Tetragonal-like Phase Research Articles

Structure evolution and energy band modulation in Ba-doped BiFeO3 thin films

Bi1−xBaxFeO3 (BBFO, x = 0, 0.03, 0.1) thin films were epitaxially grown on SrRuO3-buffered SrTiO3 (001) substrates by pulsed laser deposition. With increasing Ba content, the BBFO thin films show significantly reduced leakage currents but suppressed ferroelectric polarization. X-ray diffraction reciprocal space mappings and Raman spectra indicate a structural evolution from a rhombohedral-like to tetragonal-like phase in the BBFO thin films. Optical absorption and photoelectron spectroscopy measurements demonstrate a modulation of energy band structures in the BBFO thin films. With A-site Ba acceptor doping, the BBFO thin films exhibit a blue-shift of optical bandgap and an increase in work function. The energy positions of conduction and valence bands of the BBFO thin films have been modulated, and the Fermi level shifts down to the center of the forbidden band, but acceptor-doped BFO thin films still show n-type conduction. The presence of extra oxygen vacancies by acceptor doping is supposed to make contribution to conduction behavior. This study provides a method to manipulate the functional properties and gives insights into the physics of Ba doping in BFO thin films.

Phase Conductance of BiFeO3 Film.

In this work, the local conductance of the tetragonal-like (T-like) and rhombohedral-like (R-like) phases of epitaxial BiFeO3 film is systematically studied via conductive atomic force microscopy. At higher tip voltage, there is a mutual transition between the T-like and R-like phases, which could be attributed to the strain relaxation in the T-like phase induced by electric poling, as well as local polarization switching. The T-like phase exhibits a higher conductance, which is related to the lower interface potential barrier between the tip and film surface. Reversible low- and high-current states in the T-like phase can be tuned by polarization switching. These results will be helpful for designing novel nanoelectronic devices, such as voltage and strain sensors.

Film thickness dependence of in-plane ferroelastic domain structure in constrained tetragonal PbTiO3 films induced by isotropic tensile strain

In this study, the ferroelectric phase of PbTiO3 (PTO) thin films was grown on cubic single-crystal KTaO3 (KTO) substrates using metal–organic chemical vapor deposition. X-ray diffraction (XRD) was used to reveal an a1/a2 domain structure, which remained unchanged down to a film thickness of 2 nm. The a1 and a2 polydomains do not have a simple tetragonal symmetry because aa∥ and aa⊥ do not have the same values. The crystallographic tilt angle, α, is defined based on the rotation angle of the PTO lattice with respect to the cubic phase of KTO substrates. The in-plane tetragonal distortion (ca∥/aa∥) and α decreased with the decrease in the film thickness, following the in-plane tetragonal geometric equation: α=tan−1(ca∥aa∥)−45°. The isotropic tensile strains induced in-plane polarization directions along the [100] and [010] axes of the substrates. These axes are formed via the a1/a2 polydomain of the tetragonal-like phase. Moreover, synchrotron in-plane grazing incidence XRD and piezoelectric force microscopy were used to reveal the thickness dependency of the periodic domain width of the ferroelastic a1/a2 domain. The periodic domain width in the PTO films decreased, following Kittel's law, with the reduction in the film thickness.

Engineering Photoresponse in Epitaxial BiFe0.5Cr0.5O3 Thin Films through Structural Distortion

Multiferroic BiFe0.5Cr0.5O3 (BFCO) thin films are promising candidates for emerging optoelectronics and all-oxide solar absorbers. Yet, a thorough understanding of the structural evolution and associated changes in the functional properties of BFCO is lacking. Here, we explore thickness-dependent structural phase transitions in the epitaxial BFCO films and ascertain the impact of the accompanying crystallographic distortions on their photoresponse. The results show that the strain imposed by the substrate changes the crystal symmetry, inducing a transition from a tetragonal-like phase to a rhombohedral-like phase through a rather complex strain relaxation mechanism upon increasing film thickness. This change in crystallographic distortion also induces a shift of ∼150 meV in the bandgap. Moreover, wavelength-resolved photocurrent measurements reveal that the absorption onset is red-shifted for the tetragonal-like structure, implying light absorption up to wavelengths of 800 nm. First-principles calculations shed further light on the symmetry-induced changes in the electronic structure of the BFCO films. The crystallographic symmetry is shown to be a decisive factor in modifying the valence band maximum and conduction band minimum characteristics in the perovskite oxides, revealing an emerging type of Mott multiferroic in the BFCO system. This work provides a practical strategy to further engineer the optoelectronic properties of the multiferroic oxide films through thickness-induced phase transitions.

