[en] Near-stoichiometric Bi_1.5Zn_1.0Nb_1.5O₇ (BZN) films were deposited by rf magnetron sputtering. The relative permittivity and dielectric loss of BZN films were measured with frequencies up to 100 MHz using planar Al₂O₃/Pt/BZN/Pt and Si/SiO₂/Pt/BZN/Pt capacitor structures. BZN films with thicknesses in the range of 160 to 170 nm exhibited electric field tunable permittivities up to 220, and dielectric loss tangents less than 0.0005. A maximum applied bias field of 2.4 MV/cm resulted in a ∼55% tunability of the dielectric constant. The permittivity was independent of the measurement frequency over a wide frequency range (10 kHz-100 MHz). Above 1 MHz, losses were increasingly dominated by conductor losses of the Pt bottom electrode. Their excellent dielectric properties make BZN films attractive candidates for low-loss, medium-permittivity integrated device applications

[en] A transit-time ultrasonic flowmeter calculates a flow rate by measuring the transit time of the ultrasonic signal. In flow measurement, there are energy attenuation and noise interference, and sometimes the ultrasonic signal will fluctuate or even distort, which makes it impossible to find the reference position and obtain the correct transit time under unstable flow fields. Therefore, based on the mathematical model of the ultrasonic signal, the characteristics of the peak ratios of adjacent peaks in the ultrasonic signal are studied, and a peak ratio matching method-based signal processing method is proposed for ultrasonic flowmeters. Firstly, through the analysis of signal quality, a standard peak ratio array is set up, then the peak ratios are calculated to obtain a real-time peak ratio array in the flow measurement, and finally a matching algorithm for elements in the standard peak ratio array and elements in the real-time peak ratio array are used to find the reference position. The transit time of the ultrasonic signal can be obtained by the reference position and positive zero-crossing points. The signal processing method is implemented in real time in the system where a time-to-digital converter timing module is used to measure zero-crossing times, an analog-to-digital converter sampling module is used to sample the ultrasonic signal at the same time, and two STM32 (32-bit ARM Cortex-M Microcontroller) processors are used to analyze the signal quality and data processing. The effectiveness of this signal processing method is verified by a large number of experiments. (paper)

[en] The ability to grow VO₂ films by electrochemical methods would open a low-cost, easily scalable production route to a number of electronic devices. We have synthesized VO_x films by anodic electrodeposition of V₂O₅, followed by partial reduction by annealing in Ar. The resulting films are heterogeneous, consisting of various metallic/oxide phases and including regions with VO₂ stoichiometry. A gradual metal insulator transition with a nearly two order of magnitude change in film resistance is observed between room temperature and 140 °C. In addition, the films exhibit a temperature coefficient of resistance of ∼ −2.4%/ °C from 20 to 140 °C

[en] Epitaxial [(SrVO₃)₇/(SrTiO₃)₄]_r (SVO/STO) superlattices were grown on (0 0 1)-oriented LSAT substrates using a pulsed electron-beam deposition technique. The transport properties of the superlattices were investigated by varying the number of repetitions of the SVO/STO bilayers r (1  ⩽  r  ⩽  9). A single SVO/STO bilayer (r  =  1) was semiconducting, whereas an increase in the number of repetitions r resulted in metallic behavior in the superlattices with r  ⩾  3. The transport phenomena in the SVO/STO superlattices can be regarded as conduction through parallel-coupled SVO layers, the SVO layer embedded in the superlattices showed a great enhancement in the conductivity compared with the single SVO layer. This work provides further evidence of electronic phase separation in the SVO ultrathin layer that has been recently discovered, the SVO ultrathin layer is considered as a 2D Mott insulator with metallic and insulating phases coexisting, the coupling between SVO layers embedded in the SVO/STO superlattices creates more conduction pathways with increasing number of repetitions r, resulting in a crossover from insulating to metallic behavior. (letter)

[en] Many works have demonstrated perpendicular magnetic anisotropy in CoFe₂O₄-BiFeO₃ (CFO-BFO) composites, which is commonly believed to originate from out-of-plane compressive strain in the CFO pillars due to the lattice mismatch with the BFO matrix. Others have shown that the pillar-matrix interface in similar NiFe₂O₄-BFO composites is fully relaxed. To study the origin of the magnetic anisotropy, composite films were grown on SrTiO₃ with thicknesses ranging from 13 to 150 nm via pulsed electron deposition. In-plane compressive strain in the pillars is found for thinner samples, which induces in-plane magnetoelastic anisotropy. A model for the origin of this previously unreported strain is proposed and the results are contrasted with the thicker composite films found in the literature.

