Tunnel magnetoresistance in alumina, magnesia and composite tunnel barrier magnetic tunnel junctions

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.

Research highlights
► Transport properties of Co–Fe–B/tunnel barrier/Co–Fe–B magnetic tunnel junctions.
► Tunnel barrier consists of MgO, Al-Ox, or MgO/Al-Ox bilayer systems.
► Limitation of TMR-ratio in composite barrier tunnel junctions to Al-Ox values.
► Limitation indicates that Al-Ox layer is causing incoherent tunneling.