Microscopic Domain Structure Research Articles

Evolution of microscopic domain structure and macroscopic properties of (1-x)(Bi0.5Na0.5)TiO3-xBaTiO3 ceramic coatings

Sodium bismuth titanate-based ferroelectric materials have a promising application in modern microdevices due to their strong ferroelectricity and high Curie temperature. The domain structure plays a crucial role in the piezoelectric and ferroelectric properties of (1-x)(Bi0.5Na0.5)TiO3-xBaTiO3 ((1-x)BNT-xBT)(x=0, 0.06, 0.08, 0.10) ceramic coatings. In this paper, (1-x)BNT-xBT ceramic coatings were prepared by supersonic plasma spraying. Piezoresponse-force microscopy (PFM) was employed to gain insight into the microscopic domain evolution of (1-x)BNT-xBT ceramic coatings. Piezoelectric-force Microscopy (PFM) results show that the 0.92BNT-0.08BT ceramic coating exhibits relatively homogeneous striped domains and island domains, as well as a maximum amplitude response with an average amplitude intensity of 67.6 pm, which increases the localized piezoelectric response of the ceramic coating. The switching spectroscopy piezoelectric-force microscopy (SS-PFM) results show that the local piezoelectric coefficient (PRmax) of the 0.92BNT-0.08BT ceramic coating reaches 426.5 pm/V at 15 V, and the ferroelectric domains exhibit significant switching behavior. This study provides new ideas to further understand the relationship between microscopic electrical domains and macroscopic properties of these important lead-free piezoelectric ceramics.

Magneto-mechanical coupling effect of ferromagnetic materials: Test characteristics and theoretical model

The magneto-mechanical coupling effect of ferromagnetic materials is the result of macroscopic influence of mechanical properties on magnetic properties under the combined action of stress field and applied magnetic field, which essentially can be attributed to the change of material properties caused by the transformation of microscopic magnetic domain structures. The effect of applied magnetic field is mainly manifested as the change of magnetic moment cluster (direction and size) inside microscopic magnetic domain; The stress field mainly causes the movement and rotation of the domain walls inside the ferromagnetic material, and further causes the change of magnetic moment cluster. Taking Q235 low carbon steel as an example, the effects of tensile stress σt on some important magnetic parameters of ferromagnetic materials under different excitation intensities (i.e., H0), such as the maximum value of applied magnetic field Hmax, intrinsic coercive field Hcj, hysteresis loss power per unit volume ph and anhysteretic magnetization Man, were obtained by setting macroscopic magneto-mechanical coupling test and fully considering the influence of demagnetizing field Hd (The standard ring specimen was used for demagnetization correction of the non-standard strip specimen). Meanwhile, the basic magnetization characteristics of ferromagnetic materials were discussed by introducing the basic magnetization curve F(H0, Ban) and considering the effect of σt as the equivalent stress field Hσ based on the theory of microscopic magnetic domain and the principle of thermodynamic. In the range of 635.4 A/m ≤ H0 ≤ 928.6 A/m and 0 MPa ≤ σt ≤ 350 MPa, the theoretical calculation models Man(H0, σt) and Hσ(H0, σt) in the magneto-mechanical coupling effect were obtained, which are in good agreement with the experimental results.

The Effect of Size and Strain on Micro Stripe Magnetic Domain Structure of CoFeB Thin Films

The prerequisite for flexible magnetic electronic devices is the knowledge of the preparation technology of flexible magnetic films and the evolution of the film properties under strain. In this work, CoFeB amorphous ferromagnetic films with stripe domains were prepared on flexible polyimide (PI) substrates by oblique sputtering. The results show that oblique sputtering induces the formation of columnar crystal structure in CoFeB films, which increases the perpendicular magnetic anisotropy of the films, thus leading to the appearance of stripe magnetic domain structures. On this basis, the CoFeB films with stripe domains were processed on a microscopic scale to investigate the size effect and strain regulation on the microscopic domain structure of the magnetic films. The characterization of the magnetic domain structure shows that the stripe domain contrast is reduced by the striped structure prepared by lithography. The triangular, circular and ring patterns deflect the alignment of the stripe domain to different degrees. The experimental results show that the deflection of the stripe domains is caused by the anisotropy of the shapes produced by the different patterns and that the size of the microstructure needs to be close to the period of the stripe domains for the size effect to be significant. In addition, the strain-induced magnetoelastic anisotropy effectively rotates the orientation of the stripe domains, and the variation in domain contrast demonstrates that tensile/compressive strains vary the magnitude of the out-of-plane stray field of the film. Our results provide some insight into the modulation of the physical properties of flexible magnetic films.

