O, Ti-O hybridization. Reduced polarization value in NBT-GM ceramics with non-saturated hysteresis loops evidences the suppression of ferroelectricity. Temperature dependent magnetization measurements from R.T to 2.5K in NBTGM ceramics show the occurrence of spin-paramagnetic behavior. Susceptibility plots show some evidence for weak AFM interaction between Mn4+ ions at low temperatures below 30K. UV-Vis spectroscopy measurements reveal the indirect transition allowed band gap nature for both NBT and NBT-GM ceramics with a slight increase in band gap value for NBT-GM ceramics. Photoluminescence measurements show a strong blue emission at ∼390nm (3.18 eV) in Gd-Mn substituted NBT system, which corresponds to the radiative transition from 4T1g → 4A2g energy levels of Mn4+-ions.

Mechanism for suppression of ferroelectricity in Cd1−xCaxTiO3

A mechanism for cation-substitution-induced suppression of ferroelectricity in a perovskite-type ferroelectric oxide, CdTiO${}_{3}$, has been elucidated in terms of the relationship between the covalency and the lattice dynamics. Raman scattering experiments and first-principles calculations clarified that a change in the covalency of Cd sites due to Ca substitution selectively decreases the frequencies of the modes associated with octahedral rotations whose axes are perpendicular to the spontaneous polarization direction. The resulting destabilization of zigzag octahedral chains synchronizes the hardening of the ferroelectric soft mode, which suppresses the ferroelectricity. This result enhances our understanding of the relationship between local chemical bonds and macroscopic ferroelectricity.

Ferroelectricity and strain effects in orthorhombic YMnO3 thin films

We report on the dielectric properties of epitaxial [0 0 1]- and [1 0 0]-textured thin films of antiferromagnetic orthorhombic YMnO3 and their variation under compressive epitaxial strain. It is found that weakly strained YMnO3 films are ferroelectric with a polarization along the c-axis switchable by 90° by an external magnetic field. With reducing film thickness and increasing epitaxial strain, the ferroelectric order is progressively suppressed. Analysis of structural, dielectric and magnetic data indicates that suppression of ferroelectricity with reducing thickness is accompanied by an enlarged ratio a/b of the in-plane cell parameters of the orthorhombic structure and the appearance of a net magnetization. All results can be well-described by considering the multiferroic phase diagram of orthorhombic manganites.

Size dependent domain configuration and electric field driven evolution in ultrathin ferroelectric films: A phase field investigation

Size dependent domain configuration and its evolution under an external electric field are investigated for ultrathin ferroelectric films using an unconventional phase field method. The simulation reveals a series of domain configurations at different thicknesses, including zigzag patterns with eight variants or four variants coexisting, a vortex pattern with four variants coexisting, and a stripe pattern with two variants coexisting. When the film thickness falls below a critical value of 2.8 nm, the polarization vanishes, indicating the suppression of ferroelectricity. The evolution of domain configuration under an alternating electric field is also investigated, and the reduction in remnant polarization and coercive field with respect to decreasing thickness is observed.

Orientation dependence of the electrocaloric effect of ferroelectric bilayer thin films

An analytical thermodynamic theory is applied to investigate the electrocaloric effect of ferroelectric BaTiO 3/SrTiO 3 bilayer thin films with different orientations at room temperature. Theoretical analysis indicates that the strong electrostatic coupling between the layers results in the suppression of ferroelectricity at a critical relative thickness which occurs approximately at 50%, 23%, and 12% of SrTiO 3 fraction in the (001), (110), and (111) bilayer thin films, respectively. The ferroelectric bilayer thin films are respected to have the largest electrocaloric effect at this critical relative thickness. Moreover, the electrocaloric effect strongly depends on the orientation and the (110) oriented bilayer thin films have the largest electrocaloric effect. Consequently, control of the orientation and the relative thickness of SrTiO 3 layer can be used to adjust the electrocaloric effect of ferroelectric bilayer thin films, which may provide the potential for practical application in refrigeration devices.

Phase transitions in ultra-thin ferroelectric films and fine period multilayers

Phase transition behaviour in ferroelectric materials is strongly dependent on boundary conditions, both electrical and mechanical. In this article, we cover several topics. Beginning with thin films, we look at the suppression of ferroelectricity as film thickness is reduced, covering two situations, the first in which the polarisation decreases and eventually is suppressed, and the second where the system forms domains in order to sustain ferroelectricity. The focus then moves to superlattices, highlighting the way that electrostatic coupling dominates the behaviour of superlattices, but revealing that for very thin layers interfacial effects can communicate through this electrostatic coupling to produce new kinds of macroscopic ferroelectric behaviour.

