Spontaneous Ferroelectric Polarization Research Articles

Polar and nonpolar structures of BiCrO3 from first-principles calculations

By first-principles calculations, we find that BiCrO3 can stabilize in three structures when relaxing from different experimental crystal lattice. The experimental undetermined structure is identified to be noncentrosymmetric R3c which permits the spontaneous ferroelectric polarization. The atomic positions within the unit cell were calculated using the GGA, GGA+U, LDA and LDA+U exchange–correlation energy density functionals, then the lattice constants along with the atomic positions were relaxed to find the ground state structure. All the calculations indicate that the Pnma structure is lower in energy than R3c and C2/c.

Effect of Ferroelectric Polarization on Ionic Transport and Resistance Degradation in BaTiO3 by Phase‐Field Approach

We proposed a model to study the resistance degradation behavior of ferroelectric oxides in the presence of ferroelectric spontaneous polarization by combining the phase‐field model of ferroelectric domains and nonlinear diffusion equations for ionic/electronic transport. We took into account the nonperiodic boundary conditions for solving the electrochemical transport equations and Ginzburg–Landau equations using the Chebyshev collocation algorithm. We considered a single domain structure relative to a thin film BaTiO3 single crystal orientated to the normal of the electrode plates (Ni) in a single parallel plate capacitor configuration. The capacitor was subjected to a dc bias of 0.5 V either along the polarization direction or opposite to the polarization direction at 25°C. It is shown that the polarization bound charges at the metal/ferroelectric interface play an important role in charge carrier transport and leakage current evolution in BaTiO3 capacitor.

Conductance control at the LaAlO3/SrTiO3-interface by a multiferroic BiFeO3 ad-layer

Multilayered BiFeO3 (BFO)/LaAlO3 (LAO) thin film samples were fabricated on SrTiO3 (STO) substrates by pulsed laser deposition. In this work, the ferroelectric polarization of a multiferroic BFO ad-layer on top of the quasi-two-dimensional electron gas (2DEG) at the LAO/STO interface is used to manipulate the conductivity of the quasi-2DEG. By microstructuring the conductive area of the LAO/STO-interface, a four-point geometry for the measurement of the resistivity was achieved. Piezo force microscopy allows for imaging and poling the spontaneous ferroelectric polarization of the multiferroic layer. The resistance changes showed a linear dependence on the area scanned and a hysteretic behavior with respect to the voltages applied in the scanning process. This is evidence for the ferroelectric polarization of the multiferroic causing the resistance changes. Coupling the antiferromagnetic BFO layer to another ferromagnetic layer could enable a magnetic field control of the conductance of the quasi-2DEG at the LAO/STO interface.

Electronic, magnetic and multiferroic properties of magnetoelectric NiTiO3

The structural, electronic, magnetic, and ferroelectric properties of NiTiO3 are predicted through ab initio calculations based on the density functional theory (DFT). The theoretical structure parameters matched well with those obtained experimentally. The electronic structure results show that the antiferromagnetic (AFM) phase of LiNbO3 (LN)-type NiTiO3 has a direct band gap of 2.16eV. The calculated local magnetic moment of Ni ion is 1.61μB. The calculated Born effective charges (BECs, denoted by tensor Z*) show that the Z* of Ti and O atoms are significantly and anomalously large. Interestingly, ferroelectric spontaneous polarization is predicted to be along [111] direction with a large magnitude of 97μC/cm2. B-site Ti ions in 3d0 state dominate ferroelectric polarization of multiferroic NiTiO3, whereas A-site Ni ions having partially filled eg orbitals are considered to contribute to the antiferromagnetic properties of NiTiO3. Furthermore, the current study also found that the polar lattice distortion can induce weak ferromagnetism.

Electrical properties of BaTiO3 based – MFIS heterostructure: Role of semiconductor channel carrier concentration

Effect of semiconductor channel carrier concentration on the modifications in the electrical properties of Ag/BaTiO3/SrTiO3/ZnO Metal-Ferroelectric-Insulator-Semiconductor (MFIS) heterostructure has been investigated. Under 4 V applied voltage, low leakage current density ∼3.2 × 10−6 A/cm2, has been observed in ZnO based MFIS heterostructure, which becomes ∼5.0 × 10−6 A/cm2 for MFIS with Al:ZnO channel. Observation of counterclockwise butterfly shaped C-V behavior confirms that, hysteresis in C-V is due to spontaneous ferroelectric polarization and field effect. A device with ZnO semiconductor exhibit ∼2700% modulation which decreases to ∼800% for Al: ZnO channel with good retention behavior. Pulse induced write/erase repeatability of source/drain current confirms the usefulness of the presently studied devices for non-volatile switching memory application.

