Polarization Reversal Mechanism Research Articles

Revealing the Role of Spacer Layer in Domain Dynamics of Hf0.5Zr0.5O2 Thin Films for Ferroelectrics

AbstractHfO2‐based ferroelectrics offer promises for next‐generation nonvolatile nanoscale devices owing to excellent CMOS compatibility and robust ferroelectricity at the nanoscale. However, fundamentally understanding the mechanism of polarization reversal and domain dynamics is challenging because the role of the spacer layer at the domain level in HfO2‐based ferroelectrics remains elusive. Here, it is realized that visualization of time‐resolved dynamics of nanoscale domain configurations in the epitaxial Hf0.5Zr0.5O2 thin films using phase‐field simulations. Scale‐free domain independent switchability (namely, irreducible stable polar domain down to 1 nm lateral size) and sharp domain walls that originate from weak interactions between adjacent domains characterized by the reduced gradient energy coefficient, which unravels the mesoscale mechanism of spacer layer in domain dynamics are demonstrated. Meanwhile, it is revealed that 180° polarization switching is dominated by the nucleation of a new domain with a high nucleation density, well described by the nucleation‐limited switching model. This study not only provides a fundamental mesoscale mechanism of the spacer layer, but also stimulates further studies on the manipulation of ultra‐scaled single domain state for designing high‐density and fast‐speed HfO2‐based ferroelectric memory.

Kinetics of N- to M-Polar Switching in Ferroelectric Al1-xScxN Capacitors.

Ferroelectric wurtzite-type aluminum scandium nitride (Al1-xScxN) presents unique properties that can enhance the performance of non-volatile memory technologies. The realization of the full potential of Al1-xScxN requires a comprehensive understanding of the mechanism of polarization reversal and domain structure dynamics involved in the ferroelectric switching process. In this work, transient current integration measurements performed by a pulse switching method are combined with domain imaging by piezoresponse force microscopy (PFM) to investigate the kinetics of domain nucleation and wall motion during polarization reversal in Al0.85Sc0.15N capacitors. In the studied electric field range (from 4.4 to 5.6MVcm-1), ferroelectric switching proceeds via domain nucleation and wall movement. The currently available phenomenological models are shown to not fully capture all the details of the complex dynamics of polarization reversal in Al0.85Sc0.15N. PFM reveals a non-linear increase of both domain nucleation rate and lateral wall velocity during the switching process, as well as the dependency of the domain pattern on the polarization reversal direction. A continuously faster N- to M-polar switching upon cycling is reported and ascribed to an increasing number of M-polar nucleation sites and density of domain walls.

Unravelling the nanoscale mechanism of polarization reversal in a Hf0.5Zr0.5O2-based ferroelectric capacitor by vector piezoresponse force microscopy.

Ferroelectricity is in demand in many device concepts in electronics, energy and microsystem engineering. The performance of ferroelectrics-based devices is determined by either out-of-plane or in-plane polarization, or out-of-plane or in-plane piezoelectric strain. Real prospects for the practical implementation of innovative devices opened up after the discovery of ferroelectricity in ultrathin hafnium oxide films, due to their perfect compatibility with silicon technology. Ferroelectric properties of this material have been assigned to an orthorhombic structural phase with a single polar axis, but the spatial orientation of the polarization vector and the tensorial piezoelectric behaviour, which are inextricably coupled, still remain unknown. Herein, the rotation of the polarization vector in a Hf0.5Zr0.5O2 (10 nm) capacitor during polarization switching and the spatial distribution of longitudinal and shear piezoelectric coefficients are elucidated at the nanoscale using operando vector piezoresponse force microscopy. In most of the capacitor, a 180°-flipping of the polarization vector is observed, which is consistent with the orthorhombic phase structure. However, a rather large fraction of the capacitor is also occupied by nanoregions of ferroelastic (non-180°) switching, which is explained by the effect of the local mechanical stress. To quantify the three-dimensional piezoresponse, a novel approach exploiting the Poisson effect in artificially created non-ferroelectric regions is proposed and it shows that the shear piezoelectric coefficient is twice the longitudinal coefficient. The experimental insights entail an important step in fundamental understanding of the ferroelectric and piezoelectric properties of hafnium oxide and have great potential to trigger new versions of ferroelectric-based devices.

