The study of atomically thin ferromagnetic crystals has led to the discovery of unusual magnetic behaviour and provided insight into the magnetic properties of bulk materials. However, the experimental techniques that have been used to explore ferromagnetism in such materials cannot probe the magnetic field directly. Here, we show that ballistic Hall micromagnetometry can be used to measure the magnetization of individual two-dimensional ferromagnets. Our devices are made by van der Waals assembly in such a way that the investigated ferromagnetic crystal is placed on top of a multi-terminal Hall bar made from encapsulated graphene. We use the micromagnetometry technique to study atomically thin chromium tribromide (CrBr₃). We find that the material remains ferromagnetic down to monolayer thickness and exhibits strong out-of-plane anisotropy. We also find that the magnetic response of CrBr₃ varies little with the number of layers and its temperature dependence cannot be described by the simple Ising model of two-dimensional ferromagnetism.

对原子上薄的铁磁晶体的研究导致发现了异常的磁行为，并提供了对块状材料的磁性能的深入了解。但是，用于探索此类材料中铁磁性的实验技术无法直接探测磁场。在这里，我们证明了弹道霍尔微磁法可用于测量各个二维铁磁体的磁化强度。我们的设备是通过范德华装配制造的，将研究的铁磁晶体放置在由封装的石墨烯制成的多端子霍尔棒的顶部。我们使用微磁力技术研究原子薄的三溴化铬（CrBr ₃）。我们发现该材料在低至单层厚度的情况下仍保持铁磁性，并表现出很强的面外各向异性。我们还发现，CrBr ₃的磁响应随层数的变化很小，并且其温度依赖性无法通过二维铁磁性的简单伊辛模型来描述。