  Magnetization studies have been carried out on superconducting MgB$_2$
(T$_c$=37K) in the temperature range of 2-50K and in magnetic field up to 5
Tesla. From these measurements, values of the lower critical field H$_c1$(0),
upper critical field H$_c2$(0) at zero K are estimated to be ~300 Oe and ~12.5
Tesla, respectively, which yield a value of ~26 for the Ginzburg-Landau
parameter, k. Magnetization hysteresis loops have been obtained at various
temperatures from which the magnetization critical current density is estimated
using Bean's critical state model.
