Searcharxiv⌕ Search

arXiv subjects

Stuart A. Wolf

Publications and source records attributed to Stuart A. Wolf.

18 recordsLinked to original sources
arXiv2017

Surface morphology and superconductivity of Nb thin films by biased target ion beam deposition

One of many challenges for niobium (Nb) based superconducting devices is the improvement over the surface morphology and superconducting properties as well as the reduction of defects. We employed a novel deposition technique, i.e. biased target ion beam deposition technique (BTIBD) to prepare Nb thin films with controlled crystallinity and surface morphology. We found that the target current (ITarget) and the target bias (VTarget) were critical to the crystallinity and surface morphology of Nb films. The high target current (ITarget >500 mA and VTarget = 400 V bias) during the deposition degraded the Nb crystallinity, and subsequently reduced the critical temperature for superconductivity (Tc). VTarget was critical to the surface morphology, i.e. grain size and shape and the surface roughness. The optimized growth condition yielded very smooth film with RMS roughness of 0.4 nm that was an order of magnitude smoother than that of Nb films by sputtering process. The critical temperature for superconductivity was also close to the value of the bulk Nb. The quality of Nb film was evident in the presence of a very thin proximity layer (~ 0.8 nm). The experimental results demonstrated that the preparation of smooth Nb films with adequate superconductivity by BTIBD could serve as a base electrode for the in-situ magnetic layer or insulating layer for superconducting electronic devices.

cond-mat.mtrl-sci↗
arXiv2017

Ultrafast Electron-Lattice Coupling Dynamics in VO2 and V2O3 Thin Films

Ultrafast optical pump - optical probe and optical pump - terahertz probe spectroscopy were performed on vanadium dioxide (VO2) and vanadium sesquioxide (V2O3) thin films over a wide temperature range. A comparison of the experimental data from these two different techniques and two different vanadium oxides, in particular a comparison of the electronic oscillations generated by the photoinduced longitudinal acoustic modulation, reveals the strong electron-phonon coupling that exists in the metallic state of both materials. The low energy Drude response of V2O3 appears more susceptible than VO2 to ultrafast strain control. Additionally, our results provide a measurement of the temperature dependence of the sound velocity in both systems, revealing a four- to fivefold increase in VO2 and a three- to fivefold increase in V2O3 across the phase transition. Our data also confirm observations of strong damping and phonon anharmonicity in the metallic phase of VO2, and suggest that a similar phenomenon might be at play in the metallic phase of V2O3. More generally, our simple table-top approach provides relevant and detailed information about dynamical lattice properties of vanadium oxides, opening the way to similar studies in other complex materials.

cond-mat.str-el↗
arXiv2016

Magneto-transport and domain wall scattering in epitaxy $L1_{0}$ MnAl thin film

This work demonstrated two different kinds of magneto-transport behaviors in epitaxial $L1_{0}$ MnAl film as a function of temperature. The magneto-resistance ratio (MR) was negative and exhibited evident enhancement in the resistivity at coercive fields above 175 K. The MR enhancement was attributed to the increase of the magnetic domain walls based on the quantitative correlation between the domain density and the resistivity. Below 175 K, the MR was positive and showed a quadratic dependence on the external magnetic field, which implied that the MR was dominated by Lorentz effects.

cond-mat.mes-hall↗
arXiv2016

Spin-torque switching in large size nano-magnet with perpendicular magnetic fields

DC current induced magnetization reversal and magnetization oscillation was observed in 500 nm large size Co90Fe10/Cu/Ni80Fe20 pillars. A perpendicular external field enhanced the coercive field separation between the reference layer (Co90Fe10) and free layer (Ni80Fe20) in the pseudo spin valve, allowing a large window of external magnetic field for exploring the free-layer reversal. The magnetization precession was manifested in terms of the multiple peaks on the differential resistance curves. Depending on the bias current and applied field, the regions of magnetic switching and magnetization precession on a dynamical stability diagram has been discussed in details. Micromagnetic simulations are shown to be in good agreement with experimental results and provide insight for synchronization of inhomogenieties in large sized device. The ability to manipulate spin-dynamics on large size devices could prove useful for increasing the output power of the spin-transfer nano-oscillators (STNOs).

cond-mat.mes-hall↗
arXiv2015

Directed Self-Assembly of Epitaxial CoFe2O4-BiFeO3 Multiferroic Nanocomposites

CoFe${}_{2}$O${}_{4}$ (CFO)-BiFeO${}_{3}$ (BFO) nanocomposites are an intriguing option for future memory and logic technologies due to the magnetoelctric properties of the system. However, these nanocomposites form with CFO pillars randomly located within a BFO matrix, making implementation in devices difficult. To overcome this, we present a technique to produce patterned nanocomposites through self-assembly. CFO islands are patterned on Nb-doped SrTiO${}_{3}$ to direct the self-assembly of epitaxial CFO-BFO nanocomposites, producing square arrays of CFO pillars.

