Properties Of MgB2 Thin Films Research Articles

Roles of Fe-ion irradiation on MgB2 thin films: Structural, superconducting, and optical properties

The effects of Fe-ion irradiation on the crystal structure and superconducting properties of MgB2 thin films were investigated. Pristine samples were prepared using hybrid physical-chemical vapor deposition (HPCVD), and ion irradiation was performed at three different doses of 5 × 1013, 1 × 1014, and 2 × 1014 ions/cm2. The measured temperature-dependent resistivity showed that as the irradiation dose increased from pristine to most irradiated, the superconducting critical temperature, Tc, significantly decreased from 38.33 to 3.02 K. The crystal structures of the films were investigated by X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) measurements. The results showed that the higher the dose, the greater the change in crystal structure, such as the lattice constant and bond length. This suggests that the destruction of the crystal structure at higher doses leads to the degradation of superconductivity in the irradiated MgB2 thin films. Raman spectroscopy showed that the electron-phonon coupling constant decreased with increasing irradiation dose, which was directly related to the reduction of Tc in the samples. The optical conductivity indicates that the charge-carrier density of the σ-band plays an important role in the superconductivity of ion-irradiated MgB2.

Effects of Sn-ion irradiation on local structure and flux pinning properties of MgB2 thin films

We studied the effects of tin (Sn) ion irradiation on the local structure and flux pinning properties of 412-nm-thick MgB2 thin films. Ions with 2 MeV incident energy were irradiated onto MgB2 thin films at room temperature and various dose levels from 2 × 1012 to 7 × 1013 atoms/cm2. X-ray diffraction and extended X-ray absorption fine-structure results confirmed extended average bond lengths and increased mean-square relative displacement that correlated with crystalline disorder serving as artificial pinning centers induced by ion irradiation. While the superconducting transition temperature (Tc) exhibited a decreasing trend, the magnetization Jc (H) of the irradiated films measured at both 5 K and 20 K were remarkably enhanced compared to that of pristine films at high applied magnetic fields. The decreases in exponent β from 1.1 and 1.13 to 0.69 and 0.62 after irradiation at 5 K and 20 K, respectively, as the dose increased to 5 × 1013 atoms/cm2 indicates a shift in the pinning mechanism from weak collective pinning to strong plastic pinning according to collective pinning theory. In addition, the scaling behavior of the flux pinning force density (Fp) in the irradiated MgB2 films showed a crossover from surface pinning to normal point pinning. These results suggest that intermediate-energy irradiation by heavy ions such as Sn generates effective pinning sources for improving Jc(H) in MgB2 superconductors.

Influence of carbon-ion irradiation on the superconducting critical properties of MgB2 thin films

We investigate the influence of carbon-ion irradiation on the superconducting (SC) critical properties of MgB2 thin films. MgB2 films of two thicknesses, 400 nm (MB400nm) and 800 nm (MB800nm), were irradiated by 350 keV C ions having a wide range of fluence, 1 × 1013–1 × 1015 C atoms cm−2. The mean projected range (R p) of 350 keV C ions in MgB2 is 560 nm, thus the energetic C ions will pass through the MB400nm, whereas the ions will remain into the MB800nm. The SC transition temperature (T c), upper critical field (H c2), c-axis lattice parameter, and corrected residual resistivity (ρ corr) of both the films showed similar trends with the variation of fluence. However, a disparate behavior in the SC phase transition was observed in the MB800nm when the fluence was larger than 1 × 1014 C atoms cm−2 because of the different T cs between the irradiated and non-irradiated parts of the film. Interestingly, the SC critical properties, such as T c, H c2, and critical current density (J c), of the irradiated MgB2 films, as well as the lattice parameter, were almost restored to those in the pristine state after a thermal annealing procedure. These results demonstrate that the atomic lattice distortion induced by C-ion irradiation is the main reason for the change in the SC properties of MgB2 films.

