MgB2 Films Research Articles

Crystal orientation dependence of the critical current density in a-axis- and c-axis-oriented MgB2 films

We have investigated the critical current density for MgB2 films having various crystal orientations prepared by using a hybrid physical-chemical vapor deposition system. An enhancement of the critical current density is clearly presented in MgB2 films with an a-axis or a b-axis orientation rather than a c-axis orientation. X-ray diffraction patterns reveal a suppression of c-axis orientations while a (100) orientation becomes dominant, and the surface morphology of the a-axis-oriented film shows that the orientation of the c-axis-oriented MgB2 grains parallel to the plane of the substrate. As the a-axis orientation becomes more dominant in the MgB2 films, the field performance of the critical current density clearly becomes better. These results suggest that the synthesis of MgB2 with high ab-plane orientations is one of the keys to enhancing the critical current density in MgB2.

Comparison of critical properties in MgB2 nanometer films prepared on SiC/Si substrate

Critical superconducting properties in 100nm MgB2 film are investigated and examined in comparison with our previous results in 10nm and 50nm films. MgB2 films were prepared by sequential evaporation of boron and magnesium on SiC-buffered Si substrate followed by in situ annealing. The amount of supplied boron was controlled so as to result in the required MgB2 film thickness with excess Mg top layer. The superconducting transition temperature Tc is estimated to be 33.1K from the onset of AC diamagnetic susceptibility. The upper critical field Hc2 is also estimated from the AC susceptibility. Their temperature dependences are almost linear with |dHc2/dT|∼5.6kOe/K for the parallel magnetic field and |dHc2/dT]~3.9kOe/K for the perpendicular field. These values are larger than those in our previous 50nm film (Tc of 34.5K) in spite of a little lower transition temperature. The critical current density Jc and irreversibility field Hirr are estimated from DC magnetization hysteresis. Scaling behavior of Hirr ~ [1 – (T/Tc)2]n is examined in comparision to our previous studies in 10nm and 50nm films with different critical exponents n.

Sandwich-type MgB2/TiB2/MgB2 Josephson junctions

We present results of sandwich-type MgB2/TiB2/MgB2 superconductor–normal metal–superconductor Josephson junctions with MgB2 films grown by hybrid physical–chemical vapor deposition and TiB2 barriers, between 5 and 10 nm in thickness, grown by DC magnetron sputtering. Most junctions with an 8 nm or thicker barrier of TiB2 showed little excess current and no subgap features and can be described by the resistively-shunted-junction model. Meanwhile prominent multiple Andreev reflection characteristics such as excess currents and subgap peaks were observed in some junctions with thinner barriers. DC and AC Josephson effects were observed in agreement with the theory. The results demonstrate the possibility of leakage-free sandwich-type all-MgB2 junctions using conducting barrier materials

Effect of grain size on the electrical and magnetic properties of MgB2 thick films deposited on the Al2O3 single crystal substrates

MgB2 films having thicknesses of approximately 10 μm have been prepared on the Al2O3 single crystal substrates by using ultrasonic spray pyrolysis technique. The films fabricated were then analyzed by means of microstructural, structural, transport and magnetic properties. The influence of grain size, depending on the ex-situ and in situ heating, were investigated. The results obtained showed that the size of MgB2 grains has an effect on the transport and magnetic properties of the films. Films prepared with in situ heating have small particle size (<50 nm) and were found to be much better properties than the films prepared ex-situ heating, which have larger grains (~1 μm). The best Tc value and critical current density, Jcmag, were obtained to be ~36 K and 9.88 × 105 A cm−2 respectively for the films prepared with small grains. The results were showed that the smaller grain size and in situ heating cycles are essential points for quality thick film MgB2 fabrication.

