Spintronics is the science and technology of using spin as a vector of information in computing. Spintronics contributed to the birth of the big data era with highly sensitive hard-drive read-heads allowing dramatic data storage scaling. Magnetoresistive RAM (MRAM), the most prominent application of spintronics today, promises further increases in efficiency and performance of integrated electronic circuits. Spin valves, devices that transmit or block electrical current depending on spin orientation, are perceived as very sensitive future electronic devices that have the potency to merge in-memory processing with stochastic, neuromorphic, and quantum technologies, leading to drastically improved computing power.
Magnetic tunnel junctions (MTJ) are promising candidates for practical realization of spin valves. These are very thin devices in which carrier tunneling is controlled by the spin of the carrier. MTJs are not only important for futuristic memory devices but also interesting for studying fundamental effects such as tunneling, magnetoresistance, and spin-transfer torque. Good quality junctions at the atomic scale are of crucial importance for technological realization of MTJ devices.
One of the main remarkable characteristics of 2D materials is the ability to exhibit high homogeneity of thickness, down to atomic levels, which is hardly achievable with ordinary 3D materials deposited through conventional physical vapor deposition (PVD) growth techniques. The ultimate thickness control in 2D materials could become a critical asset for spintronics with regard to the fabrication of spin valves, where ultra-thin layers with extreme control are targeted, especially for spin-polarized electron tunnelling.
Different MJT structures, involving 2D materials. Spin-polarized charge carriers tunnel through the barrier in the middle of the device. Maëlis Piquemal-Banci et al 2017 J. Phys. D: Appl. Phys. 50 203002. Reproduced under CC 3.0 license.
Over the last years, MTJs have been the subject of intense development, with a key target being high tunnelling magnetoresistance (TMR) ratio, and more recently also switching performance. However, further improvement and downscaling has progressively unveiled issues relating to the control of the oxide barrier and the interfaces, the thermal stability, the annealing tolerance, and the robustness of the lifetime of the device. Two-dimensional materials may offer promising routes towards solving some of these issues, with layer-by-layer control of the thickness, sharp interfaces, the potential for a diffusion barrier (thermal stability), high perpendicular magnetic anisotropy, and even the possibility of new functionalities such as spin filtering.
The expectations for graphene-based MTJs were first set by calculations published in 2007. Karpan et al predicted extremely large spin polarizations close to 100%, with the resulting TMR well in excess of hundreds of percent. First experimental results followed, but with limited results due to the challenges in fabrication of high-quality graphene in those early days. In due course, however, graphene and other 2D materials showed their strength in successful experimental realizations of spin polarization TMRs.
Experimental results for TMR with graphene and other 2D materials, up until 2017. Maëlis Piquemal-Banci et al 2017 J. Phys. D: Appl. Phys. 50 203002. Reproduced under CC 3.0 license.
Experiments revealed that although mechanically exfoliated or transferred graphene can be used in such devices for basic proof of concept, direct CVD growth on ferromagnetic electrodes is a much better choice that yields high TMR. Combining high-quality (high crystallinity) epitaxial ferromagnetic Ni(111) electrodes, that act as spin sources, with directly grown multilayer graphene, yields state of the art 2D based magnetic tunnel junctions.
Aside from the construction of MTJ spin valves, graphene can be exploited to make novel devices such as spin transistors and spin logic devices, which will form the underlying architecture of future spin computers.
Development of spintronics with the aid of graphene is being pushed forward by the Spintronics Work Package of the Graphene Flagship project, of which Graphenea is part. The international group of researchers is very active, publishing dozens of scientific papers per year on progress of graphene for spintronics. For example, a line of research focuses on integrating 2D materials with magnetic materials, that are used today in memory technology, to improve technological performance of memory. This integration has resulted in improved realizations of MRAM stacks. Further, room-temperature spin logic gates were made, with applications in spin communications, as well as for magnets used in reading and writing digital information. Integration with other layered materials, such as hexagonal boron nitride and tungsten diselenide, yielded the demonstration of unprecedented new effects, such as room temperature spin-to-charge conversion and imprinted magnetism. The research is no progressing towards integration, validation, and large-scale manufacturing, on the road to low-power computing devices, from embedded memories to applications in the Internet of Things.