Spinovyy khaos eksitonnykh polyaritonov v magnitnom pole

The spin properties of exciton polaritons in a micropillar cavity placed in a static magnetic field and excited by a resonant light wave are studied theoretically. Owing to the Zeeman effect, a nonlinear polariton system has two branches of optical response that are characterized by opposite circular polarizations. An indirect mechanism of polarization reversal is predicted, according to which the current state of the system undergoes a transition to dynamical chaos, and then the alternative spin state is established spontaneously. Such spin switches, mediated by a chaotic stage, proceed in both directions near the same critical excitation amplitude, so that the sign of the circular polarization of the cavity radiation is directly determined by the intensity of the optical pump.

Polarization Reversal of Group IV-VI Semiconductors with Pucker-Like Structure: Mechanism Dissecting and Function Demonstration.

Polarization imaging presents advantages in capturing spatial, spectral, and polarization information across various spectral bands. It can improve the perceptual ability of image sensors and has garnered more applications. Despite its potential, challenges persist in identifying band information and implementing image enhancement using polarization imaging. These challenges often necessitate integrating spectrometers or other components, resulting in increased complexities within image processing systems and hindering device miniaturization trends. Here, the characteristics of anisotropic absorption reversal are systematically elucidated in pucker-like group IV-VI semiconductors MX (M=Ge, Sn; X=S, Se) through theoretical predictions and experimental validations. Additionally, the fundamental mechanisms behind anisotropy reversal in different bands are also explored. The photodetector is constructed by utilizing MX as a light-absorbing layer, harnessing polarization-sensitive photoresponse for virtual imaging. The results indicate that the utilization of polarization reversal photodetectors holds advantages in achieving further multifunctional integration within the device structure while simplifying its configuration, including band information identification and image enhancement. This study provides a comprehensive analysis of polarization reversal mechanisms and presents a promising and reliable approach for achieving dual-band image band identification and image enhancement without additional auxiliary components.

Spin Chaos of Exciton Polaritons in a Magnetic Field

The spin properties of exciton polaritons in a micropillar cavity placed in a static magnetic field and excited by a resonant light wave are studied theoretically. Owing to the Zeeman effect, a nonlinear polariton system has two branches of optical response that are characterized by opposite circular polarizations. An indirect mechanism of polarization reversal is predicted, according to which the current state of the system undergoes a transition to dynamical chaos, and then the alternative spin state is established spontaneously. Such spin switches, mediated by a chaotic stage, proceed in both directions near the same critical excitation amplitude, so that the sign of the circular polarization of the cavity radiation is directly determined by the intensity of the optical pump.

High temperature magnetic and structural transformations in Fe-Pd nanowires

The microstructure and the hysteresis properties of Fe-Pd nanowire arrays were studied in the as-electrodeposited condition at 300 K, and as functions of temperature after a thermal cycle between 300 K and 950 K. Fe36Pd64 and Fe65Pd35 nanowires, 200 nm in diameter and 7–10 μm long, were electrodeposited into commercial 26 % porosity alumina templates. Initially, the main magnetic phase in both arrays is the metastable γ-Pd(Fe) disordered phase. After a thermal cycle up to 950 K the microstructures transform into a majority ferromagnetic ordered phase (FePd3 in Fe36Pd64 and Fe3Pd in Fe65Pd35) embedding γ-Pd(Fe) remaining grains. Curie temperatures result 500 K, 550 K and 900 K for Fe3Pd, FePd3 and γ-Pd(Fe), respectively. The temperature dependence of the magnetic properties exhibits two stages: one below 500–550 K, when the two phases are ferromagnetic and another one above this temperature when only the minority γ-Pd(Fe) is ferromagnetic. Coercivity and remanence exhibit a maximum near the Curie temperature of the corresponding ordered phase. The effective magnetic anisotropy Keff remains high up to the magnetic transition (ferro to paramagnetic) of the ordered phase, due to the contribution of the magnetocrystalline anisotropy of these relatively hard phases. At higher temperature, the effective anisotropy mainly arises from the magnetostatic (shape and inter-wire dipolar interactions) contribution of the minority phase. The polarization reversal mechanism in the γ-Pd(Fe) phase is found to be the nucleation of inverse magnetic domains and the further expansion of the domain walls into the ferromagnetic grains, surrounded by the paramagnetic ordered phase.

