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Spintronics with Non-Conventional 2D Materials

Periodic Reporting for period 1 - SPIN2D (Spintronics with Non-Conventional 2D Materials)

Reporting period: 2015-05-18 to 2017-05-17

Since the discovery of graphene in 2004, a member of the two-dimensional materials (2DM) family, their amazing properties at the single layer level have attracted an enormous attention. Thanks to the plethora of available compounds, an extremely wide richness of properties that cover the full range of electrical and magnetic behaviours is expected. For spintronics, the combination of 2D layers in designed multilayer stacks can be envisaged as a way to obtain highly optimized modules thanks to the intrinsic two-dimensional nature of 2D materials that creates sharp, tuneable and free of defects interfaces. In addition, 2D layers combined altogether, will allow the realization of fully 2D advanced spintronic devices leading towards their ultimate miniaturization and engineering.
In this scenario, the main aim of SPIN2D project was to investigate the potential of 2D materials for spintronics devices and set a first step towards the future development of this new generation of spintronic multifunctional devices based on 2D building blocks. This was achieved through the fabrication of stable 2D layers of Transition Metal Dichalcogenides (TMDCs) and Transition Metal Thiophosphates (TMPCs) families, the characterization of their properties and their integration in magnetic tunnel junctions (MTJs) as the best system to test spin injection properties of 2D layers or heterostructures.
In order to investigate 2D layers potential for spintronics, we fabricated 3D crystals of different layered materials as Transition Metal Dichalcogenides (MoS2, WSe2, ZrS2, ZrSe2, WTe2…) and the antiferromagnetic Transition Metals Thiophosphates (FePS3, NiPS3, MnPSe3…) by chemical vapour transport (CVT) technique. Thin flakes down to the monolayer could be obtained for most of these materials (Figure 1). We also investigated the possibility to modify their properties towards multifunctional devices by the fabrication of multilayers stacks heterostructures or the chemical functionalization of the thin layers with spincrossover nanoparticles or Prussian Blue (PB). MoS2 functionalization with PB revealed to be of particular interest for energy application in batteries, giving rise to a publication.
The fabricated 2D materials were structurally characterized by AFM and Raman spectroscopy to determine their quality and flakes thickness. XPS characterization using synchrotron radiation was also performed in order to study their oxidation under ambient conditions. These studies are interesting by their own since the properties of these 2D materials are still mostly unexplored. Electrical characterization of the 2D layers was also performed through the fabrication of lateral devices, while vertical transport was characterized using a conductive tip AFM (CT-AFM). Finally, magnetic characterization of antiferromagnetic FePS3 2D flakes was performed by low temperature Raman spectroscopy and revealed that magnetic ordering was maintained down to the thin layers.
Next, we developed a process for implementing 2D materials in magnetic tunnel junction devices. This process allows using room temperature ferromagnetic materials as bottom electrode (e.g.: Co or NiFe), while avoiding their exposition to the air and thus their oxidation. This represents an important advance compared to state of the art MTJs fabrication with mechanically exfoliated flakes. We initially focused on MoS2 as the prototypical 2D material behind graphene, also motivated by its air stability. We successfully fabricated Co/MoS2/Co MTJs and performed magneto-transport measurements. A magnetoresistance signal up to room temperature could be detected in these devices (Figure 2). These promising results contribute to the understanding of spin injection mechanisms in MoS2 and open the way towards the implementation of other 2D materials in MTJs.
This work has already been presented in 7 international conferences or workshops and it has given rise to 2 publications. Two more publications are currently in preparation or submitted.
SPIN2D project addressed on two research areas on which worldwide scientists are betting for the development of future technologies: spintronics, which adds non-volatility and spin degree of freedom to electronics and is now identified as one of the best routes for beyond CMOS electronics and towards the development of efficient energy (zero power ICT) (it was targeted by H2020 in the ICT work programme and Graphene Flagship work package 6); and 2D materials, whose impact is well represented by the 1 billion Graphene Flagship which is EU’s biggest research/industrial initiative ever.
For spintronics, 2D materials could be used as building blocks for the development of a new class of 2D multifunctional devices, towards their ultimate miniaturization and engineering. Moreover, due to the recognized fundamental role played by interfaces in spintronics devices performances, 2D materials could represent a strong improvement thanks to their intrinsic two-dimensional nature that creates sharp, tuneable and free of defects interfaces.
In this scenario, SPIN2D project contributed to the investigation of the structural and transport properties of 2D materials that are still almost unexplored in the literature in the 2D limit (ZrS2, NiPS3, FePS3…). Moreover, it reported the successful integration of mechanically exfoliated flakes of MoS2 in magnetic tunnel junctions (MTJs) with a process that avoids bottom ferromagnetic electrode exposition to the air and thus its oxidation. This is a strong improvement compared to what reported until now in the literature for spin valves using TMDCs materials. In Co/MoS2/Co MTJs we could detect a negative MR signal up to room temperature. These results open the way towards the implementation of other 2D materials in vertical spintronic devices for the investigation of their spin injection properties, as well as for the implementation of 2D layers heterostructures, towards the fabrication of well-optimized and multifunctional spintronic devices.
Figure 2: (a) Schematic of a MTJ with MoS2 barrier. (b) MR curve measured in the junction.
Figure 1: (a) Optical microscope image and (b) AFM image of a MoS2 flake mechanically exfoliated.