立方核壳晶粒模型的分布介电效应

用立方晶粒模型模拟多面体核壳结构，得到了有效介电常数与核壳尺寸的关系。当壳层为极薄的低介电常数时，该关系表现为晶粒尺寸效应：介电峰移向低温及介电常数下降；当壳层较厚且核与壳介电常数峰不同时，核对介电常数的贡献为其尺寸平方比相关的线性叠加关系。二阶铁电相变时，分布的居里温度会使介电峰向高温移动，同时介电常数减小；分析结果发现：其顺电相介电隔离率的温度关系不能用幂律表示。壳层居里温度的线性分布与核的高斯分布结合能够得到较为平稳的介电常数温度关系。认为铁电体极化强度的平方相当于偶极子的作用，利用偶极子复介电常数关系，导出了分布式铁电相变的损耗温度关系。A model of cubic core-shell grain with the distribution of the Curie temperature is proposed for simulation of the polyhedron core-shell structure to obtain the size related dielectrics. When the shell is a very thin layer with low dielectric constant, the shell has the grain-size effect: dielectric constant decreases and the peak moves to low temperature. When the shell is thick with different dielectric peak from the core one, the contribution of the core to the dielectric constant is a linear relation to the square of it size. The distribution of the Curie temperature will move dielectric peak to high temperature with dropping dielectric constant, and the temperature relation of im-permeability in paraelectric phase can not be expressed by the power-law. Ferroelectrics will have high stability of relation of dielectric constant with temperature for the combination of the shell in linear distribution and the core in Gaussian distribution. It is the square of spontaneous polariza-tion in ferroelectrics that has the same function as dipole in dielectrics for contribution to the complex dielectric constant, and therefore a temperature dependent dielectric loss of the distri-bution of the Curie temperature is derived.

Multiferroic perovskite (Pb0.845Sm0.08Fe0.035)(Ti0.98Mn0.02)O3 with ferroelectric and weak ferromagnetic properties

We have investigated the multiferroic properties of PbTiO3 substituted on the A-site with Sm3+ and Fe2+ magnetic ions. We show that the ceramic material with composition (Pb0.845Sm0.08Fe0.035)(Ti0.98Mn0.02)O3 has a spontaneous ferroelectric polarization of about 14 μC/cm2 and weak ferromagnetism (remanent magnetization ∼2.3 emu/g) which persists even at room temperature. The dielectric constant is 190, tan δ ∼ 0.005, and the piezoelectric g33 constant is 25 mV × m/N. Temperature dependence of dielectric permittivity shows a strong anomaly at 662 K, at the paraelectric-ferroelectric transition, and a broader anomaly below 140 K where magnetization also increases, probably due to magnetoelectric coupling between ferroelectric and magnetic states.

Stable and locally stable conditions for a conical spin state in the spinel structure

A conical spin state generates the multiferroicity with both spontaneous magnetization and ferroelectric polarization, offering a great promise for the mutual control of magnetism and ferroelectricity. To clarify the stable and locally stable conditions for the conical spin order, a Monte Carlo simulation is performed on a three-dimensional spinel lattice with classical Heisenberg spins to explore the possible ground states in different parameter spaces. The simulation confirms the locally stable range of the conical spin state, which was suggested by the LKDM (Lyons, Kaplan, Dwight, and Menyuk) theory. Furthermore, it is revealed that the anisotropy plays an important role in stabilizing the conical spin order and expanding the parameter range of its existence, whereas the nearest-neighboring antiferromagnetic A-A exchange interaction enhances frustration and thereby suppresses the conical spin order. Thus, our simulation gives a necessary supplement and a reasonable improvement to the LKDM theory.

