Periodic Reporting for period 1 - SORTIR (Spin-obit torque heterostructures based on topological insulators and 2D materials)
Periodo di rendicontazione: 2023-04-01 al 2025-03-31
Due to the massive increase of data generation, data storage is a challenge in our society. Spintronics plays a key role in this field, targeting non volatility and low energy consumption. Among the solutions offered by spintronics, the magnetic random access memory (MRAM) is the only non-volatile memory allowing high endurance and very fast write operation. So far, it is considered to be the best option for non-volatile data storage operations.
The main component of the the MRAM, is the magnetic tunnel junction (MTJ) that is made of two ferromagnetic layers (FM) separated by an insulator. One of the FM stores the information with the direction of its magnetization, and the other FM is fixed and used as a reference to read the first FM. In order to change the magnetization of the first FM and therefore, write information, it is convenient to use electrical current and this is the core of MRAM technology.
In industrial MRAMs, a charge current is converted into a spin current by the first FM, that will exert a torque in the second FM. This phenomenon is known as spin-transfer torque (STT). If it is strong enough, this can lead to the switching of the second FM magnetization. The issue is that large current densities are required for this process to occur, resulting in damaging the insulator and affecting the functioning of the device.
To solve this problem, the charge to spin conversion can be achieved differently: using a high spin orbit coupling material adjacent to the first FM. With this method, the current generating the torques flows along the film stack instead of across. The result is a more reliable MRAM with a better endurance.
In parallel of this important improvements, the field of two-dimensional materials (2DMs) has led to new perspectives for downscaling and improving MRAM performances. Their 2D nature and their very weak van-der-Waals interaction between 2D layers, allows the creation of ultimately thin stacks with sharp interfaces, avoiding the usual problem of roughness and inter-diffusion that is at the origin of the degradation of the spin properties of usual materials.
Another benefit of 2D materials is their variety covering a broad range of relevant properties for spintronics. Among them, graphene shows a very low SOC resulting in long spin diffusion length3, which is of great interest to avoid spin depolarization. The 2DM family also includes high-SOC materials, known as 2D-spin-orbit materials (2D-SOM), which are relevant for charge-to-spin conversion (CSI), such as the transition metal dichalcogenides (TMDCs) and broadly studied topological insulators (TIs). Importantly, the host group pioneered the study of proximity effects between graphene and TMDCs and demonstrated large CSI in these stacks4, highlighting the possibility of combining graphene with 2D-SOMs to achieve large SOTs. Furthermore, unlike conventional heavy metals, low-symmetry TMDCs offer the possibility to generate out-of-plane damping SOTs5, which are required for continuing down-scaling1. On the other hand, TIs exhibit conducting topological surface states (TSSs) where the spin of the carriers are locked to their momentum. This results in a highly-efficient generation of spin currents with a polarization that depends on the direction of the charge current. The host group recently demonstrated the tuning of SOTs in TI heterostructures when inserting spacers between the TI and a conventional FM. Despite the advances, there are still open questions about the nature of the spin generation and how to enhance it. Moreover, recent material research has led to the discovery of 2D-FMs exhibiting a Curie temperature close or exceeding room temperature, making them promising candidates for practical nanoscale spintronics devices.
In the SORTIR project, the researcher aimed to increase the performances of usual MRAM devices by using advanced TI/graphene heterostructures.
The main component of the the MRAM, is the magnetic tunnel junction (MTJ) that is made of two ferromagnetic layers (FM) separated by an insulator. One of the FM stores the information with the direction of its magnetization, and the other FM is fixed and used as a reference to read the first FM. In order to change the magnetization of the first FM and therefore, write information, it is convenient to use electrical current and this is the core of MRAM technology.
In industrial MRAMs, a charge current is converted into a spin current by the first FM, that will exert a torque in the second FM. This phenomenon is known as spin-transfer torque (STT). If it is strong enough, this can lead to the switching of the second FM magnetization. The issue is that large current densities are required for this process to occur, resulting in damaging the insulator and affecting the functioning of the device.
To solve this problem, the charge to spin conversion can be achieved differently: using a high spin orbit coupling material adjacent to the first FM. With this method, the current generating the torques flows along the film stack instead of across. The result is a more reliable MRAM with a better endurance.
In parallel of this important improvements, the field of two-dimensional materials (2DMs) has led to new perspectives for downscaling and improving MRAM performances. Their 2D nature and their very weak van-der-Waals interaction between 2D layers, allows the creation of ultimately thin stacks with sharp interfaces, avoiding the usual problem of roughness and inter-diffusion that is at the origin of the degradation of the spin properties of usual materials.
