Colloquium: A critique on van der Waals and two-dimensional magnets

TL;DR

The Colloquium analyzes whether magnetic phenomena in truly two-dimensional van der Waals systems offer fundamentally new physics beyond conventional 2D spin models. It surveys foundational theory and experiments, emphasizing how electric-field control, strain, proximity, and stacking enable novel tunability and proximate coupling to other degrees of freedom. The authors highlight contributions in spin interactions, topology and quantum geometry, magnonic transport, and noncollinear/frustrated magnetism, while noting many predictions remain to be experimentally validated and that universality classes in 2D vdW magnets are not yet settled. Overall, 2D/vdW magnets are positioned as promising platforms for next-generation spintronics and quantum magnetic phenomena, albeit with significant challenges in materials quality, interface control, and definitive observation of topological states.

Abstract

Magnetic two-dimensional (2D) crystals were isolated about a decade ago, triggering a tremendous research activity worldwide. This colloquium raises a stiff question: what is really new about them? At first sight, they seem to be purer implementations of 2D spin models than traditional systems such as ultra-thin films. Yet, they partly realized their promises so far, and whether they give fresh perspectives on long-standing predictions in statistical physics is still an open question. Undoubtedly, they are uniquely amenable to electric-field effect, susceptible to mechanical deformation, and sensitive to moirés, for example. They represent interesting platforms for exploring, challenging, or simply revisiting a wide range of phenomena in condensed matter magnetism. This colloquium intends to offer a critical, yet not necessarily skeptical, overview of the field, clarifying what we believe could be unique with 2D magnets, related quasi-2D van der Waals magnets, and their heterostructures.

Figures (10)

• Figure 1: Top part: Historical approaches to solid-state magnetism. Arrows tentatively delimit the early development of each field. Bottom part: Illustration of some milestones in the study of 2D/vdW magnets. The role of relative atomic stacking between successive layers (altered by planar strains, lateral sliding, or moirés), of atomic substitution or applied pressure, on the orientation/length of interatomic bonds, and consequences on the superexchange interaction between spins, is highlighted at the bottom-left. Center: a heterostructure with a high-spin-orbit-interaction 2D material (WTe$_2$), and the possible thus-generated skyrmion in Fe$_3$GeTe$_2$Yang2020b. Right: magnetic proximity effect, with some spin polarization induced in the top semiconductor and a weakening of magnetism is the ferromagnet underneath (the situation for non-vdW 3D materials illustrates issues with a rough interface creating a marginal interfacial effect). Proximity is either at equilibrium (absence of external driving forces, e.g., voltage source or laser beam) or not. Left: spin-polarized current $J_\mathrm{s}$ through graphene proximitized by a vdW magnet [CrSBr Ghiasi2021, CrGeTe$_3$Yang2025] and flowed by a charge current $J_\mathrm{c}$.
• Figure 2: Spin-spin interactions, in the picture of localized spins and in the paradigm of itinerant magnetism. Direct exchange (rarely relevant in practice), anisotropic Dzyaloshinskii-Moriya exchange, superexchange (in metals, via conduction electrons, by Ruderman-Kittel-Kasuya-Yoshida interactions, or in semiconductors/insulators, via non-magnetic ions), and double-exchange (multi-valence compounds) are represented.
• Figure 3: Contour of constant magnetization exponent $\beta$ in the $(d,n)$ plane ($d$: dimensionality; $n$: spin degree of freedom). From Fisher1974. The $\beta<0$ region is nonphysical. In red: experimental data for more-or-less thin films / bulks Vaz2008.
• Figure 4: Magnon dispersion of CrBr$_3$ measured by inelastic neutron scattering along the $\Gamma$-$\Delta$, Z-F (in-plane) and $\Gamma$-Z (out-of-plane) directions. The strong anisotropy in the magnon dispersion advocates for 2D magnetism. From Samuelsen1971
• Figure 5: Graphical representation of the quantum metric tensor, ${\bf g}_{\bf k}$, and the geometrical phase ${\bm\gamma}_{\bf k}$ between two quantum states at different points in the (a) first Brillouin zone and (b) total Brillouin zone represented as a torus. The metric ${\bf g}_{\bf k}$ is a tensor and generally differs from $\delta{\bf k}$. The color gradient mimics the phase changes occurring along the path ${\bf k}\rightarrow{\bf k}+\delta{\bf k}$.