Enhancement of superconducting properties of MgB2 thin films by using oxygen annealing atmosphere

In this work, we show that significant enhancement of superconducting properties of MgB2 thin films prepared by vacuum evaporation and ex-situ annealing can be obtained when the annealing is performed in O2 atmosphere instead of the usually employed inert atmosphere (Ar in the current study). Rapid annealing in O2 significantly improves the critical transition temperature (Tc), critical current density (jc), and upper critical magnetic field (Hc2) compared to the control sample prepared in the same way, but annealed in Ar. Structural and composition analyses of the films demonstrate that the annealing in O2 atmosphere produces a sufficiently thick MgO layer on the surface of MgB2 thin films, which acts as a protective barrier preventing out-diffusion of Mg from the films during the high-temperature annealing. This consequently leads to better stoichiometry (B/Mg ratio), increased MgB2 grain size and enhanced intergrain connectivity. The higher residual resistivity of the films annealed in O2 atmosphere also indicates higher concentration of intragrain impurities, which causes a further increase of the jc and Hc.

Mechanical characterization of MgB2 thin films using nanoindentation technique

Mechanical properties of MgB2 thin films deposited by RF magnetron sputtering technique have been studied. MgB2 thin films were deposited approximately 300nm on Si substrate above room temperature, and the thin films were subsequently annealed. Great care has been taken to reduce O2 in the chamber during deposition. Structural and mechanical properties of MgB2 thin films were investigated using the Scanning Electron Microscopy (SEM), X-ray diffraction (XRD) and nanoindentation techniques. Unloading segments of nanoindentation curves analyzed using Oliver–Pharr method. Both reduced elastic modulus and nanohardness values show load dependence, i.e., indentation size effect (ISE). Proportional specimen resistance (PSR) model was used to calculate the load independent nanohardness value. Load-independent hardness value was calculated using the proportional specimen resistance (PSR) model. XRD and SEM results show that the MgB2 thin films have fine grain size. Nanoindentation results show that hardness and elastic modulus values were 11.72GPa and 178.13GPa, respectively.

Influence of annealing atmosphere on structural and superconducting properties of MgB2 thin films

Abstract Influence of an ex situ annealing temperature and atmosphere on chemical composition and structural and superconducting properties of MgB 2 thin films deposited by vacuum evaporation has been investigated. The annealing has been done in Ar, N 2 and Ar + 5%H 2 atmospheres at pressure of 700 Pa and temperature varying from 700 to 800 °C. It has been shown that annealing in Ar and N 2 atmosphere at 700–800 °C produces relatively thick MgO layer on the surface of the films, while creation of such layer is highly reduced if the annealing is done in reducing Ar + 5%H 2 atmosphere. The XPS and XRD results suggest that the MgO layer prevents out-diffusion of Mg from the film during the annealing, what assures better stoichiometry of the films as well as creation of larger MgB 2 grains. The films with the highest amount of MgO on the surface, annealed in nitrogen atmosphere, thus paradoxically exhibited the highest critical temperature of T c 0 = 34.8 K with very sharp transition width of 0.1 K.

MgB2 Thin Films on Metal Substrates for Superconducting RF Cavity Applications

Magnesium diboride is a promising material for superconducting RF (SRF) cavity applications. Compared to the currently used superconductor for SRF cavities Nb, MgB2 has the potential to achieve lower RF loss and higher acceleration field due to its higher critical temperature and thermodynamic critical magnetic field. Since the RF field only penetrates a few penetration depths into a superconductor, a superconducting coating of several hundred nanometers on a metal cavity is sufficient for superb SRF cavity performances. In this work, we report the properties of MgB2 thin films deposited by the hybrid physical–chemical vapor deposition (HPCVD) technique on different metal substrates including Nb, Mo, Ta, and stainless steel. All the films were polycrystalline, as indicated by X-ray diffractometry and scanning electron microscopy, and showed Tc∼39 K, determined by resistance versus temperature, magnetic susceptibility, and dielectric resonator measurements. MgB2 films deposited on Nb substrates polished to various degrees of smoothness exhibit similar Tc. The result is a promising step in the investigation of using MgB2 as an alternative to Nb for SRF cavities.

