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.

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We report the observation of exchange bias in (Pt/Co0.90Fe0.10)n/FeMn multilayers, with perpendicular magnetic anisotropy. We analyze the behavior of the exchange-bias field Heb as a function of temperature, FeMn thickness, number of (Pt/Co0.90Fe0.10) bilayers, and annealing conditions. Measurements carried out with field applied perpendicular to the plane of the samples indicate that the hysteresis loops present 100% of remanent magnetization, with Heb reaching 200 Oe and a tunable coercive field, depending on Co and Pt thickness ratio and on Pt buffer thickness. Furthermore, spin-valves of the form (Pt/Co0.90Fe0.10)n/NM/(Co0.90Fe0.10/Pt)m/Co0.90Fe0.10/FeMn with NM=Cu or Pt have been prepared. They exhibit two well separated hysteresis loops when the field is applied perpendicular to the plane.