Electric Polarization Reversal Research Articles

Electrical resistance switching of YbFe2O4 single crystal bulk

We report the electrical resistance switching of YbFe2O4, which is one of the electronic ferroelectrics and shows multi-step polarization reversal. The electrical resistance of the single crystal bulk of YbFe2O4 was measured along the c-axis. Three kinds of resistance states were observed: high resistance state (HRS), low resistance state (LRS), and extra low resistance state (ELRS). The as-grown sample was in HRS. The resistance state switched from HRS to LRS under an electric field of ∼180 V/cm. HRS was reproduced under the same magnitude of the electric field in the opposite direction. The switching from LRS to ELRS was assisted by a current flow in the ab-plane, which is orthogonal to the measurement axis (c-axis). The switching from LRS to ELRS was observed under an electric field of ∼140 V/cm during the resistance measurement along the c-axis after a current flow in the ab-plane for a few seconds. The resistance ratio in HRS, LRS, and ELRS was ∼2:1.5:1, which is regarded as a large value considering that the sample size was sub-millimeter order. This multi-step resistive switching is likely due to the multi-step reversal of electric polarization, accompanied by a multi-step change in Schottky barrier height between the sample and electrodes. The currents in the ab-plane are considered to release some pinnings and assist in the polarization reversal.

Ferroelectric control of magnetic anisotropy of 5d transition metal monolayers

Electric field control of magnetism using ferroelectric materials offers promising applications in low-power spintronics. We have conducted systematic density functional theory calculations to investigate the magnetic properties of epitaxial $5d$ transition metal monolayers on a ferroelectric substrate: $\mathrm{PbTi}{\mathrm{O}}_{3}$ (PTO). Our study reveals that the magnetocrystalline anisotropy energy of the osmium monolayer is significantly enhanced to 18.1 meV/Os in the upward polarization state and, moreover, the anomalous Hall coefficient of osmium/PTO film is tunable by reversing the electric polarization of the PTO substrate. Additionally, the electric polarization reversal in PTO rotates the easy magnetization axis of the iridium and platinum monolayers by ${90}^{\ensuremath{\circ}}$, which is attributed to the rearrangement of interfacial charges. Our findings suggest an efficient approach to control the magnetization direction of monoatomic layers and provide valuable insights for the development of low-energy spintronics devices.

Electric polarization reversal and nonlinear magnetoelectric coupling in the honeycomb antiferromagnet Fe4Nb2O9 single crystal

As a new magnetoelectric material, honeycomb-based antiferromagnet ${\mathrm{Fe}}_{4}{\mathrm{Nb}}_{2}{\mathrm{O}}_{9}$ has attracted a great deal of attention due to its prominent magnetoelectric (ME) coupling and high N\'eel temperature, while the physics of magnetoelectricity is far from understood. In the present study, we present our systematic investigations of the anisotropic ME effect, electric polarization reversal, and nonlinear ME effect of ${\mathrm{Fe}}_{4}{\mathrm{Nb}}_{2}{\mathrm{O}}_{9}$ single crystals, thus highlighting the phase diagram extended down to 10 K. Our results provide clear evidence for electric polarization reversal driven by magnetic field ($H$) along the [110] and [1-10] directions, respectively, while no such polarization reversal occurs as $H$ is applied along the [001] direction. The nonlinear ME effects and electric control of magnetism are unambiguously demonstrated. In addition, the angular-dependent probing reveals a 2\ensuremath{\theta} rotation of the induced electric polarization around the $c$ axis upon the rotation of magnetic field by an angle \ensuremath{\theta}. The electric polarization responses and concomitant ME coupling are well explained by means of the metal-ligand hybridization $p\text{\ensuremath{-}}d$ mechanism. This work represents an essential step forward in the understanding of ME coupling not only in this ${A}_{4}{M}_{2}{\mathrm{O}}_{9}$ honeycomb magnet.

Asymmetric Janus functionalization induced magnetization and switchable out-of-plane polarization in 2D MXene Mo2CXX'.

