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Building a Two-Dimensional Magnet One Atom at a Time

Researchers synthesized a new two-dimensional (2D) ferromagnet, chromium telluride (Cr2Te3), and measured how its electronic and magnetic properties evolve with increasing thickness and temperature. Such atomically thin magnetic materials—with magnetic properties that can be tuned by thickness and other external control parameters—would be very useful in next-generation microelectronic and spintronic applications.

“Many groups are trying to grow these transition-metal chalcogenide materials, but chromium and tellurium can be combined into so many different phases, it’s very hard to stabilize one clean phase on your substrate,” said Sung-Kwan Mo, an Advanced Light Source (ALS) staff scientist and a senior author of the study. “With this material, we have layer-by-layer control over its synthesis and obtained single-crystalline thin films on silicon substrates, so we can use different types of spectroscopy and microscopy techniques to explore their physical properties.”

In this work, the researchers used in situ molecular-beam epitaxy at ALS Beamline 10.0.1 to grow extremely thin films of Cr2Te3 (one, three, and six monolayers) with atomic-level precision. The material’s stability enabled the use of several different techniques for characterization at variable temperature: angle-resolved photoemission spectroscopy (ARPES) for electronic structure, scanning tunneling microscopy for atomic structure, and—to gain a coherent picture of the material’s magnetic properties—x-ray absorption spectroscopy at Stanford Synchrotron Radiation Lightsource (SSRL) and highly sensitive magnetometry.

The ARPES results were in excellent agreement with first-principles calculations and, combined with all the other data, suggested that a different physical model governs the ferromagnetic phenomena at the single-layer limit.

“In the thicker films, the data fit well with what’s called the Stoner model, meaning that the magnetic material is a good metal in which electrons can move freely, as in the familiar example of iron,” said Yong Zhong, an ALS collaborative postdoctoral fellow and first author of the study. “However, when the thickness is constrained to one monolayer, there is a transition to Heisenberg ferromagnetism, which means the ferromagnetism is influenced more by the localized spin interaction.”

The work shows that dimensionality is a powerful tuning knob for manipulating the properties of 2D magnetism and provides crucial information for optimizing performance in future applications.

Six panels (2 rows, 3 columns) of ARPES data, tinted blue. Top row shows spectra for 6-monolayer sample at 20, 100, and 200 K. Split bands visible at 20 K merge together at 200 K. Bottom row shows spectra for 1-monolayer sample at the same three temperatures. Bands remain split across the board.
Evolution of electronic structure in Cr2Te3 as temperature rises from 20 K to 200 K in samples of six monolayers (6 ML) and one monolayer (1 ML). The experimental results closely match first-principles calculations and highlight important changes that occur in the electronic structure as dimensionality decreases.

Y. Zhong, C. Peng, H.L. Huang, D.D. Guan, J.W. Hwang, K.H. Hsu, Y. Hu, C.J. Jia, B. Moritz, D.H. Lu, J.-S. Lee, J.-F. Jia, T.P. Devereaux, S.-K. Mo, and Z.-X. Shen, “From Stoner to local moment magnetism in atomically thin Cr2Te3,” Nat. Commun. 14, 5340 (2023), doi:10.1038/s41467-023-40997-1.