Polarization in pseudocubic epitaxial relaxed PMN-PT thin films

Understanding the relationship between structural characteristics and functional properties of complex relaxor ferroelectric thin films is of high interest for designing materials with high performances. In this work, the structure of epitaxial relaxed pulsed-laser-deposited Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-xPT; x = 25, 33, and 40) thin films on LaNiO3/SrTiO3 substrates is analyzed using a variety of diffraction and spectroscopic techniques. While based on the data obtained from high-resolution x-ray diffraction and scanning transmission electron microscopy analysis, the average structure of the PMN-xPT films is metrically cubic, micro-Raman polarimetry measurements indicate the tetragonal-like ferroelectric phase with marked preference for the polarization perpendicular to the film for all three compositions. The results of the Raman scattering analysis are supported by electromechanical properties of the samples, which clearly show that the films have a locally non-centrosymmetric structure. Furthermore, only a gradual enhancement of the electrical properties from PMN-25PT to PMN-40PT is observed, which is attributed to small tetragonal distortions that are highly similar for all three compositions.

Deterministic Dual Control of Phase Competition in Strained BiFeO3: A Multiparametric Structural Lithography Approach

The realization of a mixed-phase microstructure in strained BiFeO3 (BFO) thin films has led to numerous novel effects derived from the coexistence of the tetragonal-like monoclinic phase (T phase) and rhombohedral-like monoclinic phase (R phase). Strong strain and polarization differences between the phases should result in a high level of transformation plasticity, which enables the continuous alteration of the relative proportion of R and T states in response to external forces. Although the potential for utilizing such plasticity to control mixed-phase populations under external stimuli is evident, direct experimental evidence backed by equilibrium predictions has not yet been fully demonstrated. Here we demonstrate deterministic control of mixed-phase populations in an epitaxially strained BFO thin film through the application of localized stresses and electric fields in a reversible manner. The results illustrate and rationalize deterministic control of mixed phases in strained BFO films, which could be crucial in tuning their functional properties. The findings also highlight a new multiparametric technique in the scanning probe lithography toolbox based on tip-assisted electric and strain field manipulation of functional properties that might find application beyond the ferroelectric domain and structural phase lithography.

Density-functional theory calculation of magnetic properties of BiFeO3 and BiCrO3 under epitaxial strain

This work uses density-functional theory to model the magnetic properties of bismuth-based perovskite oxides under epitaxial strain. We augment the known transition in BiFeO3 between rhombohedral-like and tetragonal-like phases occurring at 4.2% compressive epitaxial strain with the variation in magnetic behavior near this boundary. This phase boundary coincides with a transition from G- to C-type magnetic order, as well as with a 90% decrease in the magnitude of the [001]-oriented coupling coefficients. The magnitude of iron magnetization is shown to vary by no more than 3% over the entire range of compressive strain considered. In the BiCrO3 system, we report a variation in chromium magnetization of over 20%, along with transitions from bulk G-type to regions of C-type order under tensile epitaxial strain and to F-type order under both tensile and compressive epitaxial strains. The region of F-type order stabilized under compression beyond 7.9% epitaxial strain corresponds to a “super-tetragonal” phase structurally similar to the well-known phase of BiFeO3 exhibiting spontaneous polarization on the order of 150 μC/cm2.