Intersecting multiaxial domain walls in plastic ferroelectric crystal films

Plastic ferroelectrics comprise a class of solution-processable molecular ferroelectrics, in which spherical polar molecules can rotate and/or freeze within crystal lattices. Some compounds exhibit superior multiaxial ferroelectricity and very sharp ferroelectric switching characteristics, although the domain wall (DW) characteristics have not yet been explored. Here, we report distinctive DW characteristics in drop-casted monocrystalline thin films of a typical plastic/ferroelectric $[\mathrm{AH}][\mathrm{Re}{\mathrm{O}}_{4}]$. We demonstrate that the films form ${71}^{\ensuremath{\circ}}, {109}^{\ensuremath{\circ}}$, and ${180}^{\ensuremath{\circ}}$ DWs, unique to the pseudocubic rhombohedral lattice, that emerge and compete with each other, depending on the crystallographic orientation of the films. Particularly, it is observed that all three types of DWs intersect at several single points, whose unique nature should be ascribed to the small elastic deformation. The observed domains and ${180}^{\ensuremath{\circ}}$ DWs also comprise much larger periodicity and scaling than those of conventional inorganic ferroelectric films, which should be associated with the unique polarization reversal mechanism in the compound films. These observed features reveal the exotic multiaxial ferroelectricity of soft plastic/ferroelectrics, paving the way for developing printable ferroelectronics.

(Invited, Digital Presentation) Materials Aspects of New Ferroelectrics with Simple Crystal Structure

Ferroelectric materials are defined as the materials, of which spontaneous polarization can be switched by an external electric field. Their crystal structural symmetry makes them exhibit a variety of electric properties, such as piezoelectricity, pyroelectricity, and ferroelectricity. Because of their characteristics, they are expected to be adapted for various applications, including sensors, actuators, and non-volatile memories. Over the past decades, perovskite type ferroelectric materials have occupied the central position in both fundamental studies and applications of ferroelectric materials. On the other hand, there are a few studies on ferroelectrics with other crystal structures.This regime is now changing since HfO2-based new ferroelectric materials have been discovered. The HfO2-based dielectric materials are employed as high-k insulators of the metal-oxide-semiconductor field-effect-transistors instead of the conventional SiOx gate dielectrics, suggesting the high compatibility with semiconductor technologies. Thus, discovering ferroelectricity in HfO2-based materials strongly encourages us to develop highly integrated ferroelectric devices that are difficult to fabricate with traditional perovskite-type ferroelectrics. Amid increasing interest in ferroelectric materials, ferroelectricity is demonstrated on another new (Al, Sc)N, which has a wurtzite structure. Both fluorite structure, the parent structure of HfO2-based ferroelectrics, and wurtzite structure are simple compounds, having only a single anion and cation sites in the crystal structure. This feature contrasts the complex crystal structure of conventional perovskite structure. This presentation will give a brief outline of these new ferroelectric materials and introduce our recent studies from the viewpoint of crystal chemistry. It is well-known that HfO2 undergoes successive phase transitions from monoclinic to tetragonal and tetragonal to cubic phases. However, these phases cannot show ferroelectricity because of their inversion center in the crystal structure. It is widely accepted that the ferroelectricity in HfO2-based materials originates from the metastable orthorhombic structure. This orthorhombic structure was confirmed by the convergence electron diffraction and scanning transmission electron microscopy. Among the HfO2-based materials, HfO2- ZrO2 materials are most extensively studied. However, the thickness that can exhibit ferroelectricity in these materials is limited to less than 50 nm because of their strong preference for the monoclinic structure. In order to investigate structural features of the HfO2-based materials, materials are demanded that have ferroelectricity over the wide thickness range. The Y-doped HfO2 meets the requirement, allowing us to grow the ferroelectric film over 1 μm in thickness. Furthermore, we demonstrated ferroelectricity in epitaxial films using this composition. A recent report on ferroelectricity in bulk single-crystal also employed the Y-doped HfO2 system. The ferroelectricity in the wurtzite structure has been discussed for a long time. Moriwake et al. put forward giant spontaneous polarization in wurtzite materials by calculation based on density functional theory. The proposed mechanism of polarization reversal is accompanied by the change in the outermost surface, namely a cation surface to an anion surface and vice versa. Such large polarization was demonstrated in (Al1-x Scx)N films by Fitchtner et al. They also confirmed the change in the surface by performing chemical etching. In addition to (Al1-x Scx)N films, the ferroelectricity has been confirmed in (Al1-x B x )N, (Ga1-x Sc x )N, and (Zn1-x Mg x )O. For the wurtzite structure, we can consider the virtual paraelectric BN phase, in which both anions and cations are located in the same plane. As the paraelectric phase is deemed an intermediate state during polarization reversal, easy polarization reversal is expected as the u-parameter of the wurtzite structure approaches 0.5. It is considered that the u parameter is closely related to the axial ratio of the c- and a-axes. In fact, the reduction of coercive field and remanent polarization is ascertained experimentally. The “simple compound” ferroelectrics have attracted much attention due to their unique features, e.g., outstanding compatibility to semiconductor technologies in HfO2-based materials and giant remanent polarization in wurtzite materials. However, quite a large coercive field compared to conventional ferroelectrics reduces the reliability of the devices, particularly endurance properties. Further studies and developments to unveil microstructures under and after applying a strong electric field will lead to the next application of these ferroelectrics.

