Robust templates for domain boundary engineering in ErMnO3

Emerging properties of domain boundaries define the emerging field of domain boundary engineering. For many applications, the domain boundary acts as template onto which the desired properties, such as (super-) conductivity, polarity, ferroelectricity, magnetism, are imposed. This requires for most applications that the domain structures remain unchanged under appropriate chemical doping. Hassanpour et al (2016 New J. Phys. 18 043015) have now shown, for the first time, that the magnetic and electric domain structures remain indeed robust against charge carrier doping (Ca2+ and Zr4+) of the workbench multi-ferroic ErMnO3. This opens the way into novel functionalities based on the nanostructure of ErMnO3

Ferroelastic Twinning in Minerals: A Source of Trace Elements, Conductivity, and Unexpected Piezoelectricity

Ferroelastic twinning in minerals is a very common phenomenon. The twin laws follow simple symmetry rules and they are observed in minerals, like feldspar, palmierite, leucite, perovskite, and so forth. The major discovery over the last two decades was that the thin areas between the twins yield characteristic physical and chemical properties, but not the twins themselves. Research greatly focusses on these twin walls (or ‘twin boundaries’); therefore, because they possess different crystal structures and generate a large variety of ‘emerging’ properties. Research on wall properties has largely overshadowed research on twin domains. Some wall properties are discussed in this short review, such as their ability for chemical storage, and their structural deformations that generate polarity and piezoelectricity inside the walls, while none of these effects exist in the adjacent domains. Walls contain topological defects, like kinks, and they are strong enough to deform surface regions. These effects have triggered major research initiatives that go well beyond the realm of mineralogy and crystallography. Future work is expected to discover other twin configurations, such as co-elastic twins in quartz and growth twins in other minerals

Mild and wild ferroelectrics and their potential role in neuromorphic computation

In this Perspective, two interrelated new developments are discussed. The first relates to a much better understanding of the actual movement of domain walls during switching. Ferroelectric and ferroelastic domain movements proceed via the combination of jerky and smooth displacements of domain walls. A careful separation of these two mechanisms into “wild” and “mild” is crucial for the understanding of avalanches in ferroelectrics. Avalanche switching involves jerky domain wall movements and leads to singularities in the switching current. During avalanches, domain walls enhance and localize atomic transport and generate magnetism emerging from mobile kinks in the walls. The second development is based on the transport of dopants inside domain walls during nano-fabrication of devices. Progressing domain walls in electric fields can then—mainly in the case of wild wall movements—connect defect “reservoirs” similar to synapses connecting neurons in the brain. The walls take the role of synapses, and the defect clusters take that of neurons. The combination of fast moving domain walls and chemical transport inside the walls constitutes, therefore, ingredients for memristive device elements in neuromorphic computers. This application is predicted to play a major future role in ferroelectricity

Polar domain walls trigger magnetoelectric coupling

Interface physics in oxide heterostructures is pivotal in material's science. Domain walls (DWs) in ferroic systems are examples of naturally occurring interfaces, where order parameter of neighboring domains is modified and emerging properties may develop. Here we show that electric tuning of ferroelastic domain walls in SrTiO3 leads to dramatic changes of the magnetic domain structure of a neighboring magnetic layer (La1/2Sr1/2MnO3) epitaxially clamped on a SrTiO3 substrate. We show that by exploiting the resposiveness of DWs nanoregions to external stimuli, even in absence of any domain contribution, prominent and adjustable macroscopic reactions of neighboring layers can be obtained. We conclude that polar DWs, known to exist in other materials, can be used to trigger tunable responses and may lead to new ways for manipulation of interfacial emerging properties

Elastic softening of leucite and the lack of polar domain boundaries

Elastic properties of leucite have been investigated using resonant ultrasound spectroscopy over a temperature range from 300 to 1400 K. According to these measurements, elastic moduli soften by ~50% at the Ia3d-I41/acd ferroelastic transition temperature Tc1 = 940 K relative to the value at 1400 K. A second softening is observed at Tc2 = 920 K, corresponding to the structural change from the space group I41/acd to I41/a. These elastic anomalies are analyzed in a simple model under the assumption that the transitions observed at Tc1 and Tc2 can be approximated by a single pseudoproper ferroelastic transition. The two phase transitions are accompanied by a single peak in mechanical damping attributed to the high mobility of twin walls in the intermediate phase followed by pinning in the low-temperature phase. To determine whether twin walls in tetragonal leucite are polar, resonant piezoelectric spectroscopy and second harmonic generation measurements were performed, but no evidence of polarity was found

