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Magnetic properties of a BiFe0.98Co0.02O3//LaAlO3 film (without LSMO layer). (a) Magnetization M(H) loop indicating low saturation moment of ∼4 emu/cm³. XAS and XMCD spectra of Fe (b) and Co (c) showing the L2,3 electron edges and no signal in XMCD.
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We present a comprehensive study of the physical properties of epitaxial cobalt-doped BiFeO3 films ∼50 nm thick grown on (001) LaAlO3 substrates. X-ray diffraction and magnetic characterization demonstrate high quality purely tetragonal-like (T′) phase films with no parasitic impurities. Remarkably, the step-and-terrace film surface morphology can...
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The ferroelectric phase of HfO2 is generally stabilized in polycrystalline films, which typically exhibit the highest polarization when deposited using low oxidizing conditions. In contrast, epitaxial film grown by pulsed laser deposition show low or suppressed polarization if low oxygen pressure is used. Epitaxial films are essential to better und...
Citations
... In the following work, the "dopant" atoms considered are Mn and La, simulating widely observed interdiffusion across a La 1−x Sr x MnO 3 (LSMO) interface, commonly used as a bottom electrode for BFO thin films. 23,33,34 The same interface is also studied experimentally in Section 4.6 to illustrate a number of features observed in the theoretical calculations. ...
The role of local chemical environments in the electron energy loss spectra of complex multiferroic oxides was studied using computational and experimental techniques. The evolution of the O K-edge across an interface between bismuth ferrite (BFO) and lanthanum strontium manganate (LSMO) was considered through spectral averaging over crystallographically equivalent positions to capture the periodicity of the local O environments. Computational techniques were used to investigate the contribution of individual atomic environments to the overall spectrum, and the role of doping and strain was considered. Chemical variation, even at the low level, was found to have a major impact on the spectral features, whereas strain only induced a small chemical shift to the edge onset energy. Through a combination of these methods, it was possible to explain experimentally observed effects such as spectral flattening near the interface as the combination of spectral responses from multiple local atomic environments.
... These films routinely display an increase in mixed-phase population upon increasing film thickness, driven by strain relaxation. Interestingly, in some cases where defects are incorporated throughout the film volume, the formation of the mixed phase can be completely inhibited [20][21][22]. This complex equilibrium provides a wide scope of stimuli which can locally tailor the functionality of the system. ...
In complex oxide heteroepitaxy, strain engineering is a powerful tool to obtain phases in thin films that may be otherwise unstable in bulk. A successful example of this approach is mixed-phase bismuth ferrite (BiFeO 3) epitaxial thin films. The coexistence of a tetragonal-like (T-like) matrix and rhombohedral-like (R-like) striations provides an enhanced electromechanical response, along with other attractive functional behaviors. In this paper, we compare the energetics associated with two thickness-dependent strain relaxation mechanisms in this system: domain walls arising from monoclinic distortion in the T-like phase, and the interphase boundary between the host T-like matrix and tilted R-like phases. Combining x-ray-diffraction measurements with scanning probe microscopy, we extract quantitative values using an empirical energy balance approach. The domain wall and phase-boundary energies are found to be 113 ± 21 and 426 ± 23 mJ m-2 , respectively. These numerical estimates will help us realize designer phase boundaries in multiferroics, which possess colossal responses to external stimuli, attractive for a diverse range of functional applications.
... In previous work by our group, thin-film defect-engineered BiFeO 3 25 was shown to possess a highly robust polarization for nanoscale domain structures <100 nm enabled by "designer defects." 26,27 The introduction of defects is realized by taking advantage of the unique geometry of the pulsed laser deposition (PLD) chamber, which allows a low incident flux rate and thus further drives the formation of nanoscale defective nanoregions due to the volatile nature of bismuth. The stacking-fault-like secondary defective regions as shown by the high-resolution transmission electron microscopy (TEM) images in the previous reports is suggested to be composed of tetragonal-phase β-Bi 2 O 3 based on the TEM and θ-2θ X-ray diffraction (XRD) results. ...
Robust retention of ferroelectric polarization in harsh environments is a requirement for the application of ferroelectric materials in space, liquids, and various chemical conditions. Surface screening of the polarization can significantly alter domain states and usually has a strong influence on domain stability in ferroelectrics, hindering applications that require defined and non-volatile polarization states. Here, we show that designer defects in BiFeO3 can be engineered to strongly pin domain walls, which even in harsh environments such as 100% humidity and elevated temperatures of 175 °C leads to a superior polarization retention of several years for domain sizes well below 100 nm.
