Magnetic structure and phase stability of the van der Waals bonded ferromagnet ${\mathrm{Fe}}_{3-x}{\mathrm{GeTe}}_2$

The magnetic structure and phase diagram of the layered ferromagnetic compound ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ has been investigated by a combination of synthesis, x-ray and neutron diffraction, high-resolution microscopy, and magnetization measurements. Single crystals were synthesized by self-flux reactions, and single-crystal neutron diffraction finds ferromagnetic order with moments of $1.11\left(5\right){\mu }_B/\mathrm{Fe}$ aligned along the $c$ axis at 4 K. These flux-grown crystals have a lower Curie temperature $T_{\text{c}}\approx 150$ K compared to crystals previously grown by vapor transport ($T_{\text{c}}=220$ K). The difference is a reduced Fe content in the flux-grown crystals, as illustrated by the behavior observed in a series of polycrystalline samples. As Fe content decreases, so does the Curie temperature, magnetic anisotropy, and net magnetization. In addition, Hall effect and thermoelectric measurements on flux-grown crystals suggest multiple carrier types contribute to electrical transport in ${\mathrm{Fe}}_{3-x}{\mathrm{GeTe}}_2$ and structurally similar ${\mathrm{Ni}}_{3-x}{\mathrm{GeTe}}_2$.

Figure 1

(a) Image of crystal structure with atomic positions labeled; Fe(1)-Fe(1) bonds pierce the center of each hexagon formed by Fe(2)-Ge (space group $P{6}_3/mmc$, no. 194). (b) X-ray diffraction data for an as-grown facet of ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ showing the $c$ axis normal with exclusively 00l diffraction peaks observed; a Le Bail fit (solid line) yielded the $c$-axis lattice parameter shown in the image. (c) A picture of a large single crystal.

Figure 2

Powder x-ray diffraction data for two polycrystalline samples with refined lattice parameters and compositions provided.

Figure 3

Normalized lattice parameters as a function of refined Fe content for ${\mathrm{Fe}}_{3-x}{\mathrm{GeTe}}_2$ samples [powder x-ray diffraction (p-xrd) at room temperature], including results obtained from single-crystal x-ray (sc-xrd) diffraction data collected at 173 K; $a_{\max }=4.0244\left(1\right)$ Å and $c_{\max }=16.405\left(1\right)$ Å. Vertical error bars are smaller than the size of the data markers.

Figure 4

HAADF STEM image showing $Z$ contrast. The heaviest columns (Te) show up as the brightest spots, and Fe columns show up with the dimmest contrast. Van der Waals gaps are clearly visible as black stripes. The inset shows an overlay of the lattice projection along [130]. (b) Profiles of the intensity along the dashed lines shown in (a) indicating significant disorder in the Fe(2)-Ge plane. (c) EELS integrated intensity maps showing the chemical signature of the alternating Te, Fe, and Fe-Ge planes.

Figure 5

Rocking curves of the 010 reflection from single-crystal neutron diffraction on ${\mathrm{Fe}}_{2.76}{\mathrm{Ge}}_{0.94}{\mathrm{Te}}_2$ at 10 and 157 K. The solid lines are Gaussian fits to the peaks. The image on the right shows the magnetic structure obtained by refinement of neutron diffraction data collected at 4 K (Fe = red, Te = green, Ge = blue).

Figure 6

Integrated intensities of Bragg peaks from single-crystal neutron diffraction on ${\mathrm{Fe}}_{2.76}{\mathrm{Ge}}_{0.94}{\mathrm{Te}}_2$. The increase in intensity for the 010 peak below $T_{\text{c}}$ originates from the magnetic order along $c$, whereas the intensity of the 002 peak does not change significantly with temperature.

Figure 7

Neutron powder diffraction at (a) 200 and (b) 60 K with refinements included and samples labeled by the refined compositions. The reflections from top to bottom correspond to the structural, aluminum-can, and magnetic reflections. The most intense magnetic signal was observed in the region shown in (c) and (d). Note the much increased magnetic intensity in the ${\mathrm{Fe}}_{2.9}{\mathrm{GeTe}}_2$ sample compared to the more Fe-deficient ${\mathrm{Fe}}_{2.71}{\mathrm{GeTe}}_2$.

Figure 8

(a) Temperature dependence of the magnetization $M$ divided by applied field $H$ for polycrystalline ${\mathrm{Fe}}_{3-x}{\mathrm{GeTe}}_2$ materials with various Fe concentrations as indicated by the refined Fe content in the legend and (b) anisotropic magnetization of flux-grown ${\mathrm{Fe}}_{2.76}{\mathrm{Ge}}_{0.94}{\mathrm{Te}}_2$. All data were collected upon cooling in an applied field of $H=100$ Oe.

Figure 9

(a) Magnetization loops at 2 K for three compositions of polycrystalline ${\mathrm{Fe}}_{3-x}{\mathrm{GeTe}}_2$ (refined values provided). The inset shows the magnetization at 2 K and 60 kOe for polycrystalline samples. (b) Magnetization versus applied field for single-crystalline ${\mathrm{Fe}}_{2.76}{\mathrm{Ge}}_{0.94}{\mathrm{Te}}_2$ at various temperatures, with the anisotropy demonstrated at 2 K.

Figure 10

Curie temperature as a function of lattice parameters for ${\mathrm{Fe}}_{3-x}{\mathrm{GeTe}}_2$.

Figure 11

(a) Hall resistance as a function of applied field suggests $p$-type conduction in ${\mathrm{Fe}}_{2.76}{\mathrm{Ge}}_{0.94}{\mathrm{Te}}_2$ and ${\mathrm{Ni}}_{3-x}{\mathrm{GeTe}}_2$ crystals, while in (b) the negative Seebeck coefficients indicate $n$-type conduction. The dashed line in (b) indicates the Curie temperature of the ${\mathrm{Fe}}_{2.76}{\mathrm{Ge}}_{0.94}{\mathrm{Te}}_2$ crystal.