Tutorial on STT-MRAM switching: key features of the switching dynamics and modulation knobs thereof

(Thibaut Devolder, research director at CNRS, Centre de Nanosciences et de Nanotechnologies, Université Paris-Sud, Université Paris-Saclay, France).

Disk-shaped CoFeB/MgO based magnetic tunnel junctions are the central elements of Spin Transfer Torque Magnetic Random Access Memories (STT-MRAM). In this tutorial meant for the International Memory Workshop in May 2020, I describe the key features of the dynamics of the switching of the free layer, as well as the material strategies to render the switching faster, more reliable and more energy efficient.
At first, I recall the switching dynamics in the macrospin approximation. I then describe how the switching happens when treated in the micromagnetics framework for samples of various sizes. I define the disk diameter leading to a crossover between the different switching regimes and I detail how these switching regimes evolve with temperature or when a real stack is taken into account. I introduce a semi-analytical framework to allow optimization strategies in terms of switching voltage and switching speed.
The most important metric that can still be much improved for a more efficient switching is the Gilbert damping of the free layer. The second half of the tutorial is thus devolted to the description of the phenomenology of Gilbert damping in FeCoB systems. This includes the influence of the boron content, of the encapsulation material and of possible heavy metal contaminants onto the Gilbert damping in memory-relevant free layers.

Acknowledgment: most of the work was done in collaboration with IMEC Belgium, and with the support of my PhD student Paul Bouquin.