  Fine grained polycrystalline samples of MgB(2) 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 Tc = 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 falloff in high-Tc superconductors. The current is highly
stable in time, with normalized decay rates S = -d ln(J)/d ln(t) well below
those in high-Tc materials. These results are compared with those of other
superconductors.