Strain engineering of epitaxial oxide heterostructures beyond substrate limitations

Strain engineering of epitaxial oxide heterostructures beyond substrate limitations

Strong spin-lattice coupling in tetragonal-like BiFeO3 films with thermal expansion anomalies

The evidence for a spin-lattice coupling in a tetragonal-like BiFeO3 (BFO) film is derived from thermal expansion measurements. Taking the advantage of La doping, the Néel temperature (TN) of the BFO film can be further adjusted over a broad range of temperatures. The lattice parameters exhibit anomalies near the Néel temperatures. The Bi0.8La0.2FeO3 film has a spin order-to-disorder transition at the TN without a structural phase transition, suggesting that a spin-lattice coupling drives the thermal expansion anomalies. The spin-lattice coupling can be enhanced by the coexistence of phases in the thicker BFO films. The density functional theory calculations show a smaller lattice constant of the c axis in the spin-order state than that in the spin-disorder state for the same tetragonal-like phase, which supports that the thermal expansion anomalies are a consequence of the spin-lattice coupling. Our findings may have application prospects for functional materials with controllable thermal expansions.

Phase Transition-Induced Modulation of Exchange Bias in Fe/BiFeO3 Bilayers

Many of researches indicate that epitaxial BiFeO3 (BFO) films deposited on LaAlO3 (LAO) substrate undergo strain-driven phase transition from a tetragonal-like phase (T-BFO) to a rhombohedral-like phase (R-BFO), and a mixed phase (M-BFO) that T-BFO coexists with R-BFO forms in the phase transition process. It is necessary to explore how BFO phase transition affects the exchange bias in ferromagnet (FM)/BFO bilayers. In our studies, aforementioned BFO phase transition is accomplished by varying BFO films thickness. Using 5 nm-thick Fe as ferromagnetic layer deposited on 9–354 nm-thick BFO as antiferromagnetic layer, the exchange bias in Fe/BFO bilayers exhibits that Fe/M-BFO bilayers shows smaller exchange bias than Fe/T-BFO and Fe/R-BFO bilayers. We ascribed the effect of the BFO phase transition on the exchange bias to the domain walls caused by the exchange interaction between T-BFO and R-BFO across their boundaries. Additionally, for the same reason, the coercivity also exhibits the same variation trend as the exchange bias does. Our studies will help to promote the application of controlling the ferromagnetic magnetization by the electric field.

Nanosecond Optically Induced Phase Transformation in Compressively Strained BiFeO_{3} on LaAlO_{3}.

Above-band-gap optical illumination of compressively strained BiFeO_{3} induces a transient reversible transformation from a state of coexisting tilted tetragonal-like and rhombohedral-like phases to an untilted tetragonal-like phase. Time-resolved synchrotron x-ray diffraction reveals that the transformation is induced by an ultrafast optically induced lattice expansion that shifts the relative free energies of the tetragonal-like and rhombohedral-like phases. The transformation proceeds at interfaces between regions of the tetragonal-like phase and regions of a mixture of tilted phases, consistent with the motion of a phase boundary. The optically induced transformation demonstrates that there are new optically driven routes towards nanosecond-scale control of phase transformations in ferroelectrics and multiferroics.

Enhanced piezoelectric properties in LixBi1-xNbxFe1-xO3 films contributed by low-symmetry phase and self-polarization

LixBi1-xNbxFe1-xO3 films with various Li+ and Nb5+ substitutions were fabricated onto LaNiO3-buffered Pt(111)/Ti/SiO2/Si substrates through sol-gel methods. Owing to the substitution by smaller Li+ and Nb5+ for larger Bi3+ and Fe3+ respectively, the phase structure of LixBi1-xNbxFe1-xO3 films experiences a transition from rhombohedral phase (R phase, high symmetry) to tetragonal-like phase (T-like, low-symmetry) upon the chemical pressure with Li+ and Nb5+ substitution increases. Additionally, the dielectricity, ferroelectricity and leakage behaviors of substituted LixBi1-xNbxFe1-xO3 films are also significantly improved. More importantly, the substituted LixBi1-xNbxFe1-xO3 films show significantly enhanced piezoelectric responses, especially for the Li0.05Bi0.95Nb0.05Fe0.95O3 film, whose d33 with the value of 107.5 pm/V might be compared with some lead-based piezoelectric films. The enhanced piezoelectric response of the Li0.05Bi0.95Nb0.05Fe0.95O3 film is contributed by synergistic effects of low-symmetry phase, improved ferroelectricity and downward self-polarization, where the electric field-triggered R-T phase transition and polarization vectors rotation can be easily took place upon small stimuli. Additionally, the reduction of leakage currents for substituted LixBi1-xNbxFe1-xO3 films also help to improve their dielectric, ferroelectric and piezoelectric properties.

