The study investigates the magnetic properties of monolayer chromium triiodide (CrI$_3$) using magneto-optical Kerr effect (MOKE) microscopy. The researchers demonstrate that monolayer CrI$_3$ exhibits Ising ferromagnetism with an out-of-plane spin orientation, characterized by a Curie temperature of 45 K, slightly lower than the 61 K of the bulk crystal. This suggests weak interlayer coupling. The study also reveals layer-dependent magnetic phase transitions, with monolayers showing ferromagnetic behavior, bilayers displaying antiferromagnetic behavior, and trilayers returning to ferromagnetic behavior. These findings highlight the unique properties of van der Waals crystals, such as the ability to study magnetism in isolated single crystals and the potential for novel quantum phenomena and device applications.The study investigates the magnetic properties of monolayer chromium triiodide (CrI$_3$) using magneto-optical Kerr effect (MOKE) microscopy. The researchers demonstrate that monolayer CrI$_3$ exhibits Ising ferromagnetism with an out-of-plane spin orientation, characterized by a Curie temperature of 45 K, slightly lower than the 61 K of the bulk crystal. This suggests weak interlayer coupling. The study also reveals layer-dependent magnetic phase transitions, with monolayers showing ferromagnetic behavior, bilayers displaying antiferromagnetic behavior, and trilayers returning to ferromagnetic behavior. These findings highlight the unique properties of van der Waals crystals, such as the ability to study magnetism in isolated single crystals and the potential for novel quantum phenomena and device applications.