Domain glass

Microstructural patterns of twin boundaries and tweed in ferroelastic materials display typical aspects of glasses. The patterns are complex, their dynamics follows Vogel–Fulcher statistics and their field cooling–non-field cooling hysteresis is similar to those described in this issue as ‘strain glasses’. The difference is that domain glasses do not need extrinsic defects to form. In the paraelastic phase, an intrinsic tweed pattern dominates the high temperature precursor regime. Experimentally, massive elastic precursor softening is related to polar standing waves, which are attributed to the glassy relaxation of the tweed pattern. In the ferroelastic phase we find a complex twin pattern when the sample is strained with a constant strain rate. The dynamics of the pattern formation is a-thermal at low temperatures and follows Vogel–Fulcher statistics at moderately high temperatures. It is argued that domain boundary patterns can hence evolve glass-like states while the underlying matrix remains fully crystalline without any defect induced disorder

Suppression of the ferromagnetic state in LaCoO3 films by rhombohedral distortion

Epitaxially strained LaCoO3 (LCO) thin films were grown with different film thickness, t, on (001) oriented (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 (LSAT) substrates. After initial pseudomorphic growth the films start to relieve their strain partly by the formation of periodic nano-twins with twin planes predominantly along the direction. Nano-twinning occurs already at the initial stage of growth, albeit in a more moderate way. Pseudomorphic grains, on the other hand, still grow up to a thickness of at least several tenths of nanometers. The twinning is attributed to the symmetry lowering of the epitaxially strained pseudo-tetragonal structure towards the relaxed rhombohedral structure of bulk LCO. However, the unit-cell volume of the pseudo-tetragonal structure is found to be nearly constant over a very large range of t. Only films with t > 130 nm show a significant relaxation of the lattice parameters towards values comparable to those of bulk LCO.Comment: 31 pages, 10 figure

Multiferroic materials for sensors, transducers and memory devices

Chemical compositions and basic properties of smart materials (ferroics, biferroics, multiferroics) are introduced in this paper. Single phase and composite ferroelectromagnetics are characterized in detail. Multiferroic ferroelectromagnetics are materials which are both ferromagnetic/ferrimagnetic/antiferromagnetic and ferroelectric/ferrielectric, antiferrolectric in the same phase. As a result they have a spontaneous magnetization which can be switched by an applied magnetic field, a spontaneous polarization which can be switched by an applied electric field, and often there is some coupling between those fields. The physical mechanisms of the coupling process were analyzed. In the case of the ferroelectromagnetics in general the transitions method d electrons, which are essential for magnetism, reduce the tendency for off-center ferroelectric distortion. Such materials have all the potential applications of both their parent ferroelectric and ferromagnetic materials

Landau theory applied to phase transitions in calcium orthotungstate and isostructural compounds

The pressure-driven tetragonal-to-monoclinic phase transition in CaWO4 and related scheelite-structured orthotungstates is analysed in terms of spontaneous strains. Based upon our previous high-pressure x-ray diffraction results and the Landau theory, it is suggested that the scheelite-to-fergusonite transition is of second order in nature.Comment: 14 pages, 3 figure

Role of the ferroelastic strain in the optical absorption of BiVO4

Bismuth vanadate (BiVO4) has recently been under focus for its potential use in photocatalysis thanks to its well-suited absorption edge in the visible light range. Here, we characterize the optical absorption of a BiVO4 single crystal as a function of temperature and polarization direction by reflectance and transmittance spectroscopy. The optical band gap is found to be very sensitive to temperature, and to the monoclinic-to-tetragonal ferroelastic transition at 523K. The anisotropy, as measured by the difference in absorption edge for light polarized parallel and perpendicular to the principal axis, is reduced from 0.2 eV in the high-temperature tetragonal phase to 0.1 eV at ambient temperature. We show that this evolution is dominantly controlled by the ferroelastic shear strain. These findings provide a route for further optimization of bismuth-vanadate-based light absorbers in photocatalytic devices.Comment: 6 pages, 5 figure

Biasing Effects in Ferroic Materials

In this chapter we present an overview of some important concepts related to the processes and microstructural mechanisms that produce the deformation of hysteresis loops and the loss of their symmetry characteristics in ferroelectric, ferroelastic and ferromagnetic systems. The most discussed themes include: aging and fatigue as primary mechanisms of biased hysteresis loops in ferroelectric/ferroelastic materials, imprint phenomenon as an important biasing process in ferroelectric thin films, the development of an exchange bias field and of specific spin states, such as spin canting and spin-glass-like phases, as the main causes of biased hysteresis loops in different types of magnetic heterostructures. The present discussion leads to the identification of the main differences and possible analogies in the underlying mechanisms of possible biasing effects occurring in the different ferroic systems, which can benefit the theoretical description, modelling, and engineering of multifunctional devices based on ferroic systems experiencing the internal bias phenomena