Exploring two-dimensional (2D) van der Waals materials with out-of-plane polarization and electromagnetic coupling is essential for the development of next-generation nano-memory devices. A novel class of 2D monolayer materials with predicted spin-polarized semi-conductivity, partially compensated antiferromagnetic (AFM) order, fairly high Curie temperature, and out-of-plane polarization is analyzed in this work for the first time. Based on density functional theory calculations, we systematically studied these properties in asymmetrically functionalized MXenes (Janus Mo2C)-Mo2CXX' (X, X' = F, O, and OH). Using ab initio molecular dynamics (AIMD) and phonon spectrum calculations, the thermal and dynamic stabilities of six functionalized Mo2CXX' were identified. Our DFT+U calculation results also provided a switching path for out-of-plane polarizations, where the reverse of electric polarization is driven by terminal-layer atom flipping. More importantly, strong coupling between magnetization and electric polarization originating from spin-charge interactions was observed in this system. Our results confirm that Mo2C-FO would be a novel monolayer electromagnetic material, and its magnetization can be modulated by electric polarization.

LaBr2 bilayer multiferroic moiré superlattice with robust magnetoelectric coupling and magnetic bimerons

Two-dimensional (2D) van der Waals (vdW) materials provide the versatile playground to stack two or more vdW layers for creation of superior materials with desired properties. Here we theoretically adopt a twisted stack-engineering of two LaBr2 monolayers to break space inversion symmetry for ferroelectricity and ultimately multiferroism. The enhancement and reversal of electric polarization are accompanied with the transition from interlayer ferromagnetic and antiferromagnetic orderings, demonstrating an effective magnetoelectric coupling effect with a mechanism dissimilar to that of the conventional multiferroics. Magnetization dynamics simulations show that such magnetic phase transition can excite topologically protected bimeron, and the skyrmion Hall effect can be suppressed by bilayer-bimeron stabilized in both ferromagnetic and antiferromagnetic configurations. Moreover, in the small-angle twisted moiré superlattice, the uniform polarization will evolve into a staggered domain structure, accompanied with the appearance of bimeron, which forms a significant discrepancy with the non-twisted stack-engineered multiferroic LaBr2 bilayer. This work provides a strategy for 2D multiferroic materials by twisted stack engineering of magnetic single layers.

P-orbital multiferroics in single-layer SiN

Multiferroics, coupling magnetism with electric polarization, provides special opportunities for both fundamental research and device applications. The current multiferroic research in a two-dimensional lattice is invariably focused on d-orbital based systems. We alternatively show by first-principles calculations that ideal multiferroics is present in a p-orbital based lattice of single-layer SiN. Single-layer SiN is a semiconductor exhibiting intrinsic ferromagnetism and ferroelectricity simultaneously. Its magnetism correlates with the extended p–p interaction between unpaired p orbitals of N atoms. The buckled symmetry guarantees the existence of an out-of-plane electric dipole, giving rise to the ferroelectric order. More remarkably, the ferroic orders in single-layer SiN display strongly coupled physics, i.e., the spatial distribution of magnetic moments can be well controlled by the reversal of electric polarization, thereby establishing the long-sought multiferroics with strong magnetoelectric coupling. These findings not only enrich a two-dimensional multiferroic family, but also enable a wide range of device applications.

Electron spin resonance study of spin fluctuation in multiferroic MnSb<sub>2</sub>O<sub>6</sub>

