We have fabricated thin films of a van der Waals (vdW) ferromagnetic metal Fe5GeTe2 and characterized them by measuring the anomalous Hall effect. While the bulk Fe5GeTe2 does not exhibit a perpendicular magnetic anisotropy (PMA) unlike Fe3GeTe2, PMA emerges in the thin film devices. Furthermore, the PMA is enhanced with decreasing the thickness of Fe5GeTe2. In particular, a thin film (5 unit-cell layer) device fabricated with Fe5GeTe2 quenched at 1050 K has two times larger coercive field than that prepared without quenching. Such a PMA should be useful for future vdW spintronic devices.

We have performed magnetoresistance (MR) measurements on van der Waals ferromagnetic devices using quenched- (Q-) and nonquenched- (NQ-) Fe5GeTe2 crystals. A clear butterfly-shaped hysteresis has been observed for thin-film (less than 6 unit-cell layer) Q- and NQ-Fe5GeTe2 devices, but not for thicker film ones. The switching field of the butterfly-shaped MR is consistent with the coercive filed obtained from the Hall measurements. The MR ratio of the butterfly peak reaches about 10% at maximum, which is much larger than that observed with conventional magnetic materials. Such a large MR ratio would be related to magnetic fluctuations due to the complicated magnetic structure in this material.

We have systematically studied magnetotransport properties in van der Waals (vdW) magnetic materials, (Fe1− xCo x)5GeTe2, where the magnetic phase changes from the ferromagnetic with the perpendicular magnetic anisotropy (PMA; [Formula: see text]) or with the in-plane magnetic anisotropy (IMA; x = 0.19) to the antiferromagnetic ( x = 0.46) with the PMA. We have demonstrated that such magnetic properties seen in bulk still remain even in thin film devices. An anomalous Hall resistance with magnetic hysteresis was clearly observed in the low Co substitution ([Formula: see text]). The anomalous Hall effect was still observable for x = 0.19, but the magnetic hysteresis vanishes because of the IMA. In the antiferromagnetic region, there was no anomalous Hall effect in the low magnetic field range, but a clear hysteresis was observed at 2.5 T where the spin–flop transition takes place. This hysteresis can be seen only below 30 K and monotonically decreases with increasing temperature. We argue that the defects at a specific site in this system and also the resistance upturn below 30 K could be related to the hysteric behavior at the spin–flop transition. Our findings provide a recipe for the use of (Fe1− xCo x)5GeTe2 with different Co substitutions to construct vdW magnetic devices.

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