J. H. Yen

Constrained modeling of domain patterns in rhombohedral ferroelectrics

A nonconventional phase-field model is developed to predict ferroelectric domain structures. It employs a set of field variables motivated by multirank laminates to represent energy-minimizing domain configurations, giving rise to an explicit expression of the energy-well structure. The framework is applied to domain simulation in the rhombohedral phase assuming that polarization is close to the ground states. An electromechanical self-accommodation pattern consisting of eight rhombohedral variants and an engineered domain configuration are predicted and found in good agreement with those observed in experiment.

The magnetoelectric domains and cross-field switching in multiferroic BiFeO3

BiFeO3 is an exciting multiferroic material because of its room temperature multiferrocity, excellent ferroelectric properties, and recently demonstrated electric control of antiferromagnetic domains. In this letter we report a theoretical study on the structure and evolution of magnetoelectric domains in BiFeO3. We not only observed the coupled ferroelectric and antiferromagnetic domains and demonstrated the electric control of antiferromagnetic ordering, both in consistency with experiments, but also revealed the switching of antiferromagnetic domains by mechanical stress that is yet to be explored in experiments.

Magnetoelastic domains and magnetic field-induced strains in ferromagnetic shape memory alloys by phase-field simulation

Magnetoelastic domains in ferromagnetic shape memory alloys evolve through either variant rearrangement or magnetization rotation, resulting in a large or a small magnetic field-induced strain depending on the magnitude of applied compressive stress. These phenomena are simulated in this letter using an unconventional phase-field model motivated by energy-minimizing multirank laminated domain structures. The results agree well with experiments, and confirm the analysis of Ma and Li [Appl. Phys. Lett. 90, 172504 (2007)] based on an energy minimization theory.

Pattern formation in martensitic thin films

Pattern formation in martensitic materials refers to the accommodation problem of how to mix martensitic variants coherently to minimize the strain energy. A framework motivated by energy-minimizing multirank laminated patterns is proposed to study this problem in martensitic films. It is found that the interfaces between the variants of martensite can be quite different in thin films than in bulk materials, and they typically have a simpler structure. Various intriguing and fascinating self-accommodation patterns are predicted for martensitic thin films with different orientations. The results are in good agreement with the Bhattacharya-James thin-film theory [K. Bhattacharya and R. D. James, J. Mech. Phys. Solids 47, 531 (1999)] as well as with experimental observations.

Effect of depolarization and coercivity on actuation strains due to domain switching in barium titanate

Large electrostrictive actuation in ferroelectric single crystals can be achieved through non-180° domain switching using various biasing fields and loads. The theoretical maximum actuation strain such as 1.1% for barium titanate crystals, however, has not been observed yet. In this letter, the authors propose a possible mechanism accounting for the depolarization effect to explain the significant strain reduction observed in their recent experiment. They find that a low-energy path requires the switching of 90° domains together with that of in-plane alternating layers of 180° domains formed to reduce the depolarization energy. Therefore, reduction in strain is significant for crystals with large 180° coercivities. The result is consistent with their recent experimental observations.

Operation of multiple 90° switching systems in barium titanate single crystals under electromechanical loading

Hysteresis evolution of a 5×5×2mm3 barium titanate single crystal during a combined electromechanical loading sequence reveals incomplete switching characteristics and a considerable disproportion of slope gradients at zero electric field for the measured polarization and strain hysteresis curves. A likely cause for the disproportion of gradients is the cooperative operation of multiple 90° switching systems by which “polarization-free” strain changes are induced. In addition, a theoretical study indicates that the presence of depolarization fields generated from the unshielded boundaries and/or incompatible domains within the crystal can have a substantial influence on the polarization measurement in the loading direction.

Novel Phase-Field Simulation of Microstructure in Martensitic Materials

A novel phase-field model of martensite suitable for microstructure simulation is developed here. It is motivated by the hierarchical structure of multirank laminates for establishing the rule of mixtures. As a result, a different choice of field variables, local volume fraction of laminates, is introduced to represent each martensitic variant. It provides an advantage of expressing the energy-well structure of martensitic variants in a unified fashion, instead of choosing the special polynomial expansions of conventional order parameters for a particular transformation. In addition, only two parameters are needed for microsturcture simulation. One is related to the energetic cost due to formation of the interface, and the other is the cost due to the deviation from the ground state energy. The framework is applied to the investigation of optimal microstructures for achieving large actuation strains for dome-shaped and tunnel-shaped microactuators. Finally, the framework is extended to ferroelectric domain simulation. Two cases are discussed: one is for the constrained modeling which restricts polarization remaining on the ground state, and the other is for the unconstrained modeling which allows polarization deviating from the ground state.© 2008 ASME

Micromagnetic Modeling of Released/Non-released Magnetostrictive Films

We study the magnetostrictive behavior of two types of films: one is released from the substrate but constrained from its lateral boundary and the other is attached to the substrate. The former is investigated using the recently developed micromagnetic model which is suitable for films subjected to realistic magneto-mechanical boundary conditions. The latter is modeled within a framework extended from our recent theoretical paper studying rough alloy films with composition modulation. We give an explicit form of the stress-induced magnetic field taking into account the elastic interaction of the film with the substrate. Such results are very different from those considered for attached thick films in literature.