Crossover from in-plane to perpendicular anisotropy in $Pt/CoFe/AlOx$ sandwiches as a function of Al oxidation: A very accurate control of the oxidation of tunnel barriers

By measuring the extraordinary Hall effect on a series of naturally oxidized $Pt3 nm/Co90Fe10$ 0.6 nm/Al $tAl$ samples with $0<tAl<1.2 nm,$ a first crossover from in-plane to perpendicular magnetic anisotropy is observed when $tAl$ is varied from 0 to 0.2 nm. The CoFe magnetization remains out of plane for $0.2<tAl<0.5 nm.$ When the Al thickness is further increased, a second crossover back to in plane takes place. In a series of samples in which the Al thickness is kept constant, the same behavior is observed as the time of exposure to an oxygen plasma is varied. The results clearly indicate that the degree of oxidation of Al at the $CoFe/AlOx$ interface has a dramatic effect on the magnetic anisotropy of the transition-metal layer. These sharp crossovers of anisotropy provide a very accurate and convenient way to monitor the oxidation of the tunnel barrier in magnetic tunnel junctions. This technique is also applied to characterize the oxidation kinetics of various ultrathin metallic layers as well as the aging effect in alumina barriers.