cond-mat.mtrl-sci↗
arXiv2015

Microstructural Effects of Chemical Island Templating in Patterned Matrix-Pillar Oxide Nanocomposites

The ability to pattern the location of pillars in epitaxial matrix-pillar nanocomposites is a key challenge to develop future technologies using these intriguing materials. One such model system employs a ferrimagnetic CoFe$_{2}$O$_{4}$ (CFO) pillar embedded in a ferroelectric BiFeO$_{3}$ (BFO) matrix, which has been proposed as a possible memory or logic system. These composites self-assemble spontaneously with pillars forming through nucleation at a random location when grown via physical vapor deposition. Recent results have shown that if an island of the pillar material is pre-patterned on the substrate, it is possible to control the nucleation process and determine the locations where pillars form. In this work, we employ electron microscopy and x-ray diffraction to examine the chemical composition and microstructure of patterned CFO-BFO nanocomposites. Cross-sectional transmission electron microscopy is used to examine the nucleation effects at the interface between the template island and resulting pillar.Evidence of grain boundaries and lattice tilting in the templated pillars is also presented and attributed to the microstructure of the seed island.

cond-mat.mtrl-sci↗
arXiv2015

Phase transition in bulk single crystals and thin films of VO2 by nano-infrared spectroscopy and imaging

We have systematically studied a variety of vanadium dioxide (VO2) crystalline forms, including bulk single crystals and oriented thin films, using infrared (IR) near-field spectroscopic imaging techniques. By measuring the IR spectroscopic responses of electrons and phonons in VO2 with sub-grain-size spatial resolution (~20 nm), we show that epitaxial strain in VO2 thin films not only triggers spontaneous local phase separations but also leads to intermediate electronic and lattice states that are intrinsically different from those found in bulk. Generalized rules of strain and symmetry dependent mesoscopic phase inhomogeneity are also discussed. These results set the stage for a comprehensive understanding of complex energy landscapes that may not be readily determined by macroscopic approaches.

cond-mat.str-el↗
arXiv2014

Large epitaxial bi-axial strain induces a Mott-like phase transition in VO2

The metal insulator transition (MIT) in VO2 has been an important topic for recent years. It has been generally agreed that the mechanism of the MIT in bulk VO2 is considered to be a collaborative Mott-Peierls transition, however the effect of the 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.

cond-mat.mtrl-sci↗
arXiv2013

Structural and magnetic properties of Cr-diluted CoFeB

The crystallization process and the magnetization of Cr diluted CoFeB was investigated in both ribbon samples and thin film samples with Cr content up to 30 at. %. A primary crystallization of bcc phase from an amorphous precursor in ribbon samples was observed when the annealing temperature rose to between 421 oC and 456 oC, followed by boron segregation at temperatures between 518 oC and 573 oC. The two onset crystallization temperatures showed strong dependences on both Cr and B concentrations. The impact of Cr concentration on the magnetic properties including a reduced saturation magnetization and an enhanced coercive field was also observed. The magnetizations of both ribbon samples and thin film samples were well fitted using the generalized Slater-Pauling curve with modified moments for B (-0.94 μB) and Cr (-3.6 μB). Possible origins of the enhanced coercive field were also discussed. We also achieved a damping parameter in CoFeCrB thin films at the same level as Co40Fe40B20, much lower than the value reported for CoFeCrB films previously. The results suggest a possible advantage of CoFeCrB in reducing the critical switching current density in Spin Transfer Torque Random Access Memory (STT-RAM).

cond-mat.mtrl-sci↗
arXiv2013

Metal-insulator transition induced in SrTi_{1-x}V_xO_3 thin films

Epitaxial SrTi1-xVxO3 thin films with thicknesses of ~16 nm were grown on (001)-oriented LSAT substrates using the pulsed electron-beam deposition technique. The transport study revealed a temperature driven metal-insulator transition (MIT) at 95 K for the film with x = 0.67. The films with higher vanadium concentration (x > 0.67) were metallic, and the electrical resistivity followed the T^2 law corresponding to a Fermi liquid system. In the insulating region of x < 0.67, the temperature dependence of electrical resistivity for the x = 0.5 and 0.33 films can be scaled with the variable range hopping model. The possible mechanisms behind the observed MIT were discussed, including the effects of electron correlation, lattice distortion and Anderson localization.

