F. Hunte

North Carolina State University, Raleigh, North Carolina, United States

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Publications (43)118.35 Total impact

  • [Show abstract] [Hide abstract] ABSTRACT: We report the growth of bi-epitaxial La0.7Sr0.3MnO3 (110) thin films on Si (001) substrate with cubic yttria stabilized zirconia (c-YSZ)/SrTiO3 (STO) buffer layers by pulsed laser deposition. The La0.7Sr0.3MnO3 and STO thin films were grown with a single [110] out-of-plane orientation and with two in-plane domain variants, which is confirmed by XRD and detailed TEM studies. The growth of STO on c-YSZ can be explained by the paradigm of domain matching epitaxy. The epitaxial relationship between STO and c-YSZ can be written as [110] (001) c-YSZ ‖ [ ] (110) STO (or) [110] (001) c-YSZ ‖ (110) STO. The La0.7Sr0.3MnO3 thin films are ferromagnetic with Curie temperature 324 K and showed metal to insulator transition at 285 K. The La0.7Sr0.3MnO3 thin films showed hysteresis loops in magnetoresistance when magnetic field is applied along both in-plane (110) and out-of-plane [110] directions. The highest magnetoresistance obtained in this study is −32% at 50 K and 50 kOe for in-plane configuration, whereas the room-temperature magnetoresistance is −4% at 10 kOe and −17% at 50 kOe. The hysteresis in the magnetoresistance and the controlled domain boundaries in bi-epitaxial La0.7Sr0.3MnO3 films integrated on Si can offer significant advantages over the polycrystalline counterparts.
    No preview · Article · Mar 2016 · Acta Materialia
  • [Show abstract] [Hide abstract] ABSTRACT: Magnesium diboride (MgB2) is a promising superconductor for many technical applications. Sufficient current densities at required magnetic fields, moderate operational temperature, low raw materials' cost, and an economical manufacturing process have enabled commercial development of MgB2 wires. Reacted MgB2, however, is brittle, and applications involving coils and windings with small bend radii are therefore difficult to implement. Furthermore, improvements in the critical current density are needed to expand the range of potential applications. Here, we report on the electrical behavior of novel MgB2/Ga composite wires produced such that the proximity effect enhances connectivity, allowing the high-temperature anneal typically required for in situ and ex situ MgB2 wires to be eliminated. Elimination of the high-temperature anneal simplifies MgB2 manufacturing and has the potential to create a wire that is more tolerant of bending. Here, we present critical current density and quench propagation results for MgB2/Ga composite wires sheathed in Cu.
    No preview · Article · Dec 2015 · IEEE Transactions on Applied Superconductivity
  • Y. F. Lee · R. Kumar · F. Hunte · J. Narayan · J. Schwartz
    [Show abstract] [Hide abstract] ABSTRACT: We report the epitaxial integration of defect-induced room temperature ferromagnetic insulators, Cr2O3 and MgO, with topological insulators Bi2Se3 on c-sapphire substrate by pulsed laser deposition. The structural, magnetic, and magnetotransport properties of ∼15 nm Bi2Se3 thin films are investigated on each template. The lattice misfits of Cr2O3/Bi2Se3 and MgO/Bi2Se3 are ∼16% and ∼39%, respectively, where the critical thickness for pseudomorphic growth is less than one monolayer. The insulating behavior is more pronounced due to the additional scattering of the surface states of the Bi2Se3 layer by interfacing with MgO and Cr2O3. The weak antilocalization effect from the surface states is clearly suppressed, accounting for the presence of magnetic bottom layers. This work demonstrates an effective way to study the emergence of a ferromagnetic phase in topological insulators by the magnetic proximity effect in Bi2Se3, a step toward unveiling their exotic properties.
