  Theoretical prediction of the 2nd-order magnetic transition temperature (TM)
used to be arduous. Here, we develop a first principle-based, fully automatic
structure-to-TM method for two-dimensional (2D) magnets whose effective
Hamiltonians follow the Heisenberg model. The Heisenberg exchanges, which can
be calculated to an arbitrary shell, are transferred into the Monte Carlo
calculation. Using Cr-based magnets as the showcases, we show that our method
is a powerful tool to study the 2D magnets in two aspects. First, considering
long-range exchanges enables us to identify the spin frustration in the
suspended CrTe2 monolayer, whereas the heterostructure calculations reveal that
the ferromagnetism can be recovered if the monolayer CrTe2 is grown onto
various 2D substrates. Second, we realize a high-throughput screening of novel
magnets discovered by random structure searches. Six 2D Cr chalcogenides are
selected to have high TM. Our work provides a new insight for the study of 2D
magnets and helps accelerate the pace of magnetic materials data-mining.
