Domain wall pinning and hard magnetic phase in Co-doped bulk single crystalline Fe3GeTe2

We report the effects of cobalt doping on the magnetic properties of two-dimensional van der Waals ferromagnet Fe3GeTe2. Single crystals of (Fe1-xCox)(3)GeTe2 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 the 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 H-c and the magnetic remanence to saturated magnetization ratio M-R/M-S 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.