Ferromagnetic resonance study of composite Co/Ni - FeCoB free layers with perpendicular anisotropy

TL;DR

This study investigates the magnetic properties of composite free layers with perpendicular anisotropy, using broadband ferromagnetic resonance to analyze different spacer materials and their effects on coupling and damping.

Contribution

It introduces a detailed analysis of composite Co/Ni - FeCoB free layers with various spacers, highlighting Mo as the optimal choice for spin torque memory applications.

Findings

Mo spacer provides strongest interlayer exchange coupling.

W and Mo outperform Ta in coupling strength.

Mo maintains high anisotropy and low damping.

Abstract

We study the properties of composite free layers with perpendicular anisotropy. The free layers are made of a soft FeCoB layer ferromagnetically coupled by a variable spacer (Ta, W, Mo) to a very anisotropic [Co/Ni] multilayer embodied in a magnetic tunnel junction meant for spin torque memory applications. For this we use broadband ferromagnetic resonance to follow the field dependence of the acoustical and optical excitation of the composite free layer in both in-plane and out-of-plane applied fields. The modeling provides the interlayer exchange coupling, the anisotropies and the damping factors. The popular Ta spacer are outperformed by W and even more by Mo, which combines the strongest interlayer exchange coupling without sacrificing anisotropies, damping factors and transport properties.