B. Dkhil

Crafting the magnonic and spintronic response of BiFeO3 films by epitaxial strain.

Multiferroics are compounds that show ferroelectricity and magnetism. BiFeO3, by far the most studied, has outstanding ferroelectric properties, a cycloidal magnetic order in the bulk, and many unexpected virtues such as conductive domain walls or a low bandgap of interest for photovoltaics. Although this flurry of properties makes BiFeO3 a paradigmatic multifunctional material, most are related to its ferroelectric character, and its other ferroic property--antiferromagnetism--has not been investigated extensively, especially in thin films. Here we bring insight into the rich spin physics of BiFeO3 in a detailed study of the static and dynamic magnetic response of strain-engineered films. Using Mössbauer and Raman spectroscopies combined with Landau-Ginzburg theory and effective Hamiltonian calculations, we show that the bulk-like cycloidal spin modulation that exists at low compressive strain is driven towards pseudo-collinear antiferromagnetism at high strain, both tensile and compressive. For moderate tensile strain we also predict and observe indications of a new cycloid. Accordingly, we find that the magnonic response is entirely modified, with low-energy magnon modes being suppressed as strain increases. Finally, we reveal that strain progressively drives the average spin angle from in-plane to out-of-plane, a property we use to tune the exchange bias and giant-magnetoresistive response of spin valves.

Low-Symmetry Phases in Ferroelectric Nanowires

Ferroelectric nanostructures have recently attracted much attention due to the quest of miniaturizing devices and discovering novel phenomena. In particular, studies conducted on two-dimensional and zero-dimensional ferroelectrics have revealed original properties and their dependences on mechanical and electrical boundary conditions. Meanwhile, researches aimed at discovering and understanding properties of one-dimensional ferroelectric nanostructures are scarce. The determination of the structural phase and of the direction of the polarization in one-dimensional ferroelectrics is of technological importance, since, e.g., a low-symmetry phase in which the polarization lies away from a highly symmetric direction typically generates phenomenal dielectric and electromechanical responses. Here, we investigate the phase transition sequence of nanowires made of KNbO(3) and BaTiO(3) perovskites, by combining X-ray diffraction, Raman spectroscopy, and first-principles-based calculations. We provide evidence of a previously unreported ferroelectric ground state of monoclinic symmetry and the tuning of the polarizations direction by varying factors inherent to the nanoscale.

Phenomenological theory of phase transitions in epitaxialBaxSr1−xTiO3thin films

We develop a phenomenological thermodynamic theory of ferroelectric BaTiO3 (BT) thin films epitaxially grown on cubic substrates using the Landau-Devonshire eight-order potential. The constructedmisfit-temperaturephase diagram is asymmetrical. We found that, overall view of the phase diagram depends on the values of compliances used in calculations and provide two qualitatively different diagrams. A thermodynamic path for BT film grown onto particular substrate can be found using a plot of the room-temperature tetragonal distortion (c-a)/a as a function of misfit strain.

Temperature-dependent Raman and x-ray studies of the spin-ice pyrochlore Dy 2 Ti 2 O 7 and nonmagnetic pyrochlore Lu 2 Ti 2 O 7

We present here temperature-dependent Raman, x-ray diffraction, and specific heat studies between room temperature and 12 K on single crystals of spin-ice pyrochlore compound

Control of ferroelectricity and magnetism in multi-ferroic BiFeO3 by epitaxial strain

{text{Dy}}_{2}{text{Ti}}_{2}{text{O}}_{7}

Low-temperature and high-pressure Raman and X-ray studies of pyrochlore Tb 2 Ti 2 O 7 : phonon anomalies and possible phase transition

and its nonmagnetic analog

Condensation of the atomic relaxation vibrations in lead-magnesium-niobate at T=T*

{text{Lu}}_{2}{text{Ti}}_{2}{text{O}}_{7}

Threshold fields in the dc bias dependence of dielectric responses of relaxor ferroelectric terpolymer films

. Raman data show a ``new band not predicted by factor group analysis of Raman-active modes for the pyrochlore structure in

Effect of in-plane misfit strains on dielectric and pyroelectric response of poly(vinylidene fluoride-trifluoroethylene) ferroelectric polymer

{text{Dy}}_{2}{text{Ti}}_{2}{text{O}}_{7}

In situ observation of the nanocrystal growth and their piezoelectric performance change in P(VDF-TrFE) films by hot stage piezoresponse force microscopy

, appearing below a temperature of