  Monolayer van der Waals (vdW) magnets provide an exciting opportunity for
exploring two-dimensional (2D) magnetism for scientific and technological
advances, but the intrinsic ferromagnetism has only been observed at low
temperatures. Here, we report the observation of room temperature
ferromagnetism in manganese selenide (MnSe$_x$) films grown by molecular beam
epitaxy (MBE). Magnetic and structural characterization provides strong
evidence that in the monolayer limit, the ferromagnetism originates from a vdW
manganese diselenide (MnSe$_2$) monolayer, while for thicker films it could
originate from a combination of vdW MnSe$_2$ and/or interfacial magnetism of
$\alpha$-MnSe(111). Magnetization measurements of monolayer MnSe$_x$ films on
GaSe and SnSe$_2$ epilayers show ferromagnetic ordering with large saturation
magnetization of ~ 4 Bohr magnetons per Mn, which is consistent with density
functional theory calculations predicting ferromagnetism in monolayer
1T-MnSe$_2$. Growing MnSe$_x$ films on GaSe up to high thickness (~ 40 nm)
produces $\alpha$-MnSe(111), and an enhanced magnetic moment (~ 2x) compared to
the monolayer MnSe$_x$ samples. Detailed structural characterization by
scanning transmission electron microscopy (STEM), scanning tunneling microscopy
(STM), and reflection high energy electron diffraction (RHEED) reveal an abrupt
and clean interface between GaSe(0001) and $\alpha$-MnSe(111). In particular,
the structure measured by STEM is consistent with the presence of a MnSe$_2$
monolayer at the interface. These results hold promise for potential
applications in energy efficient information storage and processing.
