Layer-dependent Ferromagnetism in a van der Waals Crystal down to the Monolayer Limit (1703.05892v2)

Abstract: Since the celebrated discovery of graphene, the family of two-dimensional (2D) materials has grown to encompass a broad range of electronic properties. Recent additions include spin-valley coupled semiconductors, Ising superconductors that can be tuned into a quantum metal, possible Mott insulators with tunable charge-density waves, and topological semi-metals with edge transport. Despite this progress, there is still no 2D crystal with intrinsic magnetism, which would be useful for many technologies such as sensing, information, and data storage. Theoretically, magnetic order is prohibited in the 2D isotropic Heisenberg model at finite temperatures by the Mermin-Wagner theorem. However, magnetic anisotropy removes this restriction and enables, for instance, the occurrence of 2D Ising ferromagnetism. Here, we use magneto-optical Kerr effect (MOKE) microscopy to demonstrate that monolayer chromium triiodide (CrI3) is an Ising ferromagnet with out-of-plane spin orientation. Its Curie temperature of 45 K is only slightly lower than the 61 K of the bulk crystal, consistent with a weak interlayer coupling. Moreover, our studies suggest a layer-dependent magnetic phase transition, showcasing the haLLMark thickness-dependent physical properties typical of van der Waals crystals. Remarkably, bilayer CrI3 displays suppressed magnetization with a metamagnetic effect, while in trilayer the interlayer ferromagnetism observed in the bulk crystal is restored. Our work creates opportunities for studying magnetism by harnessing the unique features of atomically-thin materials, such as electrical control for realizing magnetoelectronics, and van der Waals engineering for novel interface phenomena.

• The paper demonstrates that monolayer CrI₃ exhibits intrinsic Ising ferromagnetism with an out-of-plane spin orientation and a Curie temperature of 45K via MOKE microscopy.
• It reveals a layer-dependent magnetic phase transition, with bilayer CrI₃ showing suppressed magnetization and metamagnetic behavior compared to trilayer restoration of ferromagnetism.
• The findings highlight the potential of 2D vdW materials for magnetoelectronic and quantum devices, prompting further studies on interlayer coupling mechanisms.

Layer-dependent Ferromagnetism in a van der Waals Crystal down to the Monolayer Limit

This paper presents an in-depth investigation into the ferromagnetic properties of atomically-thin chromium triiodide (CrI$_3$), a van der Waals (vdW) material, extending down to the monolayer level. Employing magneto-optical Kerr effect (MOKE) microscopy, the authors assert the existence of Ising ferromagnetism in monolayer CrI$_3$ with an out-of-plane spin orientation, revealing a Curie temperature ($T_C$) of 45 K, which is modestly lower than the bulk $T_C$ of 61 K. This small decrease indicates weak interlayer coupling, a distinct attribute of vdW crystals, and suggests that ferromagnetic ordering can persist even in monolayer structures.

The research outcomes emphasize a layer-dependent magnetic phase transition in CrI$_3$. Specifically, bilayer CrI$_3$ manifests suppressed magnetization with a metamagnetic behavior, contrasting with the restored bulk-like ferromagnetic order observed in trilayers. This bifurcation across thickness levels delineates the importance of layer-dependent electronic structures and intrinsic magnetocrystalline anisotropy in influencing vdW material properties.

The implications of these findings are multifaceted. The confirmation of intrinsic magnetism in 2D materials such as CrI$_3$ opens avenues for numerous technological applications, including magnetoelectronics and quantum computing realms. The weak interaction with substrates makes CrI$_3$ an exemplary platform for studying 2D magnetism devoid of perturbations introduced by substrate coupling, unforeseeable with conventional magnetic thin films.

Moreover, the demonstrated tunability of magnetic properties through layer thickness manipulation encourages the exploration of vdW heterostructures and complex multi-layer assemblies. This could lead to novel interface phenomena and devices characterized by controllable ferromagnetic and antiferromagnetic phases in atomically-thin materials. Potential studies may delve into the integration of these materials with other 2D systems to achieve new functionalities, particularly in the context of hybrid superconducting-ferromagnetic systems.

The work also prompts a reexamination of theoretical models pertinent to 2D magnetism. Given the observed metamagnetic transition in bilayer CrI$_3$, intricacies of interlayer coupling, possibly stemming from superexchange and dipole-dipole interactions, warrant further theoretical exploration. Future research might focus on elucidating the precise mechanisms governing these interactions at atomic scales.

In summary, the paper furnishes strong evidence for intrinsic ferromagnetism in mono- and trilayer CrI$_3$, with bilayer layers exhibiting a unique antiferromagnetic coupling. This layer-dependent magnetic behavior not only entrenches CrI$_3$ as an important material for vdW magnetism studies but also sets a foundation for designing innovative magnetic devices and systems, informed by precise control of layer-stacking and material interactions.