Structural phase transitions in the van der Waals ferromagnets Fe$_x$Pd$_{y}$Te$_2$

TL;DR

Fe$_x$Pd$_y$Te$_2$ is shown to be a composition-tunable van der Waals ferromagnet with a first-order structural transition around $T \,\approx\ 360$–$420$ K. At high temperatures, $y>2$ compounds adopt a disordered tetragonal derivative of FeTe ($P4/nmm$), while below the transition the materials become incommensurately or commensurately modulated, depending on composition, with low-temperature zigzag Fe/Pd ordering and interstitial Pd behavior. Magnetic order is ferromagnetic with easy-plane moments and $T_C$ in the $98$–$180$ K range, decreasing with increasing Pd content; transport likewise shows metallic behavior with characteristic domain- and transition-related features, highlighting strong tunability of both structure and magnetism via $x$ and $y$. The work links composition to structural mood and magnetic properties, offering a platform for exploring tunable vdW ferromagnetism and associated domain phenomena in Fe$_x$Pd$_y$Te$_2$ systems.

Abstract

Here, we provide a detailed study of the crystal structure and physical properties of the recently discovered vdW ferromagnet FePd$_2$Te$_2$. We find this compound has a relatively wide width of formation, and grow single crystals with compositions Fe$_x$Pd$_{y}$Te$_2$ where $x$ ranges from 0.9 to 1.1 and $y$ from 1.8 to 2.5, respectively. Temperature-dependent X-ray diffraction and transport measurements reveal that a first-order structural transition occurs in the range of $T$ = 360-420 K. Above the transition, the compounds with Pd fraction $y>2$ adopt a disordered derivative of the tetragonal FeTe structure, with the Fe layer showing mixed Fe/Pd occupancy and the extra Pd atoms partially occupying interstitial sites. Below 370 K, the structure is incommensurately modulated. For $y<2$, the composition Fe$_{1.1}$Pd$_{1.8}$Te$_2$ has monoclinic symmetry at room temperature that is consistent with the reported structure of FePd$_2$Te$_2$. This phase undergoes a structural transition at 420 K for which the high temperature structure is yet to be determined; however, based on the similarities with the $y > 2$ compounds, we speculate that this composition also adopts a tetragonal structure above 420\,K. All compounds investigated in the Fe$_x$Pd$_{y}$Te$_2$ series show metallic behavior, with magnetic characterization indicating that they are easy-plane, hard, ferromagnets with $T_C$ spanning 98--180 K. Both the critical temperature for the structural transition and the Curie temperature are moderately suppressed with increasing Pd fraction $y$ and corresponding decreasing Fe fraction $x$, indicating that synthetic control over $x$ and $y$ paves way for the further exploration of these compounds.

Figures (5)

• Figure 1: Room temperature powder X-ray diffraction patterns of Fe$_{x}$Pd$_{y}$Te$_2$. Black marks indicate the main reflections of the low-temperature phase (P$2/n$) of $y>2$ compounds, while gray marks stand for satellite peaks up to the second order. Peak positions of FePd$_2$Te$_2$ corresponding to the structure reported by Shi et al. shi2024 are shown in red. For comparison, main reflections of the high-temperature phase (P$4/nmm$) of $y>2$ compounds are also presented. The left inset shows a close-up of Fe$_{0.9}$Pd$_{2.5}$Te$_2$ data around $2\theta\approx22^\circ$, with orange arrows indicating the satellite peaks unaccounted for in the $P2_1/m$ structural model. The inset on the right shows a photo of a typical Fe$_{x}$Pd$_{y}$Te$_2$ single crystal.
• Figure 2: Single crystal diffraction of Fe$_{0.9}$Pd$_{2.5}$Te$_2$ along (a) 100, (b) 010, and (c) 001 directions. Additional peaks at 300 K signal a structural modulation with wave vector $q \approx (-0.42\,\,0.17\,\,\frac{1}{2})$. Yellow arrows in (c) indicate peaks that split above the transition. Diffraction patterns for Fe$_{1.1}$Pd$_{1.8}$Te$_2$ along (d) 100, (e) 010, and (f) 001 directions, featuring extra intensities at $q = (\frac{1}{2}\,\,\frac{1}{2}\,\,\frac{1}{2})$. (g) Integrated intensity of the first-order satellite peak associated with the main reflection $(\bar{2}20)$ upon warming and cooling for Fe$_{0.9}$Pd$_{2.5}$Te$_2$. The fourth index on the $y$-axis refers to the order of the satellite peak, with zero standing for a main reflection. (h) Undistorted tetragonal structure of Fe$_{0.9}$Pd$_{2.5}$Te$_2$ along (100) direction, emphasizing the layered structure along the c-axis. (i) Average structure of Fe$_{1.1}$Pd$_{1.8}$Te$_2$ at 400 K. Alternating Fe and Pd chains are seen in the cleavage plane. Unit cells are shown in black lines.
• Figure 3: Anisotropic magnetization data for Fe$_x$Pd$_{y}$Te$_2$ single crystals ($x = 0.9-1.1$ and $y=2.5-1.8$). In-plane and out-of-plane measurements are represented by closed and open symbols, respectively. The in-plane orientation is $H\perp c$ for $y>2$ compounds and $H\perp [10\bar{1}]$ for Fe$_{1.1}$Pd$_{1.8}$Te$_2$, respectively. Temperature dependence of $M/H$ for (a) Fe$_{1.1}$Pd$_{1.8}$Te$_2$, (b) FePd$_{2.3}$Te$_2$, and (c) Fe$_{0.9}$Pd$_{2.5}$Te$_2$. These measurements were field-cooled with $H = 10\,$kOe. The noise around 180 K in the out-of-plane measurements for $y>2$ compounds is from zero-crossing artifacts associated with the background subtraction (see experimental details). Field-dependent magnetization isotherms measured at $T = 5\,$K (circles) and $T = 180\,$K or $T=250\,$K (squares) for (d) $y$ = 1.8, (e) $y$ = 2.3, and (f) $y$ = 2.5 samples, respectively.
• Figure 4: High-temperature $M/H$ for Fe$_x$Pd$_{y}$Te$_2$ single crystals with (a) $y > 2$ and (b) $y = 1.8$. All measurements were field-cooled with $H = 10\,$kOe applied in-plane. All compounds exhibit hysteresis on increasing (solid circles) and decreasing (open circles) temperature sweeps around the phase transition.
• Figure 5: (a) Temperature-dependent resistance upon cooling normalized to its value at 375 K (left axis), $R/R_{375}$, for Fe$_x$Pd$_{y}$Te$_2$ samples with $x = 0.9-1.1$ and corresponding $y=2.5-2.3,\, 1.8$. Resistance derivatives (right axis), $dR/dT$, are shown in dashed lines. Resistance above $T > 330\,$ K for (b) Fe$_{0.9}$Pd$_{2.5}$Te$_2$, (c) FePd$_{2.3}$Te$_2$ and (d) Fe$_{1.1}$Pd$_{1.8}$Te$_2$ measured upon warming and cooling (red and blue lines, respectively), emphasizing the hysteretic behavior of the phase transition around 360 K for $y>2$ compounds. Shaded lines are measurements collected in a subsequent warming/cooling cycle.