Periodic Reporting for period 1 - 2DMAGSPIN (Two-dimensional magnon and spin gases in magnetic Van der Waals heterostructures)
Reporting period: 2022-10-01 to 2025-03-31
The project Two-dimensional magnon and spin gases in magnetic Van der Waals heterostructures (2DMAGSPIN) aims at exploring the properties of two dimensional systems in which electron spins and/or spin waves are confined into 2 dimensions. It is proposed to fabricate nanostructures which are based on so-called Van der Waals materials. These are layered materials which can be easily exfoliated from crystals or flakes. The advantage is that they can be stacked together again into heterostructures, which allows a combination of materials with different properties. In this way we intend to use the so-called magnetic proximity effect. Here a layer of graphene is brought into contact with a layer of a ferromagnetic or antiferromagnetic material. In this way the magnetic material can imprint its properties on the graphene layer. This layer effectivly becomes ferromagnetic. In recent years we already demonstrated that this ferromagnetic graphene allows for the generation of spin currents. Here a charge current (which carries equal amounts of spin up and spin down electrons) can be converted into a spin polarized current, where the current is predominantly carried by electron spins of one direction (up or down). In the 2DMAGSPIN project we intend to take the next step and realize a fully controllable spin polarized two dimensional system, a two dimensional spin gas. We intend to reach this goal by designing and fabricating suitable Van der Waals heterostructures, where the parameters (such as electron density) can be controlled by gate electrodes. In this way we might be able to realize spin valve devices, which can be controlled by electrical electroststic gate voltages, instead of the more cumbersome control of magnetization, which requires the application of magnetic fields. Besides going towards demonstrating the potential for future devices, we will also investigate the electrical and magnetic properties of such a two dimensional spin system
The second research direction of 2DMAGSPIN is towards the realization of a two dimensional magnon gas. Magnons (or spin waves) are the elementary excitations of a ferromagnet. These consists of magnetization waves which can propagate through the magnetic material. Until now mostly three dimensional systems were studied, where the spin waves can propagate in three directions. It is the aim of this project to study the properties of magnons which are restricted to move in only 2 directions. This wil be achieved by using Van der Waals ferromagnetic and antiferromagnetic materials. Magnons will be injected into the material electrically. After they travelled through the two-dimensional magnon gas, they will be detected again electrically. In this way we will collect information how far these two-dimensional magnons can travel before they relax, but also how efficient the magnon spin transport is in these two-dimensional materials. We will continue to further restrict the motion of the magnons, and possible obtain one-dimensional magnon transport. Also we will explore if we can add additional properties, such as spin-orbit interaction, to the two-dimensional spin and/or magnon systems by making heterostructures which e.g include layers of semiconducting Van der Waals materials
The second research direction of 2DMAGSPIN is towards the realization of a two dimensional magnon gas. Magnons (or spin waves) are the elementary excitations of a ferromagnet. These consists of magnetization waves which can propagate through the magnetic material. Until now mostly three dimensional systems were studied, where the spin waves can propagate in three directions. It is the aim of this project to study the properties of magnons which are restricted to move in only 2 directions. This wil be achieved by using Van der Waals ferromagnetic and antiferromagnetic materials. Magnons will be injected into the material electrically. After they travelled through the two-dimensional magnon gas, they will be detected again electrically. In this way we will collect information how far these two-dimensional magnons can travel before they relax, but also how efficient the magnon spin transport is in these two-dimensional materials. We will continue to further restrict the motion of the magnons, and possible obtain one-dimensional magnon transport. Also we will explore if we can add additional properties, such as spin-orbit interaction, to the two-dimensional spin and/or magnon systems by making heterostructures which e.g include layers of semiconducting Van der Waals materials
These are the main achievements two years (november 2024) after the start of the project:
1) Demonstration of electrostatic control of the spin polarization of a two-dimensional electron gas in graphene. We showed that by electrostatically controlling the electron density in the graphene layer, we could tune the carriers from holes to electrons. Because the magnetic proximity effect has induced an exchange shift in the electronic bandstructure, we could change the spin polarization from about -50% to 50%. In this way we could reverse the spin polarization, without changing the magnetization itself. We did this by investigating the quantum Hall effect, which was strongly modified by the presence of electron and hole Landau levels. We are continuing the work by making and studying electrostatic spin valve structures based on this principles.
