Yuanxun Li

Strain induced magnetic anisotropy in highly epitaxial CoFe2O4 thin films

Cobalt ferrite (CoFe2O4) thin films were epitaxially grown on (001) SrTiO3 and (001) MgO by laser molecular beam epitaxy. Microstructural studies indicate that the CoFe2O4 grown on (001) SrTiO3 with compressive strain are c-oriented island growth mode with rough surface morphology, whereas the films on (001) MgO with tensile strain become c oriented with layer-by-layer mode. Magnetic property studies reveal that the compressive strained CoFe2O4 films on (001) SrTiO3 can significantly enhance out-of-plane magnetization (190emu∕cm3) with a large coercivity (3.8kOe). In contrast, the tensile strained CoFe2O4 films on (001) MgO exhibit weak magnetic anisotropy. These results suggest that strong magnetic anisotropy is highly dependent on the lattice mismatch induced strain.

Transmission line model and fields analysis of metamaterial absorber in the terahertz band

Metamaterial (MM) absorber is a novel device to provide near-unity absorption to electromagnetic wave, which is especially important in the terahertz (THz) band. However, the principal physics of MM absorber is still far from being understood. In this work, a transmission line (TL) model for MM absorber was proposed, and with this model the S-parameters, energy consumption, and the power loss density of the absorber were calculated. By this TL model, the asymmetric phenomenon of THz absorption in MM absorber is unambiguously demonstrated, and it clarifies that strong absorption of this absorber under studied is mainly related to the LC resonance of the split-ring-resonator structure. The distribution of power loss density in the absorber indicates that the electromagnetic wave is firstly concentrated into some specific locations of the absorber and then be strongly consumed. This feature as electromagnetic wave trapper renders MM absorber a potential energy converter. Based on TL model, some design strategies to widen the absorption band were also proposed for the purposes to extend its application areas.

Effects of chemical fluctuations on microstructures and properties of multiferroic BiFeO3 thin films

BiFeO3 films have been grown on SrTiO3 (001) substrates by pulsed laser deposition. It was found that oxygen partial pressure is crucial to phase purity, surface morphology, and surface chemistry. Single-phase BFO films were obtained at 1Pa O2, while Bi2O3 appeared in the films deposited at 0.01Pa as confirmed by x-ray diffractions. It was revealed that Fe2+ and metallic Bi exist in the films fabricated at 0.01Pa by x-ray photoelectron spectroscopy investigation. Owing to Fe2+ in the samples deposited at 0.01Pa, the saturation magnetization is much larger than the ones fabricated at 1Pa. A well-saturated ferroelectric hysteresis loop with a polarization of 23.6μC∕cm2 was observed in the single-phase samples. In contrast, the films deposited at 0.01Pa exhibited poor ferroelectric properties.

Epitaxial properties of ZnO thin films on SrTiO3 substrates grown by laser molecular beam epitaxy

Epitaxial ZnO thin films with different orientations have been grown by laser molecular beam epitaxy on (001)-, (011)-, and (111)-orientated SrTiO3 single-crystal substrates. The growth behavior was in situ monitored by reflection high-energy electron diffraction, and the epitaxial orientation relations were reconfirmed by ex situ x-ray diffraction measurements. In the case of ZnO on SrTiO3(001), four orthogonal domains coexisted in the ZnO epilayer, i.e., ZnO(110)‖SrTiO3(001) and ZnO[−111]‖SrTiO3⟨100⟩. For (011)- and (111)-orientated substrates, single-domain epitaxy with c axial orientation was observed, in which the in-plane relationship was ZnO[110]‖SrTiO3[110] irrespective of the substrate orientations. Additionally, the crystalline quality of ZnO on SrTiO3(111) was better than that of ZnO on SrTiO3(011) because of the same symmetry between the (111) substrates and (001) films. The obtained results can be attributed to the difference of the in-plane crystallographic symmetry. Furthermore, those align...

Enhanced dielectric characteristics of preferential (1 1 1)-oriented BZT thin films by manganese doping

Ba(Zr0.2Ti0.8)O3 (BZT) and 2 mol% Mn additional doped BZT (Mn–BZT) thin films were deposited on Pt/Ti/SiO2/Si by the pulsed laser deposition (PLD) technique under the same growth conditions. X-ray diffraction scans showed that both films were polycrystalline and preferentially (1 1 1)-oriented, and an enhanced crystallization effect was obtained after Mn doping. The parallel-plate capacitors of Au/BZT/Pt and Au/Mn–BZT/Pt were prepared to investigate the electric properties, respectively. The remanent polarization and the coercive electric field for Mn-doped BZT film were both smaller than those of undoped BZT film. Furthermore, Mn-doped BZT film exhibited a higher dielectric constant of 460 at zero bias, larger dielectric tunability of 69.0% and lower dielectric loss of 5.0‰ under an applied electric field of 615 kV cm−1 than those of undoped BZT film. The figure of merit for preferentially (1 1 1)-oriented BZT thin film was greatly enhanced from 94 to 138 by Mn doping. The enhanced dielectric behaviour by Mn doping could be mainly attributed to the decrease in electron concentration and oxygen vacancies and the reorientation of the defect complex.

