Search

Epitaxial films and ordered arrays of submicron structures of nickel and cobalt ferrites were deposited on Nb doped SrTiO3 by pulsed laser deposition. X-Ray diffraction and atomic force microscopy showed that the films have a good crystalline quality and smooth surfaces. A larger number of phonon bands was observed in the polarization dependent Raman spectra of the ferrite films than expected for the cubic spinel structures. This is explained by short range ordering of the Ni2+ (or Co2+) and Fe3+ cations at the octahedral sites inducing a lowering of the symmetry. The same behavior was also observed in the Raman spectra measured for the submicron structures, suggesting the same cation distribution as in the films. The diagonal components of the dielectric function for nickel and cobalt ferrites are determined from ellipsometry in the 0.73-5 eV photon energy range. The absorption edge was analyzed using a bandgap model and the energies for the indirect and direct optical transitions were calculated. It was found that both nickel and cobalt ferrites are indirect bandgap materials with bandgaps of 1.65 eV and 1.42 eV, respectively, while the first direct transitions lie at 2.69 eV and 1.95 eV, respectively. Magneto-optical Kerr effect spectroscopy in combination with spectroscopic ellipsometry allowed the off-diagonal elements of the dielectric tensor to be determined in the energy range from 1.7 eV to 5 eV. [ABSTRACT FROM AUTHOR]

Noncollinear spin textures such as skyrmions lead to a novel Hall effect contribution called the topological Hall effect. In recent years, research has focused on the Weyl metal SrRuO3 with topological signatures reported for SrRuO3 layers adjacent to layers with strong spin–orbit coupling. However, SrRuO3 films are known to be prone to structural modifications when interfaced with different materials. In this work the Hall effect of SrRuO3/La0.7Sr0.3MnO3 superlattices (SLs) is correlated with structural and magnetic information to study whether a topological Hall effect is induced by interfacing to ferromagnetic layers. High‐quality SLs with ultrathin layers are fabricated by pulsed laser deposition on SrTiO3 substrates. The symmetry of the SrRuO3 layers is studied by angular magnetoresistance measurements and the magnetocrystalline anisotropy by magnetization measurements. Two SLs are compared in detail, both with 8 unit cell thick SrRuO3 layers, but with La0.7Sr0.3MnO3 layers 2 and 4 unit cell thick. Only the sample with 2 unit cell thick manganite layers shows features resembling a topological Hall effect. Depending on the La0.7Sr0.3MnO3 layer thickness the SrRuO3 crystalline symmetry is orthorhombic or tetragonal. The results indicate that the topological Hall effect features arise from an intricate interplay between magnetocrystalline anisotropy and antiferromagnetic interlayer coupling. [ABSTRACT FROM AUTHOR]

We investigated the structural and magnetic properties of bare SrRuO3 (SRO) ultrathin films and SrRuO3/SrIrO3/SrZrO3 (SRO/SIO/SZO: RIZ) trilayer heterostructures between 10 and 80 K, by comparing macroscopic data using the magneto-optical Kerr effect (MOKE) and magneto-transport (anomalous Hall effect: AHE), with nanoscale fingerprints when applying noncontact scanning force microscopy (nc-SFM) and magnetic force microscopy (MFM). SRO and RIZ ultrathin films were epitaxially grown at 650 °C onto vicinal SrTiO3 (100) single-crystalline substrates to a nominal thickness of 4 and 4/2/2 unit cells (uc), respectively. Our correlated analysis allows associating topographic sample features of overgrown individual layers to their residual magnetization, as is shown here to be relevant for interpreting the macroscopic AHE data. Although the hump-like features in the AHE suggest a magnetically textured skyrmion phase to exist around 55 K associated with the topological Hall effect (THE), both our MOKE and MFM data cannot support this theory for the ultrathin films investigated. In contrast, our SFM/MFM local-scale analysis finds the local coercive field to be strongly dependent on the effective layer thickness and stoichiometry in both the SRO and RIZ samples, with a huge impact on the local band structure. In fact, it is these variations that in turn mimic a potential THE through anomalies in the AHE resistivity loops. [ABSTRACT FROM AUTHOR]

We present the complex dielectric function of BiCrO3 thin films in the energy range of 0.73-9.8 eV determined using spectroscopic ellipsometry. By analyzing the absorption onset region, it is shown that the optical bandgap of BiCrO3 is indirect, with a value of 2.27 eV. The imaginary part of the BiCrO3 dielectric function, &eh;2, calculated using density functional theory in the generalized gradient approximation with an Hubbard potential of 3 eV agrees well with the experimentally determined one. Raman spectra of both polycrystalline and epitaxial thin films of BiCrO3 are reported. The temperature dependent Raman measurements indicate a structural phase transition at ∼400 K which was confirmed also by x-ray diffraction investigations. [ABSTRACT FROM AUTHOR]

We have investigated the individual switching of nanoscale capacitors by piezoresponse force microscopy. Nanoscale epitaxial ferroelectric capacitors with terabyte per inch square equivalent density were fabricated by the deposition of top electrodes onto a pulsed laser deposited lead zirconate titanate thin film by electron beam evaporation through ultrathin anodic aluminum oxide membrane stencil masks. Using bias pulses, the nanoscale capacitors were uniformly switched and proved to be individually addressable. These film-based nanoscale capacitors might be a feasible alternative for high-density mass storage memory applications with near terabyte per inch square density due to the absence of any cross-talk effects. [ABSTRACT FROM AUTHOR]