High-Throughput Scanning Second Harmonic Generation Microscopy for Polar Materials.

The Materials Genome Initiative aims to discover, develop, manufacture, and deploy advanced materials at twice the speed of conventional approaches. To achieve this, high-throughput characterization is essential for the rapid screening of candidate materials. In this study, we developed a high-throughput scanning second harmonic generation (HTS-SHG) microscope with automatic partitioning, accurate positioning, and fast scanning that can rapidly probe and screen polar materials. Using this technique, we first investigated typical ferroelectrics, including periodically poled lithium niobate crystals and PbZr0.2 Ti0.8 O3 (PZT) thin films, whereby the microscopic domain structures were clearly revealed. We then applied this technique to a compositional-gradient (100-x)%BaTiO3 -x%SrTiO3 film and a thickness-gradient PZT film to demonstrate its high-throughput capabilities. Since SHG signal is correlated with the macroscopic remnant polarization over the probed region determined by the laser spot, it is free of artifacts arising from leakage current and electrostatic interference, while materials symmetries and domain structures must be carefully considered in the data analysis. We believe this work can help promote the high-throughput development of polar materials, and contribute to Materials Genome Initiative. This article is protected by copyright. All rights reserved.

Property enhancement in relaxor-PbTiO3 single crystals by alternating current poling: Evaluation of intrinsic and extrinsic contributions

Piezoelectric activity has always been the most concerned performance for PbTiO3-based relaxor ferroelectrics. Recently, the alternating current poling (ACP) method has been developed as a simple and economical technique to optimize piezoelectric performance. However, the mechanism of the ACP method is still debatable. In this work, we carried out the ACP on the 0.27Pb(In1/2Nb1/2)O3-0.46Pb(Mg1/3Nb2/3)O3-0.27PbTiO3 single crystal, a noticeable piezoelectric improvement of 20% has been achieved in comparison with the direct current poled sample, meanwhile, the dielectric loss is reduced by 26%. We found that the ACP sample demonstrates a uniform domain pattern with a large domain size dominated by 109° domain walls, in contrary to the complicated domain structures which contain both 109° and 71° domains in the DCP sample. With this uniform and simple domain structure, the crystal lattice is weakly constrained and very susceptive to external stimulation, resulting in enhanced piezoelectric response in ACP poled sample. Meanwhile, dielectric loss is reduced for the low domain wall density. The current work gives a comprehensive description of the ACP poled samples, including the microscopic domain structure, the quantitative estimation of contributions arise from lattice deformation and domain wall motions, and reveals the interactions between domain structure and the macro performance, which provide important guidelines for the design of high-performance ferroelectrics utilizing domain tailoring techniques.

New Role of Relaxor Multiphase Coexistence in Potassium Sodium Niobate Ceramics: Reduced Electric Field Dependence of Strain Temperature Stability.

The influence of relaxor behavior on strain behavior is less investigated in potassium sodium niobate [(K, Na)NbO3, KNN] ceramics. Here, we report novel phenomena in the temperature-dependent strain behavior with the electric field of KNN-based ceramics with relaxation characteristics. The strain temperature stability is electric field dependent below the threshold electric field: temperature-dependent strain can be effectively improved by increasing the applied electric fields, while it remains almost electric field independent above the threshold electric field. Such a macroscopic property change can be well consistent with the following microscopic domain structure evolution. Little voltage dependence is found above a certain voltage by employing voltage-dependent piezoresponse hysteresis loops and domain switching under different temperatures, implying the contribution of domain behavior to the change of strain. Ergodic polar nanoregions (PNRs) are induced by the high-density domain walls among nanodomains in the relaxor samples, as revealed by the atomic-resolution polarization mapping with Z-contrast. The facilitated domain switching due to the lowered energy barrier and nearly vanished polarization anisotropy based on the PNRs with nanoscale multiphase coexistence can promote the electric field compensation for temperature effect. This work demonstrates the contribution of relaxor behavior to the electric field dependence of strain temperature stability in KNN-based ceramics.

