The effect of nano-alumina on structural and magnetic properties of MgB₂ superconductors

Nano-Al₂O₃ doped Mg_1-xAl_xB ₂ with 0≤x≤6% were synthesized by solid state reaction at 750 °C in Fe tube encapsulation under a vacuum of 10^-5Torr. Resistance measurement shows that the T_c decreases with x and zero resistivity for x ≤ 0 and 6% are obtained at 38 and 35K, 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 MgB₂ 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 35K and in the magnetic field up to 9T 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 Al₂O₃ 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 20K. The results are discussed in terms of local-vortex instabilities caused by doping of Al₂O₃ nanoparticles.