Effect of different thickness crystalline SiC buffer layers on the ordering of MgB2 films probed by extended x-ray absorption fine structure

Extended X-ray absorption fine structure (EXAFS) spectroscopy is a powerful method to investigate the local structure of thin films. Here, we have studied EXAFS of MgB2 films grown on SiC buffer layers. Crystalline SiC buffer layers with different thickness of 70, 100, and 130 nm were deposited on the Al2O3 (0001) substrates by using a pulsed laser deposition method, and then MgB2 films were grown on the SiC buffer layer by using a hybrid physical-chemical vapor deposition technique. Transition temperature of MgB2 film decreased with increasing thickness of SiC buffer layer. However, the Tc dropping went no farther than 100 nm-thick-SiC. This uncommon behavior of transition temperature is likely to be created from electron-phonon interaction in MgB2 films, which is believed to be related to the ordering of MgB2 atomic bonds, especially in the ordering of Mg–Mg bonds. Analysis from Mg K-edge EXAFS measurements showed interesting ordering behavior of MgB2 films. It is noticeable that the ordering of Mg–B bonds is found to decrease monotonically with the increase in SiC thickness of the MgB2 films, while the opposite happens with the ordering in Mg–Mg bonds. Based on these results, crystalline SiC buffer layers in MgB2 films seemingly have evident effects on the alteration of the local structure of the MgB2 film.

Magnetic anisotropy of thin sputtered MgB2 films on MgO substrates in high magnetic fields

We investigated the magnetic anisotropy ratio of thin sputtered polycrystalline MgB2 films on MgO substrates. Using high magnetic field measurements, we estimated an anisotropy ratio of 1.35 for T = 0 K with an upper critical field of 31.74 T in the parallel case and 23.5 T in the perpendicular case. Direct measurements of a magnetic-field sweep at 4.2 K show a linear behavior, confirmed by a linear fit for magnetic fields perpendicular to the film plane. Furthermore, we observed a change of up to 12% of the anisotropy ratio in dependence of the film thickness.

The role of the substrate surface morphology in enhancing the MgB2 superconducting temperature

We hereby report on the role of the surface morphology of various substrates in the enhancement of the superconducting critical temperature of MgB2. MgB2 thin layers were grown by hybrid physical–chemical vapour deposition on silicon carbide SiC substrates/fibers and several other substrates, characterized by diverse surface morphologies. By investigating the structural, morphological and transport properties of MgB2 thin layers, the presented data show that the superconducting critical temperature Tc exceeds the bulk value only when the MgB2 films are grown on atomically flat (0001) SiC single crystals and on MgB2 bottom layers. These results further confirm the interpretation of the coalescence-driven tensile strain mechanism behind the enhancement of superconducting properties in MgB2 thin films.

Magnesium Diboride Superconducting Films Synthesized by Different Electron-Beam Annealing Currents

Considerable progress to synthesize magnesium diboride (MgB2) films by electron-beam annealing has been made. A series of MgB2 films with a Tc higher than 35 K had been fabricated. MgB2 film superconductivity is affected by electron-beam accelerating voltage, beam current, annealing duration, beam spot size and Mg/B ratio. In order to fabricate better MgB2 films, these influence factors must be intensively studied. In this paper, the influence of e-beam current on superconductivity was investigated with an accelerating voltage of 32 kV, an annealing time of 0.26 s and different annealing beam currents of 9.9 mA, 10.7 mA, 12.8 mA, 13.3 mA and 14.0 mA. The results show the MgB2 film with 12.8 mA has the highest Tc and the densest structure. Its zero-field Jc at 15K has reached 3.2×106 A/cm2. The variation of the film superconducting properties with the beam currents was grasped, which will lay the foundation for the fabrication of high quality MgB2 thin films

Properties of MgB2 films deposited on single crystal Ag layers

We have recently developed a method to deposit MgB2 thin films on single crystal Ag layers, and found that thin Ag layers reduced the the superconductivity of MgB2 not much, which may be a possible material to fabricate MgB2 SNS Josephson junctions. The single crystal Ag layers were deposited on (0001) SiC substrates, and then the MgB2 thin films were deposited on the Ag layers. All the measurements included the XRD, M–T and M–H curves. The transition temperature of MgB2 films decreases from 39K to 37K, while the thickness of Ag layers grows from 0 to 20nm, and the critical current density at 4.2K decreases from 1.76MA/cm2 to 1.24MA/cm2, too.

