Evolution of perpendicular magnetized tunnel junctions upon annealing

TL;DR

This study investigates how perpendicular magnetized tunnel junctions change structurally and magnetically during high-temperature annealing, revealing insights through ferromagnetic resonance modeling to guide future device optimization.

Contribution

It introduces ferromagnetic resonance eigenmode analysis as a diagnostic tool for structural evolution in tunnel junctions during annealing.

Findings

Anisotropies decay at different rates during annealing.

Exchange coupling fails above 375°C.

Ru spacer's effectiveness diminishes with annealing.

Abstract

We study the evolution of perpendicularly magnetized tunnel junctions under 300 to 400 $^{\circ}$C annealing. The hysteresis loops do not evolve much during annealing and they are not informative of the underlying structural evolutions. These evolutions are better revealed by the frequencies of the ferromagnetic resonance eigenmodes of the tunnel junction. Their modeling provides the exchange couplings and the layers' anisotropies within the stack which can serve as a diagnosis of the tunnel junction state after each annealing step. The anisotropies of the two CoFeB-based parts and the two Co/Pt-based parts of the tunnel junction decay at different rates during annealing. The ferromagnet exchange coupling through the texture-breaking Ta layer fails above 375$^{\circ}$C. The Ru spacer meant to promote a synthetic antiferromagnet behavior is also insufficiently robust to annealing. Based on these evolutions we propose optimization routes for the next generation tunnel junctions.