Researchers from NUS I-FIM harness light to probe the magnetisation of chromium trihalides, uncovering the previously unknown mechanism of exciton-magnetisation coupling.
In the lexicon of particle physics, elemental particles such as electrons, protons, and neutrons dominate our understanding of materials. But beyond this realm lies “exotic” quasiparticles that are less discussed. These include excitons, formed by an electron and its corresponding “hole”—the remaining void when light prompts an electron to vacate its usual position.
Venturing into the quantum world, as Albert Einstein famously put it, things get rather “spooky”. Excitons, it turns out, can interact with a material’s magnetic properties, which can result in unique characteristics and novel applications. The precise mechanics of this interaction, though, have remained blurry.
Researchers from the Institute for Functional Intelligent Materials (I-FIM) at the National University of Singapore (NUS) have broken new ground in this domain. Using chromium trihalides such as CrBr3 and CrI3 films, they have shed light on the elusive mechanism by which excitons interact with magnetism.
Despite their similarities, these chromium trihalides react to light in different ways. Deciphering this interaction and elucidating its implications could lead to faster computing speeds and vast storage capacities, all while consuming less power than conventional devices.
Let there be light
Chromium trihalides, including CrBr3 and CrI3, have emerged as materials of interest in the spintronics landscape. Part of a unique class of materials known as van der Waals crystals, chromium trihalides are two-dimensional (2D) ferromagnets known for their ability to showcase distinct electronic properties, largely influenced by their chromium electrons. These electrons not only determine the material’s magnetic nature but also its interaction with light, leading to unique optical behaviours.
However, while the interplay of light and magnetism in chromium trihalides has been observed, the underlying mechanisms remain elusive. This knowledge gap represents a hurdle in understanding light-matter interactions in the presence of a magnetic order.
In research spearheaded by Dr Maciej Koperski, a principal investigator at I-FIM, the team delved deep into the spin physics of CrBr3 and CrI3, providing critical insights into the behaviour of these materials.
While both materials share similar structural and magnetic properties, their interactions with light stand in stark contrast. “The absorption of light in insulators may lead to the formation of electron-hole pairs bound by Coulomb interactions,” explained Dr Koperski, an assistant professor at the Department of Materials Science and Engineering, NUS. “They may be described as a type of quasiparticle, often referred to as an exciton. The characteristics of excitons are essential in determining the optoelectronic and spintronic properties of the material.”
For CrBr3 and CrI3, though both materials produce excitons, the subsequent interactions of these excitons with the material’s intrinsic magnetisation vary notably. “CrBr3 exhibits a strong sensitivity to the circular polarisation of photons when absorbing light, leading to the creation of spin-polarised excitonic population in the presence of magnetisation,” said Dr Magdalena Grzeszcyk, a research fellow at NUS I-FIM. “In contrast, CrI3 does not show this sensitivity, but instead emits circularly polarised light, indicating a spin-splitting in the ground radiative state of the excitons.”
The individual responses of CrBr3 and CrI3 to light indicate that the fundamental nature of interactions between the spin of electron-holes pairs and magnetisation may lead to distinct material behaviours. “Such processes need to be understood in detail if we hope to develop applications based on hybrid materials with a complex interplay between the charge and spin of interacting particles or quasiparticles such as electrons, holes, excitons, or photons,” added Dr Koperski.
A bright future for optical devices
The study’s findings, published in Advanced Materials, illuminate the unique magnetic behaviours of the van der Waals ferromagnets, highlighting the potential to manipulate magnetisation through optical techniques.
However, the researchers suggest that there are still layers of complexities to be unravelled. “The spin-pumping experiments revealed gaps in our understanding of materials, especially those already being incorporated into device applications, mostly based on vertical tunnelling geometries,” shared Dr Koperski.
Further work will focus on making CrX3 materials stable under ambient conditions and raising the critical temperature of their ferromagnetic states for potential technological use. However, the optical spin pumping methodology is versatile, allowing for its application across a diverse range of ferromagnetic systems.
Given the strong interplay between excitons and magnetisation, there’s ample opportunity for deeper exploration. One exciting direction would be to reverse the approach—using light to alter magnetisation textures—which could pave the way for optically active devices, such as those using tunnelling light-emitting diode geometries.