A seamless 2D spintronic device by proximity effects

Spintronics is a field of technology that focuses on using the spin of electrons—an intrinsic property that can be thought of as a tiny magnetic moment—to store and process information. This concept is based on the fact that electrons, in addition to carrying electric charge, also have a “spin,” which can be oriented in different directions. These spins can be used to carry information in ways that traditional charge-based electronics cannot. To make efficient spintronic devices, however, you need materials that can generate, transport, and manipulate these electron spins effectively.

Spin Currents and Materials

Spintronic devices require materials that can not only carry electric current but also “spin current,” which means a current made up of moving spins rather than just moving charges. Two kinds of materials are commonly used for this purpose:

1. Ferromagnetic (FM) materials: These materials have a built-in magnetization (a kind of “magnetic memory”) and can generate spin currents where the spins are aligned with the direction of the magnetization.
2. Spin-orbit materials: These materials can convert electric charge currents into spin currents through mechanisms like the Spin Hall Effect (SHE) and the Rashba-Edelstein Effect. These effects occur because of the way the movement of electrons interacts with the atomic structure, causing the electrons to acquire spin in addition to their charge.

The problem of spin lifetime and transport

While these materials can generate and transport spin currents, there is a limitation: spin lifetime. The spin lifetime refers to how long the electron spins stay aligned in one direction before they lose their coherence and are scattered. Two effects, Magnetic Exchange Coupling and Spin-Orbit Coupling, can reduce this spin lifetime in materials, making it harder to transport spins efficiently over long distances. This means that designing materials for spintronics requires balancing these interactions carefully to maintain long spin lifetimes while still achieving useful spin currents.

Proximity effects in materials

To overcome these challenges, scientists use something called proximity effects. A proximity effect happens when one material (say, a metal or semiconductor) is placed next to another material, and some of the properties of the second material “transfer” or influence the first material. For example, a non-magnetic material can acquire magnetic properties or change its behavior when in close contact with a magnetic material. This is useful for designing new materials that combine the best properties of different ones.

Van der Waals heterostructures—a type of material made by stacking thin layers of materials with weak interactions between them—are especially useful for exploiting proximity effects. These heterostructures allow researchers to finely control the interactions between layers, which can enhance the spin properties needed for spintronic devices.

Graphene and proximity Effects

One particularly interesting material is graphene—a single layer of carbon atoms arranged in a honeycomb pattern. Graphene is known for having very high carrier mobility, meaning that electrons can move through it very easily, and it also has a long spin diffusion length, meaning spins can travel long distances before losing coherence. However, graphene also has a low intrinsic spin-orbit coupling, meaning it doesn’t naturally generate spin currents through mechanisms like the Spin Hall Effect.

However, graphene is a great platform to test proximity effects, where you can introduce spin-orbit or magnetic effects by placing it next to other materials. This is important because, by adding layers of certain materials, you can modify graphene’s properties without changing the graphene itself. This makes graphene a very versatile material for spintronic devices.

The proximity effects of CGT

Now, a team of researchers has created ¹ a new graphene-based spintronic device using the proximity effect. Specifically, they put graphene in contact with a ferromagnetic van der Waals semiconductor, Cr₂Ge₂Te₆ (CGT). This material, when in contact with graphene, can induce both Magnetic Exchange Coupling (MEC) and Spin-Orbit Coupling (SOC) in graphene, turning it into a material that can generate, transport, and manipulate spin currents efficiently.

The experiment set up a lateral spin valve, a type of device used to study spin currents. The researchers found that, above a certain temperature (the Curie temperature of CGT), the graphene showed a Spin Hall Effect (SHE) due to the induced SOC. Below this temperature, in addition to the SHE, the graphene also exhibited MEC, which further enhanced spin injection (putting spins into the graphene) and helped with the generation of spin currents.

Creating a seamless 2D spintronic device

One of the key achievements of this research is the construction of a seamless 2D lateral spin valve that only uses graphene, without needing any other ferromagnetic metals. This device shows efficient spin generation, transport, and detection by using the proximity effects from two separate flakes of CGT to induce MEC in specific regions of the graphene. The researchers were able to control the spin currents by switching the magnetization of the CGT flakes, confirming this with a spin precession experiment—a way to measure the spin behaviour in the device.

Finally, because both MEC and SOC are present in the system, the graphene showed an anomalous Hall effect (AHE), which is a signature of spin-polarized currents.

In conclusion, graphene, when in contact with certain ferromagnetic materials, can be engineered to carry and manipulate spin currents effectively. By exploiting proximity effects, the researchers showed that graphene could be made to behave like a spintronic material while retaining its natural properties like high mobility. This research moves us closer to creating fully functional, efficient, and low-power spintronic devices based on graphene, without needing bulky or complex ferromagnetic materials.

The work also highlights the promise of graphene-based heterostructures in spintronics, which could lead to new, faster, and more efficient technologies for electronic devices.

Author: César Tomé López is a science writer and the editor of Mapping Ignorance

Disclaimer: Parts of this article may have been copied verbatim or almost verbatim from the referenced research paper/s.