Layer-Controlled laws of electron transport in two-dimensional ferromagnets

Recent discoveries of intrinsic two-dimensional ferromagnetism in layered compounds drive intense research on their fundamental properties and spintronic applications in atomically thin materials. In these materials, ferromagnetism engenders remarkable transport and optical phenomena and can be controlled electrically. However, coupling of carriers with magnetic moments in lateral charge transport is yet to be explored. Here, we report layer-dependent electron transport in two-dimensional rare-earth ferromagnets MSi2 , formed by silicene, evolving from an antiferromagnetic metal in the bulk to a ferromagnetic semiconductor in one monolayer. Small integer numbers of monolayers (1, 2, etc.) result in qualitatively different transport regimes following simple analytical laws. Most remarkably, a gap opening in the monolayer limit gives rise to a colossal negative magnetoresistance, depending exponentially on the magnetic field. The results call for applications in nanoelectronics and stimulate studies of fundamental spin behaviors.