Julia Slutsker

Self-assembled multiferroic nanostructures in the CoFe2O4-PbTiO3 system

The effect of substrate orientation on the morphologies of epitaxial self-assembled nanostructures was demonstrated using multiferroic 0.67PbTiO3-0.33CoFe2O4 thin films. The two-phase composite films were grown by pulsed laser deposition on single crystal SrTiO3 substrates having (001) and (110) orientations. The nanostructures of both orientations consisted of vertical rod- or platelet-like columns of CoFe2O4 dispersed in a PbTiO3 matrix. For the (001) orientation the platelet habits were parallel to the {110} planes, whereas for the (110) orientation the platelets were parallel to the {111} planes. The differences were explained using a thermodynamic theory of heterophase structures.

Martensitic transformation in constrained films

Polydomain micro- and nano-structures are natural products of phase transformations in solids. The trend to minimize the energy of internal long-range fields (magnetic, electric or mechanical) leads to the formation of the arrangement of domains of different phases or differently oriented domains of the same phase [1–3]. Transformations of bulk materials usually result in formation of complex irregular polydomain structures, which are difficult to control. However, the advance of thin film technology makes it possible to obtain well- controlled polydomain structures with desirable properties. The objective of this paper is to show how controlled structures of elastic domain in constrained single crystalline layers are developed and how these structures can be designed by engineering of a constraint.

Thermodynamic analysis and phase field modeling of domain structures in bilayer ferroelectric thin films

The domain structure in bilayer ferroelectric films was studied by using an analytical thermodynamic theory and computer simulations utilizing the phase field model. It is demonstrated that in a low applied field, a self-poled state can be produced in a bilayer film with one layer in a polydomain state and the other layer in a single-domain state. The presence of the layer with a polydomain structure results in a higher dielectric constant and lower coercive field as compared to a bilayer in a single-domain state. The increase in the applied field results in the transition to a single-domain state in the whole bilayer. The thermodynamic potentials of the layers and the energies of electrostatic and elastic interactions determine the critical fields which can control the transitions between single-domain and polydomain states as well as polarization switching of bilayers. Different thermodynamic hysteresis loops are obtained depending on the relationship between these critical fields and the amplitude of t...

Contribution of substrate to converse piezoelectric response of constrained thin films

The converse piezoelectric response of a thin film constrained by a substrate is analyzed in different geometries under various boundary conditions. We considerthe effects of elastic deformation of the substrate on the total displacement of thefilm surface induced by the electric field. The change of film thickness and the bending curvature of a film/substrate couple are calculated. For a thin film island clamped on a large thick substrate, the theoretical estimation of the piezoresponse, including a local bending in the vicinity of the island/substrate interface, is in agreement with the finite element calculation.

Epitaxial self-assembly of multiferroic nanostructures

The morphology and structure of interfaces in PbTiO3–CoFe2O4 films on SrTiO3 substrates of various orientations are studied. It is found that for film with thickness more than 100 nm with columnar two-phase morphology, the average orientations of the interfaces between the phases reside on the {110}, {111}, and {112} planes for films normal to substrates oriented along ⟨001⟩, ⟨110⟩, and ⟨111⟩ correspondingly. These macroscopic interfaces consist of {111} nanofacets. The study of very thin film less than 30 nm shows that the interfaces between nanosize phases are {111} planes. Available theoretical results on morphology of multiferroic thin film nanostructure allow us to conclude that for thick films (>100 nm), two-phase columnar morphology (shape of constituent phases, their arrangement, and average crystallographic orientations of the interfaces) is essentially determined by the trend for minimization of elastic energy, while the trend to minimize the energy of interfaces results in faceting interfaces a...

