We report on the coupling between ferroelectric and magnetic order parameters in a nanostructured BaTiO3-CoFe2O4 ferroelectromagnet. This facilitates the interconversion of energies stored in electric and magnetic fields and plays an important role in many devices, including transducers, field sensors, etc. Such nanostructures were deposited on single-crystal SrTiO3 (001) substrates by pulsed laser deposition from a single Ba-Ti-Co-Fe-oxide target. The films are epitaxial in-plane as well as out-of-plane with self-assembled hexagonal arrays of CoFe2O4 nanopillars embedded in a BaTiO3 matrix. The CoFe2O4 nanopillars have uniform size and average spacing of 20 to 30 nanometers. Temperature-dependent magnetic measurements illustrate the coupling between the two order parameters, which is manifested as a change in magnetization at the ferroelectric Curie temperature. Thermodynamic analyses show that the magnetoelectric coupling in such a nanostructure can be understood on the basis of the strong elastic interactions between the two phases.

The recent discovery of ferromagnetism above room temperature in low-temperature-processed MnO(2)-ZnO has generated significant interest. Using suitably designed bulk and thin-film studies, we demonstrate that the ferromagnetism in this system originates in a metastable phase rather than by carrier-induced interaction between separated Mn atoms in ZnO. The ferromagnetism persists up to approximately 980 K, and further heating transforms the metastable phase and kills the ferromagnetism. By studying the interface diffusion and reaction between thin-film bilayers of Mn and Zn oxides, we show that a uniform solution of Mn in ZnO does not form under low-temperature processing. Instead, a metastable ferromagnetic phase develops by Zn diffusion into the Mn oxide. Direct low-temperature film growth of Zn-incorporated Mn oxide by pulsed laser deposition shows ferromagnetism at low Zn concentration for an optimum oxygen growth pressure. Our results strongly suggest that the observed ferromagnetic phase is oxygen-vacancy-stabilized Mn(2-x)Zn(x)O(3-delta.).

We report on the discovery of a lead-free morphotropic phase boundary in Sm doped BiFeO 3 with a simple perovskite structure using the combinatorial thin film strategy. The boundary is a rhombohedral to pseudo-orthorhombic structural transition which exhibits a ferroelectric (FE) to antiferroelectric (AFE) transition at approximately Bi 0.86 Sm 0.14 FeO 3 with dielectric constant and out-of-plane piezoelectric coefficient comparable to those of epitaxial (001) oriented Pb(Zr,Ti)O 3 (PZT) thin films at the MPB. The discovered composition may be a strong candidate of a Pb-free piezoelectric replacement of PZT.

ZnO thin films have been grown heteroepitaxially on epi-GaN/sapphire (0001) substrates. Rutherford backscattering spectroscopy, ion channeling, and high resolution transmission electron microscopy studies revealed high-quality epitaxial growth of ZnO on GaN with an atomically sharp interface. The x-ray diffraction and ion channeling measurements indicate near perfect alignment of the ZnO epilayers on GaN as compared to those grown directly on sapphire (0001). Low-temperature cathodoluminescence studies also indicate high optical quality of these films presumably due to the close lattice match and stacking order between ZnO and GaN. Lattice-matched epitaxy and good luminescence properties of ZnO/GaN heterostructures are thus promising for ultraviolet lasers. These heterostructures demonstrate the feasibility of integrating hybrid ZnO/GaN optoelectronic devices.

We have investigated structural phase transitions across a ferroelectric-to-antiferroelectric phase boundary in epitaxial ͑001͒ oriented Bi ͑1−x͒ Sm x FeO 3 thin films. For the Sm 3+ concentration of 0.1Յ x Յ 0.14, we observe short-range antiparallel cation displacements, verified by the appearance of localized 1 4 ͕011͖ weak reflections in the selected area electron diffraction patterns. At the critical composition of x = 0.14, the system adopts a complex nanoscale domain mixture with appearance of 1 4 ͕011͖, 1 2 ͕011͖, 1 2 ͕010͖, and 1 2 ͕111͖ reflections and an incommensurate phase bridging the rhombohedral and orthorhombic phases. For compositions 0.14Ͻ x Ͻ 0.2, orientational twin domains coupled with antiphase oxygen octahedral tilts, identified by 1 2 ͕hkl͖ weak superstructure are observed. The above systematic changes in the microstructure as a function of Sm 3+ doping are linked to the macroscopic functional properties.

Magnesium incorporation during the molecular beam epitaxy growth of wurtzite GaN is found to invert the Ga-polar (0001) face to the N-polar face. The polarity is identified based on the two different sets of reconstructions seen on the film prior to and after about 1 monolayer Mg exposure. The inversion boundary is seen to lie on the (0001) plane from transmission electron microscopy images, and a structural model is presented for the inversion. On the Ga-polar face, Mg is also seen to stabilize growth in the N-rich regime.

Articles you may be interested inStrain relaxation of epitaxial ( Ba 0.6 Sr 0.4 ) ( Zr 0.3 Ti 0.7 ) O 3 thin films grown on SrTiO 3 substrates by pulsed laser deposition

We report direct observation of controlled and reversible switching of magnetic domains using static (dc) electric fields applied in situ during Lorentz microscopy. The switching is realized through electromechanical coupling in thin film Fe(0.7)Ga(0.3)/BaTiO(3) bilayer structures mechanically released from the growth substrate. The domain wall motion is observed dynamically, allowing the direct association of local magnetic ordering throughout a range of applied electric fields. During application of approximately 7-11 MV/m electric fields to the piezoelectric BaTiO(3) film, local magnetic domains rearrange in the ferromagnetic Fe(0.7)Ga(0.3) layer due to the transfer of strain from the BaTiO(3) film. A simulation based on micromagnetic modeling shows a magnetostrictive anisotropy of 25 kPa induced in the Fe(0.7)Ga(0.3) due to the strain. This electric-field-dependent uniaxial anisotropy is proposed as a possible mechanism to control the coercive field during operation of an integrated magnetoelectric memory node.