    No preview · Article · Sep 2015 · Journal of Applied Physics
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    Y.F. Lee · R. Kumar · F. Hunte · J. Narayan · J. Schwartz
    [Show abstract] [Hide abstract] ABSTRACT: We have synthesised Bi2Se3 epitaxial thin films on c-sapphire substrates, where Se-related defects and strains are controlled precisely during pulsed laser deposition. This allows us to tune electrical and magnetotransport properties and probe the role of defects and strains as a function of processing conditions systematically. The defect microstructure has been studied in detail using high resolution X-ray diffraction and high-angle annular dark field scanning transmission electron microscopy. Magnetotransport measurements show a strong dependence on microstructure which is associated with the Se-content. With higher Se content, the film experiences large compressive strain along the [0 0 1] direction which is accompanied by the partial suppression of one family of twin domain formation. As a result, the insulating behavior becomes more pronounced at a low temperature which is understood in terms of the quantum correlation induced by electron–electron interactions. The compressive strain enhances spin–orbit coupling and topological characteristics. These results shed light on the importance of controlling the intrinsic defects during the growth of Bi2Se3 thin films, providing an effective way to suppress the bulk conductivity and establish the correlation between microstructure and strain.
    Full-text · Article · Aug 2015 · Acta Materialia
  • [Show abstract] [Hide abstract] ABSTRACT: We report on the epitaxial growth and magnetic properties of Cr2O3 thin films grown on r-sapphire substrate using pulsed laser deposition. The X-ray diffraction (XRD) (2θ and Φ) and TEM characterization confirm that the films are grown epitaxially. The r-plane ( 01 1 ¯ 2 ) of Cr2O3 grows on r-plane of sapphire. The epitaxial relations can be written as [ 01 1 ¯ 2 ] Cr2O3 ‖ [ 01 1 ¯ 2 ] Al2O3 (out-of-plane) and [ 1 ¯ 1 ¯ 20 ] Cr2O3 ‖ [ 1 ¯ 1 ¯ 20 ] Al2O3 (in-plane). The as-deposited films showed ferromagnetic behavior up to 400 K but ferromagnetism almost vanishes with oxygen annealing. The Raman spectroscopy data together with strain measurements using high resolution XRD indicate that ferromagnetism in r-Cr2O3 thin films is due to the strain caused by defects, such as oxygen vacancies.
    No preview · Article · May 2015 · Journal of Applied Physics
  • [Show abstract] [Hide abstract] ABSTRACT: Intrinsic defects in Bi2Se3 topological insulators tend to produce a high carrier concentration and current leakage through the bulk material. Bi2Se3 thin films were grown by hybrid physical chemical vapor deposition on (0001) Al2O3 substrates with high Se vapor pressure to reduce the occurrence of Se vacancies as the main type of defect. Consequently, the carrier concentration was reduced to ∼5.75 × 1018 cm−3 comparable to reported carrier concentration in Bi2Se3 thin films. Magnetotransport measurements were performed on the films and the data were analyzed for weak anti-localization using the Hikami-Larkin-Nagaoka model. The estimated α and l ϕ values showed good agreement with the symplectic case of 2-D transport of topological surface states in the quantum diffusion regime. The temperature and angular dependence of magnetoresistance indicate a large contribution of the 2-D surface carriers to overall transport properties of Bi2Se3 thin film.
    No preview · Article · Feb 2015 · Journal of Applied Physics
  • [Show abstract] [Hide abstract] ABSTRACT: We report on the epitaxial growth and magnetic properties of antiferromagnetic and magnetoelectric (ME) Cr2O3 thin films deposited on cubic yttria stabilized zirconia (c-YSZ)/Si(001) using pulsed laser deposition. The X-ray diffraction (2ϴ and Φ) and TEM characterizations confirm that the films were grown epitaxially. The Cr2O3(0001) growth on YSZ(001) occurs with twin domains. There are four domains of Cr2O3 with in-plane rotation of 30° or 150° from each other about the [0001] growth direction. The epitaxial relation between the layers is given as [001]Si ‖ [001]YSZ ‖ [0001]Cr2O3 and [100]Si ǁ [100]YSZ ǁ [101¯0] Cr2O3 or [112¯0] Cr2O3. Though the bulk Cr2O3 is an antiferromagnetic with TN = 307 K, we found that the films exhibit ferromagnetic like hysteresis loops with high saturation and finite coercive field up to 400 K. The thickness dependent magnetizations together with oxygen annealing results suggest that the ferromagnetism (FM) is due to oxygen related defects whose concentration is controlled by strain present in the films. This FM, in addition to the intrinsic magneto-electric properties of Cr2O3, opens the door to relevant spintronics applications.