... This may be attributed to the Schottky barrier height reduction with the increasing T′-phase fraction in mixed-phase BFO films, where an enlarged band gap is induced because of this strained T′-phase structure. 27,69 This could be also related to the oxygen octahedral tilt and, further, the variation of the bond angle and orbital overlapping from this strained nonequilibrium T′-BFO. Besides, strain gradients at the R′−T′-phase boundaries can also redistribute the nonequilibrium carriers excited by an external stimulus, such as the photon. ...
Phase-pure epitaxial bismuth ferrite (BiFeO3, BFO) thin films with a homogeneous mixed-phase structure were synthesized on (001)-oriented lanthanum aluminate (LaAlO3, LAO) substrates using chemical solution deposition. The phase development of the BFO thin film and its leakage current characteristics have been systematically investigated as a function of thickness (number of spin-coated layers) and the heat treatment process (heating temperature and dwell time). The results show that the tetragonal-like (T′) phase fraction changes dramatically from 35% (45 nm thick single layer) to 10% (250 nm thick four-layer films). In a two-layer film (80 nm) configuration, the T′-phase fraction was further tuned. When annealing at 640 °C for 30 min, this mixed-phase BFO film, despite its high T′-phase fraction (28%), shows the lowest leakage current (<0.1 A/cm² at <500 kV/cm), comparable to 200 nm pulsed laser deposition-grown pure R-BFO thin films. In contrast to the observed bulk-limited Ohmic or space-charge-limited-conduction (SCLC)-predominant mechanism in pure R′-phase and low T′-phase fraction BFO thin films, the high T′-phase fraction (∼28%) mixed-phase BFO film displays an interface-limited Schottky emission to an SCLC mechanism transition with increasing electric field.
... However, the poling process results in the formation of different surface charges, including ferroelectric polarization charges, screening charges 17,18 and injection charges. 19,20 As is known, polarization charges and screening charges play an important role in the process of polarization switching, which determines the stability of ferroelectric domain structures 21,22 and further affects data storage density and signal stability. 23,24 In the past few years, some experimental and theoretical works have been reported on the effects of the domain structure and polarization charges and screen charges on the surface potential of materials. ...
The origin of an injected charge and its temperature dependence in ferroelectric Pb(Zr0.52Ti0.48)O3 (PZT) thin films is studied by multimode scanning probe microscopy. During the poling process in scanning probe microscope (SPM) measurement, which is a local bias applied by using a conductive tip on a film’s surface to induce polarization orientation, a strong charge injection is always observed in oxide ferroelectric films; therefore, the surface potential is dominated by injection charge rather than polarization and screening charge. The surface potential shows an increase with the increase in the applied bias and saturation at a higher bias, which is much higher than the coercive field in PZT films. The positive surface potential shows a clear increase after oxygen plasma treatment, suggesting that the injection behavior is significantly enhanced. Subsequent heating could recover the surface condition to the initial state. Charge injection could be weakened but could not be completely eliminated by heat treatment. The current results suggest that charge injection behavior could not be easily relaxed, and a careful control of the localized poling process using an SPM conductive tip is required especially for studying the charge state on the surfaces of ferroelectric thin films.
... Thin films of 2% cobalt-doped T' phase BFO thin films with a thickness of 60 nm were grown on a (001) LAO substrates by pulsed laser deposition, with a 3 nm-thick La 0.67 Sr 0.33 MnO 3 (LSMO) bottom electrode inserted between the substrate and the film (see Methods). The clear atomic steps in a representative topography image in Fig. 1a show a high-quality surface of the thin film and no R-like striped domains are found, consistent with previous studies on similar films 44,45 . Figure 1b presents the corresponding IP piezoresponse force microscopy (PFM) phase image. ...
... Figure 1b presents the corresponding IP piezoresponse force microscopy (PFM) phase image. The striped IP domains oriented along <110> -type directions indicate a monoclinic structure (M c ) [46][47][48] , in agreement with previous X-ray diffraction (XRD) reciprocal space mapping (RSM) data 44 . In Fig. 1c, the high-angle XRD θ−2θ scan exhibits only (00 l) peaks yielding an out-of-plane (OOP) lattice parameter c = 4.67 Å. ...
Ferroelectric materials possess a spontaneous polarization that is switchable by an electric field. Robust retention of switched polarization is critical for non-volatile nanoelectronic devices based on ferroelectrics, however, these materials often suffer from polarization relaxation, typically within days to a few weeks. Here we exploit designer-defect-engineered epitaxial BiFeO3 films to demonstrate polarization retention with virtually no degradation in switched nanoscale domains for periods longer than 1 year. This represents a more than 2000% improvement over the best values hitherto reported. Scanning probe microscopy-based dynamic switching measurements reveal a significantly increased activation field for domain wall movement. Atomic resolution scanning transmission electron microscopy indicates that nanoscale defect pockets pervade the entire film thickness. These defects act as highly efficient domain wall pinning centres, resulting in anomalous retention. Our findings demonstrate that defects can be exploited in a positive manner to solve reliability issues in ferroelectric films used in functional devices. The use of ferroelectric materials in memory device applications is held back by low retention times. Here, the authors demonstrate that by intentionally introducing defective nanoregions which increase the activation field for domain wall motion, retention times larger than a year can be achieved.
