Fine grained polycrystalline samples of MgB2 superconductor were synthesize
d from the elements to contain < 5 % of impurity phases, according to x-ray
powder diffractometry. The superconductive transition was sharp with a mid
point 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 magn
etization) decreases approximately linearly with T, in contrast to the quas
i-exponential fall-off in high-T-c superconductors. The current is highly s
table 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 othe
r superconductors.