We demonstrate that epitaxial strain engineering is an efficient method to manipulate the ferromagnetic and ferroelectric properties in BiFeO(3)-CoFe(2)O(4) columnar nanocomposites. On one hand, the magnetic anisotropy of CoFe(2)O(4) is totally tunable from parallel to perpendicular controlling the CoFe(2)O(4) strain with proper combinations of substrate and ferroelectric phase. On the other hand, the selection of the used substrate allows the growth of the rhombohedral bulk phase of BiFeO(3) or the metastable nearly tetragonal one, which implies a rotation of the ferroelectric polar axis from [111] to close to the [001] direction. Remarkably, epitaxy is preserved and interfaces are semicoherent even when lattice mismatch is above 10%. The broad range of sustainable mismatch suggests new opportunities to assemble epitaxial nanostructures combining highly dissimilar materials with distinct functionalities.

3D single-crystalline, well-aligned GaN-InGaN rod arrays are fabricated by selective area growth (SAG) metal-organic vapor phase epitaxy (MOVPE) for visible-light water splitting. Epitaxial InGaN layer grows successfully on 3D GaN rods to minimize defects within the GaN-InGaN heterojunctions. The indium concentration (In ∼ 0.30 ± 0.04) is rather homogeneous in InGaN shells along the radial and longitudinal directions. The growing strategy allows us to tune the band gap of the InGaN layer in order to match the visible absorption with the solar spectrum as well as to align the semiconductor bands close to the water redox potentials to achieve high efficiency. The relation between structure, surface, and photoelectrochemical property of GaN-InGaN is explored by transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), Auger electron spectroscopy (AES), current-voltage, and open circuit potential (OCP) measurements. The epitaxial GaN-InGaN interface, pseudomorphic InGaN thin films, homogeneous and suitable indium concentration and defined surface orientation are properties demanded for systematic study and efficient photoanodes based on III-nitride heterojunctions.

Electron-energy-loss spectroscopy is used to map composition and electronic states in epitaxial La 2/3 Ca 1/3 MnO 3 ͑LCMO͒ films of various thicknesses grown on SrTiO 3 ͑001͒ and ͑110͒ substrates. For relatively thick films ͑Ն20 nm͒, epitaxial tensile strain in ͑001͒ films promotes a compositional La/Ca gradient across the film thickness, being the interface La rich, while the relaxed ͑110͒ films are chemically homogeneous. In contrast, much thinner ͑001͒ and ͑110͒ LCMO films display a different La/Ca distribution, being La rich at the free surface. The observed distinct thickness-dependent composition gradient behavior reflects a balance between strain-induced elastic energy minimization and kinetic effects during growth. © 2009 American Institute of Physics. ͓DOI: 10.1063/1.3211130͔Mixed-valence ferromagnetic manganite films have been the object of much attention in recent years due to their potential applications in spintronics. 1,2 However, expectations have been lowered by the negligible room-temperature magnetoresistance observed in tunnel junctions. 3 Although the reasons for this behavior are not yet fully known, it has been suggested that they may be linked to electronic phase separation. 4 Whether the latter is a pure electronic effect or related to inhomogeneous chemical distributions can be suitably explored by electron energy-loss spectroscopy ͑EELS͒. For instance, electronic states can be mapped out by direct determination of local Mn oxidation state at the nanometric scale. [5][6][7][8] On the other hand, mappings of the chemical composition can be achieved offering chemical information at the nanometric scale [5][6][7][8][9] We recently reported 8 a comparative characterization, both from the microstructural and chemical points of view, of relatively thick ͑ϳ80 nm͒ LCMO layers grown on STO in order to determine which differences accounted for the fact that ͑110͒ LCMO films display enhanced magnetic properties when compared to their ͑001͒ counterparts. While homogeneous chemical composition was associated to the contribution of plastic defects as strain-relieve mechanism in the more relaxed ͑110͒ films, a Ca 2+ ion migration toward free surface, and the concomitant Mn m+ oxidation state variation was observed in the more stressed ͑001͒ films.In this letter, we have extended the characterization of the microstructure, local stoichiometry variations, and Mn oxidation state of ͑001͒ and ͑110͒ LCMO films within a range of layer thicknesses between 3.5 and 40 nm. The results confirm the same trend for LCMO films with t ϳ 40 nm as those with t ϳ 80 nm previously reported. 8 However, most importantly, we assess that for ultrathin LCMO films ͑t Ͻ 20 nm͒, a new scenario is revealed with significant differences regarding cation distribution across the film in comparison with thicker layers, while keeping an orthorhombic crystalline phase in contrast with structural changes reported by Qin et al. 10 The ͑001͒ and ͑110͒ LCMO films with thicknesses ͑t͒ ranging from 3.5 to 82 nm were grown by rf sputtering at 8...

Ferroelectric (FE) and ferromagnetic (FM) materials engineered in horizontal heterostructures allow interface-mediated magnetoelectric coupling. The so-called converse magnetoelectric effect (CME) has been already demonstrated by electric-field poling of the ferroelectric layers and subsequent modification of the magnetic state of adjacent ferromagnetic layers by strain effects and/or free-carrier density tuning. Here we focus on the direct magnetoelectric effect (DME) where the dielectric state of a ferroelectric thin film is modified by a magnetic field. Ferroelectric BaTiO3 (BTO) and ferromagnetic CoFe2O4 (CFO) oxide thin films have been used to create epitaxial FE/FM and FM/FE heterostructures on SrTiO3(001) substrates buffered with metallic SrRuO3. It will be shown that large ferroelectric polarization and DME can be obtained by appropriate selection of the stacking order of the FE and FM films and their relative thicknesses. The dielectric permittivity, at the structural transitions of BTO, is strongly modified (up to 36%) when measurements are performed under a magnetic field. Due to the insulating nature of the ferromagnetic layer and the concomitant absence of the electric-field effect, the observed DME effect solely results from the magnetostrictive response of CFO elastically coupled to the BTO layer. These findings show that appropriate architecture and materials selection allow overcoming substrate-induced clamping in multiferroic multi-layered films.