Temperature dependent scaling behavior of 0.67PMN–0.33PT relaxor ferroelectric ceramics

Polarization reversal mechanisms are the key to understand the ferroelectric properties. The scaling behaviors of ferroelectric loops reveal the dynamic processes of ferroelectric domain's nucleation, growth, and reversal. Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN-PT) at morphotropic phase boundary (MPB) exhibits excellent piezoelectric and electromechanical properties. In this paper, the dynamic scaling behaviors of 0.67PMN-0.33PT ceramics at variable temperatures were investigated. Results indicate that, at the high electric field, the remanent polarization Pr, coercive field Ec, and hysteresis loops area <A> decrease monotonically with the increase of temperature. While at a low electric field, they first increase and then decrease with increasing temperature. The scaling exponents a and b increase with the increase of temperature, indicating that the frequency and electric amplitude dependence of polarization reversal is enhanced with temperature.

Reduction of depolarization field effect on ferroelectric switching process in semiconductor–relaxor ferroelectric composite

Temperature-dependent dynamic ferroelectric hysteresis of semiconductor–relaxor ferroelectric (0–3) type composite {0.30(ZnO)–0.70[(Bi0.5Na0.5)0.94Ba0.06TiO3 (BNBTO)]} has been investigated using polarization–electric field (P–E) loops, current density–electric field (J–E) curves, and temperature-dependent dielectric permittivity. It is well known that the polarization reversal mechanism can be explained by the concept of ferroelectric domain switching kinetics, which depends strongly on the temperature. The present work ascribes the role of polar nanoregion induced thermal depolarization field on the temperature-dependent ferroelectric hysteresis loop along with polarization reversal mechanism. The present composite exhibits unique ferroelectric switching behavior above the thermal depolarization temperature (∼100 °C), which is observed in P–E and J–E loops. The depolarization field-induced pinched P–E loops of a BNBTO solid solution above Td (∼100 °C) have been significantly overcome by the incorporation of semiconductor (ZnO) particles, which extensively described the underlying mechanism in the present context. In addition, the temperature-dependent polarization reversal mechanism displays unique two-stage processes [low-T (&amp;lt;100 °C) and high-T (&amp;gt;100 °C)] for the minor loops (∼30 and 40 kV) and saturated loops (∼45 kV) as described by the electric field–temperature phase diagram. The present results may provide a distinct way to Bi0.5Na0.5TiO3-based solid solutions for high-temperature piezoelectric applications.

Effect of Film Microstructure on Domain Nucleation and Intrinsic Switching in Ferroelectric Y:HfO2 Thin Film Capacitors

AbstractOne of the general features of ferroelectric systems is a complex nature of polarization reversal, which involves domain nucleation and motion of domain walls. Here, time‐resolved nanoscale domain imaging is applied in conjunction with the integral switching current measurements to investigate the mechanism of polarization reversal in yttrium‐doped HfO2 (Y:HfO2)—currently one of the most actively studied ferroelectric systems. More specifically, the effect of film microstructure on the nucleation process is investigated by performing a comparative study of the polarization switching behavior in the epitaxial and polycrystalline Y:HfO2 thin film capacitors. It is found that although the epitaxial Y:HfO2 capacitors tend to switch slower than their polycrystalline counterparts, they exhibit a significantly higher nucleation density and rate, suggesting that this is a rate‐limiting mechanism. In addition, it is observed that under the external fields approaching the activation field value, the switching kinetics can be described equally well by the nucleation limited switching and the Kolmogorov‐Avrami‐Ishibashi models for both types of capacitors. This signifies convergence of two different mechanisms implying that the polarization reversal proceeds via a homogeneous nucleation process unaffected by the film microstructure, which can be considered as approaching the intrinsic switching limit.