Transport abnormity and its modulations via gating effect and light illumination at the SrNbO3/SrTiO3 interface

LaAlO3/SrTiO3-based two-dimensional electron gas (2DEG) has been extensively studied because of its intriguing physical properties and potential application prospect. However, seldom researches have related their extraordinary macroscopic transport phenomena to the microscopic domain structure of SrTiO3. This requires some unique technique like scanning superconducting quantum interference device (SQUID) microscopy. In this work, we developed a different 2DEG system at the interface of SrNbO3 thin film and SrTiO3. Using only the electrical methods, we found a pronounced hysteresis behavior in the resistance versus temperature curves, marked by the appearance/disappearance of two resistance peaks in the heating/cooling process. In sharp contrast to the conventional gate effect, the resistance peak grows under positive electric biases applied to backgate with conducting interface being grounded. In addition, a weak light (0.04 mW, 405 nm) can completely eliminate the two resistance anomalies. After a systematic analysis, we attribute the resistance anomaly to the cubic-tetragonal transition of bulk SrTiO3 and surface SrTiO3. The present work presents a promising demonstration to get mesoscopic information on oxide interface via transport behaviors.

Microscopic Manipulation of Ferroelectric Domains in SnSe Monolayers at Room Temperature.

Two-dimensional (2D) van der Waals ferroelectrics provide an unprecedented architectural freedom for the creation of artificial multiferroics and nonvolatile electronic devices based on vertical and coplanar heterojunctions of 2D ferroic materials. Nevertheless, controlled microscopic manipulation of ferroelectric domains is still rare in monolayer-thick 2D ferroelectrics with in-plane polarization. Here we report the discovery of robust ferroelectricity with a critical temperature close to 400 K in SnSe monolayer plates grown on graphene and the demonstration of controlled room-temperature ferroelectric domain manipulation by applying appropriate bias voltage pulses to the tip of a scanning tunneling microscope (STM). This study shows that STM is a powerful tool for detecting and manipulating the microscopic domain structures in 2D ferroelectric monolayers, which are difficult for conventional approaches such as piezoresponse force microscopy, thus facilitating the hunt for other 2D ferroelectric monolayers with in-plane polarization with important technological applications.

Composition-dependent phase evolution and enhanced electrostrain properties of (Bi0.5Na0.5)TiO3–BaTiO3–Bi(Li0.5Ta0.5)O3 lead-free ceramics

Exploring lead-free materials to substitute the commercialized PZT-based compounds is in great demand for the development of miniaturized and nonhazardous actuator devices. Here, a new ternary lead-free (1-x)(Bi0.5Na0.5)1-yBayTiO3-xBi(Li0.5Ta0.5)O3 (BNBTy-xBLT) solid solution system is designed to optimize the electrostrain performance of the well-studied BNT ceramics, and the effects of BLT and Ba substitutions on phase evolution as well as electrical performances of BNT ceramics are systematically investigated. All the compositions prepared by solid-state sintering process exhibit similar pseudocubic phase with rhombohedral/tetragonal distortions in nanoregions. Through optimizing the BLT and Ba contents, the composition with 6.9 mol% Ba and 0.8 mol% BLT exhibits the maximum unipolar strain of ∼0.39% (equivalent normalized strain d33*∼654 pm/V, 60 kV/cm). Moreover, the origin of the excellent electrostrain response is emphatically analyzed from microscopic (domain structure evolution) and macroscopic (TF-R shifting) perspectives, which results from the electric-field-induced reversible relaxor-ferroelectric phase transition. These results suggest the composition-optimized BNT-BT-BLT system as a potential alternate to lead-based systems in actuator applications.

Ferroelectric critical size and vortex domain structures of PbTiO3 nanodots: A density functional theory study

The ferroelectric critical size and microscopic domain structure of PbTiO3 nanodots with unit cells of N × N × N (N = 1–3) have been investigated by ab initio (first-principles) density functional theory calculations. Nanodots with PbO and TiO surface terminations are investigated, and the ground state of TiO-terminated nanodots is found to be paraelectric regardless of the size. However, for PbO-terminated nanodots, the ferroelectric state is energetically favorable even in the smallest nanodot, indicating the absence of an intrinsic critical size for ferroelectricity in the nanodot structure. Moreover, the distributions of polarizations in nanodots with different sizes are analyzed. The vortex polarizations rotating around both the central [001] axis and diagonal [11¯1] directions of nanodots can stably exist. The vortex polarization arises from the opposite rotation between the cations and anions around the [001] and the [11¯1] directions of nanodots, respectively. On the other hand, the toroidal moments of vortex polarizations both around the [001] and [11¯1] directions increase with the increment of nanodot size, and these vortex polarizations are energetically favorable in small and large nanodots, respectively.

