  Using magnetron sputtering, we have prepared Co-Fe-B/tunnel barrier/Co-Fe-B
magnetic tunnel junctions with tunnel barriers consisting of alumina, magnesia,
and magnesia-alumina bilayer systems. The highest tunnel magnetoresistance
ratios we found were 73% for alumina and 323% for magnesia-based tunnel
junctions. Additionally, tunnel junctions with a unified layer stack were
prepared for the three different barriers. In these systems, the tunnel
magnetoresistance ratios at optimum annealing temperatures were found to be 65%
for alumina, 173% for magnesia, and 78% for the composite tunnel barriers. The
similar tunnel magnetoresistance ratios of the tunnel junctions containing
alumina provide evidence that coherent tunneling is suppressed by the alumina
layer in the composite tunnel barrier.