The magnetic materials with a chiral crystallographic lattice have hold neither inversion center nor mirror plane, leading to the emergence of Dzyaloshinskii-Moriya interaction and exotic physical phenomena like skyrmion, multiferroicity, and chiral solition lattice. The trigonal oxide MnSb<sub>2</sub>O<sub>6</sub> is recognized as a novel chiral-lattice helimagnet with unusual multiferroic properties, where magnetic field enables the selecting of a single ferroelectric domain and a slight tilting of field direction can trigger the reversal of electric polarization. Single crystal of MnSb<sub>2</sub>O<sub>6</sub> is prepared by the flux method. The magnetic susceptibility at 2 K shows a linear field dependent behavior except in the low field region. The magnetization shows a deviation from linearity at around 0.2 T for <i>H</i>⊥<i>c</i>, while a step-like anomaly is observed at about 1 T for <i>H</i>//<i>c</i>, suggesting the domain selection and spin-flop transition, respectively. The electron spin resonance parameters, such as the resonance field, the g-factor and the linewidth Δ<i>H</i>, are obtained by performing single Lorentzian line. Interestingly, the resonance field shows a distinct, anisotropic temperature dependent behavior when further cooling, the resonance field shifts towards the lower field direction for <i>H</i>⊥<i>c</i>, while it shifts towards higher field direction for <i>H</i>//<i>c</i>. Excluding several mechanisms for this FM-like temperature dependent behavior of the resonance field, combining the ground state of spiral phase and its unique multiferroic properties, we suggest that the spiral magnetic structure of the ground state of MnSb<sub>2</sub>O<sub>6</sub> forms a conical magnetic structure under external magnetic field. Based on this, we can speculate the variation of ferroelectric polarization intensity with moderate and higher magnetic field. Moreover, the critical fitting of the ESR linewidth gives an unusual small critical index, <i>p</i> = 0.49 for <i>H</i>⊥<i>c</i> and <i>p</i> = 0.54 for <i>H</i>//<i>c</i>, implying that the magnetism possesses a two-dimensional characteristic and competitive interaction.

Multistep stochastic mechanism of polarization reversal in orthorhombic ferroelectrics

A stochastic model of electric field-driven polarization reversal in orthorhombic ferroelectrics is advanced, providing a description of their temporal electromechanical response. The theory accounts for all possible parallel and sequential switching events. Application of the model to the simultaneous measurements of polarization and strain kinetics in a lead-free orthorhombic (K,Na)NbO3-based ferroelectric ceramic over a wide timescale of 7 orders of magnitude allowed identification of preferable polarization switching paths, fractions of individual switching processes, and their activation fields. Particularly, the analysis revealed substantial contributions of coherent non-180{\deg} switching events, which do not cause macroscopic strain and thus mimic 180{\deg} switching processes.

Ferroelectricity in novel one-dimensional P42-InSeI nanowires

With the increasing demands for the miniaturization of ferroelectricity-enabled electronic devices, it is highly desirable that the ferroelectricity would exist in low-dimensional materials. However, those ferroelectrics, especially one-dimensional (1D) ones, are still limited. Herein, based on the first-principles calculations, 1D ferroelectricity in novel InSeI nanowires with noncentrosymmetric P42 space group is explored, which could possess three different paths for the reversal of electric polarization. The minimum energy barrier of 97 meV/f.u. for the polarization reversal, comparable with those of other low-dimensional ferroelectrics, could be achieved through a unique four-step concerted process. According to ab initio molecular dynamics simulations, the Curie temperature of the proposed P42-InSeI nanowires is estimatedto about 866 K, above room temperature. The electronic structure and carrier mobility of the nanowires are also studied, demonstrating that they could be promising materials in nanoelectronics such as high-density nonvolatile memories due to their stable array structures formed by van der Waals interactions.

Magnetoelectric phase diagram and magnetic field-induced reversal of electric polarization in Ba0.5Sr1.5Mg1.6Co0.4Fe12O22 single crystal

Multiferroic Y-type hexaferrites are single-phase compounds in which the electric polarization (P) is induced by a low magnetic field (H). In this study, Y-type hexaferrite Ba0.5Sr1.5Mg1.6Co0.4Fe12O22 single crystal was synthesized using Na2CO3-Fe2O3 flux. Their structural, magnetic, and magnetoelectric properties were investigated. The different transition temperatures between the H⊥c and H//c configurations showed that the direction of the applied H was important for the spin configurations. The M-H loops exhibited that the field-induced transitions could occur at room temperature (RT). A first-order magnetic phase transition around 250 K was unveiled from the Arrott plots. The P was non-zero at zero H below 50 K. By studying the magnetic field dependence of the dielectric properties at selected temperatures, a magnetoelectric phase diagram exhibiting the evolution of the spin-induced ferroelectric order with the Hab and temperature (T) was revealed. The magnetoelectric phase diagram showed that the polarization was reversed by the magnetic fields below 150 K where the H-induced ferroelectric order was maintained in a zero magnetic field.