Dielectric properties and tunability of cubic pyrochlore Bi1.5MgNb1.5O7 thin films

The Bi1.5MgNb1.5O7 thin films with cubic pyrochlore structure were prepared onto Pt-coated sapphire substrates by rf magnetron sputtering deposition from a stoichiometric target. Dielectric measurements indicated that the Bi1.5MgNb1.5O7 thin films exhibited low dielectric loss of ∼0.0018–0.004, medium dielectric constant of ∼86, and superior tunable dielectric properties at room temperature. A bias field of 1.6 MV/cm resulted in the maximum voltage tunability of 39%. A brief discussion is given on the enhanced tunability compared to Bi1.5ZnNb1.5O7 thin films. The low loss and superior tunability make Bi1.5MgNb1.5O7 thin films promising for potential tunable capacitor applications.

Much simplified ion-beam assisted deposition-TiN template for high-performance coated conductors

A much simplified template, i.e., two nonsuperconducting layers between the superconducting YBa2Cu3O7−δ (YBCO) and the polycrystalline metal substrate, has been developed for high-performance coated conductors by using biaxially aligned TiN as a seed layer. A combination of a thin TiN (∼10 nm by ion-beam assisted deposition) layer and an epitaxial buffer LaMnO3 layer (∼120 nm) allows us to grow epitaxial YBCO films with values of full width at half-maximum around 3.5° and 1.7° for the ϕ-scan of (103) and rocking curve of (005) YBCO, respectively. The YBCO films grown on electropolished polycrystalline Hastelloy using this two-layer template exhibited a superconducting transition temperature of 89.5 K, a critical current density of 1.2 MA/cm2 at 75.5 K, and an α value (proportional factor of critical current density Jc∼H−α) of around 0.33, indicating a high density of pinning centers and an absence of weak links.

Fabrication and characterization of pulsed laser deposited HfO2 films for high-k gate dielectric applications

We investigated the structural and electrical properties of HfO2 films fabricated by the pulsed laser deposition technique. HfO2 films were deposited directly on n-Si (100) substrates and Pt coated silicon substrates, respectively, at 300°C in a 20 Pa N2 ambient, and in situ post-annealed in a 20 Pa N2 ambient. X-ray diffraction indicates that films post-annealed at temperatures more than 500°C exhibit a polycrystalline monoclinic structure. High-resolution transmission electron microscopy images clearly show that an interfacial layer (IL) between an amorphous HfO2 layer and the Si substrate exists. An x-ray photoelectron spectroscopy measurement was performed to identify this IL as nonstoichiometric Hf-silicate. The dielectric constant of amorphous HfO2 was determined to be about 26 by measuring the Pt/HfO2/Pt capacitor structures. Capacitance–voltage measurements show that a small equivalent oxide thickness of 1.26 nm for the 5 nm HfO2 film on the n-Si substrate, with a leakage current of 2.2 mA cm−2 at 1 V gate voltage was obtained.

Study on magnetic properties of low temperature sintering M-Barium hexaferrites

The effects of Zn2+ and Ti4+ substitutions on the microstructure and properties of low temperature sintered M-type barium hexaferrites Ba(ZnTi)xFe12−2xO19 have been studied in order to adapt the development of low temperature cofired ferrites technology and produce circulators with a multilayer process. It is found that part of Zn2+ ions can enter into 2b sublattice and the saturation magnetization of the samples decrease when x increases. The additive of 3–5 wt % Bi2O3⋅B2O3⋅SiO2 glass lowers the sintering temperature to about 900 °C, which is ideal for cofiring with silver paste. Scanning electron microscope and x-ray diffraction analysis show that the samples have excellent crystalline grains with a uniform size about 1.0 μm. A high density of 4.85 g/cm3 is obtained in the samples sintered at 900 °C with 5 wt % glass additive. Magnetic measurements show that the saturation magnetization reaches 63.5 emu/g (about 308 kA/m) at 900 °C and increases as the sintering temperature arises.

Great enhancement of pyroelectric properties for Ba0.65Sr0.35TiO3 films on Pt–Si substrates by inserting a self-buffered layer

(100)-Ba0.65Sr0.35TiO3 (BST) films were deposited on Pt/Ti/SiO2/Si substrates using a low-temperature self-buffered layer. X-ray diffraction and atomic force microscope investigations show that the microstructure of BST films strongly depends on surface morphology of annealed self-buffered layer. The mechanism of nucleus formation and the growth initiation of BST films on self-buffered layers were proposed. It was found that the pyroelectric properties of BST films can be greatly enhanced. The pyroelectric coefficient and material merit figure of (100)-BST films are 1.16×104 μC m−2 K−1 and 2.18×10−4 Pa−1/2, respectively. The detectivity of 9.4×107 cm Hz1/2 W−1 was obtained in the (100)-BST film capacitors thermally isolated by 500 nm SiO2 films.