[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] 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] 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] Many novel superconducting compounds such as the high T_c oxides are intrinsically inhomogeneous systems by virtue of the superconductivity being closely related to the carrier density which is in turn provided in most cases by doping. An inhomogeneous structure is thus created by the statistical nature of the distribution of dopants. At the same time doping also leads to pair-breaking and, consequently, to a local depression of T_c. This is a major factor leading to inhomogeneity. As a result, the critical temperature is spatially dependent: T_c=T_c(r). The 'pseudogap' state is characterized by several energy scales: T*, T_c*, and T_c . The highest energy scale (T*) corresponds to phase separation (at T< T*) into a mixed metallic-insulating structure. Especially interesting is the region T_c*>T>T_c where the compound contains superconducting 'islands' embedded in a normal metallic matrix. As a result, the system is characterized by a normal conductance along with an energy gap structure, anomalous diamagnetism, unusual a.c. properties, an isotope effect, and a 'giant' Josephson proximity effect. An energy gap may persist to temperatures above T_c* caused by the presence of a charge density wave (CDW) or spin density wave (SDW) in the region T>T_c* but less than T*, whereas below T_c* superconducting pairing also makes a contribution to the energy gap (T_c* is an 'intrinsic' critical temperature). The values of T*, T_c*, T_c depend on the compound and the doping level. The transition at T_c into the dissipationless (R=0) macroscopically coherent state is of a percolation nature

[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] The metal insulator transition (MIT) in vanadium dioxide (VO₂) has been an important topic for recent years. It has been generally agreed upon that the mechanism of the MIT in bulk VO₂ is considered to be a collaborative Mott-Peierls transition, however, the effect of strain on the phase transition is much more complicated. In this study, the effect of the large strain on the properties of VO₂ films was investigated. One remarkable result is that highly strained epitaxial VO₂ thin films were rutile in the insulating state as well as in the metallic state. These highly strained VO₂ films underwent an electronic phase transition without the concomitant Peierls transition. Our results also show that a very large tensile strain along the c-axis of rutile VO₂ resulted in a phase transition temperature of ∼433 K, much higher than in any previous report. Our findings elicit that the metal insulator transition in VO₂ can be driven by an electronic transition alone, rather the typical coupled electronic-structural transition.

[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).

[en] Epitaxial MnAl films with a high chemical ordering were synthesized and characterized during a series of irradiations by 2 MeV protons (H⁺). The chemical ordering was first reduced to a minimum at a total fluence (TF) of 1 × 10¹⁵ H⁺/cm², and consequently was recovered at the final total fluence of 2 × 10¹⁵ H⁺/cm². We attributed the recovery of chemical ordering to thermal effects and the enhanced diffusion caused by the high energy protons. In addition, the damages by the protons have little effect on the magnetic scattering processing in MnAl characterized by the anomalous Hall effect.