Topological ferroelectric phases by mode-selective phonon excitation

Topological structural phases (TSPs) of nontrivial winding numbers are of long-standing fundamental relevance in the theory of phase transition. However, for decades, no TSPs have been discovered in technological ferroelectric bulk solids. Here we formulate a generally applicable scheme to create TSPs in ferroelectrics. The approach consists in selectively exciting phonon modes to produce the targeted TSPs and can be employed to generate TSPs that do not naturally exist in bulk materials. We further demonstrate the effectiveness of this approach by creating in bulk ${\mathrm{SrTiO}}_{3}$ three TSP structures, namely the flower phase, the tetragonal-like vortex phase, and the orthorhombiclike vortex phase. These topological phases lead to the discovery of interesting properties, which include (i) a marked difference in the excitation stiffness of different topological phases and (ii) the existence of symmetry breaking between vortex phases.

In-situ growth of high-quality epitaxial BiFeO3 thin film via off-axis RF magnetron sputtering

This work demonstrates an in-situ heteroepitaxial growth of BiFeO3 thin film on the SrRuO3-buffered (100) SrTiO3 substrate via off-axis radio-frequency (RF) magnetron sputtering. The coexistence of a dominant rhombohedral-like phase and a small amount of tetragonal-like phase of BiFeO3 thin film with a good (100) orientation was revealed. Furthermore, the fabricated BiFeO3 thin film exhibited the enhanced electrical properties, e.g. a reduced leakage current (J < 3.2 × 10−3 A/cm2) and a low loss (tanδ < 0.087), as well as a large intrinsic polarization of 2Pr ∼ 124 μC/cm2 with a coercive field of 2Ec ∼ 408 kV/cm. This finding can provide a simple and feasible in-situ sputtering technique for the high-quality thin films.

Phase transitions in BiFeO3 nanoislands with enhanced electromechanical response

We propose a novel method to drive phase transitions by etching strained BiFeO3 thin films to nanoislands. Atomic force microscopy (AFM) measurements reveal that the amount of rhombohedral-like (R) phase increases as the BiFeO3 thin films with tetragonal-like (T) matrix are etched to nanoislands and larger fraction of R phase can be obtained with the size reduction from 1 μm to 200 nm, indicating the T to R phase transitions induced by partial release of substrate clamping. Using piezoresponse force microscopy (PFM), it is demonstrated that rhombohedral-like (R) to tetragonal-like (T) phase transitions can be reversibly achieved under DC electric field in BFO nanoislands. Large electromechanical response has been observed in BFO nanoislands as well. This approach can be extended to other strained oxide films and provide guidance for the development of high-performance electromechanical lead-free materials.

Effect of Zn and Ti Co-doping on structure and electrical properties of BiFeO3 ceramics

In this paper, the effect of Zn and Ti co-doping on the structure, dielectric, ferroelectric and magnetic properties of BiFeO3 (BFO) ceramics have been studied. Co-doped BFO ceramics with different doping concentrations are prepared by using the sol-gel method. The structure, morphology and electrical properties of co-doped BFO ceramics are studied by using X-ray diffraction (XRD), scanning electron microscopy, impedance analyzer and ferroelectric test analysis system. Furthermore, the vibrating sample magnetometer (VSM) is used to investigate the magnetic properties. The structural transition from rhombohedral to tetragonal-like phase is observed with a high level of co-doping (x = 2.5%). In addition, higher content of Zn and Ti results in enhanced dielectric constant (ε) and reduced dielectric loss (tanδ). The highest remnant polarization of 0.4 μC/cm2 and coercive electric field of 15.5 kV/cm is demonstrated at x = 2.5%. The enhanced dielectric and ferroelectric properties can be attributed to the formation of defect complexes due to the Zn and Ti substitution.