Thermodynamics of polydomain ferroelectric bilayers and graded multilayers

The equilibrium domain structure and its evolution under an electric field in ferroelectric bilayers and graded multilayers are considered. The equilibrium bilayer is self-poled and contains single-domain and polydomain (with 180° domains) layers. The polarization of a graded multilayer proceeds by movement of wedge-like domains as a result of progressive transformation of polydomain layers to a single-domain state. The theory provides the principal explanation of dielectric behavior of graded ferroelectric films.The equilibrium domain structure and its evolution under an electric field in ferroelectric bilayers and graded multilayers are considered. The equilibrium bilayer is self-poled and contains a single-domain and a polydomain (with 180 domains) layers. The polarization of a graded multilayer proceeds by movement of wedge-like domains as a result of progressive transformation of polydomain layers to a single-domain state. The theory provides the principal explanation of peculiarities of dielectric behavior of graded ferroelectric films and can be applied to graded ferromagnetics and ferroelastics.

Deformation of adaptive materials. Part III: Deformation of crystals with polytwin product phases

Abstract Deformation as a result of solid–solid phase transformations, particularly martensitic, is the subject of this paper. The thermodynamic theory of elastic domains is applied to a transformation with a polytwin product phase which consists of two or more different domains (twins) forming plane–parallel alternations. The volume fractions of the phases and different twin components in the product phase are obtained as functions of applied stress (strain) and temperature. It is shown that during stress- or strain-induced transformation a twin fraction in martensite plates changes and a product phase becomes incompatible with an initial phase. This fact leads to the intrinsic instability of a transformational deformation, which appears as a negative Youngs modulus under strain-controlled deformation and a thermodynamic hysteresis under stress-controlled deformation. The deformation is irreversible also due to microstructure irreversibility of the transformation from a polytwin state to a single-domain state. Superelastic deformation due to martensitic transformations is discussed as an application of the presented theory.

Deformation of adaptive materials. Part I. Constrained deformation of polydomain crystals

Abstract Deformation of an adaptive heterophase crystal with variable microstructure is considered. Under mechanical constraint the phase transformation in single component systems result in an equilibrium two-phase mixture. A typical equilibrium microstructure of a constrained crystal is a polydomain, i.e. an alternation of the plane-parallel layers, or domains, of the parent and product phases with a special crystallographic orientation of interfaces between domains. The relative fractions of the domains are determined by the external conditions. The free energy of a polydomain is a non-convex function of constrained strain. Therefore, the stress–strain relation at displacement controlled deformation of the polydomain is characterized by a negative Young’s modulus. If deformation proceeds under stress control, a hysteretic stress–strain curve on loading and unloading should be observed instead of a negative stress–strain slope. Besides this thermodynamic hysteresis, anelastic hysteresis appears at a constant strain-rate deformation, if the microstructure relaxes more slowly than the strain changes.

Morphological transitions of elastic domain structures in constrained layers

The phase transformation in a constrained layer is the subject of this article. The formation and evolution of polydomain microstructure under external stress in the constrained layer are investigated by phase-field simulation and analytically using homogeneous approximation. As a result of simulation, it has been shown that the three-domain hierarchical structure can be formed in the epitaxial films. Under external stress there are two types of morphological transitions: from the three-domain structure to the two-domain one and from the hierarchical three-domain structure to the cellular three-domain structure. The results of phase-field simulation are compared with conclusions of homogenous theory and with available experimental data.

Deformation of adaptive heterophase materials

A crystal which can be in two possible phase states is considered. During tensile extension the crystal is deformed elastically. After a certain amount of elastic strain a phase transformation begins. For each fixed level of strain an equilibrium mesostructure is established, which corresponds to a minimum in the free energy of the crystal. The equilibrium mesostructure consists of plane, parallel layers of a product phase separated by layers of an initial phase. The product phase itself consists of two or more different domains (twins) forming plane, parallel alternations. The volume fractions of the phases and of different twin components in the product phase are functions of strain and temperature. Above a critical temperature the product phase is a single domain (untwinned). The stress‐strain curve which reflects the evolution of the equilibrium mesostructure is calculated. For deformation under a strain control the calculated equilibrium stress‐strain curve has a section with negative slope that corr...