Strain-induced phase transitions in multiferroic BiFeO3[formula omitted] epitaxial film

Intriguing rich phase transitions and multiferroic behavior in (110)-oriented BiFeO3 epitaxial films are studied by first-principles calculations. We demonstrate that a BiFeO3(110)c film undergoes complicated monoclinic–monoclinic–orthorhombic (Ma–Mb–O) phase transitions depending on the epitaxial strain conditions. During the phase transition, ferroelectric spontaneous polarization and a magnetic moment due to weak ferromagnetism rotate in different ways, leading to different anisotropies in the magnetoelectric response of the BiFeO3(110)c film. Electronic structure analysis reveals that applying compression worsens the resistance to current leakage of the BiFeO3 film.

First principles calculations of magnetism, dielectric properties and spin–phonon coupling in double perovskite Bi2CoMnO6

First principles electronic structure calculations have been performed for the double perovskite Bi2CoMnO6 in its non-centrosymmetric polar state using the generalized gradient approximation plus the Hubbard U approach. We find that the ferromagnetic state is more favored compared to the ferrimagnetic state with both Co and Mn in high spin states. The calculated dynamical charge tensors are anisotropic reflecting a low-symmetry structure of the compound. The magnetic structure dependent phonon frequencies indicate the presence of a weak spin–phonon coupling. Using the Berry phase method, we obtain a spontaneous ferroelectric polarization of 5.88 μC cm−2, which is close to the experimental value observed for a similar compound, Bi2NiMnO6.

Electronic, magnetic and dielectric properties of multiferroic MnTiO3

Abstract

Cell thickness dependence of liquid crystal parameters

Ferroelectric liquid crystal parameters, spontaneous polarization, and transition temperature were studied as a function of cell thickness. These parameters were found to increase with increasing cell thickness, but an exception was observed for the transition temperature in the case of a thin cell. A simple Landau model is presented to interpret the theoretical and experimental observations. The anomalous behavior is attributed to the electroclinic effect and is explained using the Landau model.

Ferroelectricity and ferromagnetism in EuTiO3nanowires

We predicted the ferroelectric-ferromagnetic multiferroic properties of EuTiO3 nanowires and generated the phase diagrams in coordinates of temperature and wire radii. The calculations were performed within the Landau-Ginzburg-Devonshire theory with phenomenological parameters extracted from tabulated experimental data and first principles calculations. Since bulk EuTiO3 is antiferromagnetic at temperatures lower than 5.5 K and paraelectric at all temperatures, our goal was to investigate the possibility of inducing the ferroelectric and ferromagnetic properties of EuTiO3 by reducing the bulk to nanosystems. Our results indicate that ferroelectric spontaneous polarization of ~0.1-0.5C/m2 is induced in EuTiO3 nanowires due to the intrinsic surface stress, which is inversely proportional to the nanowire radius. The spontaneous polarization exists at temperatures lower than 300 K, for the wire radius less than 1 nm and typical surface stress coefficients ~ 15 N/m. Due to the strong biquadratic magnetoelectric coupling, the spontaneous polarization in turn induces the ferromagnetic phase at temperatures lower than 30 K for 2 nm nanowire, and at temperatures lower than 10 K for 4 nm nanowire in EuTiO3. Thus we predicted that the EuTiO3 nanowires can be the new ferroelectric-ferromagnetic multiferroic.

Bias voltage effect on electron tunneling across a junction with a ferroelectric–ferromagnetic two-phase composite barrier

The effect of bias voltage on electron tunneling across a junction with a ferroelectric–ferromagnetic composite barrier is investigated theoretically. Because of the inversion symmetry breaking of the spontaneous ferroelectric polarization, bias voltage dependence of the electron tunneling shows significant differences between the positive bias and the negative one. The differences of spin filtering or tunnel magnetoresistance increase with the increasing absolute value of bias voltage. Such direction preferred electron tunneling is found intimately related with the unusual asymmetry of the electrical potential profile in two-phase composite barrier and provides a unique change to realize rectifying functions in spintronics.

Stability of spontaneous polarization in ultrathin improper ferroelectrics

The competition between the substrate-induced strain and the electrostatic polarization in improper ferroelectric films (hexagonal YMnO${}_{3}$) has been studied using optical second-harmonic generation techniques. Deposited on a (111) surface of yttria-stabilized zirconia substrate, the films exhibit (1) strong strain with negligible polarization, (2) relaxation accompanied by the nonlinear buildup of the polarization, and (3) bulklike polar behavior, in the order of increasing film thickness. The role of the substrate is distinctively different from that in proper ferroelectrics.

