Seminar : The switching paths of spin transfer torque magnetic random access memories

Location : IMT Atlantique (Room C02-113)

Visio :  https://imt-atlantique.zoom.us/j/95590634556

Lecturer : Paul BOUQUIN, PIM post-doc

Abstract :

"In  spin  transfer  torque  random  access  memories  (STTMRAM),  the  magnetization  of  a  thin  ferromagnetic  layer  is  reversed  under the action of a polarized spin current. We study the switching path that the STTMRAM undergo. The first results are micromagnetic simulations of the switching. We study the impact of the diameter of the  device  on  the  switching  path.  From  these  numerical  calculations  we  predict for  devices  between  20  and  100  nm  at  room temperature a switching path composed of a coherent phased followed by a domain wall nucleation and motion. It is the switching path expected in our forthcoming measurements. The domain wall dynamics observed in the micromagnetic simulations present complex Walker oscillations that are not understood from the domain wall models of the state of the art. Therefore, I present a more complete model for the domain wall dynamics within a STTMRAM which takes into account the exact geometry of the system. In this geometry the elasticity terms act as  a  new  effective  field  called  the  stretch  field.  The  stretch  field  plays  a  key  role  in  the  wall  dynamics  and  explains  the  complex Walker oscillations. The conditions under which these effects can be measured are also predicted by our new model. Our measurements are performed on state-of-the-art STTMRAM based on perpendicular magnetic tunnel junction. The diameter of  the  devices  varies  between  26  and  200  nm.  We  characterize  our  devices  by  magnetometry,  ferromagnetic  resonance  and electrical time-resolved measurements of the switching path. The switching path in our time-resolved measurements presents the signatures of an initial coherent phase and of a domain wall motion.  This  is  in  agreement  with  the  simulated  switching  path.  The  complex  Walker  oscillations  predicted  by  our  models  are measured  in  specific  devices  with  an  ultrasoft  free  layer,  but  not  in  our  most  standard  stack.  This  highlight  the  interest  of  our analytical models for understanding the behavior of application-oriented devices."

Presentation : 2021 10 28 Séminaire Paul BOUQUIN