SOUSA Ricardo

This paper investigates laser and heavy ion irradiation effects on Perpendicular Magnetic Anisotropy Spin Transfer Torque Magnetic Tunnel Junction devices (PMA STT-MTJ). The Radiative campaign will take place at the Université Catholique de Louvain (UCL) facility in April 2020. The considered devices consist of STT p-MTJs purely magnetic memories and they were fabricated using the most advanced CoFeB-MgO MTJ technology. Single-event upset (SEU) tolerance and modification of magnetic properties will be deeply investigated and presented in the final paper.

To improve the read/write margin in perpendicular spin transfer torque magnetic random access memory (STT-MRAM), a concept of the double-magnetic tunnel junction (MTJ) MRAM cell is proposed in which the STT efficiency can be changed between read and write modes. In conventional double-MTJ stacks, the storage layer magnetization is submitted to two additive STT contributions, one from the reference layer below the bottom tunnel barrier and the other from an additional polarizing layer above the top tunnel barrier. In the proposed stack, the magnetization of the top polarizing layer can be switched between the read and write mode by domain wall propagation or spin orbit torque. This allows us to maximize the STT on the storage layer during write and minimize it during read. The associated advantages are a lower write current, reduced read disturb, maximal magnetoresistance amplitude during read, and faster read thanks to larger read current. We report here the experimental demonstration of this concept on perpendicular double-MTJ stacks.

In this study, a new type of compact magnetic memristor is demonstrated.

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In magnetic spin transfer memories, the magnetization of a ferromagnetic thin film is flipped under the effect of a polarized current. In this manuscript, we study the way this reversal occurs, called the reversal path. After having established the basic theoretical concepts and performed a state of the art of the reversal path, I present the results of our micromagnetic simulations. We have studied the rollover path as a function of the device diameter. These numerical calculations predict a rollover composed of a coherent phase followed by the nucleation and propagation of a domain wall. This rollover path is expected for devices the 20 to 100 nm at room temperature, thus in our future measurements. The domain wall propagation observed in the simulations exhibits complex Walker oscillations that are not explained by the state-of-the-art models. Also I present a more complete wall dynamics model, where the exact geometry of the system is taken into account. In this geometry the elasticity of the wall gives rise to a new field that we call the stretching field. This stretching field plays a crucial role in the wall dynamics and will allow us to understand and predict the complex Walker oscillations. Our measurements are performed on the latest generation of magnetic spin transfer memory devices, based on a perpendicular anisotropy magnetic tunnel junction. The diameter of our devices varies between 26 and 200 nm. We perform magnetometry, ferromagnetic resonance and time-resolved electrical measurements of the switching. The measured turning path in the latter exhibits the signatures of a coherent initial phase followed by a domain wall shift, as calculated in our simulations. The strong Walker oscillations predicted by our models are observed for specific samples where the free layer has few defects, but not in our most standard samples. This highlights the value of our analytical work in understanding rollover in devices for industrial applications.

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Perpendicular STT-MRAM temperature limits have been investigated in this paper by characterizing different simple and composite storage layer's thicknesses as well as different capping materials and annealing temperature conditions. Temperature measurements showed the importance of tuning precisely the thickness of the storage layer in order to choose between low consumption, high magnetic immunity, high working temperature, TMR and data retention in which the composite storage layer appeared to be the best compromise. The different thicknesses and stacks have then been compared to the main industrial applications to match thickness and performances.

A novel multi-functional antiferromagnetic coupling layer (MF-AFC) combining Ru and W is revealed to realize an extremely thin (3.8 nm), back-end-of-line compatible as well as magnetically and electrically stable perpendicular synthetic antiferromagnetic layer (pSAF), essential for spintronic memory and logic device applications. In addition to achieving antiferromagnetic RKKY coupling, this MF-AFC also acts as a Boron sink and texture-breaking layer. A detailed optimization of the thickness of the various involved layers has been carried out to obtain extremely thin-pSAF reference layer with stable magnetic properties, which enables the realization of sub-20 nm STT-MRAM cells. Two important advantages are provided by this ultrathin reference layer: the easing of the reference layer etching and the minimization of the dipolar field acting on the storage layer magnetization.

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This manuscript presents highlights of research I contributed to on magnetic random access memory (MRAM) since arriving at the CEA-Grenoble Spintec laboratory in 2002. This work involved the development of MRAM concepts using magnetic field and spin transfer torque (STT) writing methods and the materials necessary to demonstrate their principle of operation. After briefly summarizing my career path in the first chapter, I introduce areas that would benefit from the adoption of non-volatile memory in particular as embedded memory. The second chapter reviews the advantages of using the current flow through the cell in thermally assisted writing using magnetic fields and STT. It focuses on the heating dynamics and methods to determine the temperature increase inside a magnetic tunnel junction pillar. The next chapter, reviews solutions to write and reverse the magnetization direction at the ultimate speed limit defined by the magnetization precession frequency in the GHz range. It also discusses the tunnel junction barrier breakdown at short pulse widths. This is followed by a close look at the advantages of perpendicular magnetic anisotropy materials for MRAM, as the concept most likely to gain industry-wide adoption, and the material developments necessary to scale cells below 20nm diameter. Finally, I summarize our results in the context of possible future research directions in the field of MRAM.