Ferroelectric polarization reversal in multiferroic MnWO4 via a rotating magnetic field up to 52 T

A long-standing issue in multiferroic ${\mathrm{MnWO}}_{4}$ is the electric polarization reversal when magnetic field is applied along the magnetic easy axis $(H||u)$. In this work, we performed comprehensive ferroelectric polarization measurements on ${\mathrm{MnWO}}_{4}$ in fields up to 52 T and with rotating field direction in the $ac$ plane. When the field is rotated from $H||c$ to $H||u$, the induced high-field phase $(\ensuremath{-}P||b)$ favors a sign reversal relative to the low-field $+P||b$ phase, which is attributed to the vector spin chirality changing from one side of the spin connecting vector to the other. Particularly, further slight deviation of the field from the magnetic easy axis towards $H||a$ causes an abrupt reversal of the high-field phase from $\ensuremath{-}P||b$ to $+P||b$. We assume that this unusual angular dependence of polarization behavior can be understood by magnetic control of the helicity of the chiral plane with opposite conical spin structures. In addition, we also propose electric-field control of the chiral plane close to the ferroelectric phase transition at high fields. This study results in an overall understanding of this important multiferroic material and sheds light on field-induced polarization switching in related compounds.

Spin-Current Control by Induced Electric Polarization Reversal in Ni/hBN/Ni: A Cross-Correlation Material

We undertook an ab-initio study of hexagonal boron nitride (hBN) sandwiched between Ni(111) layers to examine the interface of this material structure. We considered Ni(111)/hBN/Ni(111) with a slab with three Ni atomic layers to determine the exact atom arrangement at the interface. The density functional theory calculations for 36 stacking arrangements, which are doubled with respect to the magnetic alignment of slabs in an anti-parallel configuration (APC) and parallel configuration (PC), revealed that the number of formed weak chemical bonds, in the pd-hybridization between the N and Ni atoms, is decisive. A maximum of two pd-hybridization bonds stabilized the structure, with APC proving to be the most favorable magnetic alignment, in line with the results of previous experimental studies. In the lowest energy state, an induced magnetic moment at an N site appears when N is moved closer to one of the Ni atoms. Interestingly, the moment direction is switched by the position of the N layer in the resulting bi-stable state with electrical polarization when APC is chosen. The transmission probability calculation of Ni/hBN/Ni having the determined interface structure at the center of the junction exhibits a spin-filtering effect where the spin-polarized current is controlled by the electric field when a field-induced reversal of the polarization is realized.

Reversibility of spin-induced electric polarization in multiferroic hexaferrites

Hexaferrites with noncollinear conical magnetic structures are among the most interesting and promising single-phase multiferroics. One puzzle remaining unsolved is why the spin-induced electric polarization can be either reversible or nonreversible by applying magnetic fields. We have unraveled a solution to this puzzle by a systematic study in the multiferroic hexaferrite ${\mathrm{Ba}}_{0.3}{\mathrm{Sr}}_{1.7}{\mathrm{Co}}_{2}{\mathrm{Fe}}_{11}{\mathrm{AlO}}_{22}$ where the electric polarization is reversible at low temperatures but nonreversible at high temperatures. Neutron diffraction results reveal that the rotation of spin cones with applied $ab$-plane magnetic field takes distinct paths at 150 and 305 K: in-plane and out-of-plane rotation, respectively. A theoretical analysis based on the spin current model confirms that the reversal of electric polarization is caused by the in-plane rotation. Our study clarifies the mechanism underlying the reversibility of spin-induced electric polarization in multiferroic hexaferrites.

