Crossovers from in-plane to perpendicular anisotropy in magnetic tunnel junctions as a function of the barrier degree of oxidation

Pt/Co/M sandwiches (M=Al, Ta, Cr) were prepared by sputtering. The M capping layer was oxidized either by natural oxidation in air or by exposure to an oxygen plasma. By measuring the extraordinary Hall effect in these systems, we observed that the magnetic anisotropy of the Co electrode is extremely sensitive to the degree of oxidation of the capping layer. In particular, when M=Al, the magnetic anisotropy is found in plane when the ${\mathrm{AlO}}_x$ layer is under or overoxidized whereas it is out of plane when this oxide layer is stochiometric alumina. The conditions of oxidation which give the perpendicular anisotropy in ${\mathrm{Pt/Co/AlO}}_x$ are found to be the same than those which lead to a maximum of specular reflection at ${\mathrm{Co/AlO}}_x$ interface in ${\mathrm{Co/Ru/Co/Cu/Co/AlO}}_x$ synthetic spin-valves. This crossover of anisotropy therefore provides a very useful way for controlling the oxidation of tunnel barriers. Furthermore, this phenomenon can be used to study the aging of naturally oxidized tunnel junctions exposed to air, and it is also observed with other capping materials such as Ta or Cr.