Electric-field-induced crossover of polarization reversal mechanisms in Al1−x Sc x N ferroelectrics

Scandium-doped aluminum nitride, Al1−x Sc x N, represents a new class of displacive ferroelectric materials with high polarization and sharp hysteresis along with high-temperature resilience, facile synthesizability and compatibility with standard CMOS fabrication techniques. The fundamental physics behind the transformation of unswitchable piezoelectric AlN into switchable Al–Sc–N ferroelectrics depends upon important atomic properties such as local structure, dopant distributions and the presence of competing mechanism of polarization switching in the presence of an applied electric-field that have not been understood. We computationally synthesize Al1−x Sc x N to quantify the inhomogeneity of Sc distribution and phase segregation, and characterize its crystal and electronic structure as a function of Sc-doping. Nudged elastic band calculations of the potential energy surface and quantum molecular dynamics simulations of direct electric-field-driven ferroelectric switching reveal a crossover between two polarization reversal mechanisms—inhomogeneous nucleation-and-growth mechanism originating near Sc-rich regions in the limit of low applied fields and nucleation-limited-switching in the high-field regime. Understanding polarization reversal pathways for these two mechanisms as well as the role of local Sc concentration on activation barriers provides design rules to identify other combinations of dopant elements, such as Zr, Mg etc. to synthesize superior AlN-based ferroelectric materials.

Accessing nanoscopic polarization reversal processes in an organic ferroelectric thin film.

Towards eliminating toxic substances from electronic devices, Croconic Acid (CA) has great potential as a sublimable organic ferroelectric material. While studies on CA thin films are just beginning to emerge, its capability to be integrated in nanodevices remains unexplored. We demonstrate at the laterally nanoscopic scale robust ferroelectric switching of a stable enduring polarization at room temperature in CA thin films, without leakage. The challenging ferroelectric characterization at the nanoscale is performed using a unique combination of piezoresponse force microscopy, polarization switching current spectroscopy and concurrent strain response. This helps rationalize the otherwise asymmetric polarization-voltage hysteresis due to background noise limited undetectable switching currents, which are statistically averaged in macrojunctions but become prevalent at the nanoscale. Apart from successfully estimating the nanoscopic polarization in CA thin films, we show that CA is a promising lead-free organic ferroelectric towards nanoscale device integration. Our results, being valid irrespective of the ferroelectrics' nature; organic or inorganic, pave the way for fundamental understandings and technological applications of nanoscopic polarization reversal mechanisms.

Primary insight into the cathode strengthened electrons transport and nitrous oxide reduction during hydrogenotrophic denitrification in bioelectrochemical system (BES)

Long time for acclimating biocathode and low efficiency of electrons transport owing to the high cathodic over-potentials were the main challenges for hydrogenotrophic denitrification in bioelectrochemical system (BES). The current study was aimed to reveal the mechanism of polarity reversal and porous polypyrrole (PPy) modification on facilitating the electrons transfer from hydrogen to nitrate reduction chain and nitrous oxide reduction. The results showed the highest accumulated NADH concentration was achieved in the group of combination, which was 7.5%, 18.2% and 48.5% higher compared with the PPy group, reversal group and control group, respectively. Moreover, significant increases of nitrous oxide reductase (Nos) concentrations were detected in the groups of cathode strengthening compared with the control group. In addition, specific microorganisms related with electrons transport and nitrous oxide reduction were enriched by polarity reversal and PPy modification. Overall, the cathode strengthening strategies could both improve electrons transport and nitrous oxide reduction in hydrogenotrophic denitrification process.

Multistep stochastic mechanism of polarization reversal in rhombohedral ferroelectrics

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

Ab initio molecular dynamics simulations of the ferroelectric-paraelectric phase transition in sodium nitrite