... However, increasing the thickness to greater than ∼30 nm generally induces a strain relaxation mechanism, whereby a complex phase mixture consisting of the metastable TA and a more stable RA-like phase (thus named owing to its similarity in structure to the bulk rhombohedral BFO phase) is formed. 22) This mixed-phase ensemble has been shown to have markedly enhanced piezoresponses, 24) bulk photovoltaic responses, 25) interesting magnetic properties, 26) and in addition, through the electric-field-induced interconversion of mixed-phase regions into pure TA, 27) the mixedphase BFO shows attractive electrochromic properties. 13) Thus far, the phase-field control of various isomorphs of BFO and the tuning of their proportions have been achieved through modification of physical parameters. ...
The stability of the "T-like" (T') phase in BiFeO3 films grown on LaAlO3(001) is investigated. We show that the T' phase can be stabilized for thicknesses >70 nm under ultralow incident flux conditions in pulsed laser ablation growth. This low flux results in a low growth rate; thus, the sample is held at high temperatures (>600 °C) for much longer than is typical. Transmission electron microscopy and X-ray diffraction analysis suggest that such growth conditions favor the formation of nanoscale "defect pockets", which apply a local compressive strain of ~1.8%. We propose that the cumulative effect of local stresses induced by such "designer defects" maintains macroscale strain coherence mechanical boundary conditions, which then preserves the T' phase to thicknesses beyond conventional wisdom. Finally, by intentionally introducing an amorphous phase at the film-substrate interface, it is shown that the mixed-phase proportion can be tuned for a given thickness.
In ferroic materials, giant susceptibilities can be realized at artificially constructed phase boundaries through deterministic manipulation of the order parameter. Here, emergent ferroelectric structural phase evolution behavior is demonstrated through a synergistic combination of A‐site doping and strain engineering. Using chemical solution deposition derived (001)‐oriented Sm‐substituted bismuth ferrite (Bi1‐xSmxFeO3) films as a prototypical system, a morphotropic phase boundary comprising a coexistence of four distinct crystallographic phases is uncovered. These ferroelectric, polar, and nonpolar phases form a nanoscale mixture without the presence of crystallographically hard boundaries. The system thus possesses the ability to show both polarization rotation and extension, effectively releasing the polarization from its crystallographic constraint. Consequently, both robust ferroelectric properties and giant electromechanical responses are obtained. For instance, the optimized composition with x = 0.14 has a remnant polarization of 2Pr = 103 µC cm⁻² and electromechanical response 175% that of undoped BFO. These findings showcase the tremendous potential of synthetic phase boundaries, particularly in the context of lead‐free functional multiferroics.
Mimicking and replicating the function of biological synapses with engineered materials is a challenge for the 21st century. The field of neuromorphic computing has recently seen significant developments, and new concepts are being explored. One of these approaches uses topological defects, such as domain walls in ferroic materials, especially ferroelectrics, that can naturally be addressed by electric fields to alter and tailor their intrinsic or extrinsic properties and functionality. Here, we review concepts of neuromorphic functionality found in ferroelectric domain walls and give a perspective on future developments and applications in low-energy, agile, brain-inspired electronics and computing.
Room-temperature out-of-plane two-dimensional ferroelectrics have promising applications in miniaturized non-volatile memory appliances. The feasible manipulation of polarization switching significantly influences the memory performance of ferroelectrics. However, conventional high-voltage-induced polarization switching inevitably generates charge injection or electric breakdown, and large-mechanical-loading-induced polarization switching may damage the structure of ferroelectrics. Hence, decreasing critical voltage/loading for ferroelectric polarization reversal is highly required. Herein, using atomic force microscopy experiments, the ferroelectric domain switching via both electric field and mechanical loading was demonstrated for an ultrathin (∼4.1 nm) CuInP2S6 nanoflake. The relevant threshold voltage/loading for polarization switching was ∼ -5 V/1095 nN, resulting from the electric field and flexoelectric effect, respectively. Finally, the electrical-mechanical coupling was adopted to reduce the threshold voltage/loading of CuInP2S6 significantly. It can be explained by the Landau-Ginzburg-Devonshire double-well model. This effective way for easily tuning the polarization states of CuInP2S6 opens up new prospects for mechanically written and electrically erased memory devices.
