  Owing to the strong spin-orbit coupling and the related fascinating physical
properties, heavy 5d transition-metals exhibit desirable application prospects.
However, up to now, the 5d magnetic materials are still very limited,
especially very rare for tungsten. In this work, we theoretically predict a
two-dimensional multiferroic W3Cl8 monolayer. Intrinsic 5d magnetism of
tungsten is activated by the W ions' fractional valence in a breathing Kagome
lattice of reduced effective dimension. A coplanar Y-type antiferromagnetism
composed by ferromagnetic W3 trimers is confirmed as the magnetic ground state.
The spontaneous ferroelectric polarization mainly originates from the ion
displacement induced by the breathing distortion of Kagome lattice. An
intrinsic magneto-optical Kerr effect with sizable Kerr angle can be observed
to detect this trimeric Y-type antiferromagnetism, and it depends strongly on
the detailed magnetic order. Thereby, we propose a general scheme for realizing
more 5d magnetism in two-dimensional multiferroic systems.
