We have investigated structural phase transitions across a ferroelectric-to-antiferroelectric phase boundary in epitaxial ͑001͒ oriented Bi ͑1−x͒ Sm x FeO 3 thin films. For the Sm 3+ concentration of 0.1Յ x Յ 0.14, we observe short-range antiparallel cation displacements, verified by the appearance of localized 1 4 ͕011͖ weak reflections in the selected area electron diffraction patterns. At the critical composition of x = 0.14, the system adopts a complex nanoscale domain mixture with appearance of 1 4 ͕011͖, 1 2 ͕011͖, 1 2 ͕010͖, and 1 2 ͕111͖ reflections and an incommensurate phase bridging the rhombohedral and orthorhombic phases. For compositions 0.14Ͻ x Ͻ 0.2, orientational twin domains coupled with antiphase oxygen octahedral tilts, identified by 1 2 ͕hkl͖ weak superstructure are observed. The above systematic changes in the microstructure as a function of Sm 3+ doping are linked to the macroscopic functional properties.

Bismuth ferrite (BiFeO3) is one of the very few known single-phase multiferroic materials. While the bulk compound is rhombohedral (R), the discovery of an epitaxial strain-induced structural transition into a so-called 'super tetragonalphase' (T-phase) in this material incited a flurry of research activity focused on gaining an understanding of this phase transition and its possible functionalities. This metastable phase of BiFeO3 is also multiferroic, with giant ferroelectric polarization and coexisting antiferromagnetic order, but above all it is the strain relaxation-induced phase mixtures and their outstanding piezoelectric and magnetoelectric responses which continue to intrigue and motivate the physicist and materials scientist communities. Here, we review the research into the T-phase and mixed-phase BiFeO3 system. We begin with a brief summary of the history of the T-phase and an analysis of the structure of the various phases reported in the literature. We then address important questions regarding the symmetry and octahedral rotation patterns and the (as yet underexplored) important role of chemistry in the formation of the metastable T-phase. We follow by describing the phase transitions in this material, and how these may hold promise for large magnetoelectric responses. Finally we point out some experimental challenges inherent to the study of such a system, and potential pathways for how they may be overcome. It is our intention with this work to highlight important issues that, in our opinion, should be carefully considered by the community in order to use this fascinating materials system for a new paradigm of functionality.

A new paradigm of domain wall nanoelectronics has emerged recently, in which the domain wall in a ferroic is itself an active device element. The ability to spatially modulate the ferroic order parameter within a single domain wall allows the physical properties to be tailored at will and hence opens vastly unexplored device possibilities. Here, we demonstrate via ambient and ultrahigh-vacuum (UHV) scanning probe microscopy (SPM) measurements in bismuth ferrite that the conductivity of the domain walls can be modulated by up to 500% in the spatial dimension as a function of domain wall curvature. Landau-Ginzburg-Devonshire calculations reveal the conduction is a result of carriers or vacancies migrating to neutralize the charge at the formed interface. Phase-field modeling indicates that anisotropic potential distributions can occur even for initially uncharged walls, from polarization dynamics mediated by elastic effects. These results are the first proof of concept for modulation of charge as a function of domain wall geometry by a proximal probe, thereby expanding potential applications for oxide ferroics in future nanoscale electronics.

We present investigations on the resistive switching effect in SrRuO3∕PbZr0.2Ti0.8O3∕Pt ferroelectric capacitors. Using a conductive atomic force microscope, the out-of-plane piezoelectric response and the capacitive and resistive current were simultaneously measured as a function of applied bias voltage. We observed two independent switching phenomena, one attributed to the ferroelectric switching process and the other to resistive switching.We show that I-V curves alone are not sufficient in ferroelectric materials to clarify the underlying switching mechanism and must be used with sufficient caution.

