Physical properties of the van der Waals bonded ferromagnet Fe$_{3-x}$GeTe$_2$

Fe$_3$GeTe$_2$ is an itinerant ferromagnetic with a layered structure held together by van der Waals bonds. The material has been synthesized using a flux-growth technique that results in large single crystals suitable for neutron scattering, and its magnetic structure and phase diagram have been investigated. The flux-grown crystals possess a Curie temperature $T_C \approx$ 150$\,$K, which is less than that reported for polycrystalline Fe$_3$GeTe$_2$ with $T_C \approx$ 230$\,$K. The difference is explained by intrinsic Fe-deficiency in these single crystals. This talk will summarize the physical properties of the flux grown single crystals and a series of polycrystalline samples with varying concentrations of Fe, which reveal how Fe content is correlated to structural parameters and $T_C$. In combination with the magnetic properties, Hall effect and thermoelectric data reveal that Fe$_{3-x}$GeTe$_2$ compounds are multi-carrier type, itinerant ferromagnets. Research supported by the US DOE, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division.