Adam Hauser

Characterization of electronic structure and defect states of thin epitaxial BiFeO3 films by UV-visible absorption and cathodoluminescence spectroscopies

UV-visible absorption and cathodoluminescence spectra of phase-pure epitaxial BiFeO3 thin films grown on SrTiO3(001) substrates by ultrahigh vacuum sputtering reveal a bandgap of 2.69–2.73eV for highly strained ∼70nm thick BiFeO3 films. This bandgap value agrees with theoretical calculations and recent experimental results of epitaxial BiFeO3 films, demonstrating only minimal bandgap change with lattice distortion. Both absorption and cathodoluminescence spectra show defect transitions at 2.20 and 2.45eV, of which the latter can be attributed to defect states due to oxygen vacancies.

Nanoscale scanning probe ferromagnetic resonance imaging using localized modes

The discovery of new phenomena in layered and nanostructured magnetic devices is driving rapid growth in nanomagnetics research. Resulting applications such as giant magnetoresistive field sensors and spin torque devices are fuelling advances in information and communications technology, magnetoelectronic sensing and biomedicine. There is an urgent need for high-resolution magnetic-imaging tools capable of characterizing these complex, often buried, nanoscale structures. Conventional ferromagnetic resonance (FMR) provides quantitative information about ferromagnetic materials and interacting multicomponent magnetic structures with spectroscopic precision and can distinguish components of complex bulk samples through their distinctive spectroscopic features. However, it lacks the sensitivity to probe nanoscale volumes and has no imaging capabilities. Here we demonstrate FMR imaging through spin-wave localization. Although the strong interactions in a ferromagnet favour the excitation of extended collective modes, we show that the intense, spatially confined magnetic field of the micromagnetic probe tip used in FMR force microscopy can be used to localize the FMR mode immediately beneath the probe. We demonstrate FMR modes localized within volumes having 200 nm lateral dimensions, and improvements of the approach may allow these dimensions to be decreased to tens of nanometres. Our study shows that this approach is capable of providing the microscopic detail required for the characterization of ferromagnets used in fields ranging from spintronics to biomagnetism. This method is applicable to buried and surface magnets, and, being a resonance technique, measures local internal fields and other magnetic properties with spectroscopic precision.

Manipulation of Magnetically Labeled and Unlabeled Cells with Mobile Magnetic Traps

A platform of discrete microscopic magnetic elements patterned on a surface offers dynamic control over the motion of fluid-borne cells by reprogramming the magnetization within the magnetic bits. T-lymphocyte cells tethered to magnetic microspheres and untethered leukemia cells are remotely manipulated and guided along desired trajectories on a silicon surface by directed forces with average speeds up to 20 microm/s. In addition to navigating cells, the microspheres can be operated from a distance to push biological and inert entities and act as local probes in fluidic environments.

Electric field-tunable BaxSr1−xTiO3 films with high figures of merit grown by molecular beam epitaxy

We report on the dielectric properties of BaxSr1−xTiO3 (BST) films grown by molecular beam epitaxy on epitaxial Pt bottom electrodes. Paraelectric films (x ≲ 0.5) exhibit dielectric losses that are similar to those of BST single crystals and ceramics. Films with device quality factors greater than 1000 and electric field tunabilities exceeding 1:5 are demonstrated. The results provide evidence for the importance of stoichiometry control and the use of a non-energetic deposition technique for achieving high figures of merit of tunable devices with BST thin films.

Correlation between stoichiometry, strain, and metal-insulator transitions of NdNiO3 films

The interplay of film stoichiometry and strain on the metal-insulator transition (MIT) and Hall coefficient of NdNiO3 films grown under different conditions is investigated. Unstrained lattice parameters and lattice mismatch strains are evaluated for films grown under a range of growth pressures and on different substrates. It is shown that both the temperature of the MIT and the Hall coefficient in the metallic phase are highly sensitive to film strain. In films grown with lower oxygen/total growth pressures, very large compressive in-plane strains can be obtained, which can act to suppress the MIT. Both the Hall coefficient and the temperature of the MIT are relatively insensitive to growth pressure, provided that films under the same strain are compared. The results support an itinerant picture of the transition that is controlled by the Ni eg bands, and that is relatively insensitive to changes in film stoichiometry.

Tuning bad metal and non-Fermi liquid behavior in a Mott material: Rare-earth nickelate thin films.