We have investigated heteroepitaxial films of Sm-doped BiFeO 3 with a Sm-concentration near a morphotropic phase boundary. Our high-resolution synchrotron X-ray diffraction, carried out in a temperature range of 25°C to 700°C, reveals substantial phase coexistence as one changes temperature to crossover from a low-temperature PbZrO 3 -like phase to a high-temperature orthorhombic phase. We also examine changes due to strain for films exhibiting anisotropic misfit between film and substrate. Additionally, thicker films exhibit a substantial volume collapse associated with the structural transition that is suppressed in thinner films.

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It has been well established that both in bulk at ambient pressure and for films under modest strains, cubic SrCoO 3−δ (δ < 0.2) is a ferromagnetic metal. Recent theoretical work, however, indicates that a magnetic phase transition to an antiferromagnetic structure could occur under large strain accompanied by a metal-insulator transition. We have observed a strain-induced ferromagnetic to antiferromagnetic phase transition in SrCoO 3−δ films grown on DyScO3 substrates, which provide a large tensile epitaxial strain, as compared to ferromagnetic films under lower tensile strain on SrTiO3 substrates. Magnetometry results demonstrate the existence of antiferromagnetic spin correlations and neutron diffraction experiments provide a direct evidence for a G-type antiferromagnetic structure with Neél temperatures between TN ∼ 135 ± 10 K and ∼ 325 ± 10 K depending on the oxygen content of the samples. Therefore, our data experimentally confirm the predicted strain-induced magnetic phase transition to an antiferromagnetic state for SrCoO 3−δ thin films under large epitaxial strain.The broad range of transition metal oxide functionalities, including superconductivity, magnetism, and ferroelectricity, can be tuned by the careful choice of parameters such as strain, oxygen content, or applied electric and magnetic fields 1-9 . This tunability makes transition metal oxide materials ideal candidates for use in developing novel information and energy technologies 10,11 . SrCoO 3 is a particularly interesting system for investigation. SrCoO 3−δ has long been studied due to its propensity to form oxygen-vacancy-ordered structures as the oxygen content is decreased. The system undergoes well-defined structural phase transitions between distinct topotactic phases, from a cubic perovskite phase at SrCoO 3 to brownmillerite SrCoO 2.5 . The ties between the structural and functional properties of the material are obvious as a magnetic phase transition from ferromagnetic (FM) SrCoO 3.0 with T C = 280-305 K to antiferromagnetic (AFM) SrCoO 2.5 with T N = 570 K accompanies the structural transition 5,[12][13][14] . This is similar to the case of SrFeO 3−δ , which has also been demonstrated to undergo oxygen vacancy ordering with magnetic phase transitions related to the structure and Fe charge ordering [17][18][19] .In addition to oxygen stoichiometry, other possibilities, such as strain or applied magnetic or electric fields, may be used to tune the system. Lee and Rabe have simulated the effect of epitaxial strain on SrCoO 3.0 and predict a large polar instability resulting in a dependence of the magnetic structure on strain 20,21 . Their results show that the magnetic state can be controlled by the amount of compressive or tensile strain applied. An AFM-FM transition is predicted at both, tensile strain of ∼2.0 % and compressive strain of approximately -0.8 %, which is caused through the Goodenough-Kanamori rules as a consequence of simultaneous structural phase transitions between phases with different distortions and rotational pa...

Abstract:In specific solid-state materials, under the right conditions, collections of magnetic dipoles are known to spontaneously form into a variety of rather complex geometrical patterns, exemplified by vortex and skyrmion structures. While theoretically, similar patterns should be expected to form from electrical dipoles, they have not been clearly observed to date: the need for continued experimental exploration is therefore clear. In this article we report the discovery of a rather complex domain arrangement that has spontaneously formed along the edges of a thin single crystal ferroelectric sheet, due to surface-related depolarizing fields.Polarization patterns are such that nanoscale 'flux-closure' loops are nested within a larger mesoscale flux closure object. Despite the orders of magnitude differences in size, the geometric forms of the dual-scale flux closure entities are rather similar.