The Rayleigh law in silicon doped hafnium oxide ferroelectric thin films

A wealth of studies have confirmed that the low‐field hysteresis behaviour of ferroelectric bulk ceramics and thin films can be described using Rayleigh relations, and irreversible domain wall motion across the array of pining defects has been commonly accepted as the underlying micro‐mechanism. Recently, HfO2 thin films incorporated with various dopants were reported to show pronounced ferroelectricity, however, their microscopic domain structure remains unclear till now. In this work, the effects of the applied electric field amplitude, frequency and temperature on the sub‐coercive polarization reversal properties were investigated for 10 nm thick Si‐doped HfO2 thin films. The applicability of the Rayleigh law to ultra‐thin ferroelectric films was first confirmed, indicating the existence of a multi‐domain structure. Since the grain size is about 20–30 nm, a direct observation of domain walls within the grains is rather challenging and this indirect method is a feasible approach to resolve the domain structure. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

Phase field modeling of flexoelectricity in solid dielectrics

A phase field model is developed to study the flexoelectricity in nanoscale solid dielectrics, which exhibit both structural and elastic inhomogeneity. The model is established for an elastic homogeneous system by taking into consideration all the important non-local interactions, such as electrostatic, elastic, polarization gradient, as well as flexoelectric terms. The model is then extended to simulate a two-phase system with strong elastic inhomogeneity. Both the microscopic domain structures and the macroscopic effective piezoelectricity are thoroughly studied using the proposed model. The results obtained show that the largest flexoelectric induced polarization exists at the interface between the matrix and the inclusion. The effective piezoelectricity is greatly influenced by the inclusion size, volume fraction, elastic stiffness, and the applied stress. The established model in the present study can provide a fundamental framework for computational study of flexoelectricity in nanoscale solid dielectrics, since various boundary conditions can be easily incorporated into the phase field model.

Magnetic force microscopy using tip magnetization modulated by ferromagnetic resonance

In magnetic force microscopy (MFM), the tip–sample distance should be reduced to analyze the microscopic magnetic domain structure with high spatial resolution. However, achieving a small tip–sample distance has been difficult because of superimposition of interaction forces such as van der Waals and electrostatic forces induced by the sample surface. In this study, we propose a new method of MFM using ferromagnetic resonance (FMR) to extract only the magnetic field near the sample surface. In this method, the magnetization of a magnetic cantilever is modulated by FMR to separate the magnetic field and topographic structure. We demonstrate the modulation of the magnetization of the cantilever and the identification of the polarities of a perpendicular magnetic medium.

Complete vertical M-H loop shift in La0.7Sr0.3MnO3/SrRuO3 thin film heterostructures

In the current work, we have epitaxially integrated La0.7Sr0.3MnO3/SrRuO3 (LSMO/SRO) BLs with the technologically important substrate Si (100) using pulsed laser deposition. Interestingly, at 4 K, under the magnetic field sweep of ±1500 Oe, a complete vertical M-H loop shift is observed in the sample prepared with 180 nm SRO thickness, which is unusual. This vertical shift persists even up to a field sweep range of ±6000 Oe, at which point the shift disappears and a symmetrical hysteresis loop centered at the origin is observed. In contrast, at the same temperature, under the same field sweep range, we observe a normal M-H loop (no or little vertical shift) from the sample with 45 nm SRO thickness. In both the cases, the LSMO thickness was held constant at ∼100 nm. It appears that SRO moment is frozen in place in the latter case, providing a clear demonstration of the effect that biasing layer (SRO) thickness can have on the magnetic characteristics of bilayer films. We attribute this vertical shift to the strong interplay between the uniaxial magnetocrystalline anisotropy and microscopic interface domain structure.

Dual-tip magnetic force microscopy with suppressed influence on magnetically soft samples

Standard magnetic force microscopy (MFM) is considered as a powerful tool used for magnetic field imaging at nanoscale. The method consists of two passes realized by the magnetic tip. Within the first one, the topography pass, the magnetic tip directly touches the magnetic sample. Such contact perturbs the magnetization of the sample explored. To avoid the sample touching the magnetic tip, we present a new approach to magnetic field scanning by segregating the topological and magnetic scans with two different tips located on a cut cantilever. The approach minimizes the disturbance of sample magnetization, which could be a major problem in conventional MFM images of soft magnetic samples. By cutting the cantilever in half using the focused ion beam technique, we create one sensor with two different tips—one tip is magnetized, and the other one is left non-magnetized. The non-magnetized tip is used for topography and the magnetized one for the magnetic field imaging. The method developed we call dual-tip magnetic force microscopy (DT-MFM). We describe in detail the dual-tip fabrication process. In the experiments, we show that the DT-MFM method reduces significantly the perturbations of the magnetic tip as compared to the standard MFM method. The present technique can be used to investigate microscopic magnetic domain structures in a variety of magnetic samples and is relevant in a wide range of applications, e.g., data storage and biomedicine.

