Prediction of 
MnSiTe3
 as an intrinsic layered half-metal

Half-metallic materials allow electrons with one spin orientation to conduct through, while completely blocking electrons with the other spin orientation. Here we use a multiscale approach to predict that ${\mathrm{MnSiTe}}_{3}$ within the transition metal trichalcogenide (TMT) family is an intrinsic layered half-metal both in bulk form and at the ultrathin two-dimensional (2D) limit. We first use first-principles calculations to demonstrate that the system is a van der Waals crystal with weak interlayer coupling, allowing ready separation of a monolayer. Next we show that the half-metallicity of the bulk crystal is preserved also for a freestanding monolayer, with highly desirable ferromagnetic interlayer coupling. The ferromagnetic order of the monolayer is further shown to originate from the interplay of the exchange interaction and Ruderman-Kittel-Kasuya-Yosida coupling mediated by the itinerant carriers, while the singly spin-polarized and long-range nature of the latter is largely responsible for the unusually large next-next-nearest ferromagnetic coupling among the Mn ions. These findings, together with the latest developments surrounding ferromagnetism in 2D TMTs, may offer immense application potentials in nanomagnetic and spintronic devices.