This paper reports on the first ab initio molecular dynamics study of the ferroelectric sodium nitrite, shedding light on its ferroelctric-paraelectric phase transition. The remnant polarization ${P}_{r}$ was calculated using a Mulliken population analysis and maximally localized Wannier functions. Especially the Wannier based model is in outstanding agreement with experimental findings and previous Berry phase calculations. The simulations predict a ferroelectric Curie temperature ${T}_{c}$ between 425 and $450\phantom{\rule{4pt}{0ex}}\mathrm{K}$ in excellent agreement with the experimental value of $437\phantom{\rule{4pt}{0ex}}\mathrm{K}$. In addition, the anomalous lattice behavior (shrinking of the $c$ axis) during the phase transition is reproduced. Furthermore, the analysis of the phase transition revealed a combined displacive and order-disorder mechanism. The crystal field effect in the material could be quantified by investigating the molecular dipoles based on the maximally localized Wannier functions and the intermolecular charge transfer by analyzing the Mulliken charges. In agreement with earlier experimental and theoretical findings, the polarization reversal mechanism was found to be dominated by a $c$-axis rotation of the nitrite ions. The molecular insight into such a simple and prototypical material serves as a basis for a further development of more complex crystalline ferroelectrics, using a design principle inspired by ${\mathrm{NaNO}}_{2}$.

Electric field and frequency dependent scaling behavior of dynamic hysteresis in relaxor-based ferroelectric 0.71Pb(Mg1/3Nb2/3)O3–0.29PbTiO3 single crystal

The evolution of the dynamic hysteresis with electrical field amplitude (E0) and frequency (f) in the relaxor-based ferroelectric 0.71Pb(Mg1/3Nb2/3)O3–0.29PbTiO3 (PMN-29PT) single crystal has been investigated systematically. Our results showed that the electric field dependent scaling relationship in PMN-29PT single crystal can be divided to three regions, and the hysteresis area <A> follows the power law <A>∝fαE0β in the low and high E0 regions, but the power law is not obeyed in the intermediate region due to the complex collective contributions of 180° and non-180° domains. The frequency dependent scaling relationship can be separated into two regions, and presents a unique behavior when the field level E0 is equal or lower than the coercive field Ec. The hysteresis area decreases continually with the increase of frequency when E0 < Ec, while <A> first increases then decreases for high E0 situation (E0≥2Ec). Related electrical field and frequency dependent polarization reversal mechanisms are proposed based on the experimental observations.

L10-FeNi ordered phase in AC electrodeposited iron-nickel biphasic nanowires

Nanowire arrays with nominal composition FexNi100-x (x = 0, 18, 53, 93, 100) have been synthesized by AC electrodeposition into the cylindrical pores of alumina templates. Except for the composition Fe53Ni47, nanowires are single-phase, consisting of small grains of the A1-FeNi disordered phase. Near the equiatomic nominal composition nanowires are biphasic, consisting of grains of the A1-FeNi disordered phase (γ-FeNi phase) as well as grains of the L10-FeNi ordered phase (γ″-FeNi phase). Even in nanowires with high Fe content the α-Fe bcc phase is absent. The coercive field and reduced remanence dependences on composition are non monotonic but go through a local maximum near the equiatomic composition. Biphasic nanowires behave as a magnetic single phase, exhibiting a unique switching field for polarization reversal in the entire range between 5 K and 300 K. The linear dependence of the coercive field on temperature and the dependence of the coercive field on the angle between the nanowire long axis and the applied field are consistent with a polarization reversal mechanism controlled by the nucleation by curling and expansion of inverse domains, assisted by thermal fluctuations and the applied field. The effective uniaxial anisotropy resulting from these measurements suggests that the shape anisotropy makes a small contribution to the high coercive field observed (170 mT). The effective uniaxial magnetocrystalline constant measured at room temperature, 130–170 k Jm−3, may be explained by considering nucleation at favorable sites in an exchange-hardened γ-FeNi soft phase.

Nanoscopic studies of domain structure dynamics in ferroelectric La:HfO2 capacitors

Visualization of domain structure evolution under an electrical bias has been carried out in ferroelectric La:HfO2 capacitors by a combination of Piezoresponse Force Microscopy (PFM) and pulse switching techniques to study the nanoscopic mechanism of polarization reversal and the wake-up process. It has been directly shown that the main mechanism behind the transformation of the polarization hysteretic behavior and an increase in the remanent polarization value upon the alternating current cycling is electrically induced domain de-pinning. PFM imaging and local spectroscopy revealed asymmetric switching in the La:HfO2 capacitors due to a significant imprint likely caused by the different boundary conditions at the top and bottom interfaces. Domain switching kinetics can be well-described by the nucleation limited switching model characterized by a broad distribution of the local switching times. It has been found that the domain velocity varies significantly throughout the switching process indicating strong interaction with structural defects.