Production and characterization of exchange coupled Co/Pt and Co/Pd based pseudo spin-valves

Pseudo spin-valves (PSVs) based on giant and tunneling magnetoresistance (GMR and TMR, respectively) heterostructures with perpendicular magnetic anisotropy (PMA) are attracting an ever increasing attention due their potential use in magnetic random access memories based on the spin transfer torque effect (STT-MRAMs), being also of interest for other applications including magnetic field sensors and logic ports. However, the increasing demand for miniaturization of magnetic devices requires to specifically address the effect of size confinement, down to the nanoscale, on PSVs with perpendicular anisotropy. In this context, the NANOREST project has addressed the problem of enhancing the magnetic stability of nanostructures, to be employed as electrodes in spin-valves of the MRAM architecture. The approach is to build both the reference layer (RL) and the free layer (FL) as two-phase dots in which a key role in determining the magnetic behaviour is played by the interface exchange coupling. In this contribution,we present the activitieson perpendicular [Co/M] (M=Pt, Pd) based PSV aimed at establishing the best conditions to obtain both the deposition of continuous multilayers and the shaping of the same systems in the form of nanosized dot arrays by using FIB based processes. To this purpose a [Co/M]n/FeCo/Cu/FeCo/[Co/M]n PSVs structure with PMA is proposed, in which a key role is played by the exchange coupling at the soft/hard FeCo/[Co/M] interface. By properly changing the thickness of Co and M layers and the number of repetition n, the PMA can be finely modulated thus allowing two electrodes with different coercivity to be obtained, as required in a SV system. The results showed that, when a [Co/Pd]n layer is used as the hard phase in the continuous bilayers, the desired rigid exchange- coupled system, consisting of hard and soft phases with PMA, was obtained. However, when two bilayers with different coercivity are chosen as RL and FL and assembled in a PSV stack, the behavior of such a system was not a simple superposition of the hysteresis loops of the individual electrodes (as expected for ideal PSV stacks), the overall shape suggesting a strong dipolar coupling between the constituent layers across the Cu spacer. The effect of the nanostructuring process on the magnetic properties of patterned bilayers and PSV systems will be illustrated.