Growth, Structure Analysis and Anisotropic Superconducting Properties of MgB₂Single Crystals

Abstract

Here we report the growth of sub-millimeter MgB 2 single crystals of various shapes under high pressure in Mg-B-N system. Structure refinement using a singlecrystal X-ray diffraction analysis gives lattice parameters a=3.0851 (5) . It was found that as sintering temperature increased, the sample quality was improved, but for the heat treatment above 950 o C the gold started to melt and to adhere strongly to MgB 2. Preparation of high-density bulk MgB 2 materials at a higher temperature (950-1250 o C) and 3.5 GPa was reported in BN crucibles 7) . However, the samples prepared at 1250 o C apparently showed inhomogeneous structure in macroscopic scale, indicating the occurrence of contamination from the crucible wall. PACSThese results are not quite favorable for crystal growth of MgB 2 under high-pressure.In this letter, we report the successful growth of high-quality MgB 2 single crystals under high pressure in the Mg-B-N system. Using the crystals, we carried out structure refinement and measured anisotropic superconducting properties of magnesium diboride. were used for refinement after absorption correction by a ψ-scan. The structure was refined by the full-matrix least-square procedure.The temperature dependence of the magnetization M(T) was measured by a SQUID (MPMS XL, Quantum Design) for 37 crystals of different shape (total weight 0.25mg) aligned with the c-axis perpendicular to the sample holder. In-plane electrical resistivity was measured for several plate-like crystals of typical size of 500x100x20 µm 3 in a four-probe configuration using a low frequency (17.8Hz) ac technique with a voltage resolution of less than 0.3nV. To study the anisotropic superconducting properties, the resistivity measurements were performed in the magnetic fields up to 7Tesla applied parallel and perpendicular to the Mg and B planes.The Mg-B-N system has been extensively studied during last 40 years [8][9][10] . In this system Mg and Mg-containing compounds act as the catalysts for the 4 transformation of boron nitride (BN) from hexagonal to cubic form under high pressure and the formation of MgB 2 and MgB 6 crystals has been observed 11). These compounds were formed as the intermediate phases in Mg-BN system, during the synthesis of cubic boron nitride, but MgB 2 was decomposed after prolong heat treatment.We found that the lower limit for MgB 2 crystal growth in "pressure- , using polycrystalline and powdered sample, refined structural data were previously unavailable. It has been proved to be very difficult to avoid the formation of impurity phases (MgO and MgB 4 ) in polycrystalline samples and thus the precise structure refinement is almost impossible. In this study well-formed single crystals were chosen for structure analysis to minimize of absorption effects. The refined crystallographic data, atomic coordinates, and thermal parameters are listed in Table 1 .In the final part of this letter we briefly characterize the superconducting properties of our MgB 2 single crystals. .In Fig.5(a) we present the resistive supe...

Mechanically reinforced MgB2 wires and tapes with high transport currents

“…In a quite recently reported thin film sample extraordinary high current densities of J c > 10 5 Acm -2 at 4.2 K, 10 T, and an irreversibility field of 21 T (4.2 K) was measured and H c2 = 40 T was confirmed [12]. Investigations on single crystals confirmed the anisotropic superconductivity in MgB 2 [13].…”

“…I c degradations observed above some sample specific strain limits, Fig. 13 Field dependence of the critical currents of an ex-situ MgB 2 /Fe/SS wire with 51% steel content for axial strains of 0%, 0.3% and 0.6% 0.2% for the Fe sheath, 0.4 % for the Fe/SS25 sheath and 0.6% for the Fe/SS35 sheath, however were irreversible and have to be attributed to a damage of the superconductor caused by formation of cracks in the filament.…”

Strong competition between the δl and δTc flux pinning mechanisms in MgB2 doped with carbon containing compounds

“…Intergrain boundary pinning 12 and point defect pinning 13 are the two main important pinning mechanisms used to improve j c . In high-type-II superconductors, where is the Ginzburg-Landau parameter, the most important elementary interaction between the vortices and the pinning centers is the core interaction.…”

Hall effect inc-axis-orientedMgB2thin films

“…The onset T c was 39.2 K and had a narrow transition width of ∼ 0.15 K, as judged from the 10 to 90% superconducting transition. At 40 K, ρ xx was 3.4 µΩ cm, giving a residual resistivity ratio [RRR = ρ xx (300K)/ρ xx (40K)] of 3, which was smaller than the value observed for MgB 2 single crystals [7,8]. A very small (less than 0.5%) magnetoresistance was observed in the normal state at 5 T.…”

“…However, many experimental results still remain controversial because of the relatively high anisotropic nature of this compound. Based on measurements of the upper critical field for different crystal planes of MgB 2 single crystals [7,8] and of highly c-axis-oriented thin films [9], they have confirmed the anisotropic nature of MgB 2 superconductor. These results strongly suggest that the physical properties of MgB 2 should be investigated by either using single crystals or high-quality thin films having preferred orientations.…”