Evidence of Ferromagnetism and Ultrafast Dynamics of Demagnetization in an Epitaxial FeCl₂ Monolayer

The development of two-dimensional (2D) magnetism is driven not only by the interest of low-dimensional physics but also by potential applications in high-density miniaturized spintronic devices. However, 2D materials possessing a ferromagnetic order with a relatively high Curie temperature (T_c) are rare. In this paper, the evidence of ferromagnetism in monolayer FeCl₂ on Au(111) surfaces, as well as the interlayer antiferromagnetic coupling of bilayer FeCl₂, is characterized by using spin-polarized scanning tunneling microscopy. A Curie temperature (T_c) of ∼147 K is revealed for monolayer FeCl₂, based on our static magneto-optical Kerr effect measurements. Furthermore, temperature-dependent magnetization dynamics is investigated by the time-resolved magneto-optical Kerr effect. A transition from one- to two-step demagnetization occurs as the lattice temperature approaches T_c, which supports the Elliott-Yafet spin relaxation mechanism. The findings contribute to a deeper understanding of the underlying mechanisms governing ultrafast magnetization in 2D ferromagnetic materials.