  We report the effects of cobalt doping on the magnetic properties of
two-dimensional van der Waals ferromagnet Fe3GeTe2. Single crystals of
(Fe{1-x}Cox)3GeTe2 with x=0-0.78 were successfully synthesized and
characterized with x-ray diffraction, energy dispersive x-ray spectroscopy and
magnetization measurements. Both the Curie-Weiss temperature and ferromagnetic
(FM) ordered moment of Fe3GeTe2 are gradually suppressed upon Co doping. A kink
in zero-field-cooling low field M(T) curve which is previously explained as an
antiferromagnetic transition is observed for samples with x=0-0.58. Our
detailed magnetization measurements and theoretical calculations strongly
suggest that this kink is originated from the pinning of magnetic domain walls.
The domain pinning effects are suddenly enhanced when the doping concentration
of cobalt is around 50%, both the coercive field Hc and the magnetic remanence
to saturated magnetization ratio MR/MS are largely improved and a hard magnetic
phase emerges in bulk single crystal samples. The strong doping dependent
magnetic properties suggest more spintronic applications of Fe3GeTe2.