Heterostructures consisting of PbZr0.2Ti0.8O3 and PbZr0.4Ti0.6O3 epitaxial films on a SrTiO3 (100) substrate with a SrRuO3 bottom electrode were prepared by pulsed laser deposition. By using the additional interface provided by the ferroelectric bilayer structure and changing the sequence of the layers, the content of dislocations and elastic domain types was varied in a controlled manner. The resulting microstructure was investigated by transmission electron microscopy. Macroscopic ferroelectric measurements have shown a large impact of the formation of dislocations and 90° domain walls on the ferroelectric polarization and dielectric constant. A thermodynamic analysis using the Landau–Ginzburg–Devonshire approach that takes into account the ratio of the thicknesses of the two ferroelectric layers and electrostatic coupling is used to shed light on the experimental data. [ABSTRACT FROM AUTHOR]

Electrical properties, i.e., dielectric hysteresis and leakage current, of epitaxial Pb(Zr0.2Ti0.8)O3 films with bottom SrRuO3 electrode and different metals as top contact were investigated. The leakage current is largely insensitive to the work function of the top metal but increases with decreasing electronegativity as well as with decreasing number of electrons on the d-shell of the top metal. The best rectifying properties are obtained for metals with complete d-shell (Cu, Au, Ag, Pd), while the metals with few electrons on the d-shell (Ta, Cr) form Ohmic-like contacts. [ABSTRACT FROM AUTHOR]

Heterostructures consisting of Pb[Zr.sub.0.2][Ti.sub.0.8][O.sub.3] and Pb[Zr.sub.0.4][Ti.sub.0.6][O.sub.3] epitaxial films on a SrTi[O.sub.3] (100) substrate with a SrRu[O.sub.3] bottom electrode are prepared by pulsed laser deposition. Macroscopic ferroelectric measurements have a major impact on the formation of dislocations and 90 [degree] domain walls on the ferroelectric polarization and dielectric constant.

The electrical properties like dielectric hysteresis and leakage current of epitaxial Pb([Zr.sub.0.2][Ti.sub.0.8])[O.sub.3] films with bottom SrRu[O.sub.3] electrode and different metals as top contact are studied. The current is mostly insensitive to the work function of the top metal but has increased with the decreasing electronegativity as well as with decreasing number of electrons on the d-shell of the top metal.

Ferromagnetic multilayers with asymmetric interfaces and perpendicular magnetic anisotropy can stabilize non-trivial magnetic structures due to interfacial Dzyaloshinskii-Moriya interactions. Magnetic interlayer coupling between ferromagnetic layers separated by non-magnetic insulating spacers is an additional important ingredient for the properties of multilayers. We addressed the magnetic coupling between ferromagnetic SrRuO3 epitaxial layers separated by ultrathin spacers of SrIrO3/SrZrO3, so that inversion symmetry is broken at the top and bottom interfaces. Major and minor magnetization loops allowed us to assess the type and strength of the magnetic coupling. We inferred that the magnetic coupling of SrRuO3 layers through non-magnetic insulating interlayers (≤1.6 nm thick) is weakly ferromagnetic. [ABSTRACT FROM AUTHOR]

Interaction of conduction electrons with non-coplanar or non-collinear spin textures in metals can result in intriguing phenomena, among which a topological Hall effect. Here, we report on topological Hall effect manifestation in epitaxial heterostructures of thin metallic layers of two prototypical ferromagnetic oxides, namely [ABSTRACT FROM AUTHOR]

Orbital ordering has been less investigated in epitaxial thin films, due to the difficulty to evidence directly the occurrence of this phenomenon in thin film samples. Atomic resolution electron microscopy enabled us to observe the structural details of the ultrathin LaVO3 films. The transition to orbital ordering of epitaxial layers as thin as ≈4 nm was probed by temperature-dependent Raman scattering spectroscopy of multilayer samples. From the occurrence and temperature dependence of the 700 cm-1 Raman active mode it can be inferred that the structural phase transition associated with orbital ordering takes place in ultrathin LaVO3 films at about 130 K. [ABSTRACT FROM AUTHOR]

The degradation of the functional properties of epitaxial oxide films and the performance of related devices have often been attributed to the presence of so-called interfacial 'dead layers'. Extensive efforts have been made to understand the origin of such dead layers and to avoid their formation. However, the results of these efforts have not been fully satisfactory, largely as a result of the complex origin of dead layers. Here, the dead layer is studied in a sample that contains hillocks of nominally La0.7Sr0.3MnO3 (LSMO) sandwiched between a ferroelectric PbTiO3 film and a Nb-doped SrTiO3 substrate using aberration-corrected (scanning) transmission electron microscopy, nanobeam electron magnetic circular dichroism spectroscopy, and related techniques. The results here reveal the presence of a 5 nm thick layer in the LSMO hillocks, close to the LSMO/Nb-SrTiO3 interface, which exhibits distinct structural and physical properties. The atomic, electronic, and magnetic structures and local chemistry of the interfacial layer are determined. It is found that octahedral rotations are suppressed in ultrathin regions at the edges of the hillocks, providing evidence for a strong effect of the adjacent Nb-SrTiO3 and PbTiO3. The formation of the dead layer is discussed in the light of lattice strain and valence changes of Mn ions. [ABSTRACT FROM AUTHOR]