    No preview · Article · Sep 2014 · Applied Physics Letters
  • [Show abstract] [Hide abstract] ABSTRACT: In this paper, we report on the epitaxial integration of room temperature lead-free ferroelectric BaTiO3 thin (∼1050 nm) films on Si (100) substrates by pulsed laser deposition technique through a domain matching epitaxy paradigm. We employed MgO and TiN as buffer layers to create BaTiO3/SrRuO3/MgO/TiN/Si (100) heterostructures. C-axis oriented and cube-on-cube epitaxial BaTiO3 is formed on Si (100) as evidenced by the in-plane and out-of-plane x-ray diffraction, and transmission electron microscopy. X-ray photoemission spectroscopic measurements show that Ti is in 4(+) state. Polarization hysteresis measurements together with Raman spectroscopy and temperature-dependent x-ray diffraction confirm the room temperature ferroelectric nature of BaTiO3. Furthermore, laser irradiation of BaTiO3 thin film is found to induce ferromagnetic-like behavior but affects adversely the ferroelectric characteristics. Laser irradiation induced ferromagnetic properties seem to originate from the creation of oxygen vacancies, whereas the pristine BaTiO3 shows diamagnetic behavior, as expected. This work has opened up the route for the integration of room temperature lead-free ferroelectric functional oxides on a silicon platform.
    No preview · Article · Sep 2014 · Journal of Applied Physics
  • [Show abstract] [Hide abstract] ABSTRACT: Recent electrical measurements have accessed transport in the topological surface state band of thin exfoliated samples of Bi2Se3 by removing the bulk n-type doping by contact with thin films of the molecular acceptor F4-TCNQ. Here we report on the film growth and interfacial electronic characterization of F4-TCNQ grown on Bi2Se3. Atomic force microscopy shows wetting layer formation followed by 3D island growth. X-ray photoelectron spectroscopy is consistent with this picture and also shows that charge transferred to the molecular layer is localized on nitrogen atoms. Ultraviolet photoelectron spectroscopy shows a work function increase and an upward shift of the valence band edge that suggest significant reduction in carrier density at the Bi2Se3 surface.
    No preview · Article · Jul 2014 · The Journal of Physical Chemistry C
  • [Show abstract] [Hide abstract] ABSTRACT: We report the bi-epitaxial growth of ZnO and resistance switching characteristics of Pt/ZnO/TiN-based heterojunction devices fabricated on Si(001) substrates by pulsed laser deposition. The structural properties of the heterostructures characterized by XRD (θ-2θ, φ scans) and TEM confirm that the ZnO films having hexagonal wurtzite structure (six-fold symmetry) grow bi-epitaxially on the TiN buffer layer (four-fold symmetry). The Pt(111) grows epitaxially on ZnO(0001). The epitaxial relationship between the various films is given as (111)Pt ‖ (0001)ZnO ‖ (001)TiN ‖ (001)Si and [100]TiN ‖ [100]Si, [21¯1¯0]ZnO ‖ [110]TiN or [101¯0]ZnO ‖ [110]TiN, and [101¯]Pt ‖ [21¯1¯0]ZnO. The effect of ZnO growth temperature on the electrical properties of Pt/ZnO/TiN devices is studied and correlated with the microstructure of the ZnO/TiN interface. The Pt/ZnO/TiN devices exhibited good bi-polar resistance switching characteristics at voltages as low as ±1 V.
