  To date only a few two-dimensional (2D) magnetic crystals were experimentally
confirmed, such as CrI3 and CrGeTe3, all with very low Curie temperatures (TC).
High-throughput first-principles screening over a large set of materials yields
89 magnetic monolayers including 56 ferromagnetic (FM) and 33 antiferromagnetic
compounds. Among them, 24 FM monolayers are promising candidates possessing TC
higher than that of CrI3. High TC monolayers with fascinating electronic phases
are identified: (i) quantum anomalous and valley Hall effects coexist in a
single material RuCl3 or VCl3, leading to a valley-polarized quantum anomalous
Hall state; (ii) TiBr3, Co2NiO6 and V2H3O5 are revealed to be half-metals. More
importantly, a new type of fermion dubbed type-II Weyl ring is discovered in
ScCl. Our work provides a database of 2D magnetic materials, which could guide
experimental realization of high-temperature magnetic monolayers with exotic
electronic states for future spintronics and quantum computing applications.