[en] We use THz time-domain spectroscopy to investigate the far-infrared properties of vanadium dioxide thin films, strain-engineered through epitaxial growth on (100)_R TiO₂ substrates. The films exhibit a large uniaxial tensile strain along the rutile c-axis. X-ray diffraction measurements reveal a structural transition temperature of 340 K, whereas independent THz conductivity measurements yield a metal–insulator transition temperature of 365 K along c_R. Analysis of these results suggests a Mott–Hubbard behavior along the c_R-axis. Along c_R the conductivity is approximately 5500 (Ω cm)⁻¹, comparable to bulk single crystals. The tensile strain leads to remarkably uniform cracking oriented along the rutile c-axis, resulting in a large conductivity anisotropy in our single-crystal epitaxial thin films. We discuss our results in the context of previous measurements and calculations of the properties of VO₂, under different strain conditions. This work demonstrates the potential of strain engineering to tune the properties of complex materials while also serving as a powerful discriminatory tool for probing microscopic responses. (paper)

[en] Epitaxial NbO₂ thin films were synthesized on Al₂O₃ (0001) substrates via reactive bias target ion beam deposition. X-ray diffraction and Raman spectra were used to confirm the tetragonal phase of pure NbO₂. Through XPS, it was found that there was a ∼1.3 nm thick Nb₂O₅ layer on the surface and the bulk of the thin film was NbO₂. The epitaxial relationship between the NbO₂ film and the substrate was determined. Electrical transport measurement was measured up to 400 K, and the conduction mechanism was discussed

[en] Complex oxide epitaxial film growth is a rich and exciting field, owing to the wide variety of physical properties present in oxides. These properties include ferroelectricity, ferromagnetism, spin-polarization, and a variety of other correlated phenomena. Traditionally, high quality epitaxial oxide films have been grown via oxide molecular beam epitaxy or pulsed laser deposition. Here, we present the growth of high quality epitaxial films using an alternative approach, the pulsed electron-beam deposition technique. We demonstrate all three epitaxial growth modes in different oxide systems: Frank-van der Merwe (layer-by-layer); Stranski-Krastanov (layer-then-island); and Volmer-Weber (island). Analysis of film quality and morphology is presented and techniques to optimize the morphology of films are discussed.

[en] Surface oxidation of the bottom ferromagnetic (FM) electrode, one of the major detrimental factors to the performance of a magnetic tunnel junction (MTJ), is difficult to avoid during the fabrication process of the MTJ's tunnel barrier. Since Co rich alloys are commonly used for the FM electrodes in MTJs, overoxidation of the tunnel barrier results in the formation of a CoO antiferromagnetic (AF) interface layer which couples with the bottom FM electrode to form a typical AF/FM exchange bias (EB) system. In this work, surface oxidation of the CoFe and CoFeB bottom electrodes was detected via magnetometry measurements of EB characterizations including the EB field, training effect, uncompensated spin density, and enhanced coercivity. Variations in these parameters were found to be related to the surface oxidation of the bottom electrode, among them the change in coercivity is most sensitive. Annealed samples show evidence for an oxygen migration back to the MgO tunnel barrier by annealing.

[en] In this study, ferromagnetic MnAl films were prepared by alternating Al/Mn quasi-monolayer deposition using a novel biased target ion beam deposition (BTIBD) technique. XRD results showed that the magnetic τ phase was well formed in MnAl thin films (∼10 nm), which grew epitaxially on single crystal MgO (001) substrates. The optimized saturation magnetization was ∼394 emu/cc. Furthermore, we observed a thickness-dependent uniaxial anisotropy in ferromagnetic MnAl films, which was attributed to the change of the tetragonal lattice distortion as a function of film thickness. The relationship between the film thicknesses and saturation magnetizations suggested the existence of a magnetically dead layer ∼2.7 nm with an extrapolated saturation moment around 523 emu/cc (∼1.90 μ_B/Mn). This value has exceeded the experimental value in bulk materials and is close to the theoretically predicted magnetization (∼1.975 μ_B/Mn).