    No preview · Article · Jun 2014 · Journal of Applied Physics
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    [Show abstract] [Hide abstract] ABSTRACT: High strength dispersion strengthened (DS) Ag/Al alloys with various Al content are studied as candidates for sheathing Bi2Sr2CaCu2O8Cx (Bi2212) wire. The Ag/Al alloys are fabricated by powder metallurgy and internally oxidized in pure oxygen. The time and temperature of the internal oxidation heat treatment is varied to maximize the strength after undergoing the Bi2212 partial melt process (PMP). Vickers micro-hardness number (HVN), room temperature tensile behavior, optical and scanning electron microscopy, ion channeling contrast imaging using a focused ion beam and electrical resistivity measurements are used to characterize the alloys. An Ag/0.2wt%Mg (Ag/Mg) alloy is used for comparison. Results show that internal oxidation at 650–700 �C for 4 h produces the highest HVN for the DS Ag/Al alloy; when oxidized at 675 �C for 4 h the HVN, yield strength and tensile strength of the DS Ag/Al are 50% higher than the corresponding values of Ag/Mg. Microstructural observations show that Al2O3 precipitates play the main role in strengthening the DS Ag/Al alloy. The alloy retains its fine grain structure and strength after PMP heat treatment.
    Full-text · Article · Oct 2013 · Superconductor Science and Technology
  • [Show abstract] [Hide abstract] ABSTRACT: One of the remaining challenges for the implementation of commercial Ag-alloy-sheathed Bi2Sr2CaCu2O8+x (Bi2212) wires in high-field superconducting magnets is quench protection. To develop an effective quench protection system, it is important to understand the conditions that must be avoided during a quench so that the conductor is not degraded. While these conditions are understood for NbTi and Nb3Sn, they are conductor specific and there remains a lack of data and understanding of the limiting conditions for Bi2212 wires. Here, quenches are induced in short strands and small coils of Bi2212 round wires at 4.2 K. The quench conditions are varied to identify the threshold conditions resulting in wire degradation. These conditions are quantified in terms of the maximum temperature, the maximum time rate of change of the temperature, and the maximum temperature spatial gradient along the length of the wire. It is found that the time rate of change of the temperature (thermal shock) is not a primary driver for degradation but that both the maximum temperature and its spatial gradient play a key role. It is not clear, however, whether the temperature gradient along the length of the wire, or radially from the center of the wire to the surface, dominates. It is also found that threshold values for these parameters vary between different Bi2212 wires and, thus, must be identified for the specific wire to be used in a magnet system. Implications of these results on quench protection are discussed.
    No preview · Article · Oct 2013 · IEEE Transactions on Applied Superconductivity
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    [Show abstract] [Hide abstract] ABSTRACT: The significant amount of energy stored in a large high-field superconducting magnet can be sufficient to destroy the coil in the event of an unprotected quench. For magnets based on high-temperature superconductors (HTSs), such as Bi2Sr2CaCu2Ox (Bi2212) and YBa2Cu3O7−x (YBCO), quench protection is particularly challenging due to slow normal zone propagation. A previous computational study showed that the quench behavior of HTS magnets is significantly improved if the turn-to-turn electrical insulation is thermally conducting, enhancing 3-D normal zone propagation. Here, a new doped-titania electrical insulation with high thermal conductivity is evaluated. The thermal conductivity of the insulation is measured at cryogenic temperatures, and its chemical compatibility with Bi2212 round wires is determined. Thin layers of the insulation are deposited onto the surface of Bi2212 and YBCO wires, which are then wound into small coils to study the quench behavior. Results show that the critical current and homogeneity of Bi2212 coils are improved relative to coils reacted with mullite insulation. Relative to similar coils with conventional insulation (mullite for Bi2212 and Kapton for YBCO), the turn-to-turn quench propagation is increased by a factor of 2.8 in Bi2212 coils at 4.2 K and self-field and by a factor of 2.5 in YBCO coils at 4.2 K and 5 T. These results indicate that doped-titania insulation may significantly improve Bi2212 and YBCO coils. Increased normal zone propagation velocity enhances quench detection and quench protection, and the thinness of the insulation relative to the most common alternatives increases the magnet winding pack current density and reduces the coil specific heat.
