Evidence of Ferromagnetism and Ultrafast Dynamics of Demagnetization in an Epitaxial FeCl 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 () are rare. In this paper, the evidence of ferromagnetism in monolayer FeCl on Au(111) surfaces, as well as the interlayer antiferromagnetic coupling of bilayer FeCl, is characterized by using spin-polarized scanning tunneling microscopy. A Curie temperature () of ∼147 K is revealed for monolayer FeCl, 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 , 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.