Antipolar phase in multiferroic BiFeO3at high pressure

The crystal and magnetic structures of BiFeO${}_{3}$ have been studied at high pressures up to 8.6 GPa by means of neutron powder diffraction at ambient temperature. Upon compression, a reduction of the spontaneous ferroelectric polarization is evidenced from the analysis of atomic displacements in the rhombohedral phase. A structural phase transition from the polar rhombohedral $R$3$c$ phase to the antipolar orthorhombic Pbam phase with antiferroelectic character of atomic displacements was revealed at $P$ = 3 GPa. The lattice parameters of the Pbam phase at $P$ = 4.1 GPa are $a$ = 5.5177(4), $b$ = 11.159(1) and $c$ = 7.7550(6) \AA{}. The variation of the structural parameters of both phases as a function of pressure was determined. The initially incommensurate G-type antiferromagnetic structure of BiFeO${}_{3}$ becomes commensurate due to the structural phase transition.

Symmetry analysis for the Ruddlesden-Popper systems Ca3Mn2O7and Ca3Ti2O7

We perform a symmetry analysis of the zero-temperature instabilities of the tetragonal phase of Ca3Mn2O7 and Ca3Ti2O7 which is stable at high temperature. We introduce order parameters to characterize each of the possible lattice distortions in order to construct a Landau free energy which elucidates the proposed group-subgroup relations for structural transitions in these systems. We include the coupling between the unstable distortion modes and the macroscopic strain tensor. We also analyze the symmetry of the dominantly antiferromagnetic ordering which allows weak ferromagnetism. We show that in this phase the weak ferromagnetic moment and the spontaneous ferroelectric polarization are coupled, so that rotating one of these ordering by applying an external electric or magnetic field one can rotate the other ordering. We discuss the number of different domains (including phase domains) which exist in each of the phases and indicate how these may be observed.

Integration between LiNbO<sub>3</sub> Ferroelectric Film and AlGaN/GaN System

LiNbO3 film (LNO)/AlGaN/GaN heterostructure was fabricated epitaxially. The preferable C+ oriented domains in LNO film lead to the formation of the spontaneous ferroelectric polarization. As a result, the sheet electron concentration of the 2DEG (ns) decreased from 1.13×1013 cm-2 to 1.04×1013 cm-2 when a LNO film deposited on the AlGaN/GaN. The ns decreased nonlinearly with decreasing the temperature. Additionally, the electron mobility for the LNO/AlGaN/GaN heterostructure decreased greatly compared with that for AlGaN/GaN heterostructure, which was caused by the non-uniform domain structure in the LNO film. By external bias switching the ferroelectric polarization, the relative enhancement of the 2DEG, about 7.68×1011/cm2, could be accessible from capacitance-voltage measurement. These results indicated that ferroelectric films combined with AlGaN/GaN would hold promise for next-generation GaN-based memory devices.

Switching of a substitutional-iron/oxygen-vacancy defect complex in ferroelectric PbTiO3from first principles

A combination of first-principles calculations and atomistic simulations was used to study the energy barriers for reorientation of the Fe${}_{\mathrm{ }\mathrm{Ti}}^{\ensuremath{'}}$-V${}_{\mathrm{O}}^{\ifmmode\bullet\else\textbullet\fi{}\ifmmode\bullet\else\textbullet\fi{}}$ defect complex in tetragonal ferroelectric PbTiO${}_{3}$ via migration of the oxygen vacancy next to the iron substitutional. It was found that the barrier for reorientation strongly depends on the position of the migrating oxygen vacancy with respect to the iron atom and the surrounding spontaneous ferroelectric polarization. The obtained results indicate that the defect complex can play a different role in the switching of the spontaneous ferroelectric polarization in PbTiO${}_{3}$ by 90${}^{\ifmmode^\circ\else\textdegree\fi{}}$ or 180${}^{\ifmmode^\circ\else\textdegree\fi{}}$.

Comparative Study of PbTiO3 and SrTiO3 (100) Thin Films Lattice Dynamics and Ferroelectric Properties in a Nonempirical Model of Polarizable Ions

The work devoted to the comparative study of the lattice dynamics, dielectric permittivities, frequencies of unstable ferroelectric and rotation modes and spontaneous polarization of PbTiO3 and SrTiO3 thin films with different thickness.