  Due to the atomically thin nature of monolayer and few-layer van der Waals
magnets, the undesired background signal from the substrate can have
significant contribution when characterizing their magnetic properties. This
brings challenges in accurately determining the magnitude of the magnetic
moment of the epitaxially grown van der Waals magnets on bulk substrates. In
this paper, we discuss the impact of the background subtraction method for
accurately determining the magnetic moments in such systems. Using the recently
reported intrinsic two-dimensional (2D) van der Waals ferromagnet MnSe${_2}$ as
an example, we show that a normal diamagnetic background subtraction method in
analyzing the bulk magnetometry measurement will result in an unexpectedly
large magnetic moment (greater than ~10 {\mu}${_B}$ per formula unit). Through
our systematic growth study, we identify an additional paramagnetic signal due
to unintentional Mn doping of the substrate. To extract the correct magnetic
moment, a paramagnetic background should also be considered. This yields a
total magnetic moment of ~4 {\mu}${_B}$ per formula unit in monolayer
MnSe${_2}$, which is in close agreement to the theoretically predicted value.
