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In 2017, researchers showed that a single atomic layer of chromium triiodide (CrI3) could sustain ferromagnetic order. The result was notable not because it produced a practical room-temperature magnet, but because magnetic behavior changed sharply as the crystal was reduced to one, two or three layers.
How researchers made and detected an atom-thin magnet
CrI3 is a layered van der Waals crystal: its atomic sheets are held together relatively weakly compared with the bonds within each sheet. Researchers mechanically isolated thin flakes from a bulk crystal using adhesive tape, a technique commonly called exfoliation.
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They used magneto-optical Kerr effect (MOKE) microscopy to detect magnetic order. In this method, light reflected from a magnetic material changes polarization; mapping that change lets researchers observe its magnetic response. The 2017 study described monolayer CrI3 as an Ising ferromagnet, with spins oriented perpendicular to the sheet. As the authors put it, “Here we use magneto-optical Kerr effect microscopy to demonstrate that monolayer chromium triiodide (CrI3) is an Ising ferromagnet with out-of-plane spin orientation.” (Huang et al., Nature, 8 June 2017.)
What changed as CrI3 gained layers?
The magnetic response depended on thickness; adding a layer did not simply make the same ferromagnet stronger. Huang and co-authors reported these behaviors in their 2017 Nature study:
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| CrI3 thickness | Reported magnetic behavior | Curie temperature |
|---|---|---|
| One layer (monolayer) | Out-of-plane Ising ferromagnetism | 45 K — Huang et al., Nature, 2017 |
| Two layers (bilayer) | Suppressed magnetization with a metamagnetic effect | Not stated in the cited study |
| Three layers (trilayer) | Interlayer ferromagnetism restored, as in bulk | Not stated in the cited study |
| Bulk crystal | Ferromagnetism | 61 K — Huang et al., Nature, 2017 |
A metamagnetic response means the magnetic state can change under an applied field; in the bilayer, the researchers observed that behavior alongside suppressed magnetization. The reported Curie temperatures are cryogenic: they are not evidence of room-temperature operation.
Why ferromagnetism in one atomic layer mattered
In a two-dimensional material, thermal fluctuations make it difficult for an ideal isotropic Heisenberg magnet to maintain magnetic order at finite temperature. CrI3 is not isotropic: its magnetic anisotropy favors spins pointing in particular directions, here out of the plane. That directional preference allows the system to behave as an Ising ferromagnet and sustain order in a monolayer.
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The Nature paper summarized the point this way: “Magnetic anisotropy removes this restriction, however, and enables, for instance, the occurrence of two-dimensional Ising ferromagnetism.” (Huang et al., Nature, 8 June 2017.) The significance was therefore both experimental and physical: the study demonstrated magnetic order in a true single-layer crystal and showed that the details of interlayer coupling could produce different behavior when sheets were stacked.
What the finding could enable—and what it did not show
The researchers pointed to possible future directions in sensing, data storage, magnetoelectronics and studies of interface phenomena. These were research opportunities, not demonstrated commercial devices. The work established that atomically thin CrI3 can be magnetic under cryogenic conditions; it did not show that all two-dimensional materials are magnetic or that a practical, room-temperature atom-thin magnet was ready for use.
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The original news report appeared in Chemistry World on 12 June 2017, after the primary study was published in Nature on 8 June 2017.
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