The objective of this thesis was the modeling and experimental demonstration of the read and write functionalities of a new thermally assisted magnetoresistive random access memory structure, the self-referenced MRAM. The magnetic stacking of the self-referenced MRAM is obtained from that of the thermally assisted MRAM by removing the reference antiferromagnetic layer, thus replacing the trapped reference layer by a free reading layer. By indirectly switching the magnetization of the readout layer via an external field, the magnetization direction of the trapped storage layer, and thus the stored logic level, can be measured in-situ. Thanks to the possibility of individually programming the two magnetic layers, the self-referencing MRAM can be considered as a magnetic logic unit, combining memory functionality with comparative logic in one device, which opens up new fields of application. The functionality of the read and write modes of the self-referencing MRAM has been demonstrated experimentally on a first set of samples. However, the required fields proved to be incompatible with an application in a functional industrial product. In order to optimize the required fields for writing and reading, a macrospin model, inspired by the Stoner-Wohlfarth model of magnetization reversal, was developed. By introducing the phenomena of magnetostatic coupling, RKKY and exchange between ferromagnetic and antiferromagnetic materials, a general form of the energy applicable to any MRAM magnetic stack has been obtained. A low field strength writing mode, based on the magnetostatic coupling between the read and storage layers, has been predicted by the model and experimentally demonstrated on a new batch of samples. An excellent agreement was obtained between the model and the experimental measurements. In order to study the reproducibility of writing, the influence of thermal activation was introduced by calculating the energy barriers related to the magnetic transitions performed during writing, and then compared to the experimental measurements of the writing probability of a new batch of samples. Once again, excellent agreement was obtained between the model and the experiment. Using the developed and validated model, a roadmap defining the magnetic stacks allowing to keep low operating fields for memory points up to 45 nm has been established. Due to fundamental technological limitations in field-switched MRAMs, it appeared essential to increase the individual storage capacity of each memory point to achieve higher storage density. A new angular storage method exploiting the mobility of the readout layer magnetization has been explored. Using the previously developed model, suitable samples were produced and experimentally demonstrated a storage capacity of up to 4 bits per individual memory point. However, the required operating ranges were found to be far above what is compatible with an industrial application. Using the model, a new writing method was proposed and allowed to establish a second roadmap towards the 45 nm technology node. Double barrier mirror structures were then studied, with an experimental demonstration of the feasibility of their fabrication, as well as their functionalities. In particular, a low-field writing mode, similar to the one observed in single-barrier self-referencing MRAMs, has been obtained. Finally, the adaptation of angular storage to these mirror structures has been modeled, leading to the proposal of a method allowing to store up to 8 bits per memory point.

The influence of the bottom and top magnetic electrodes thicknesses on both perpendicular anisotropy and transport properties is studied in (Co/Pt)/Ta/CoFeB/MgO/FeCoB/Ta magnetic tunnel junctions. By carefully investigating the relative magnetic moment of the two electrodes as a function of their thicknesses, we identify and quantify the presence of magnetically dead layers, likely localized at the interfaces with Ta, that is, 0.33 nm for the bottom electrode and 0.60 nm for the top one. Critical thicknesses (spin-reorientation transitions) are determined as 1.60 and 1.65 nm for bottom and top electrodes, respectively. The tunnel magnetoresistance ratio reaches its maximum value, as soon as both effective (corrected from dead layer) electrode thicknesses exceed 0.6 nm.

In the context of increasing the storage density of magnetoresistive random access memories (MRAMs), materials with perpendicular magnetic anisotropy are particularly interesting because they possess very high anisotropy. However, this increase in anisotropy also induces an increase in write power consumption. A new concept of thermally assisted writing has been proposed by the SPINTEC laboratory. The principle is to design a very stable structure at room temperature, but which loses its anisotropy when heated, thus facilitating writing. The aim of this thesis is to experimentally validate this concept. The first chapters are devoted to the optimization of materials with perpendicular anisotropy such as (Co/Pt), (Co/Pd) and (Co/Tb) multilayers. Their integration in a magnetic tunnel junction is then presented. The evolution of the anisotropy in temperature, a crucial parameter for the proper functioning of the thermal assistance, has also been studied. Finally, it is shown that the thermally assisted writing is particularly efficient: the developed structures present a reduced writing consumption compared to conventional structures and a high stability at room temperature.

This thesis is devoted to the integration of a perpendicular anisotropy polarizer in a magnetic tunnel junction with planar magnetizations. By the effect of the spin transfer coming from the perpendicular polarizer, it is possible to induce oscillations of the magnetization of the free layer. These ultra-fast oscillations of the order of a picosecond, can be used as a writing mode in a magnetic MRAM cell. This type of writing is called precessional writing. We have optimized functional structures while keeping good electrical and magnetic qualities. Writing tests on nanopillars have validated the concept of precessional writing, thus opening a door to the understanding of the various phenomena related to tunneling and magnetization dynamics.