Pinning enhancement in MgB2 superconducting thin films by magnetic nanoparticles of Fe2O3

MgB2 thin films were fabricated on r-plane Al2O3 (\({1} \overline{{1}} {0} {2})\) substrates. First, deposition of boron was performed by rf magnetron sputtering on Al2O3 substrates and followed by a post-deposition annealing at 850 °C in magnesium vapour. In order to investigate the effect of Fe2O3 nanoparticles on the structural and magnetic properties of films, MgB2 films were coated with different concentrations of Fe2O3 nanoparticles by spin coating process. The magnetic field dependence of the critical current density J c was calculated from the M–H loops and magnetic field dependence of the pinning force density, f p(b), was investigated for the films containing different concentrations of Fe2O3 nanoparticles. The critical current densities, J c, in 3T magnetic field at 5 K were found to be around 2·7 × 104 A/cm2, 4·3 × 104 A/cm2, 1·3 × 105 A/cm2 and 5·2 × 104 A/cm2 for films with concentrations of 0, 25, 50 and 100% Fe2O3, respectively. It was found that the films coated with Fe2O3 nanoparticles have significantly enhanced the critical current density. It can be noted that especially the films coated by Fe2O3 became stronger in the magnetic field and at higher temperatures. It was believed that coated films indicated the presence of artificial pinning centres created by Fe2O3 nanoparticles. The results of AFM indicate that surface roughness of the films significantly decreased with increase in concentration of coating material.

Deteriorated superconductivity of MgB2 films due to Al diffusion from Al2O3 substrates: Thermodynamic perspective

Recently we reported the growth of MgB2 films by pyrolysis of decaborane (B10H14) in Mg vapor. In this article, we discuss the intrinsic problem at the MgB2/Al2O3 interface, which was encountered during our optimization of the growth temperature. Mg is a stronger reductant than Al, and takes out oxygen from Al2O3. Increasing the growth temperature, this reaction becomes noticeable and leads to the formation of MgO and Mg1−xAlxB2, which deteriorates the properties of resultant MgB2 films. Our observations present a general issue in the choice of substrate and barrier materials in thin film growth and/or junction fabrication of compounds containing reactive cations such as Mg.

Photoinduced operation and diagnostic of superconductivity in the MgB2 films

The occurrence of the optical second harmonic generation (SHG) at fundamental wavelength of Er:glass nanosecond 1,540 nm stimulated by two coherent CO2 microsecond laser beams at wavelengths 10.6 and 5.3 μm was observed for the MgB2 films. The SHG signal has shown its sensitivity to the occurrence of the superconducting (SC) phase transition. The SHG achieves its maximum at temperatures higher with respect to the SC transition. The effect is reversible with respect to the photo induced treatment. After the switching off of the external photoinducing treatment the effect almost disappears. However, this still strongly affects the SC properties of the MgB2 thin layers, as the critical current is increased by 50 % after illumination.

Suppression of superconductivity in epitaxial MgB2 ultrathin films

MgB2 ultrathin films have potential to make sensitive superconducting devices such as superconducting single-photon detectors working at relatively high temperatures. We have grown epitaxial MgB2 films in thicknesses ranging from about 40 nm to 6 nm by using the hybrid physical-chemical vapor deposition method and performed electrical transport measurements to study the thickness dependence of the superconducting critical temperature Tc. With reducing film thickness d, although a weak depression of the Tc has been observed, which could be attributed to an increase of disorder (interband impurity scattering) in the film, the Tc retains close to the bulk value of MgB2 (39 K), being about 35 K in the film of 6 nm thick. We show that this result, beneficial to the application of MgB2 ultrathin films and in accordance with recent theoretical calculations, is in contrast to previous findings in MgB2 films prepared by other methods such as co-evaporation and molecular-beam epitaxy, where a severe Tc suppression has been observed with Tc about one third of the bulk value in films of ∼5 nm thick. We discuss this apparent discrepancy in experiments and suggest that, towards the ultrathin limit, the different degrees of Tc suppression displayed in currently obtained MgB2 films by various techniques may arise from the different levels of disorder present in the film or different extents of proximity effect at the film surface or film-substrate interface.