2) We have have studied magnon (spin wave) transport in quasi two-dimensional CrPS4, an antiferromagnetic Van der Waals material. We observed a strong dependence of the magnon spin conductivity on the applied magnetic field, which we could relate to the dependence of the ferromagnetic and antiferromagnetic magnon modes on the magnetic field. Moreover, we observed that the magnon spin relaxation length can reach values up to 1 micrometer. This offers opportunities for controllable magnon transistors (see (3)).
3) We fabricated and studied magnon spin transistors, where the magnon spin transport is controlled by magnon injection and generation by a third gate electrode. We studied two different geometries. We showed that we could control the magnon transport by both electrical injection or extration of magnons, as well as the thermal generation of magnon currents and magnon accumulation. We alse determined the efficiencies of both mechanisms.
1) Demonstration of electrostatic control of the spin polarization of a two-dimensional electron gas in graphene. We showed that by electrostatically controlling the electron density in the graphene layer, we could tune the carriers from holes to electrons. Because the magnetic proximity effect has induced an exchange shift in the electronic bandstructure, we could change the spin polarization from about -50% to 50%. In this way we could reverse the spin polarization, without changing the magnetization itself. We did this by investigating the quantum Hall effect, which was strongly modified by the presence of electron and hole Landau levels. We are continuing the work by making and studying electrostatic spin valve structures based on this principles.
2) We have have studied magnon (spin wave) transport in quasi two-dimensional CrPS4, an antiferromagnetic Van der Waals material. We observed a strong dependence of the magnon spin conductivity on the applied magnetic field, which we could relate to the dependence of the ferromagnetic and antiferromagnetic magnon modes on the magnetic field. Moreover, we observed that the magnon spin relaxation length can reach values up to 1 micrometer. This offers opportunities for controllable magnon transistors (see (3)).
3) We fabricated and studied magnon spin transistors, where the magnon spin transport is controlled by magnon injection and generation by a third gate electrode. We studied two different geometries. We showed that we could control the magnon transport by both electrical injection or extration of magnons, as well as the thermal generation of magnon currents and magnon accumulation. We alse determined the efficiencies of both mechanisms.
The research of 2DMAGSPIN is curiousity driven, but it also has links with future applications. This is based on the technology used, where we make the thinnest possible electronic devices, which are based on only a few layers of Van der Waals materials. We show that we can fabricate different spintronic functionalities, which can also be controlled electrostatically by a gate voltage.
The possibility to stack materials with different functionalities, and, if required, twist them at arbrbitrary angles, opens up a new avenue for future device concepts. It should be noted however that we intend to restrict ourselves to exfoliated materials. This is because the quality is usually higher than 2D materials which are made with large througput methods. Also exfoliation under protective atmosphere allows us to make beyond state of the art dedicated Van der Waals heterostructures, with clean interfaces. With the results mentioned above, we made the first steps in those direction.
, e.g. demonstating gate controllable spin polarization, which has never been shown before, and which might be used in electrostatically controlled spin valve devices. From the fundamental science direction, we can now explore materials and devices which have not been studied before. For example, building on our obtained results, we are working towards a 100% spin polarized two-dimensional electron gas, which is predicted to have special properties. Also a coupled 2D electronic spin gas coupled to a 2D magnon spin gas is in our opinion within the possibilites of the project.
The possibility to stack materials with different functionalities, and, if required, twist them at arbrbitrary angles, opens up a new avenue for future device concepts. It should be noted however that we intend to restrict ourselves to exfoliated materials. This is because the quality is usually higher than 2D materials which are made with large througput methods. Also exfoliation under protective atmosphere allows us to make beyond state of the art dedicated Van der Waals heterostructures, with clean interfaces. With the results mentioned above, we made the first steps in those direction.
, e.g. demonstating gate controllable spin polarization, which has never been shown before, and which might be used in electrostatically controlled spin valve devices. From the fundamental science direction, we can now explore materials and devices which have not been studied before. For example, building on our obtained results, we are working towards a 100% spin polarized two-dimensional electron gas, which is predicted to have special properties. Also a coupled 2D electronic spin gas coupled to a 2D magnon spin gas is in our opinion within the possibilites of the project.