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ability to synthesize and characterize 2D materials with long-range magnetic order was notably absent. Such materials hold great promise for potential applications in spintronics, [6][7][8][9][10][11] topological superconductivity, [12,13] vdW heterostructures, [14,15] and memory storage. [16,17] Hypothetical 2D magnets defy the Mermin-Wagner theorem, [18] which states that long-range magnetic order cannot exist at finite temperature within the 2D isotropic Heisenberg model. [19] Hence, magnetic anisotropy is a necessary prerequisite to realizing stable 2D magnetism, and has been demonstrated to exist in several layered materials over the last few years. [3,4,[20][21][22][23][24][25][26] Among these, the chromium trihalides CrX 3 (X = Cl, Br, I) have been the most extensively studied, all of which display intralayer ferromagnetic (FM) order and either FM or antiferromagnetic (AFM) interlayer coupling depending on the choice of halide and crystal thickness. [27] Despite their extensive characterization, chromium trihalides suffer from a lack of air and moisture stability and possess relatively low transition temperatures, limiting their applications. On the other hand, CrSBr has emerged [28][29][30][31] as an air-stable layered magnetic material possessing A-type AFM ordering with a comparatively high Néel temperature (T N ≈ 132 K).A variety of analytical tools have been used to characterize CrSBr, including magnetometry, [32,33] magneto-transport, [32,33] 2D materials can host long-range magnetic order in the presence of underlying magnetic anisotropy. The ability to realize the full potential of 2D magnets necessitates systematic investigation of the role of individual atomic layers and nanoscale inhomogeneity (i.e., strain) on the emergence of stable magnetic phases. Here, spatially dependent magnetism in few-layer CrSBr is revealed using magnetic force microscopy (MFM) and Monte Carlo-based simulations. Nanoscale visualization of the magnetic sheet susceptibility is extracted from MFM data and force-distance curves, revealing a characteristic onset of both intra-and interlayer magnetic correlations as a function of temperature and layer-thickness. These results demonstrate that the presence of a single uncompensated layer in odd-layer terraces significantly reduces the stability of the low-temperature antiferromagnetic (AFM) phase and gives rise to multiple coexisting magnetic ground states at temperatures close to the bulk Néel temperature (T N ). Furthermore, the AFM phase can be reliably suppressed using modest fields (≈16 mT) from the MFM probe, behaving as a nanoscale magnetic switch. This prototypical study of few-layer CrSBr demonstrates the critical role of layer parity on field-tunable 2D magnetism and validates MFM for use in nanomagnetometry of 2D materials (despite the ubiquitous absence of bulk zero-field magnetism in magnetized sheets).The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.202201000.

Efficient control of photons is enabled by hybridizing light with matter. The resulting light-matter quasi-particles can be readily programmed by manipulating either their photonic or matter constituents. Here, we hybridized infrared photons with graphene Dirac electrons to form surface plasmon polaritons (SPPs) and uncovered a previously unexplored means to control SPPs in structures with periodically modulated carrier density. In these periodic structures, common SPPs with continuous dispersion are transformed into Bloch polaritons with attendant discrete bands separated by bandgaps. We explored directional Bloch polaritons and steered their propagation by dialing the proper gate voltage. Fourier analysis of the near-field images corroborates that this on-demand nano-optics functionality is rooted in the polaritonic band structure. Our programmable polaritonic platform paves the way for the much-sought benefits of on-the-chip photonic circuits.