Interlayer coupling in ferroelectric bilayer and superlattice heterostructures

Ferroelectric multilayers and superlattices have gained interest for dynamic random access memory (DRAM) applications and as active elements in tunable microwave devices in the telecommunications industry. A number of experimental studies have shown that these materials have many peculiar properties which cannot be described by a simple series connection of the individual layers that make up the heterostructures. A thermodynamic analysis is presented to demonstrate that ferroelectric multilayers interact through internal elastic, electrical, and electromechanical fields and the strength of the coupling can be quantitatively described using Landau theory of phase transformations, theory of elasticity, and principles of electrostatics. The theoretical analysis shows that compositional variations across ferroelectric bilayers result in a broken spatial inversion symmetry that can lead to asymmetric thermodynamic potentials favoring one ferroelectric ground state over the other. Furthermore, the thermodynamic modeling indicates that there is a strong electrostatic coupling between the layers that leads to the suppression of ferroelectricity at a critical paraelectric layer thickness for ferroelectric-paraelectric bilayers. This bilayer is expected to have a gigantic dielectric response similar to the dielectric anomaly near Curie-Weiss temperature in homogeneous ferroelectrics at this critical thickness.

PHASE TRANSFORMATION CHARACTERISTICS OF FERROELECTRIC-PARAELECTRIC BILAYERS

ABSTRACT A thermodynamic analysis is presented to demonstrate that ferroelectric multilayers interact through internal elastic, electrical, and electromechanical fields and the “strength” of the coupling can be quantitatively described using Landau theory of phase transformations, theory of elasticity, and principles of electrostatics. The theoretical analysis indicates that there is a strong electrostatic coupling between the layers that may lead to the modification of the phase transformation characteristics of ferroelectric superlattices and multilayer heterostructures. It also results in the suppression of ferroelectricity at a critical paraelectric layer thickness for ferroelectric-paraelectric bilayers.

Incomplete ferroelectricity in SrTi 18O 3

Motivated by recent experiments demonstrating suppression of ferroelectricity with pressure in SrTi18O3, the dynamics of the phase transition mechanism are reinvestigated within a nonlinear polarizability model. For temperatures far above the phase transition polar micro domains are formed which increase in size with decreasing temperature to freeze out at Tc without forming long range order. Experimentally, soft mode dynamics are simultaneously observed, evidencing that displacive and order/disorder features coexist. In the ferroelectric phase both components persist whereby an incomplete and inhomogeneous ferroelectric state is formed.

Dielectric anomaly due to electrostatic coupling in ferroelectric-paraelectric bilayers and multilayers

A thermodynamic model is presented that describes the polarization and the dielectric response of ferroelectric-paraelectric bilayers and multilayers. It is shown that a strong electrostatic coupling between the layers results in the suppression of ferroelectricity at a critical paraelectric layer thickness. The bilayer is expected to have a gigantic dielectric response similar to the dielectric anomaly near Curie–Weiss temperature in homogeneous ferroelectrics at this critical thickness. A numerical analysis is carried out for a pseudomorphic (001) BaTiO3∕SrTiO3 heteroepitaxial bilayer on (001) SrTiO3 and a stress-free BaTiO3∕SrTiO3 bilayer. Complete polarization suppression and a dielectric peak are predicted to occur at approximately 66% and 14% of SrTiO3 in these two systems, respectively.

Misfit dislocations in nanoscale ferroelectric heterostructures

We present a quantitative study of the thickness dependence of the polarization and piezoelectric properties in epitaxial (001) PbZr0.52Ti0.48O3 films grown on (001) SrRuO3-buffered (001) SrTiO3 substrates. High-resolution transmission electron microscopy reveals that even the thinnest films (∼8nm) are fully relaxed with a dislocation density close to 1012cm−2 and a spacing of approximately 12 nm. Quantitative piezoelectric and ferroelectric measurements show a drastic degradation in the out-of-plane piezoelectric constant (d33) and the switched polarization (ΔP) as a function of decreasing thickness. In contrast, lattice-matched ultrathin PbZr0.2Ti0.8O3 films that have a very low dislocation density show superior ferroelectric properties. Supporting theoretical calculations show that the variations in the strain field around the core of the dislocation leads to highly localized polarization gradients and hence strong depolarizing fields, which result in suppression of ferroelectricity in the vicinity of a dislocation.