Study of IF Bandwidth of ${\hbox{MgB}}_{2}$ Phonon-Cooled Hot-Electron Bolometer Mixers

A noise bandwidth (NBW) of 6-7 GHz was obtained for hot-electron bolometer (HEB) mixers made of 10 nm MgB2 films. A systematic investigation of the (IF) gain bandwidth as a function of the MgB2 film thickness (30, 15, and 10 nm) is also presented. The gain bandwidth (GBW) of 3.4 GHz was measured for a 10 nm film, corresponding to a mixer time constant of 47 ps. For 10 nm films a reduction of the GBW was observed with the reduction of the critical temperature (Tc). Experimental data were analyzed using the two-temperature model. From the theoretical analysis, the electron-phonon time (τe - ph), the phonon escape time (τesc) and the electron and phonon specific heats (ce, cph) were extrapolated giving the first model for HEB mixers of MgB2 films.

Thickness dependence of grain growth orientation in MgB2films fabricated by hybrid physical-chemical vapor deposition

We have investigated the effect of thickness of the MgB2 film on the grain growth direction as well as on their superconducting properties. films of various thicknesses were fabricated on c-cut substrates at a temperature of by using hybrid physical-chemical vapor deposition (HPCVD) technique. The superconducting transition temperature () was found to increase with increase in the thickness of the film. X-ray diffraction analysis revealed that the orientation of grains changed from c-axis to a-axis upon increasing the thickness of the film from 0.6 to 2.0 . grains of various orientations were observed in the microstructures of the films examined by scanning electron microscopy. It is observed that at high magnetic fields the 2.0--thick film exhibit considerably larger critical current density () as compared to 0.6--thick film. The results are discussed in terms of an intrinsic-pinning in similarly as intrinsic-pinning occurring in high-Tc cuprate superconductors with layered structure.

Superconducting MgB2 rapid single flux quantum toggle flip flop circuit

Using self-shunted MgB2/MgO/MgB2 Josephson junctions, we have designed, fabricated, and tested a rapid single flux quantum toggle flip flop (TFF) circuit. The junctions used MgB2 films grown by hybrid physical-chemical vapor deposition with MgO barrier layer and insulating layer deposited by RF magnetron sputtering. The result showed the frequency-division function of the TFF circuit, evidenced by the output voltage being half the input voltage, up to 63 GHz at 20 K and 180 GHz at 3.2 K, which demonstrates the potential of high operating speeds and high working temperatures in MgB2 superconducting integrated circuits.

Hybrid Physical–Chemical Vapor Deposition of Ultrathin $\hbox{MgB}_{2}$ Films on MgO Substrate With High $T_{C}$ and $J_{C}$

MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> has been recognized as a candidate material for fabricating superconducting nanowire single photon detectors with the expectation of having an operating temperature of the detector at 10-20 K. Preparation of ultrathin MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films with high superconducting transition temperature T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> is the first crucial step to achieve this goal. Here we present the hybrid physical-chemical vapor deposition of MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> superconducting films with thicknesses around 10 nm. The films are epitaxially grown on (111) MgO substrates. The T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> of the films, including for a 6-nm-thick film, is found to be above 34 K, close to the bulk value of MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (39 K). The normal-state resistivity at 42 K of a 10-nm-thick film is shown to be 3.1 μΩ cm, indicating that the film is relatively clean. The superconducting critical current density J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> of the films has also been measured on strip lines patterned in the films. The J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> of the 10-nm-thick film reaches 7.7 ×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 2 K, so far the highest value reported for MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films of the same thickness. The high T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> , low residual resistivity, and high J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> of the ultrathin MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films on MgO substrates suggest that these films have the potential in developing MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -based superconducting nanowire single photon detectors or other related superconducting devices.