AC driven magnetic domain quantification with 5 nm resolution.

As the magnetic storage density increases in commercial products, e.g. the hard disc drives, a full understanding of dynamic magnetism in nanometer resolution underpins the development of next-generation products. Magnetic force microscopy (MFM) is well suited to exploring ferromagnetic domain structures. However, atomic resolution cannot be achieved because data acquisition involves the sensing of long-range magnetostatic forces between tip and sample. Moreover, the dynamic magnetism cannot be characterized because MFM is only sensitive to the static magnetic fields. Here, we develop a side-band magnetic force microscopy (MFM) to locally observe the alternating magnetic fields in nanometer length scales at an operating distance of 1 nm. Variations in alternating magnetic fields and their relating time-variable magnetic domain reversals have been demonstrated by the side-band MFM. The magnetic domain wall motions, relating to the periodical rotation of sample magnetization, are quantified via micromagnetics. Based on the side-band MFM, the magnetic moment can be determined locally in a volume as small as 5 nanometers. The present technique can be applied to investigate the microscopic magnetic domain structures in a variety of magnetic materials, and allows a wide range of future applications, for example, in data storage and biomedicine.

Complex structural-ferroelectric domain walls in thin films of hexagonal orthoferrites RFeO3 (R = Lu, Er)

Hexagonal orthoferrites have recently attracted much attention as possible high-temperature ferromagnetic ferroelectrics. The ferroelectric domain structure of hexagonal RMnO3, their antiferromagnetic structural analogies, has recently shown an atypical and complicated behavior. Hexagonal RFeO3 are expected to exhibit similar domain structure that should coexist with weak ferromagnetic order and may represent a material with a unusual magnetoelectric interaction. In this report, we discuss microscopic ferroelectric domain structure of hexagonal orthoferrites in a thin-film state and demonstrate a distinct and unusual improper ferroelectric behavior of these oxide materials.

Magnetoelectric effect driven by magnetic domain modification in LuFe2O4.

The magnetocapacitance effect was investigated using impedance spectroscopy on single crystals of LuFe(2)O(4). The intrinsic impedance response could be separated from the interfacial response and showed a clear hysteresis loop below T(Ferri)∼240 K under the magnetic field. The neutron diffraction experiment under the magnetic field proves the origin of the dielectric property related to the motion of the nanosized ferromagnetic domain boundary. These results imply that the modification of the microscopic domain structure is responsible for the magnetoelectric effect in LuFe(2)O(4).

Visualization of magnetic domain structure changes induced by interfacial strain in CoFe2O4/BaTiO3 heterostructures

We visualized the evolution of the magnetic domain structure in CoFe2O4 films on different structural phases of BaTiO3 substrates with {0 0 1} orientation, using variable temperature magnetic force microscopy. When BaTiO3 underwent transitions from rhombohedral to orthorhombic to and from orthorhombic to tetragonal structures with increasing temperature, only local variations to the magnetic domains were observed. At the BaTiO3 tetragonal–cubic transition however, the magnetic domain size increased by 75% with an overall decrease in stray field contrast (33% decrease) because of reorientation of the magnetization to the in-plane directions. The reorientation of magnetization during the tetragonal to cubic phase transition of BaTiO3 was induced by release of asymmetric interfacial strain between the CoFe2O4 film and the BaTiO3 substrate, as non-uniformly distributed a and c surface ferroelectric domains in the BaTiO3 tetragonal phase disappeared at the paraelectric BaTiO3 cubic phase transition. Strain analysis based on macroscopic magnetization measurements was correlated with the microscopic magnetic domain structure to help understand the coupling in two-phase multiferroicheterostructure mediated by interfacial strain.

Ferroelectric domain structure of highly textured BiFeO3 microcrystal films prepared by hydrothermal method

Microcrystal BiFeO3 films have been prepared on Nb doped SrTiO3 substrate with the hydrothermal method. The microcrystals in the film are highly textured and c-oriented with the (101)/(011) planes as their preferential side growth facets. These crystal facets intersect the film plane along either x or y direction. Microscopic domain structures are studied by combining the information of both in-plane and out-of-plane piezoresponse microscopy (PFM) measurements. Both 71° and 109° domain walls have been observed. Ordered stripe domains are found in the valley areas between two adjacent grains, which are ascribed to the compressive strain exerted during growth by the neighboring grains.