Another benefit of 2D materials is their variety covering a broad range of relevant properties for spintronics. Among them, graphene shows a very low SOC resulting in long spin diffusion length3, which is of great interest to avoid spin depolarization. The 2DM family also includes high-SOC materials, known as 2D-spin-orbit materials (2D-SOM), which are relevant for charge-to-spin conversion (CSI), such as the transition metal dichalcogenides (TMDCs) and broadly studied topological insulators (TIs). Importantly, the host group pioneered the study of proximity effects between graphene and TMDCs and demonstrated large CSI in these stacks4, highlighting the possibility of combining graphene with 2D-SOMs to achieve large SOTs. Furthermore, unlike conventional heavy metals, low-symmetry TMDCs offer the possibility to generate out-of-plane damping SOTs5, which are required for continuing down-scaling1. On the other hand, TIs exhibit conducting topological surface states (TSSs) where the spin of the carriers are locked to their momentum. This results in a highly-efficient generation of spin currents with a polarization that depends on the direction of the charge current. The host group recently demonstrated the tuning of SOTs in TI heterostructures when inserting spacers between the TI and a conventional FM. Despite the advances, there are still open questions about the nature of the spin generation and how to enhance it. Moreover, recent material research has led to the discovery of 2D-FMs exhibiting a Curie temperature close or exceeding room temperature, making them promising candidates for practical nanoscale spintronics devices.
In the SORTIR project, the researcher aimed to increase the performances of usual MRAM devices by using advanced TI/graphene heterostructures.
The main approach of this project was to develop a new type of heterostructures: BST/graphene/FM. BST is a TI that we use for charge to spin conversion. The goal of making this structure was to benefit from BST TSSs in order to optimize his spin to charge conversion in the device. The first challenge was to preserve these TSSs during the whole process of fabrication. Benefiting from the experience developed in the lab concerning the growth of the TI and the researcher’s experience on manipulating 2D interfaces, we could succeed in preserving the TSSs of the TI while completing the full heterostructure. We could confirm the existence of TSS by electrical characterization (Hall measurements) showing very low charge density. We then performed second harmonics electrical measurements showing a 25-fold increase in the field-like spin-orbit torque efficiency at room-temperature compared to BST/Co., with elevated efficiencies across a wide temperature range.
This underscores the benefits of a stable, atomically smooth interface for efficient spin torque generation. Importantly, the temperature dependence of the field-like and damping-like SOT components further suggests that both TSSs and graphene may influence SOT mechanisms. These findings highlight the value of interfacial engineering with van der Waals materials, revealing new possibilities for achieving temperature-robust, high-efficiency SOTs tailored for advanced spintronic applications. Our work opens new avenues for precisely tuning interfacial properties in TI-based heterostructures, crucial for next-generation magnetic memory and logic devices.
The researcher also worked in close collaboration with Abert Fert's Lab in France in order to achieve large-scale SOT-MRAM devices.
Together with Albert Fert's Lab, the researcher developed methods using Pulsed Laser Deposition (PLD) to achieve TMDCs/Co SOT-MRAM, for scalability and repeatability that is lacking using the usual exfoliation and transfers of 2D materials. The result was functioning devices with sizable SOT signals measured by second harmonic electrical characterizations.
This is a major step in the development of 2D based MRAM towards industry.
This underscores the benefits of a stable, atomically smooth interface for efficient spin torque generation. Importantly, the temperature dependence of the field-like and damping-like SOT components further suggests that both TSSs and graphene may influence SOT mechanisms. These findings highlight the value of interfacial engineering with van der Waals materials, revealing new possibilities for achieving temperature-robust, high-efficiency SOTs tailored for advanced spintronic applications. Our work opens new avenues for precisely tuning interfacial properties in TI-based heterostructures, crucial for next-generation magnetic memory and logic devices.
The researcher also worked in close collaboration with Abert Fert's Lab in France in order to achieve large-scale SOT-MRAM devices.
Together with Albert Fert's Lab, the researcher developed methods using Pulsed Laser Deposition (PLD) to achieve TMDCs/Co SOT-MRAM, for scalability and repeatability that is lacking using the usual exfoliation and transfers of 2D materials. The result was functioning devices with sizable SOT signals measured by second harmonic electrical characterizations.
This is a major step in the development of 2D based MRAM towards industry.
Concerning the BST/Graphene/FM heterostructure:
It is a major step for fundamental physics towards applications. This is the first time that we achieve a heterostructure Ti.Gr.Co while preserving the TI's TSSs leading in unprecedented SOT magnitudes in this type of structures.
This is of high interest as this could lead to very efficient technologies for data storage.
Concernant the TMDCs/Co SOT-MRAM:
This is a step towards applications as we found a way to scale efficiently the production of efficient 2D based MRAMs.
It is a major step for fundamental physics towards applications. This is the first time that we achieve a heterostructure Ti.Gr.Co while preserving the TI's TSSs leading in unprecedented SOT magnitudes in this type of structures.
This is of high interest as this could lead to very efficient technologies for data storage.
Concernant the TMDCs/Co SOT-MRAM:
This is a step towards applications as we found a way to scale efficiently the production of efficient 2D based MRAMs.