# Two-dimensional magnets: magnetism, one layer at a time

> A staircase of CrI₃ stacks: a single layer is an Ising ferromagnet, two layers make a layered antiferromagnet that a gate can flip, four layers between graphene filter spins with a record tunnelling magnetoresistance, and the bulk is a ferromagnet again. Around it: why one layer needs an easy axis to order, how Kerr microscopy sees it, and metallic Fe₃GeTe₂ gated to room temperature.

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- Updated: 2026-10-14
- Category: Graphical abstracts
- Method: Graphical abstract
- Starting materials: Layered van der Waals magnets (CrI₃, Cr₂Ge₂Te₆, Fe₃GeTe₂) thinned to a few atomic layers
- Product: Magnetic order that changes layer by layer, and devices built from it
- Key result: Tc 45 K (monolayer) vs 61 K (bulk CrI₃) (Huang et al. 2017); TMR up to 19,000 % with four-layer CrI₃ barriers (Song et al. 2018); Fe₃GeTe₂ bulk Tc 205 K, gated to room temperature (Deng et al. 2018)
- Figure: 1920 × 1080 px, 384 editable layers (2 generated objects with settings)

## Steps

1. **Why it is hard**: The Mermin–Wagner theorem forbids order of isotropic spins in 2D at finite temperature; magnetic anisotropy lifts the ban (Huang et al. 2017; Gong et al. 2017)
2. **One layer**: Kerr microscopy shows monolayer CrI₃ is an Ising ferromagnet with out-of-plane spins, Curie temperature 45 K (Huang et al. 2017)
3. **Two layers**: Bilayer CrI₃ is a layered antiferromagnet; a magnetic field or a gate voltage switches it to ferromagnetic (Huang et al. 2017; Huang et al. 2018)
4. **Four layers**: CrI₃ between graphene contacts is a spin-filter tunnel barrier: up to 19,000 % magnetoresistance with four layers at low temperature, from the layer-by-layer antiferromagnetic order (Song et al. 2018)
5. **Bulk and beyond**: Bulk CrI₃ orders at 61 K with ferromagnetic interlayer coupling (Huang et al. 2017); metallic Fe₃GeTe₂ (bulk 205 K) stays ferromagnetic down to one layer and an ionic gate raises its Tc to room temperature (Deng et al. 2018)

## Uses

- Graphical abstracts on 2D magnetism and van der Waals magnets
- Talks on spintronics and magnetic tunnel junctions from 2D materials
- Teaching the Mermin–Wagner theorem and magnetic anisotropy

## References

1. Huang et al., “Layer-dependent ferromagnetism in a van der Waals crystal down to the monolayer limit”, Nature 2017. https://doi.org/10.1038/nature22391 (primary)
2. Gong et al., “Discovery of intrinsic ferromagnetism in two-dimensional van der Waals crystals”, Nature 2017. https://doi.org/10.1038/nature22060 (primary)
3. Huang et al., “Electrical control of 2D magnetism in bilayer CrI3”, Nat. Nanotechnol. 2018. https://doi.org/10.1038/s41565-018-0121-3 (primary)
4. Song et al., “Giant tunneling magnetoresistance in spin-filter van der Waals heterostructures”, Science 2018. https://doi.org/10.1126/science.aar4851 (primary)
5. Deng et al., “Gate-tunable room-temperature ferromagnetism in two-dimensional Fe3GeTe2”, Nature 2018. https://doi.org/10.1038/s41586-018-0626-9 (primary)

Credit: Drawn by LabCanvas after Huang et al., Nature 2017 (doi:10.1038/nature22391) and Nat. Nanotechnol. 2018 (doi:10.1038/s41565-018-0121-3); Song et al., Science 2018 (doi:10.1126/science.aar4851); Gong et al., Nature 2017 (doi:10.1038/nature22060); Deng et al., Nature 2018 (doi:10.1038/s41586-018-0626-9).

## Questions

**What does the “Two-dimensional magnets: magnetism, one layer at a time” template show?** A staircase of CrI₃ stacks: a single layer is an Ising ferromagnet, two layers make a layered antiferromagnet that a gate can flip, four layers between graphene filter spins with a record tunnelling magnetoresistance, and the bulk is a ferromagnet again. Around it: why one layer needs an easy axis to order, how Kerr microscopy sees it, and metallic Fe₃GeTe₂ gated to room temperature.

**What are the steps shown in “Two-dimensional magnets: magnetism, one layer at a time”?** 1. Why it is hard: The Mermin–Wagner theorem forbids order of isotropic spins in 2D at finite temperature; magnetic anisotropy lifts the ban (Huang et al. 2017; Gong et al. 2017). 2. One layer: Kerr microscopy shows monolayer CrI₃ is an Ising ferromagnet with out-of-plane spins, Curie temperature 45 K (Huang et al. 2017). 3. Two layers: Bilayer CrI₃ is a layered antiferromagnet; a magnetic field or a gate voltage switches it to ferromagnetic (Huang et al. 2017; Huang et al. 2018). 4. Four layers: CrI₃ between graphene contacts is a spin-filter tunnel barrier: up to 19,000 % magnetoresistance with four layers at low temperature, from the layer-by-layer antiferromagnetic order (Song et al. 2018). 5. Bulk and beyond: Bulk CrI₃ orders at 61 K with ferromagnetic interlayer coupling (Huang et al. 2017); metallic Fe₃GeTe₂ (bulk 205 K) stays ferromagnetic down to one layer and an ionic gate raises its Tc to room temperature (Deng et al. 2018).

**What are the starting materials and the product?** Starting materials: Layered van der Waals magnets (CrI₃, Cr₂Ge₂Te₆, Fe₃GeTe₂) thinned to a few atomic layers. Product: Magnetic order that changes layer by layer, and devices built from it.

**Which papers is “Two-dimensional magnets: magnetism, one layer at a time” based on?** Huang et al., “Layer-dependent ferromagnetism in a van der Waals crystal down to the monolayer limit”, Nature 2017 (doi:10.1038/nature22391); Gong et al., “Discovery of intrinsic ferromagnetism in two-dimensional van der Waals crystals”, Nature 2017 (doi:10.1038/nature22060); Huang et al., “Electrical control of 2D magnetism in bilayer CrI3”, Nat. Nanotechnol. 2018 (doi:10.1038/s41565-018-0121-3); Song et al., “Giant tunneling magnetoresistance in spin-filter van der Waals heterostructures”, Science 2018 (doi:10.1126/science.aar4851); Deng et al., “Gate-tunable room-temperature ferromagnetism in two-dimensional Fe3GeTe2”, Nature 2018 (doi:10.1038/s41586-018-0626-9).

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