Periodic Reporting for period 1 - SOT-2DvdW (Spin-Orbit Torque in 2D van der Waals Heterostructures)
Reporting period: 2018-07-18 to 2020-07-17
Most of the electronic devices (processors and memories) inside mobile phones, computers, laptops, etc., are fabricated by stacking different kinds of materials: semiconductors, insulators, and metals. The interface between these materials is a very important region where crystal defects, impurities, and charges accumulate leading to unwanted effects and reducing the performance of these devices. This problem has been eventually overcome thanks to the discovery of two-dimensional materials such as Graphene. In bulk (3D), Graphene layers stack on top of each other like a sandwich forming Graphite. The forces that keep these layers together are so weak that allow them to be exfoliated from the bulk and be transferred to the top of another material. The catalog of layered materials has grown enormously in the last few years, and now it is possible to combine 2D materials showing not only conducting or insulating properties but also very exotic properties such as superconductivity, magnetism, non-trivial topology, etc. This project aims to study the microscopic effects taking place at the interface between these materials where different properties combine leading to novel physical phenomena. More specifically, we want to understand better the interface between 2D magnets and 2D semiconductors with strong spin-orbit coupling. These materials are at the heart of a novel way of spin manipulation called spin-orbit torque which may allow the fabrication of high-performance low-consumption magnetic random-access memories (MRAM).
From the beginning, the project has been focused on understanding the electronic and magnetic properties of the recently discovered Chromium Trihalides 2D ferromagnets (CrI3, CrBr3, and CrCl3). Magnets can store information in the spins of electrons. Manipulating the spins externally allows for controlling such information. We have performed first-principles (density functional theory) calculations on these Chromium Trihalides and developed new methodologies to study the effect of Coulomb interactions and substrate effects on the magnetic properties of these materials. The main results achieved are summarized below.
Understanding antiferromagnetic (AFM) interlayer coupling in few-layer CrI3 samples has been the goal of a bunch of publications since the isolation of this material in 2017 by the groups of Prof. Pablo Jarillo-Herrero at MIT, and Prof. Xiadong Xu at University of Washington. This layered material couple ferromagnetically (FM; the electron spins couple parallel) across its layers in bulk but become antiferromagnetic (anti-parallel) in ultrathin samples. We were one of the first theoretical groups in reporting the role of the stacking configuration in the interlayer magnetism. These materials are exfoliated at room temperature. At this temperature the layers stack on top of each other following a very different pattern than at low temperatures. This modifies the coupling between Cr atoms at different layers leading to a different magnetic coupling (also known as super-super-exchange mechanism).
The AFM coupling in few-layer CrI3 is so small that it is possible to tune the interlayer coupling from AFM to FM by external perturbations. Experimental works by the groups of Prof. Jie Shan at Cornell University, and Prof. Xiadong Xu at University of Washington demonstrated the possibility to tune the interlayer magnetism by electrostatic doping. We demonstrated that this transition is faster in the case of electron doping, as experimentally observed, driven by the formation of magnetic polarons. When an electron is added to a semiconducting AFM bilayer, this usually orders its spin ferromagnetically with respect to the other layer to reduce locally the gap, which is energetically favored. This electron acts as a seed for the formation of small ferromagnetic patches which finally evolve to a ferromagnetic transition. Our calculations showed that this mechanism was not energetically favored in the case of hole doping.
Finally, we are studying the effect of Coulomb interactions in the magnetic properties of Chromium Trihalides. Electron-electron interactions have strong effects in the magnetic properties and eventually may lead to the destruction of magnetism or a FM-AFM transition. All these effects have important implications in the Curie temperature of these materials (the Curie temperature is the temperature at which the spins of the electrons align giving rise to ferromagnetism, only below this temperature the material behaves as a real ferromagnet). One of the most important research lines right now regarding 2D ferromagnets is to find ways of increasing their Curie temperature. Coulomb interactions can be easily tuned by modifying the substrates (Silicon Oxide, Graphite, Boron Nitride, Metals) or by external fields. Besides, the modifications in the Coulomb interactions affect the spin-wave (magnon) spectrum of these materials (temperature excites the electron spins forming a spin-wave). We are able to study these spin-waves for different Coulomb parameters which is connected with recent experiments able to observe these spin-waves in 2D ferromagnets and antiferromagnets.
Understanding antiferromagnetic (AFM) interlayer coupling in few-layer CrI3 samples has been the goal of a bunch of publications since the isolation of this material in 2017 by the groups of Prof. Pablo Jarillo-Herrero at MIT, and Prof. Xiadong Xu at University of Washington. This layered material couple ferromagnetically (FM; the electron spins couple parallel) across its layers in bulk but become antiferromagnetic (anti-parallel) in ultrathin samples. We were one of the first theoretical groups in reporting the role of the stacking configuration in the interlayer magnetism. These materials are exfoliated at room temperature. At this temperature the layers stack on top of each other following a very different pattern than at low temperatures. This modifies the coupling between Cr atoms at different layers leading to a different magnetic coupling (also known as super-super-exchange mechanism).
The AFM coupling in few-layer CrI3 is so small that it is possible to tune the interlayer coupling from AFM to FM by external perturbations. Experimental works by the groups of Prof. Jie Shan at Cornell University, and Prof. Xiadong Xu at University of Washington demonstrated the possibility to tune the interlayer magnetism by electrostatic doping. We demonstrated that this transition is faster in the case of electron doping, as experimentally observed, driven by the formation of magnetic polarons. When an electron is added to a semiconducting AFM bilayer, this usually orders its spin ferromagnetically with respect to the other layer to reduce locally the gap, which is energetically favored. This electron acts as a seed for the formation of small ferromagnetic patches which finally evolve to a ferromagnetic transition. Our calculations showed that this mechanism was not energetically favored in the case of hole doping.
Finally, we are studying the effect of Coulomb interactions in the magnetic properties of Chromium Trihalides. Electron-electron interactions have strong effects in the magnetic properties and eventually may lead to the destruction of magnetism or a FM-AFM transition. All these effects have important implications in the Curie temperature of these materials (the Curie temperature is the temperature at which the spins of the electrons align giving rise to ferromagnetism, only below this temperature the material behaves as a real ferromagnet). One of the most important research lines right now regarding 2D ferromagnets is to find ways of increasing their Curie temperature. Coulomb interactions can be easily tuned by modifying the substrates (Silicon Oxide, Graphite, Boron Nitride, Metals) or by external fields. Besides, the modifications in the Coulomb interactions affect the spin-wave (magnon) spectrum of these materials (temperature excites the electron spins forming a spin-wave). We are able to study these spin-waves for different Coulomb parameters which is connected with recent experiments able to observe these spin-waves in 2D ferromagnets and antiferromagnets.
The use of 2D magnets in spin-orbit torque devices and other type of memory devices is still in its infancy. One of the most important advances going on in this field is the possibility to transport spins between distant electrodes through spin-waves. This makes insulating 2D magnets (FM and AFM) a new platform for low-consumption spintronic devices. There is still a long way to go regarding their application due to lack of chemical stability in ambient conditions or their very low Curie temperatures. However, this is a hot topic which is evolving very fast and new 2D magnets are being discovered showing better properties to be used as devices.