Monoclinic-orthorhombic first-order phase transition in K<inf>2</inf>ZnSi<inf>5</inf>O<inf>12</inf>leucite analogue; Transition mechanism and spontaneous strain analysis

Hydrothermally synthesised K2ZnSi5O12 has a polymerized framework structure with the same topology as leucite (KAlSi2O6, tetragonal I41/a), which has two tetrahedrally coordinated Al3+ cations replaced by Zn2+ and Si4+. At 293K it has a cation-ordered framework P21/c monoclinic structure with lattice parameters a = 13.1773(2)A, b = 13.6106(2) A, c = 13.0248(2)A, = 91.6981(9). This structure is isostructural with K2MgSi5O12, the first cation-ordered leucite analogue characterised. With increasing temperature, the P21/c structure transforms reversibly to cation-ordered framework orthorhombic Pbca. This transition takes place over the temperature range 848-863K where both phases coexist; there is an 1.2% increase in unit cell volume between 843K (P21/c) and 868K (Pbca), characteristic of a first-order, displacive, ferroelastic phase transition. Spontaneous strain analysis defines the symmetry- and non-symmetry related changes and shows that the mechanism is weakly first order; the two-phase region is consistent with the mechanism being a strain-related martensitic transition

Domain switching dynamics in ferroelastic and ferroelastic/ferroelectric perovskites

PhDA comprehensive study of domain switching process in different ferroelastic and ferroelastic/ferroelectric perovskite structured ceramics has been performed. The effects of thermal fluctuations on domain switching dynamics were investigated in the ferroelastic and in the ferroelectric case under static and dynamic electric and mechanical conditions. In the ferroelastic case, domain switching behaviour was investigated for different compositions, using different types of mechanical tests. Compression tests were carried out to characterize the ferroelastic properties, such as coercive stress, hysteresis loop and irreversible strain. Creep experiments were performed to study the domain switching time dependence at different stress levels. Domain switching kinetics during creep was characterized by implementing a rate model, based on thermal activation rate theory, which allowed the activation volume to be estimated. A Rayleigh-type analysis was performed to study the effects of stress amplitude, loading rate, temperature and composition on ferroelastic switching. Rayleigh-type relationships were proposed to fit the results and the rate model developed was applied to quantify the effect of the loading rate on the Rayleigh loops. Alternative methodologies were developed to assess the effects of rate and temperature on the coercive stress, providing original sets of data. A further application of the rate model provided an estimation of the activation parameters (volume and enthalpy). In PZT 5A at the coercive field the activation volume was calculated to be 2.44 nm3, with a reasonable consistency with the value obtained from creep tests (7.49 nm3). In the ferroelectric case, domain switching was studied by generating P-E and butterfly hysteresis loops and by analysing creep-relaxation curves. In creep experiments, the polarization and the strain were measured simultaneously, during the application of a constant electric field. An insight into the evolution of domain structure and on domain switching mechanisms was gained, highlighting analogies and differences with the ferroelastic case. Experiments at different frequencies, allowed the activation volume to be estimated at the coercive field (77 nm3). The relatively large value indicates small rate dependence and suggests a domain structure with broad and mobile domain walls, being the preferred sites for the nucleation

Thermodynamics of multicaloric effects in multiferroic materials; application to metamagnetic shape memory alloys and ferrotoroidics

We develop a general thermodynamic framework to investigate multicaloric effects in multiferroic materials. This is applied to the study of both magnetostructural and magnetoelectric multiferroics. Landau models with appropriate interplay between the corresponding ferroic properties (order parameters) are proposed for metamagnetic shapememory and ferrotoroidic materials, which, respectively, belong to the two classes of multiferroics. For each ferroic property, caloric effects are quantified by the isothermal entropy change induced by the application of the corresponding thermodynamically conjugated field. The multicaloric effect is obtained as a function of the two relevant applied fields in each class of multiferroics. It is further shown that multicaloric effects comprise the corresponding contributions from caloric effects associated with each ferroic property and the crosscontribution arising from the interplay between these ferroic properties. This article is part of the themed issue 'Taking the temperature of phase transitions in cool materials