Suppression of magnetic ordering in Fe-deficient ${\mathrm{Fe}}_{3-x}\mathrm{Ge}{\mathrm{Te}}_2$ from application of pressure

Two-dimensional van der Waals magnets with multiple functionalities are becoming increasingly important for emerging technologies in spintronics and valleytronics. Application of external pressure is one method to cleanly explore the underlying physical mechanisms of the intrinsic magnetism. In this paper, the magnetic, electronic, and structural properties of van der Waals-layered, Fe-deficient ${\mathrm{Fe}}_{3-x}\mathrm{Ge}{\mathrm{Te}}_2$ are investigated. Magnetotransport measurements show a monotonic decrease in the Curie temperature $\left(T_C\right)$ and the magnetic moment with increasing pressure up to 13.9 GPa. The electrical resistance of ${\mathrm{Fe}}_{3-x}\mathrm{Ge}{\mathrm{Te}}_2$ shows a change from metallic to a seemingly nonmetallic behavior with increasing pressure. High-pressure angle dispersive powder x-ray diffraction shows a monotonic compression of the unit cell and a reduction of the volume by $\sim 25\%$ with no evidence of structural phase changes up to 29.4(4) GPa. We suggest that the decrease in the $T_C$ due to pressure results from increased intralayer coupling and delocalization that leads to a change in the exchange interaction.

Figure 1

Ambient pressure temperature-dependent magnetization measurement showing $T_C=155\mathrm{K}$ and easy axis along $c$. Closed symbols are with the magnetic field along the $c$ axis, and open symbols are with the field on the $ab$ plane. The inset: Ball-and-stick model showing ${\mathrm{Fe}}_{3-x}\mathrm{Ge}{\mathrm{Te}}_2$ crystal structure from the side view where the green, orange, and purple balls represent Te, Fe, and Ge atoms, respectively.

Figure 2

Room-temperature XRD measurements showing the compression of the unit-cell parameters and volume. There is no indication of a phase transition, and the $c$ axis is compressed about 4% more than the $a$ axis by 30 GPa.

Figure 3

Resistance of ${\mathrm{Fe}}_{3-x}\mathrm{Ge}{\mathrm{Te}}_2$ measured at a series of increasing pressures. (a) Schematic of the designer DAC used for electrical measurements. (b) Normalized resistance measurements for cross comparison (offset for clarity). The change in slope is an indication of the Curie temperature in each trace. With increasing pressure, the paramagnetic region changes from metallic to nonmetallic.

Figure 4

MR measurements. (a) Negative MR at 5 K for several pressures showing the greatest relative change at 11 GPa. (b) Negative MR at 7.8 GPa showing changes in shape (linear versus sublinear) as a function of temperature due to magnon scattering. $R_{\mathrm{xy}}\left(H\right)$ scans showing changes in magnitude as a function of temperature and pressure where (c) the AHE component shows suppression of magnitude from compression of the $\mathrm{F}{\mathrm{e}}_{2.75}\mathrm{Ge}{\mathrm{Te}}_2$ crystal and (d) shows the raw data as a function of temperature at 4.1 GPa where the signal is dominated by the linear ordinary Hall component above $T_C$ and a low-field saturation below $T_C$.

Figure 5

(a)-(b) $T_C$ determined from AHE data. $S$ is defined as the initial slope of the $R_{\mathrm{xy}}$($H<1$ kOe) curve. The derivative shows a dip near/at $T_C$ where arrows indicate the local minimum of a Lorentzian fit (this fitting is also used to determine error bar for $T_C$). The plotted data are offset for clarity. There is no measurable transition temperature above 13.9 GPa, therefore, an arrow denoting $T_C$ at 16.2 GPa is not shown in (b). (c) $P-T$ phase diagram showing $T_C$ determined by these approaches. The dashed line is a parabolic fit of the data to 13.9 GPa for a guide to the eye. The $T_C$ shows a monotonic decrease at a decay rate of 7.4 K/GPa. The contour regions show $\mathrm{M}{\mathrm{R}}^{*}=d^2\mathrm{MR}/d{H}^2$ at $H=60\mathrm{kOe}$ (in units of ${10}^{-12}\mathrm{\Omega }/\mathrm{O}{\mathrm{e}}^2$) where the change from zero is consistent with indications of ferromagnetic ordering. Pressure error bars are determined via a difference of pressure before and after temperature cycles, and details are discussed in the Appendix.

Figure 6

(a)-(b) $T_C$ as a function of unit-cell parameters. The red and blue data are the results of pressure measurements whereas the green circles show a variation in $x$ from 0 to 0.3 for ${\mathrm{Fe}}_{3-x}\mathrm{Ge}{\mathrm{Te}}_2$. The variation in the a lattice parameter is consistent for all three sets of measurements whereas the $c$ axis parameter is not.

Figure 7

Rutherford backscattering spectra from $\mathrm{F}{\mathrm{e}}_{2.75}\mathrm{Ge}{\mathrm{Te}}_2$ sample. Symbols are experimental points, whereas solid lines are results of rump-code simulations. For clarity, only every tenth experimental point is depicted. Surface edges of Fe, Ge, and Te are marked by arrows.

Figure 8

Ambient pressure measurements of $\mathrm{F}{\mathrm{e}}_{2.75}\mathrm{Ge}{\mathrm{Te}}_2$ with (a) showing XRD with a preferred texture along the $c$ axis. The inset: Photograph of $\mathrm{F}{\mathrm{e}}_{2.75}\mathrm{Ge}{\mathrm{Te}}_2$ sample used in measurements (left) and 2D diffraction image showing polycrystallinity in the sample (right). Scale bar is 5 mm. Black lines are from the image plate. (b) $M$($H$) measurements at 5 K showing magnetic anisotropy along the $c$ axis of the crystal.

Figure 9

(a) XRD plotted as a function of pressure showing compression of the unit cell with higher 2θ angles. (b) Rietvield refinement of powdered $\mathrm{F}{\mathrm{e}}_{2.75}\mathrm{Ge}{\mathrm{Te}}_2$ at 0.7 GPa. †, *, and • are $\mathrm{FeT}{\mathrm{e}}_2$, Au pressure marker, and Re gasket, respectively. All other unlabeled indexed peaks are the sample. The ^ symbol is indicating solidification of the neon gas at higher pressure.

Figure 10

Magnetoresistance measurements at a select pressure of 4.1 GPa and select temperature of 5 K. (a) Raw electrical resistance data as a function of applied magnetic field showing nonsymmetric MR about the zero field. (b) Symmetrized and (c) antisymmetrized data showing both $R_{\mathrm{xx}}$ and $R_{\mathrm{xy}}$ contributions in the raw $R\left(H\right)$ measurement.

Figure 11

(a)–(f) Antisymmetrized $R_{\mathrm{xy}}$($H$) measurements as a function of temperature at a given pressure. $R_{\mathrm{xy}}$ curve transitions from a nonlinear saturating curve indicative of ferromagnetism below $T_C$ to a linear nonsaturating curve above $T_C$.

Figure 12

Pressure measurements using the magnetometer showing a drop in $T_C$ with pressures up to 0.7 GPa.