This work elucidates unconventional metallic behavior and metal-insulator transitions in a strongly correlated materials system. Resistances that exceed the Mott-Ioffe-Regel limit (known as bad metal behavior) and non-Fermi liquid behavior are ubiquitous features of the normal state of many strongly correlated materials. We establish the conditions that lead to bad metal and non-Fermi liquid phases in NdNiO3, which exhibits a prototype bandwidth-controlled metal-insulator transition. We show that resistance saturation is determined by the magnitude of Ni eg orbital splitting, which can be tuned by strain in epitaxial films, causing the appearance of bad metal behavior under certain conditions. The results shed light on the nature of a crossover to a non-Fermi liquid metal phase and provide a predictive criterion for Anderson localization. They elucidate a seemingly complex phase behavior as a function of film strain and confinement and provide guidelines for orbital engineering and novel devices.

Tailoring resistive switching in Pt/SrTiO3 junctions by stoichiometry control

Resistive switching effects in transition metal oxide-based devices offer new opportunities for information storage and computing technologies. Although it is known that resistive switching is a defect-driven phenomenon, the precise mechanisms are still poorly understood owing to the difficulty of systematically controlling specific point defects. As a result, obtaining reliable and reproducible devices remains a major challenge for this technology. Here, we demonstrate control of resistive switching based on intentional manipulation of native point defects. Oxide molecular beam epitaxy is used to systematically investigate the effect of Ti/Sr stoichiometry on resistive switching in high-quality Pt/SrTiO3 junctions. We demonstrate resistive switching with improved state retention through the introduction of Ti- and Sr-excess into the near-interface region. More broadly, the results demonstrate the utility of high quality metal/oxide interfaces and explicit control over structural defects to improve control, uniformity, and reproducibility of resistive switching processes. Unintentional interfacial contamination layers, which are present if Schottky contacts are processed at low temperature, can easily dominate the resistive switching characteristics and complicate the interpretation if nonstoichiometry is also present.

Temperature-dependence of the Hall coefficient of NdNiO3 thin films

The Hall coefficient of epitaxial NdNiO3 films is evaluated in a wide range of temperatures, from the metallic into the insulating phase. It is shown that for temperatures for which metallic and insulating regions co-exist, the Hall coefficient must be corrected for the time-dependence in the longitudinal resistance, which is due to a slow evolution of metallic and insulating domains. The positive Hall and negative Seebeck coefficients, respectively, in the metallic phase are characteristic for two bands participating in the transport. The change in the sign of the Hall coefficient to negative values in the insulating phase is consistent with the suppression of the contribution from the large hole-like Fermi surface, i.e., the formation of a (pseudo)gap due to charge ordering.

Superfluid density of superconductor-ferromagnet bilayers

We report the first measurements of the effective superfluid density nS(T)∝λ−2(T) of superconductor-ferromagnet (SC/FM) bilayers, where λ is the effective magnetic field penetration depth. Thin Nb∕Ni bilayers were sputtered in ultrahigh vacuum in quick succession onto oxidized Si substrates. Nb layers are 102A thick for all samples, while Ni thicknesses vary from 0to100A. TC determined from λ−2(T) decreases rapidly as Ni thickness dNi increases from zero to 15A, then it has a shallow minimum at dNi≈25A. λ−2(0) behaves similarly, but has a minimum several times deeper. In fact, λ−2(0) continues to increase with increasing Ni thickness long after TC has stopped changing. We argue that this indicates a substantial superfluid density inside the ferromagnetic Ni films.

Growth control of GaAs nanowires using pulsed laser deposition with arsenic over-pressure

Using pulsed laser ablation with arsenic over-pressure, the growth conditions for GaAs nanowires have been systematically investigated. The single-crystal structure and geometry of the nanowires have been characterized for various growth conditions. Arsenic over-pressure with As2 molecules was introduced into the system by thermal decomposition of polycrystalline GaAs to control the stoichiometry and shape of the nanowires during growth. GaAs nanowires exhibit a variety of geometries under varying arsenic over-pressures, which can be understood by different growth processes via a vapor?liquid?solid mechanism. Without As2 over-pressure, branched growth of GaAs with uncontrollable size and geometry was observed due to the decomposition of GaAs nanowires, producing metallic Ga which serves as catalysts for the branched growth of GaAs on the nanowire surfaces. Under optimal As2 over-pressure, single-crystal GaAs nanowires with uniform diameter, small diameter distribution, length over 20??m and thin surface oxide layer were obtained and used for I?V characterization.