Orienting anisometric pores in ferroelectrics: Piezoelectric property engineering through local electric field distributions

Ferroelectrics are a technologically important class of materials that are used in actuators, sensors, transducers, and memory devices. Introducing porosity into these materials offers a method of tuning functional properties for certain applications, such as piezo- and pyroelectric sensors and energy harvesters. However, the effect of porosity on the polarization switching behavior of ferroelectrics, which is the fundamental physical process determining their functional properties, remains poorly understood. In part, this is due to the complex effects of porous structure on the local electric field distributions within these materials. To this end, freeze-cast porous lead zirconate titanate (PZT) ceramics were fabricated with highly oriented, anisometric pores and an overall porosity of 34 vol.%. Samples were sectioned at different angles relative to the freezing direction, and the effect of pore angle on the switching behavior was tracked by measuring simultaneously the temporal polarization and strain responses of the materials to high-voltage pulses. Finite-element modeling was used to assess the effect of the pore structure on the local electric field distributions within the material, providing insight into the experimental observations. It is shown that increasing the pore angle relative to the applied electric field direction decreases the local electric field, resulting in a reduced domain-wall dynamic and a broadening of the distribution of switching times. Excellent longitudinal piezoelectric (${d}_{33}=630\phantom{\rule{0.28em}{0ex}}\mathrm{pm}/\mathrm{V}$) and strain responses (${S}_{\mathrm{bip}}=0.25%$ and ${S}_{\mathrm{neg}}=0.13%$, respectively), comparable to the dense material (${d}_{33}=648\phantom{\rule{0.28em}{0ex}}\mathrm{pm}/\mathrm{V}$, ${S}_{\mathrm{bip}}=0.31%$, and ${S}_{\mathrm{neg}}=0.16%$), were found in the PZT with anisometric pores aligned with the poling axis. Orienting the pores perpendicular to the poling axis resulted in the largest reductions in the effective permittivity (${\ensuremath{\varepsilon}}_{33}^{\ensuremath{\sigma}}=200$ compared to ${\ensuremath{\varepsilon}}_{33}^{\ensuremath{\sigma}}=4100$ for the dense PZT at 1 kHz), yielding the highest piezoelectric voltage coefficient (${g}_{33}=216\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\phantom{\rule{0.28em}{0ex}}\mathrm{Vm}/\mathrm{N}$) and energy-harvesting figure of merit (${d}_{33}{g}_{33}=73\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}\phantom{\rule{0.28em}{0ex}}{\mathrm{m}}^{2}/\mathrm{N}$). These results demonstrate that a wide range of application-specific properties can be achieved by careful control of the porous microstructure. This work provides an understanding of the interplay between the local electric field distribution and polarization reversal in porous ferroelectrics, which is an important step towards further improving the properties of this promising class of materials for sensing, energy harvesting, and low-force actuators.

First-principle investigation on multiferroicity and interfacial coupling of nonstoichiometric tetragonal La2/3Sr1/3MnO3/BiCoO3 interface

The energetically stable tetragonal nonstoichiometric La2/3Sr1/3MnO3/BiCoO3 (LSMO/BCO) interface, whose chemical formula is (BiO)3(CoO2)4/(LaO)3(SrO)1(MnO2)3, was investigated by using first-principle calculations. A magnetoelectric coupling effect, i.e., an electric control of the magnetism, was approved by a maximum variation of 17.9% in magnetic moments tuned by the electric polarization reversal. The interfacial coupling was controlled by the polar shifts of the interfacial Co and/or O atoms. A huge variation in spin-polarization from 6.3% to 72.7% was achieved upon switching the electric polarization. These findings are useful for magnetoelectric coupled spintronic device applications.

Coupled multiferroic domain switching in the canted conical spin spiral system Mn2GeO4

Despite remarkable progress in developing multifunctional materials, spin-driven ferroelectrics featuring both spontaneous magnetization and electric polarization are still rare. Among such ferromagnetic ferroelectrics are conical spin spiral magnets with a simultaneous reversal of magnetization and electric polarization that is still little understood. Such materials can feature various multiferroic domains that complicates their study. Here we study the multiferroic domains in ferromagnetic ferroelectric Mn2GeO4 using neutron diffraction, and show that it features a double-Q conical magnetic structure that, apart from trivial 180o commensurate magnetic domains, can be described by ferromagnetic and ferroelectric domains only. We show unconventional magnetoelectric couplings such as the magnetic-field-driven reversal of ferroelectric polarization with no change of spin-helicity, and present a phenomenological theory that successfully explains the magnetoelectric coupling. Our measurements establish Mn2GeO4 as a conceptually simple multiferroic in which the magnetic-field-driven flop of conical spin spirals leads to the simultaneous reversal of magnetization and electric polarization.