    Full-text · Article · Oct 2013 · IEEE Transactions on Applied Superconductivity
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    Y F Lee · F Wu · R Kumar · F Hunte · J Schwartz
    [Show abstract] [Hide abstract] ABSTRACT: Epitaxial thin films heterostructures of topological insulator candidate Sr3SnO (SSO) are grown on a cubic yttria-stabilized zirconia (c-YSZ)/Si (001) platform by pulsed laser deposition. X-ray and electron diffraction patterns confirm the epitaxial nature of the layers with cube-on-cube orientation relationship: (001)[100]SSO∥(001)[100]c-YSZ∥(001)[100]Si. The temperature dependent electrical resistivity shows semiconductor behavior with a transport mechanism following the variable-range-hopping model. The SSO films show room-temperature ferromagnetism with a high saturation magnetization, and a finite non-zero coercivity persisting up to room temperature. These results indicate that SSO is a potential dilute magnetic semiconductor, presumably obtained by controlled introduction of intrinsic defects.
    Full-text · Article · Sep 2013 · Applied Physics Letters
  • Liyang Ye · Frank Hunte · Justin Schwartz
    [Show abstract] [Hide abstract] ABSTRACT: The development of high field superconducting magnets using high temperature superconductors (HTSs) is progressing for high energy physics, nuclear magnetic resonance and energy storage applications. Yet the key issue of quench protection remains unresolved, primarily due to the slow normal zone propagation velocity (NZPV) in HTS magnets. High magnetic field may affect the quench behavior through two opposing effects: increased NZPV may result due to reduced critical temperature and current sharing temperature, but decreased NZPV may result due to reduced critical current density and thus operating current. At present it is unclear which effect dominates. Here, a series of quench experiments at high magnetic field on multilayer wind-and-react Bi2Sr2CaCu2Ox (Bi2212) coils addresses this question. The two- and three-dimensional quench behavior is investigated in a magnetic field up to 20 T at 4.2 K. With increasing magnetic field, the minimum quench energy decreases significantly. The NZPV also decreases with magnetic field, but only up to about 8 T. For magnetic fields above 8 T, the NZPV is independent of magnetic field up to at least 20 T. Thus, at low field the NZPV is dominated by the decreasing critical current density, whereas at higher magnetic field the competing effects of decreasing critical current density and decreasing temperature margin offset each other.
    No preview · Article · Mar 2013 · Superconductor Science and Technology
  • Honghai Song · Frank Hunte · Justin Schwartz
    [Show abstract] [Hide abstract] ABSTRACT: YBa2Cu3O7–x (YBCO) coated conductors are emerging as an important option for magnets for energy systems and experimental science. One of the remaining challenges for YBCO superconducting magnets is quench protection, i.e. ensuring that the YBCO is not damaged due to a fault condition. One key issue is understanding the underlying causes of degradation during a quench. Here, the microstructure of a quenched, degraded sampled is compared to that of an unquenched control sample. To facilitate microstructural analysis of the YBCO surface, the Cu stabilizer and Ag cap layer were removed by etching. Reactions between the Cu etchant and YBCO proved to be a signature of Ag/YBCO delamination. Two types of pre-existing defects were identified as initiation points of degradation. Defects on the conductor edge resulting in delaminated Ag lead to dendritic flux avalanches and high local heating, which cause further Ag delamination. This self-propagating effect results in dendritic Ag delamination, which is seen through etchant–YBCO reactions. Defects within the YBCO layer result in breaches in the protective Ag layer such that Cu etchant penetrates and reacts with the YBCO. Energy-dispersive X-ray spectroscopy analysis showed similar reactions as in the edge degradation but also showed pure Ag particles, which indicates that the local temperature was sufficient to cause localized Ag melting.
