Synopsis

# Solving a Magnetic Puzzle

Physics 13, s42
Spectroscopic measurements explain why a van der Waals ferromagnet displays different magnetic behavior in its layered and bulk forms.

Van der Waals materials—crystals made of atomic layers held together by weak van der Waals forces—exhibit unique magnetic properties that could be harnessed in spintronic devices. These properties can change dramatically depending on how many layers the crystal comprises—a behavior that current models of magnetism in these materials cannot explain. Now, a team led by Liuyan Zhao of the University of Michigan and Rui He of Texas Tech University has solved this puzzle for one such compound, chromium triiodide $\left({\text{CrI}}_{3}\right)$, using spectroscopic measurements.

${\text{CrI}}_{3}$ monolayers were recently discovered to be ferromagnetic. This finding was remarkable, since ferromagnetism was thought to be suppressed in two dimensions. But samples comprising two or a few layers of the same material turned out to be antiferromagnetic, a behavior difficult to reconcile with that of bulk ${\text{CrI}}_{3}$, which is ferromagnetic.

Zhao, He, and their colleagues studied the properties of bulk ${\text{CrI}}_{3}$ by measuring how the sample shifted the wavelength of incident polarized laser light. This technique allowed them to probe specific phonons—vibrations that reveal the crystal structure—and magnons—spin waves that reveal the spin arrangement in the solid. They found that bulk ${\text{CrI}}_{3}$ does not have a simple ferromagnetic structure but is instead in a mixed state. Like few-layer films, ${\text{CrI}}_{3}$ layers closest to the surface have an antiferromagnetic arrangement: atomic spins are aligned within each layer, but the orientation flips from one layer to the next. Layers further from the surface, however, are ferromagnetic, explaining the material’s bulk properties. The researchers say that understanding the material’s magnetic properties may help to find ways to control them via external stimuli, such as strain or pressure.

This research is published in Physical Review X.

–Matteo Rini

Matteo Rini is the Deputy Editor of Physics.

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