Eu-doping effects on the structural and multiferroic properties of YMn0.5Cr0.5O3

Y1−xEuxMn0.5Cr0.5O3 (x = 0.1–1.0) oxides were synthesized in a polycrystalline form by a conventional solid-state reaction method. X-ray diffraction patterns revealed that the whole series of compounds featured a simple distorted perovskite structure with the orthorhombic space group Pbnm. With increasing Eu content, the shorter a and c axes increased, while the longest b axis changed little. The ferrimagnetic transition temperature TN increased gradually to x = 0.8 before declining with a further increase in x. A clear correlation between TN and the negative molecular field coefficient within the Cr3+ sublattice indicated that an antiferromagnetic exchange interaction between the Cr3+ cations plays a dominant role in the magnetic ordering. By reversing the sign of the poling field, the occurrence of electric polarization reversal at temperatures around TN implied a possible multiferroic state in the samples. From the dielectric point of view, the disappearance of the dielectric constant anomaly at TN might be related to the high AC conductivity of the samples, which is a common feature of orthochromites.

Tunneling electroresistance of MgZnO-based tunnel junctions

We investigated the tunneling electroresistance (TER) in metal/wurtzite-MgZnO/metal junctions for applications in nonvolatile random-access memories. A resistive switching was detected utilizing an electric-field cooling at ±1 V and exhibited a TER ratio of 360%–490% at 2 K. The extracted change in the average barrier height between the two resistance states gave an estimation of the MgZnO electric polarization at 2.5 μC/cm2 for the low-temperature limit. In addition, the temperature-dependent TER ratio and the shift of the localized states energies at the barrier interface supported the ferroelectric behavior of the MgZnO tunnel-barrier. From the first-principles calculations, we found a similar effect of the barrier height change coming from the reversal of ZnO electric polarization. The possibility of using metal electrodes and lower growth temperatures, in addition to the ferroelectric property, make the ZnO-based memory devices suitable for CMOS integration.

Magnetic Reversal of Electric Polarization with Fixed Chirality of Magnetic Structure in a Chiral-Lattice Helimagnet MnSb_{2}O_{6}.

The correlation between magnetic and dielectric properties has been investigated for the single crystal of the chiral triangular-lattice helimagnet MnSb_{2}O_{6}. We found that the spin-spiral plane in the ground state has a considerable tilting from the (110) plane and that the sign of the spin-spiral tilting angle is coupled to the clockwise or counterclockwise manner of spin rotation and accordingly to the sign of magnetically induced electric polarization. This leads to unique magnetoelectric responses such as the magnetic-field-induced selection of a single ferroelectric domain as well as the reversal of electric polarization just by a slight tilting of the magnetic field direction, where the chiral nature of the crystal structure plays a crucial role through the coupling of the chirality between the crystal and magnetic structures. Our results demonstrate that crystallographic chirality can be an abundant source of novel magnetoelectric functions with coupled internal degrees of freedom.

Simultaneous resonant x-ray diffraction measurement of polarization inversion and lattice strain in polycrystalline ferroelectrics.

Structure-property relationships in ferroelectrics extend over several length scales from the individual unit cell to the macroscopic device, and with dynamics spanning a broad temporal domain. Characterizing the multi-scale structural origin of electric field-induced polarization reversal and strain in ferroelectrics is an ongoing challenge that so far has obscured its fundamental behaviour. By utilizing small intensity differences between Friedel pairs due to resonant scattering, we demonstrate a time-resolved X-ray diffraction technique for directly and simultaneously measuring both lattice strain and, for the first time, polarization reversal during in-situ electrical perturbation. This technique is demonstrated for BaTiO3-BiZn0.5Ti0.5O3 (BT-BZT) polycrystalline ferroelectrics, a prototypical lead-free piezoelectric with an ambiguous switching mechanism. This combines the benefits of spectroscopic and diffraction-based measurements into a single and robust technique with time resolution down to the ns scale, opening a new door to in-situ structure-property characterization that probes the full extent of the ferroelectric behaviour.