  Exploring and controlling magnetism in two-dimensional (2D) layered magnetic
crystals, as well as their inclusion in heterogeneous assemblies, provide an
unprecedented opportunity for fundamental science and technology. To date,
however, there are only a few known intrinsic 2D magnets. Here we predict a
novel 2D intrinsic magnet, yttrium sulfur selenide (YSSe), using
first-principles calculations. The magnetism of this transition metal
dichalcogenide originates from the partially-filled $3p$- and $4p$-orbitals of
the chalcogens, unlike other known intrinsic magnets where magnetism arises
from the partially-filled $3d$- and $4f$-orbitals. The unconventional magnetism
in YSSe is a result of a unique combination of its structural and electronic
properties. We further show that a lack of mirror symmetry results in
piezoelectric properties, while the broken space- and time-symmetry ensures
valley polarization. YSSe is a rare magnetic-piezoelectric material that can
enable novel spintronics, valleytronics, and quantum technologies.
