Van der Waals (vdW) magnets provide a compelling experimental platform for testing theories of two-dimensional magnetism, spin-wave excitations, and magnetic anisotropy. Mechanical exfoliation of vdW crystals—one of the simplest methods for reaching the atomic limit—yields flakes with distinct layer numbers, enabling systematic investigation of thickness-dependent magnetic interactions.

However, conventional techniques such as magnetometry, neutron, or X-ray scattering cannot detect magnetic signals in atomically thin samples. Reflective magnetic circular dichroism (RMCD) helps overcome this limitation. I will first introduce this optical technique, widely used to probe magnetic responses in 2D magnets.

The second part of the talk will focus on our study of atomically thin VBr₃ (1–4 layers), a vdW antiferromagnet with a bulk Néel temperature of 26.5 K. Remarkably, bilayer VBr₃ exhibits a squared hysteresis loop, the hallmark of ferromagnetism. Even more striking is the emergence of double-hysteresis or "wasp-waist" loops in trilayer and tetralayer samples. All the investigated several-layer samples show a critical temperature comparable to that of the bulk, suggesting weak interlayer coupling. We will discuss plausible layered spin configurations that align with these experimental observations. The complex magnetic structure, which our group recently determined using neutron diffraction on a bulk sample, can serve as a basis for interpreting these results.