Thermally robust synthetic antiferromagnetic fixed layers containing FeCoB for use in STT-MRAM devices

Abstract

In this work we measure the bilinear and biquadratic interlayer exchange coupling, J1 and J2, for several different synthetic antiferromagnetic structures containing FeCoB both before and after annealing at 350°C, with Ru spacer layers ranging in thicknesses from 0.4 to 1 nm. We show that when FeCoB is on top of the Ru spacer layer, in a Co/Ru/FeCoB trilayer structure, it is able to maintain antiferromagnetic coupling after annealing at 350°C, for Ru spacer layer thicknesses of 1 nm or less. This is in contrast to coupling in FeCoB/Ru/FeCoB trilayer structures that becomes strongly ferromagnetically coupled after annealing above 300°C. This difference is thought to be caused by boron diffusion into the spacer layer, which in turn enhances the diffusion of magnetic atoms into the spacer layer. We also show that coupling in a FeCoB/Co/Ru/FeCo/FeCoB structure remains AFC after annealing at 350°C, for Ru spacer layer thicknesses of 0.8 nm or less. This is thought to be caused by the Co and FeCo layers acting as diffusion barriers and reducing the diffusion of boron into the Ru spacer layer during the annealing process. We also show that, in a Co/Ru/Co trilayer structure, J1 can be increased in magnitude by replacing the top Co layer with FeCo, for spacer layer thicknesses of 0.5 nm or less. After these samples have been annealed at 350°C, the replacement of the top Co layer with FeCo was found to result in a two fold increase in the magnitude of J1, for Ru spacer layer thicknesses of 0.5 nm or less.