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Collective excitations of bound electron–hole pairs, i.e., excitons, are ubiquitous in condensed matter systems, and it has been shown that they can strongly couple to other degrees of freedom, such as spin, orbital, and lattice. Among van der Waals materials with pronounced excitonic responses, CrSBr has attracted significant interest due to a very large energy shift of its fundamental bright exciton (at 1.36 eV) under an external magnetic field. This effect has been associated with an increased interlayer electronic hopping when the magnetic order is switched from antiferromagnetic to ferromagnetic by an external magnetic field, enabling its optical detection. In this work, we report the observation of a second bright excitonic resonance (at 1.76 eV), displaying a 5-fold enhancement of the magnetically induced energy shift to ∼100 meV, which we associate to a decreased spatial localization and increased interband nature compared to the fundamental exciton. Moreover, we show how the light-matter interaction reaches the ultrastrong regime where this exciton hybridizes with the cavity modes of photons confined to CrSBr flakes, forming polaritons with a Rabi splitting of ℏΩR≃ 372 meV, of the same order of magnitude as the one reported for the first exciton. These results expand the understanding of the relationship between the optical response and band structure of CrSBr and clarify the essential ingredients for optimizing magneto-electronic coupling for applications.

The realization of above room-temperature ferromagnetism in the two-dimensional (2D) magnet Fe5GeTe2represents a major advance for the use of van der Waals (vdW) materials in practical spintronic applications. In particular, observations of magnetic skyrmions and related states within exfoliated flakes of this material provide a pathway to the fine-tuning of topological spin textures via 2D material heterostructure engineering. However, there are conflicting reports as to the nature of the magnetic structures in Fe5GeTe2. The matter is further complicated by the study of two types of Fe5GeTe2crystals with markedly different structural and magnetic properties, distinguished by their specific fabrication procedure: whether they are slowly cooled or rapidly quenched from the growth temperature. In this work, we combine X-ray and electron microscopy to observe the formation of magnetic stripe domains, skyrmion-like type-I, and topologically trivial type-II bubbles, within exfoliated flakes of Fe5GeTe2. The results reveal the influence of the magnetic ordering of the Fe1 sublattice below 150 K, which dramatically alters the magnetocrystalline anisotropy and leads to a complex magnetic phase diagram and a sudden change of the stability of the magnetic textures. In addition, we highlight the significant differences in the magnetic structures intrinsic to slow-cooled and quenched Fe5GeTe2flakes.