Properties of MgB2 Thin Films Deposited on Different Substrates Prepared by Ex-Situ Annealing Process

MgB2 thin films were deposited on MgO (100) substrate and r-plane Al2O3\((1\bar{1}02)\) substrate by ex-situ annealing of boron film in magnesium vapor. The thickness of ex-situ annealed MgB2 films is approximately 600 nm according to data observation by ellipsometer. The magnetic properties of samples were determined using a vibrating sample magnetometer. The magnetic field dependence of the critical current density Jc was calculated from M–H loops and also the magnetic field dependence of Fp was compared for the different temperature ranges from 5 to 25 K. The critical current density Jc was found to be around 1.0×106 A/cm2 and 1.7×106 A/cm2 in zero field at 5 K for MgB2 films deposited on MgO and r-plane Al2O3 substrates, respectively. It was found that the critical current density of the film deposited on MgO became stronger than that of r-plane Al2O3 in the magnetic field. The superconducting transition temperature was determined by ac susceptibility measurement using physical properties measurement system. ac susceptibility measurements for MgB2 films deposited on MgO and r-plane Al2O3 substrates were performed as a function of temperatures at constant frequency and ac field amplitude in the absence of dc bias field. The critical current densities as a function of temperature were estimated from the ac susceptibility data.

Improved superconducting properties of MgB2 thin films fabricated by ultrasonic spray pyrolysis method at high temperature

High quality MgB2 superconducting thin films have been successfully prepared by 2.4MHz ultrasonic spray pyrolysis (USP) system on single crystal Al2O3 (001) substrates. The microstructure, electrical and magnetic properties of approximately 500–600nm thick films were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) in conjunction with the energy dispersive X-ray analysis (EDX), resistance versus temperature (R–T) and magnetization measurements (M–H) under different magnetic fields and transport critical current density (Jc). Films were first heat treated in situ in the spraying chamber with an extra Mg powder during deposition to compensate excess evaporation of Mg from the films and then additionally heat treated in Ar atmosphere at 700°C for a short time. According to the results obtained, orientation on any particular direction for the crystal growth was not seen. Homogeneous, highly dense and highly smooth surface morphology and low resistance have been achieved under optimum conditions. Optimally treated films exhibited relatively high transport critical current density of 2.37×105Acm−2. These results have been also compared with the Jcmag results calculated from the M–H curves. The electrical resistance property of the best samples was obtained to be 39.5 and 37.4K for Tc and Tzero, respectively.

Artificial Pinning Centers on MgB2 Superconducting Thin Films Coated by FeO Nanoparticles

MgB2 thin films were fabricated on MgO (100) single crystal substrates. First, deposition of boron was performed by rf magnetron sputtering on MgO substrates and followed by a post deposition annealing at 850 °C in magnesium vapor. In order to investigate the effect of FeO nanoparticles on magnetic properties of MgB2 thin films, the films were coated with different concentrations of FeO nanoparticles by spin coating process. The magnetic field dependence of the critical current density \(J_{\mathrm{c}}\) was calculated from the M–H loops and also magnetic field dependence of the pinning force density \(f_{\mathrm{p}}(b)\) was determined for the films containing different concentrations of FeO nanoparticles. The values of the critical current density \(J_{\mathrm{c}}\) in zero field at 5 K was found to be around 1×106 A/cm2 for pure MgB2 film, 1.4×106 for MgB2 film coated with 25 %, 7.2×105 for MgB2 film coated with 33 %, 9.1×105 for MgB2 film coated with 50 % and 1.1×106 A/cm2 for MgB2 film coated with 100 %. It was found that the film coated with 25 % FeO nanoparticles has slightly enhanced critical current density and it can be noted that especially the film coated with 25 % FeO became stronger in the magnetic field. The films coated with FeO were successfully produced and they indicated the presence of artificial pinning centers created by FeO nanoparticles. The superconducting transition temperature of the film coated with 25 % FeO nanoparticles was determined by moment–temperature (M–T) measurement to be 34 K which is 4 K higher than that of the pure film.

