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Development of self-referenced thermally assisted magnetic random access memory cells (MRAM)

Vendredi 19 décembre 2014 10:00 - Duree : 2 heures
Lieu : CEA Grenoble

Orateur : Soutenance de Thèse de Quentin STAINER

The goal of this thesis was to model and demonstrate experimentally the functionalities of a new thermally assisted magn etic random access memory structure, the self-referenced MRAM. The self-referenced MRAM stack is obtained from the thermally assisted MRAM one by removing the reference antiferromagnetic, effectively replacing the pinned reference layer by a free layer : the sense layer. By remotely switching the sense layer magnetization, by means of an external field, the storage layer magnetization direction, and as such the stored bit state, can be probed in-situ. Due the possibility to program both magnetic layers individually, self-referenced MRAM can be operated as a Magnetic Logic Unit, combining in-stack the storage and exclusive-or logic functions and thereby opening new application ranges. The read and write functionality of self-referenced MRAM were experimentally demonstrated on a first batch of samples. However, the field requirements were found to be higher than the target requirements for fully functional industrial products. In order to optimize the read and write field requirements, we developed a macrospin model based on the Stoner-Wohlfarth model of magnetization reversal. By introducing magnetostatic, RKKY and ferromagnet/antiferromagnet exchange coupling phenomena, we calculated a general form of the energy for any type of MRAM magnetic stack. A previously proposed highly efficient switching mode, relying on the magnetostatic interactions between the sense and the storage layer, was effectively predicted by the model and experimentally demonstrated in new samples. An excellent agreement was obtained between the model and the experimental results. Increasing the stiffness of the storage layer was found to be critical in order to minimize the read field requirements at decreasing patterning dimensions. Material developments were performed to maximize the RKKY coupling in the synthetic ferrimagnet storage layer. In order to study the reproducibility of the write operation, the influence of thermal activation was modelled by calculating energy barriers and transition paths and compared with on-the-fly measurements of switching probabilities on the new set of samples with a stiffer storage layer. Again, an excellent agreement was obtained between the model and the experiments. Based on the model developed, we built a roadmap describing the magnetic stack to use, that allows a downscaling of the self-referenced MRAM down to 45 nm while conserving manageable field requirements. Due to fundamental limitations in field-induced switching MRAM technology, reaching higher densities was found to require increasing the individual storage capacity of each MTJ, i.e. storing multiple bits per unit cell. A new angle-based storage method taking advantage of the sense layer free magnetization was investigated. Using the magnetic model developed previously, suitable samples were designed and allowed to experimentally demonstrate up to 4 bits per single MTJ. The field requirements were however found to be substantially higher than those compatible with a fully functional product. A new write method, predicted by the model, was investigated and exploited in the building of a second roadmap down to 45 nm. Mirrored dual barrier structures were then studied, with the experimental demonstration of their manufacturability and functionality. Notably, a highly efficient write mechanism similar to that observed in single barrier self-referenced MRAM was obtained. Finally, the adaptation of angle-based storage to these dual barrier devices was modeled, leading to the proposition of a method to store up to 8 bits per single cell.

Contact : rachel.mauduit@cea.fr

Discipline évènement : (Physique)
Entité organisatrice : (INAC/SPINTEC)
Nature évènement : (Soutenance de thèse)
Site de l'évènement : Site CEA sans badge requis

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