Tunneling investigation of the electron scattering effect on the momentum-dependent energy gap distribution in MgB2

We have studied the effect of electron scattering on the momentum-dependent energy gap distributions in MgB2 by measuring planar tunnel junctions made from epitaxial MgB2 thin films on different substrates, of different thicknesses, with different junction interface properties, and different counter-electrode materials. The phonon features in the tunneling spectra indicated that the native oxide barrier is mainly MgO with estimated barrier thickness ranging from 1.1 to 1.5 nm and the average barrier height from 1.7 to 2.6 eV. When tunneling into the ab-plane occurred in very clean films, both the π (∼1.8 meV) and σ (∼7.2 meV) gaps were observed with fine structures in the conductance peaks, indicating a distribution of gap values due to anisotropic electron-phonon interaction. The σ gap was enhanced (∼7.9 meV) in MgB2 thin films on SiC substrates which had Tc values over 40 K due to epitaxial tensile strain. As the MgB2 film thickness decreased from 100 nm to 33 nm, the π gap distribution range was narrowed from ∼1.7 meV to ∼1.4 meV, and the fine peak structures disappeared due to the reduced electron mean free path limited by the film thickness. The fine peak structures were also gradually smeared out when the junction resistance increased. The results show that the electron scattering, either from the MgB2 film or the junction interface or barrier, can smear out the gap distribution structures in the tunneling spectra. Deterioration of the MgB2 film surface was also found to cause an increase in the π gap value, likely due to an enhancement of interband scattering.

Nanostructure characterization of Ni and B layers as artificial pinning centers in multilayered MgB2/Ni and MgB2/B superconducting thin films

Research on the MgB2/Ni and MgB2/B multilayer films fabricated by an electron beam (EB) evaporation technique have been extensively carried out. The critical current density, Jc of MgB2/Ni and MgB2/B multilayer films in parallel fields has been suggested to be higher than that of monolayer MgB2 film due to introducing the artificial pinning centers of nano-sized Ni and B layers. Nanostructure characterization of the artificial pinning centers in the multilayer films were examined by transmission electron microscopy (TEM) and scanning TEM (STEM-energy dispersive X-ray spectroscopy (STEM-EDS))–EDS to understand the mechanism of flux pinning. The growth of columnar MgB2 grains along the film-thickness direction was recognized in the MgB2/Ni multilayer film, but not in the MgB2/B multilayer film. Nano-sized Ni layers were present as crystalline epitaxial layers which is interpreted that Ni atoms might be incorporated into the MgB2 lattice to form (Mg,Ni)B2 phase. On the other hand, nano-sized B layers were amorphous layers. Crystalline (Mg,Ni)B2 layers worked more effectively than amorphous B-layers, providing higher flux-pinning force that resulted in higher Jc of the MgB2/Ni multilayer film than the MgB2/B multilayer film.

Preparing Ultrathin MgB&lt;sub&gt;2&lt;/sub&gt; Thin Film by &lt;i&gt;Ex Situ&lt;/i&gt; Annealing of Mg-B Precursor Film

Ultrathin MgB2 film is essential for the fabrication of MgB2 superconducting single photon detectors (SSPDs). In this paper, we prepared 20nm and 10nm MgB2 film using ex-situ annealing of Mg-B multilayer method. The precursor films were prepared by electron beam evaporation. A flowing Mg vapour and H2 was introduced in the annealing process to keep MgB2 thin film thermodynamically stable. The annealing temperature was between 680 and 740 and annealing time was 1-10min. 20nm MgB2 films on MgO(111) substrates had the critical temperature (Tc) of 32.2K. The films grew along c-axes direction. As the substrate changed to SiC(001) and Al2O3(001), Tc decreased to 30.3K and 10.2K respectively. For 10nm MgB2 film on SiC(001) substrate, Tc was 24.2K. The self-field critical current density for 10nm and 20nm film on SiC(001) substrate was 2.1×106A/cm2 and 2.3×106A/cm2, respectively. AFM image showed that the film had a flat surface with mean roughness of 0.899nm for 10nm MgB2 film.