Flux pinning properties of MgB2 thin films on Al tape substrates deposited by electron beam evaporation

Flux pinning properties have been investigated in two kinds of MgB2 thin films deposited on Al tapes by electron beam evaporation: One is a stoichiometric composition and the other is a slightly B-rich composition. The values of critical current density Jc in both MgB2 thin films on Al tape substrates at 10K in the magnetic field parallel to the c-axis are higher than those in MgB2 thin films on Si and Al2O3 substrates prepared by the same method. Both the MgB2 thin films on Al tapes show the large peaks for a magnetic field, B//c in the field-angular dependence of Jc. This result indicates that the MgB2 thin films have the c-axis correlated pinning centers. Scaling analysis in the reduced macroscopic pinning force density versus the reduced magnetic field at 20K implies that a main pinning center in both the MgB2 thin films is grain boundaries. In addition, it was suggested that a nonstoichiometric MgB2 thin film has additional pinning centers which act effectively in a high magnetic field.

Flux pinning properties of MgB2 thin films on Ti buffered substrate prepared by molecular beam epitaxy

Transport properties of the MgB2 thin films on Si, MgO and ZnO substrates with Ti buffer layer prepared by molecular beam epitaxy were investigated to clarify effects of the substrates and the Ti buffer layer on flux pinning. The critical current density Jc of each sample shows different dependence on magnetic fields parallel to c-axis. However, the scaling analysis of the macroscopic pinning force for all the measured samples implies that the grain boundaries work as the dominant pinning centers for B//c. The pinning parameter for MgB2/Ti/Si estimated from the electric field E vs. the current density J characteristics shows the highest value among all the measured samples. This result is attributed to the high density of grain boundaries caused by the effect of both the Ti buffer and Si substrate in the growth process. Therefore, the selection of substrates and buffer layer strongly affects the flux pining properties of MgB2 thin films and plays an important role in the determination of performance for superconducting devices and wires.

Surface morphology and thickness dependence of the properties ofMgB2 thin films by hybrid physical–chemical vapor deposition

The influence of growth parameters in hybrid physical–chemicalvapor deposition (HPCVD) on surface morphologies ofMgB2 thinfilms, and the dependence of their properties on film thickness are systematically studied. As the HPCVDMgB2 films grow in the Volmer–Weber mode, where islands coalesce into continuousfilms, the grain size has an important influence on the film morphology. We foundthat films deposited with higher growth rates have larger grains, and a largergrain size appears to correspond to a larger film roughness. Under optimizeddeposition conditions, the surface root mean square (RMS) roughness is 1.2 nm over a1 µm × 1 µm area for a 100 nm thick film. The surface morphology does notdirectly affect the structural and superconducting properties of theMgB2 films. The resultsof ultrathin MgB2 films, as thin as 10 nm, with excellent superconducting properties are also presented in thispaper.

Investigation of critical properties in MgB2/SiC/Si thin films prepared under varied conditions

Critical magnetic properties of MgB2 thin films grown on SiC/Si substrate are investigated with different preparation conditions. MgB2 films were prepared by sequential evaporation of boron and magnesium on SiC buffered Si substrate. The substrate temperature and the amount of supplied boron was varied and influence of varied conditions is investigated. The maximum superconducting transition temperature obtained is 34.5 K. The upper critical field Hc2 is estimated from the onset of AC diamagnetic susceptibility. The temperature dependence is generally linear, being fitted with |dHc2/dT| around 3 ∼ 7 kOe/K depending on the substrate temperature during in situ annealing as well as the magnetic field direction. The critical current density is evaluated from DC magnetization hysteresis to be more than 106 A/cm2 at lower temperatures. Irreversibility field scales with [1 — (T/Tc)2]n with critical exponents n of 3 ∼ 7. Correlations among critical properties, crystal qualities and sample preparation conditions are discussed.

Properties of MgB2 films fabricated under different conditions by ex-situ annealing of Mg/B multilayer precursor

The effects of annealing temperature, annealing time and film thickness on the properties of MgB2 thin films prepared by ex-situ annealing of Mg/B multilayer have been studied. The films with the thickness of 250 nm annealed at temperatures as low as 400 ℃ exhibited a superconducting transition. The optimal annealing condition of 250 nm thick MgB2 films regarding to the transition temperature, Ｔc, was 750 ℃ for 20—30 min. Ｔc became lower as the film thickness decreased and the Ｔc of thinner MgB2 films was influenced more greatly by the annealing temperature. With reducing the thickness of each single layer, we were able to fabricate the 20 nm thick MgB2 superconducting film with a Tonsetc of 31K.