Crackling noise and avalanches in minerals

Abstract: The non-smooth, jerky movements of microstructures under external forcing in minerals are explained by avalanche theory in this review. External stress or internal deformations by impurities and electric fields modify microstructures by typical pattern formations. Very common are the collapse of holes, the movement of twin boundaries and the crushing of biominerals. These three cases are used to demonstrate that they follow very similar time dependences, as predicted by avalanche theories. The experimental observation method described in this review is the acoustic emission spectroscopy (AE) although other methods are referenced. The overarching properties in these studies is that the probability to observe an avalanche jerk J is a power law distributed P(J) ~ J−ε where ε is the energy exponent (in simple mean field theory: ε = 1.33 or ε = 1.66). This power law implies that the dynamic pattern formation covers a large range (several decades) of energies, lengths and times. Other scaling properties are briefly discussed. The generated patterns have high fractal dimensions and display great complexity

Ferroelectric switching in ferroelastic materials with rough surfaces

Abstract: Electric switching of non-polar bulk crystals is shown to occur when domain walls are polar in ferroelastic materials and when rough surfaces with steps on an atomic scale promote domain switching. All domains emerging from surface nuclei possess polar domain walls. The progression of domains is then driven by the interaction of the electric field with the polarity of domain boundaries. In contrast, smooth surfaces with higher activation barriers prohibit effective domain nucleation. We demonstrate the existence of an electrically driven ferroelectric hysteresis loop in a non-ferroelectric, ferroelastic bulk material

Avalanches from charged domain wall motion in BaTiO3 during ferroelectric switching

We report two methods for direct observations of avalanches in ferroelectric materials during the motion of domain walls. In the first method, we use optical imaging techniques to derive changes in domain structures under an electric field. All changes occur through small jumps (jerks) that obey avalanche statistics. In the second method, we analyze jerks by their displacement current. Both methods reveal a power law distribution with an energy exponent of 1.6, in agreement with previous acoustic emission measurements, and integrated mean field theory. This new combination of methods allows us to probe both polarization and strain variations during the motion of domain walls and can be used for a much wider class of ferroelectrics, including ceramic samples, than acoustic emission

Infrared spectra of Si-O overtones, hydrous species, and U ions in metamict zircon: radiation damage and recrystallization

Radiation damage and recrystallization in natural zircons have been studied by analysing Si-O stretching overtones/combinations, hydrous species, and U-ion spectra in the frequency region between 1200 and 11 000 cm -1. The effects of radiation are characterized by a dramatic variation of intensity, a decrease in frequencies of multi-phonon bands (e.g., Si-O stretching overtones), a change of spectral profile of OH species, a formation of new OH species, and new signals related to U ions. The formation of new anisotropic OH species in the crystalline regions of metamict zircon is observed and this could account for the different thermal behaviour of OH species between metamict zircon and titanite during high-temperature annealing. The results imply systematic modifications of the local environments of the OH and U ions in the damage process. Both U 4+ and U 5+ spectra show dramatic variations during metamictization. We observe, for the first time, that as a result of radiation damage, the U 5+ signals near 6668 and 9030 cm -1 become undetectable at a dose of around 1.5×1018 α-events g -1 while extra lines near 6650 and 8969 cm -1 appear. These variations are interpreted as radiation-induced local modifications in crystalline regions. The general shape of the U-ion spectrum of the crystalline zircon is somehow still preserved in highly damaged zircon. A decomposed zircon, consisting of ZrO2, SiO2, and ZrSiO4, shows spectral features different from those of metamict zircon samples. Thermal annealing of a highly damaged zircon leads to recovery of the structure of zircon, indicated by spectral changes of multi-phonon bands and U ions, accompanied with the appearance of new OH species. The results confirm that the recrystallization process in heavily damaged zircon involves the decomposition of metamict ZrSiO4 into SiO2 and ZrO2 near 1100 K and the significant crystal growth of ZrSiO4 near 1400 K as indicated by the recovery of Si-O stretching overtones and U 4+ and U 5+ bands.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/48894/2/c21219.pd

Current vortices and magnetic fields driven by moving polar twin boundaries in ferroelastic materials

Abstract: Ferroelastic twin boundaries often have properties that do not exist in bulk, such as superconductivity, polarity etc. Designing and optimizing domain walls can hence functionalize ferroelastic materials. Using atomistic simulations, we report that moving domain walls have magnetic properties even when there is no magnetic element in the material. The origin of a robust magnetic signal lies in polar vortex structures induced by moving domain walls, e.g., near the tips of needle domains and near domain wall kinks. These vortices generate displacement currents, which are the origin of magnetic moments perpendicular to the vortex plane. This phenomenon is universal for ionic crystals and holds for all ferroelastic domain boundaries containing dipolar moments. The magnetic moment depends on the speed of the domain boundary, which can reach the speed of sound under strong mechanical forcing. We estimate that the magnetic moment can reach several tens of Bohr magnetons for a collective thin film of 1000 lattice planes and movements of the vortex by the speed of sound. The predicted magnetic fields in thin slabs are much larger than those observed experimentally in SrTiO3/LaAlO3 heterostructures, which may be due to weak (accidental) forcing and slow changes of the domain patterns during their experiments. The dynamical multiferroic properties of ferroelastic domain walls may have the potential to be used to construct localized magnetic memory devices in future