Structural, magnetic, and ferroelectric properties of T-like cobalt-doped BiFeO3 thin films

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 be fully recovered following a local electric-field-induced rhombohedral-like to T′ phase transformation. Local switching spectroscopy experiments confirm the ferroelectric switching to follow previously reported transition pathways. Critically, we show unequivocal evidence for conduction at domain walls between polarization variants in T′-like BFO, making this material system an attractive candidate for domain wall-based nanoelectronics.

Tetragonal-Like Phase in Core–Shell Iron Iron-Oxide Nanoclusters

Two sizes of iron/iron-oxide (Fe/Fe-oxide) nanoclusters (NCs) of 10 and 35 nm diameters were prepared using a cluster deposition technique. Both these NCs displayed X-ray diffraction (XRD) peaks due to body-centered cubic (BCC) Fe0 and magnetite-like phase. 57Fe Mössbauer spectroscopy measurements confirmed (a) the core–shell nature of the NCs, (b) the Fe-oxide shell to be nanocrystalline and partially oxidized beyond magnetite, and (c) the Fe-oxide spins are significantly canted. In addition to the BCC Fe and magnetite-like phases, a phase similar to tetragonal σ-Fe-Cr (8% Cr) was clearly evident in the larger NC, based on XRD. The origin of the tetragonal-like phase in the larger NC could be due to significant distortion of the Fe0 core lattice planes; subtle peaks due to this phase were also apparent in the smaller NC. Unambiguous evidence for the presence of such a phase was not clear from X-ray photoelectron spectroscopy, vibrating sample magnetometry, X-ray magnetic circular dichroism, or transmission electron microscopy. 57Fe Mössbauer spectroscopy, however, provided support for a non-BCC Fe0 phase in the strongest sixth and the first resonance lines of the Fe0 sextet. To our knowledge, this is the first report of a tetragonal-like phase in the Fe/Fe-oxide core–shell systems.

Reversible transition between coherently strained BiFeO3 and the metastable pseudotetragonal phase on (LaAlO3)0.3(Sr2AlTaO6)0.7 (001)

Coherently strained BiFeO3 epitaxial films deposited on (001)-oriented (LaAlO3)0.3(Sr2AlTaO6)0.7 have a tetragonal crystal form, a stress-distorted version of the rhombohedral phase. A conversion from coherent BiFeO3 to a new, tilted pseudotetragonal phase with the c/a ratio exceeding 1.2 is observed beyond the critical thickness of 60 nm. X-ray reciprocal space maps display that this highly elongated metastable structure is monoclinically distorted by ∼0.2° and exhibits an out-of-plane tilt of ∼3°. These observations are at odds with traditional understandings that a coherent epilayer should turn into its parent structure upon increasing the thickness, providing a new insight into the strain relaxation mechanism of epitaxial films. We show that in the heating and cooling cycles, the transition between these two phases is completely reversible and is associated with the alleviation of thermal stress. Our results reveal that the coherent BiFeO3 epilayer with tetragonal symmetry stabilized by moderate compressive strain behaves as a structural bridge that links the thermally stable rhombohedral phase and the metastable tetragonal-like phase with a giant axial ratio. Moreover, the finding of a BiFeO3 phase mixture in our study extends the threshold in-plane strain of the stress-driven morphotropic phase boundary to a value as low as −2.3%.

Atomic-level study ofBiFeO3under epitaxial strain

Structural and thermal properties of $\mathrm{BiFe}{\mathrm{O}}_{3}$ under compressive epitaxial strain are investigated using a shell model fitted to first-principles calculations. We show that a model developed for the bulk describes properly the behavior of the compound as function of the strain, including the appearance of tetragonallike phase with a large $c/a$ ratio. The obtained temperature-strain phase diagram reproduces several features observed experimentally in thin films. Molecular dynamic simulations show that morphotropic phase boundary separating the $R$-like and $T$-like regions is temperature independent but with different phases along the transition region. The microscopic analysis of the temperature-strain phase diagram emphasizes the relevance of the interplay between polarization, oxygen octahedron rotations, and strain.