cond-mat.str-el↗
arXiv2013

Dimensional-Crossover-Driven Mott Insulators in SrVO3 Ultrathin Films

High-quality epitaxial SrVO3 (SVO) thin films of various thicknesses were grown on (001)-oriented LSAT substrates by pulsed electron-beam deposition technique. Thick SVO films (~25 nm) exhibited metallic behavior with the electrical resistivity following the T2 law corresponding to a Fermi liquid system. We observed a temperature driven metal-insulator transition (MIT) in SVO ultrathin films with thicknesses below 6.5 nm, the transition temperature TMIT was found to be at 50 K for the 6.5 nm film, 120 K for the 5.7 nm film and 205 K for the 3 nm film. The emergence of the observed MIT can be attributed to the dimensional crossover from a three-dimensional metal to a two-dimensional Mott insulator, as the resulting reduction in the effective bandwidth W opens a band gap at the Fermi level. The magneto-transport study of the SVO ultrathin films also confirmed the observed MIT is due to the electron-electron interactions other than localization.

cond-mat.str-el↗
arXiv2013

Transport behavior and electronic structure of phase pure VO2 thin films grown on c-plane sapphire under different O2 partial pressure

We grew highly textured phase pure VO2 thin films on c-plane Al2O3 substrates with different oxygen partial pressure. X-ray absorption and photoemission spectroscopy confirm the identical valence state of vanadium ions despite the different oxygen pressure during the deposition. As the O2 flow rate increases, the [010] lattice parameter for monoclinic VO2 was reduced and coincidently distinctive changes in the metal- semiconductor transition (MST) and transport behaviors were observed despite the identical valence state of vanadium in these samples. We discuss the effect of the oxygen partial pressure on the monoclinic structure and electronic structure of VO2, and consequently the MST.

cond-mat.mtrl-sci↗
arXiv2011

VO2 nanosheets: controlling the THz properties through strain engineering

We investigate far-infrared properties of strain engineered vanadium dioxide nanosheets through epitaxial growth on a (100)R TiO2 substrate. The nanosheets exhibit large uniaxial strain leading to highly uniform and oriented cracks along the rutile c-axis. Dramatic anisotropy arises for both the metal-insulator transition temperature, which is different from the structural transition temperature along the cR axis, and the metallic state conductivity. Detailed analysis reveals a Mott-Hubbard like behavior along the rutile cR axis.

cond-mat.str-el↗
arXiv2011

Ordering of dopants and potential increase in Tc to near room temperature

This paper describes a novel method to increase the resistive Tc of cuprate superconductors to values that might approach room temperature if the method is applied to the so-called underdoped regime. This involves ordering the dopants that provide the holes as well as provide pair breaking. The strategy is to separate the doped regions with lower Tc from metallic but undoped regions with the higher Tc.

cond-mat.supr-con↗
arXiv2011

Inhomogeneous Superconducting State and intrinsic Tc : Near Room Temperature Superconductivity in the Cuprates

Doped cuprates are inhomogeneous superconductors. The concept of an intrinsic critical temperature, Tcintr.$\equiv$ Tc*, whose value greatly exceeds that for the resistive Tcres.$\equiv$ Tc, is supported by a number of experimental studies, including those performed recently. These data are discussed in this review. The anomalous diamagnetism observed at Tcres.$\equiv$ <T< Tc* is a manifestation of the presence of superconducting clusters embedded into a normal metallic matrix. The value of intrinsic critical temperature in some cuprates reaches a value which is close to room temperature. The a.c. properties of such inhomogeneous systems are discussed.

cond-mat.supr-con↗
arXiv2010

Detection of bottom ferromagnetic electrode oxidation in magnetic tunnel junctions by magnetometry measurements

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, over-oxidation 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 exchange-bias characterizations including the EB field, training effect, uncompensated spin density, and coercivity. Variations of these parameters were found to be related to the surface oxidation of the bottom electrode, among them the change of coercivity is most sensitive. Annealed samples show evidence for an oxygen migration back to the MgO tunnel barrier by annealing.

cond-mat.mtrl-sci↗
arXiv2009

Target bias voltage effect on properties of magnetic tunnel junctions by biased target ion beam deposition

Magnetic tunnel junctions (MTJ) with AlOx barrier were fabricated by a deposition tool called Biased Target Ion Beam Deposition (BTIBD) using low energy ion source (0-50 eV) and voltage biased targets. The BTIBD system applies bias voltage directly and only on the desired targets, providing enough sputtering energy and avoiding "overspill" contamination during film deposition. The successful deposition of AlOx-MTJs demonstrated the capability of BTIBD to make multilayer structures with good film quality. MTJ thin film surface roughness and intermixing between layers are among the key problems leading to low TMR performance. Here by studying the bias voltage effect on MTJ properties via the measurement of Neel coupling field and TMR, we suggest that the lower bias voltage reduces the intermixing that occurs when top CoFe free layer is deposited on AlOx barrier, but produces relatively high surface roughness. On the other hand, higher energy deposition enhances both interlayer mixing and surface flattening. Such understanding of bias voltage effects on film properties could be used to optimize the MTJ performances.

cond-mat.mtrl-sci↗