The effect of disorder caused by MeV heavy ion
irradiation on the tunnel magnetoresistance (TMR) of magnetic
tunnel junctions is investigated. Two types of tunnel junctions,
viz. Co/Al2O3/Ni80Fe20 and Co/Gd-doped
Al2O3/Ni80Fe20 are studied. Upon 70 MeV Si ion
irradiation at a fluence of $5\times10^{11}$ ions/cm2, the undoped
junctions show relatively small but irreversible change, while the
Gd-doped junctions show a huge reduction, in TMR. In both cases
junctions were completely destroyed by 200 MeV Ag ion irradiation
at a fluence of $1 \times 10^{11}$ ions/cm2. The results are
attributed to the modification of the barrier and the neighboring
interfaces caused by the high energy density deposited by incident
ions.