Effect of Rb and Cs doping on superconducting properties ofMgB2 thin films

A Rutherford backscattering spectrometry (RBS) study has found thatconcentrations up to 7 at.% of Rb and Cs can be introduced to a depth of∼700 Å inMgB2 thin films by annealing in quartz ampoules containing the elemental alkali metals at<350 °C. No significant change in transition temperature(Tc) (determined resistively) was observed, in contrast to an earlier report of very highTcs (>50 K) from susceptibilitymeasurements on MgB2 powders. The lack of a significant change inTc and intra-granular carrier scattering suggests that Rb and Cs diffuse into the film, but do not enterthe grains. Instead, the observed changes in the electrical properties, including a significant drop inJc and anincrease in Δρ(ρ300−ρ40), arise from a decrease in inter-granular connectivity due to segregation of the heavyalkaline metals and other impurities (i.e. C and O) introduced into the grain boundaryregions during the anneals.

Change in magnetic properties of MgB2 thin films after irradiation by 200MeV Au ion beam

Abstract The SHI irradiation induced effects on magnetic properties of MgB2 thin films are reported. The films having thickness 300–400 nm, prepared by hybrid physical chemical vapor deposition (HPCVD) were irradiated by 200 MeV Au ion beam (Se ∼ 23 keV/nm) at the fluence 1 × 1012 ion/cm2. Interestingly, increase in the transition temperature Tc from 35.1 K to 36 K resulted after irradiation. Substantial enhancement of critical current density after irradiation was also observed because of the pinning provided by the defects created due to irradiation. The change in surface morphology due to irradiation is also studied.

Microwave properties of MgB2 thin films prepared in situ by thermal evaporation combined with sputtering

Superconducting MgB2 thin films were prepared in situ using a combination of rf magnetron sputtering of B and thermal evaporation of Mg. The films exhibited Tc of up to 36 K. The microwave measurements were performed on 14 × 14 mm2 films using both Cu-shielded and Nb-shielded sapphire puck resonators at the frequency of 18.8 GHz. The hf surface resistance (RS) and the change of the hf surface reactance (ΔXS) were determined. The films exhibited low RS matching the literature results for high-quality MgB2 films. Below 3K RS reached 3-5 µΩ which was the resolution limit of our measurement. The temperature dependences of both RS and ΔXS were in good agreement with BCS theory. From the RS(T) dependence we obtained an energy gap Δ(0) ≈ 3 meV. The measured variation of the London penetration depth with temperature, ΔλL(T), was also in good agreement with the BCS model. Using the BCS relation between the energy gap and the penetration depth we fitted our experimental ΔλL(T) data and obtained λL(0) values, which ranged for different films from 85 to 100 nm.

The effect of oxygenation on the superconducting properties of MgB2 thin films

The effects of oxygen on the superconducting properties of thin films of MgB2 have been studied. It is found that after annealing for short times in flowing O2 at 400°C, complex oxides of Mgx(ByO)z are formed that cause a “two-step” behavior in the resistively determined superconducting transition. These oxides are not stable at room temperature and decay into MgO and B2O3 precipitates. As the complex oxides decay so the original Tc behavior of the films is restored to its preoxygenation level.

Properties of MgB2 thin films with carbon doping

We have studied structural and superconducting properties of MgB2 thin films doped with carbon during the hybrid physical-chemical vapor deposition process. A carbon-containing precursor metalorganic bis(methylcyclopentadienyl)magnesium was added to the carrier gas to achieve carbon doping. As the amount of carbon in the film increases, the resistivity increases, Tc decreases, and the upper critical field increases dramatically as compared to clean films. The self-field Jc in the carbon doped film is lower than that in the clean film, but Jc remains relatively high to much higher magnetic fields, indicating stronger pinning. Structurally, the doped films are textured with columnar nano-grains and highly resistive amorphous areas at the grain boundaries. The carbon doping approach can be used to produce MgB2 materials for high magnetic-field applications.