The effect of nano-alumina on structural and magnetic properties of MgB2superconductors

Nano-Al 2 O 3 doped Mg 1− x Al x B 2 with 0≤ x ≤6% were synthesized by solid state reaction at 750 °C in Fe tube encapsulation under a vacuum of 10 −5 Torr. Resistance measurement shows that the T c decreases with x and zero resistivity for x = 0 and 6% are obtained at 38 and 35 K, respectively. XRD measurement shows that the lattice parameter and cell volume also decrease monotonically with increasing doping levels. From this we infer that the Al has been substituted in the lattice of MgB 2 at Mg sites. Resistivity measurement shows a systematic decrease in T c with doping which also confirms the substitution of Al. Magnetization studies in the temperature range from 4 to 35 K and in the magnetic field up to 9 T shows a significant increase in the irreversibility field ( H irr ), critical current density ( J c ) and remanent magnetization ( M R ) with increasing concentration of the Al 2 O 3 nanoparticle. At low fields we have observed large vortex instabilities (known as a vortex avalanche) associated with all doped samples. The vortex-avalanche effect is reduced with increasing temperature and vanishes near 20 K. The results are discussed in terms of local-vortex instabilities caused by doping of Al 2 O 3 nanoparticles.