Ti-doping effects on magnetic properties of dense MgB₂ bulk superconductors

We have studied the effects of Ti doping on the magnetic properties of MgB₂ superconducting bulks. The trapped field, which was obtained by field-cooled magnetization, $B_T^{FC}$, was about 3.6 T at 13 K at the surface of the single Ti5–20% doped MgB₂ bulks, which was about 1.3 times larger than that of the non-doped bulk. The $B_T^{FC}$ of 4.6 T was achieved at 14.1 K in the centre of the doubly stacked Ti-doped MgB₂ bulks. The remanent magnetic flux density, which corresponds to the trapped field by zero-field cooled magnetization, $B_T^{ZFC}$, was comparable with the absolute value of coercive force with a very small vortex creep rate of about 2% over 40 h. These results suggested that the MgB₂ bulk was an excellent 'quasi-permanent' magnet. The critical current density, $J_c$, under magnetic field was also enhanced by Ti doping; under 3 T at 20 K, the $J_c$ of $4.0\times {10}^3$ A cm⁻² for the pristine sample was enhanced to that of 1.6–1.8 $\times {10}^4$ A cm⁻² for the Ti-doped samples. The irreversibility field exceeded 5 T at 20 K for the Ti-doped samples. The existence of nanometric unreacted B and strongly Mg-deficient Mg-B particles and TiB₂ layers at the periphery of Ti precipitates was suggested in the Ti-doped bulks by microscopic observation. The improvement of the vortex pinning properties in Ti-doped MgB₂ originated from the creation of the nanometric nonsuperconducting particles and TiB₂ layers acting as strong vortex pinning centres. (paper)