    No preview · Article · Dec 2012 · Acta Materialia
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    [Show abstract] [Hide abstract] ABSTRACT: High-strength high-elastic-modulus dispersion-strengthened (DS) silver aluminum alloys are studied for sheath-ing Bi 2 Sr 2 CaCu 2 O 8+x (Bi2212) round wire. DS is an effective method for producing a fine grain metallurgical structure that is resistant to softening during high-temperature heat treatment. Here, DS Ag/0.5-wt.% Al (AgAl) alloy sheet is produced using powder metallurgy and is compared with Ag/0.2-wt.% Mg (AgMg) alloy, which is currently the most common alloy used for Bi2212 wire. Room temperature (RT), 77-and 4.0-K tensile tests, Vickers microhardness, optical microscopy, field emission scanning electron microscopy, and electrical resistivity measurements are compared. Furthermore, Bi2212/AgMg and Bi2212/AgAl wires are produced and compared for short-sample and coil I c (4.2 K; self-field). It is found that the AgAl solid wire shows high yield stress and ultimate tensile strength in the annealed condition at both RT and 4.0 K, as well as significant ductility at 4.0 K. Electrical transport measurements show that the Bi2212/AgAl wires perform as well or better than Bi2212/AgMg wires. Furthermore, no leakage is observed after partial melt processing (PMP) of Bi2212/AgAl spirals. After PMP, the Bi2212/AgAl wire not only has yield and tensile stresses slightly higher than those of the Bi2212/AgMg wire but also exhibits > 2% elongation, which is several times higher than that of Bi2212/AgMg. Index Terms—Dispersion-strengthened (DS) alloys, high-temperature superconductor, tensile properties, transport measurements.
    Full-text · Article · Feb 2012 · IEEE Transactions on Applied Superconductivity
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    [Show abstract] [Hide abstract] ABSTRACT: This paper reviews the status of high temperature superconductors for high field magnets for future devices such as a high energy LHC or a muon collider. Some of the primary challenges faced for the implementation of systems are discussed. Two conductor technologies, Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ and YBa$_2$Cu$_3$O$_{7-\delta}$, have emerged as high field conductor options, but their relative advantages and disadvantages for high field magnets are quite different. These are reviewed from an engineering perspective, including coil manufacturing, electromechanical behaviour and quench behaviour. Lastly, the important roles of "system pull" upon conductor and magnet technology development, and of interactions between the materials and magnet communities for accelerating development, are discussed.
    Full-text · Article · Aug 2011
  • [Show abstract] [Hide abstract] ABSTRACT: The quench behavior of Bi<sub>2</sub>Sr<sub>2</sub>CaCu<sub>2</sub>O x (Bi2212) wire is investigated through numerical simulations. This work is part of the U.S. Very High Field Superconducting Magnet Collaboration (VHFSMC). Numerical simulations are carried out using a one-dimensional computational model of thermal transport in Bi2212 composite wires. A quench is simulated by introducing heat in a section of the wire, and the voltage and temperature are monitored as function of time and position. The quench energy, normal zone propagation velocity, and spatial distribution of temperature are calculated for varying transport current and applied magnetic field. The relevance of these simulations in defining criteria for experimental measurements is discussed.
    No preview · Article · Jul 2011 · IEEE Transactions on Applied Superconductivity
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    [Show abstract] [Hide abstract] ABSTRACT: The Hc2(T) of a set of four carbon-doped MgB2 films grown on both SiC and Al2O3 substrates by HPCVD from methane CH4 at flow rates from 7 to 10 sccm were measured in fields up to 65T. Compared to early metalorganic C sources which generated high Hc2(0), these films have much lower resistivities and higher connectivities. The curvature of Hc2(T) derived from low current four point magnetoresistance shows upturn at low temperatures, which is consistent with the dominance of pi-band scattering in the theory of dirty two-gap superconductivity. Hc2^|(0) > 60T is close to the paramagnetic limit of ˜ 66T for the 10 sccm film on SiC, though still a little lower than for the previously used metalorganic (C6H7)2Mg. Differences in the Hc2(T) behavior between films grown on the two substrates are attributed to variations in strain fields produced by the substrate coupled to the film at growth.
    Full-text · Article · Mar 2011 · Superconductor Science and Technology