  Fe$_{3}$GeTe$_{2}$ is a two-dimensional van der Waals material with a
ferromagnetic ground state and a maximum transition temperature
$T_{\mathrm{c}}\sim225$ K. However, when Fe$_{3}$GeTe$_{2}$ is synthesized
lower values of $T_{\mathrm{c}}$ are often reported. This is attributed to a
deficiency in the Fe at the 2c site in the crystal structure. Here we
investigate the effect of Fe deficiency and the substitution of Co for Fe on
the magnetic properties of this system. We have synthesized both
polycrystalline material and single crystals by chemical vapor transport and
the flux method, with the largest crystals obtained using the flux method.
Cobalt substitution at the Fe site is found to significantly reduce the
magnetic transition temperature. Crystals of Fe$_{3}$GeTe$_{2}$ grown by
chemical vapor transport with $\sim 8\%$ excess Fe in the starting materials
display an optimum Fe content and magnetic transition temperature.
