Pulsed Plasma Treatment of Magnesium Diboride System for Formation of Superconducting Regions

The discovery of superconductivity in MgB2, with Tc as high as 39 K, has created great excitement about the possibility of using this material in many practical applications. Soon after this discovery, the first superconducting MgB2 thin films were successfully demonstrated. The goal of the studies undertaken by present authors is to synthesize for the first time, the superconducting MgB2 layer from a liquid phase (without conventional furnace annealing), using ion implantation and transient melting processes (TMP). For TMP one can use short laser, electron, ion and plasma pulses. In the present work Mg ions of energy 30-100 keV and doses of (0.7-1.0)x1018 Mg/cm2 were implanted from MEVVA type source into polycrystalline boron substrates. For TMP, 3 argon plasma pulses (in ms range) of fluence 0.8 J/cm2 to 2.5 J/cm2 were used as the second step of the procedure. Monitoring intensities of the Mg I 516.73 nm, Mg I 517.27 nm, Mg I 518.36 nm, and Mg II 448.1 nm lines observed in plasma by optical spectroscopy it was possible to show that there was a substantial amount of magnesium atoms and ions just above the melted substrate surface. Intensities of the Mg lines were proportional to the pulse fluence in the 1-3 J/cm2 range. The latter finding qualitatively supports our expectation that a plum of Mg vapor/ions exerts a pressure on the liquid surface sufficient to prevent any significant loss of magnesium from the irradiated Mg-B system. The samples were characterized by: magnetic measurements, magnetically modulated microwave absorption (MMMA), electric conductivity and SIMS measurements. Thus far the best results were obtained for 50 keV Mg ions energy, the dose of 7x1017Mg/cm2 and pulses of 2.5 J/cm2. The results obtained can be summarized as follows: - he critical temperature Tc (onset) for the measured samples is equal 26 K (with ΔTc=3 K from resistivity measurements). -To evaluate the critical current densities and the size of superconducting region for the measured samples we applied the method of reverse magnetic hysteresis leg elaborated by Angadi. The results obtained, namely, Jc=1·105 [A/cm2] for the critical current density and r=1.94 mm for the radius of superconducting region (comparable with the sample radius r=2 mm). - Obtained results shows relatively good superconducting properties of the sample with the superconducting phase above the percolation threshold. - Strong superconducting evidence for the sample is confirmed by MMMA method as well as four-point transport measurements. (author)