High temporal stability of supercurrents in MgB₂ materials

Fine grained polycrystalline samples of MgB₂ superconductor were synthesized from the elements to contain < 5 % of impurity phases, according to x-ray powder diffractometry. The superconductive transition was sharp with a midpoint T_c = 38.5 K. The magnetization in the vortex state was studied as a function of applied field H, temperature T and time t. From the equilibrium magnetization, the London penetration depth was obtained. The supercurrent density J(T, H, t) in the vortex state (derived from the irreversible magnetization) decreases approximately linearly with T, in contrast to the quasi-exponential fall-off in high-T_c superconductors. The current is highly stable in time, with normalized decay rates S = - d ln(J)/d ln(t) well below those in high-T_c materials. These results are compared with those of other superconductor.