Competing magnetic phases in Cr$_{3+δ}$Te$_4$ are spatially segregated

TL;DR

This work reveals that Cr$_{3+ ormalfont ext{δ}}$Te$_4$ with δ near -0.1 intrinsically contains two monoclinic, spatially segregated phases: a ferromagnetic phase A and an antiferromagnetic phase B. Using a combination of single-crystal neutron diffraction, neutron powder diffraction, X-ray diffraction, TEM, and DFT, the authors show that these phases form a fine intergrowth (≈100 nm domains) whose magnetostrictive strains act in opposite directions and couple across interfaces. The δ ≈ -0.26 crystal, by contrast, exhibits predominantly FM order, linking Cr content and vacancy distribution to the magnetic ground state. The results explain prior observations of coexisting magnetic orders and emphasize strain as a critical control parameter for magnetic ordering temperatures and anisotropies in Cr$_{1+x}$Te$_2$, with implications for thin-film behavior and topological transport phenomena.

Abstract

Cr$_{1+x}$Te$_2$ is a self-intercalated vdW system that is of current interest for its room-temperature FM phases and tunable topological properties. Early NPD measurements on the monoclinic phase Cr$_3$Te$_4$ ($x=0.5$) presented evidence for competing FM and AFM phases. Here we apply neutron diffraction to a single crystal of Cr$_{3+δ}$Te$_4$ with $δ=-0.10$ and discover that it consists of two distinct monoclinic phases, one with FM order below $T_{\rm C} \approx 321$ K and another that develops AFM order below $T_{\rm N} \approx 86$ K. In contrast, we find that a crystal with $δ=-0.26$ exhibits only FM order. The single-crystal analysis is complemented by results obtained with NPD, XPD, and TEM measurements on the $δ=-0.10$ composition. From observations of spontaneous magnetostriction of opposite sign at $T_{\rm C}$ and $T_{\rm N}$, along with the TEM evidence for both monoclinic phases in a single thin ($\approx$ 100 nm) grain, we conclude that the two phases must have a fine-grained ($\lesssim$ 100 nm) intergrowth character, as might occur from high-temperature spinodal decomposition during the growth process. Calculations of the relaxed lattice structures for the FM and AFM phases with DFT provide a rationalization of the observed spontaneous magnetostrictions. Correlations between the magnitude and orientation of the magnetic moments with lattice parameter variation demonstrate that the magnetic orders are sensitive to strain, thus explaining why magnetic ordering temperatures and anisotropies can be different between bulk and thin-film samples, when the latter are subject to epitaxial strain. Our results point to the need to investigate the supposed coexistence FM and AFM phases reported elsewhere in the Cr$_{1+x}$Te$_2$ system, such as in the Cr$_5$Te$_8$ phase ($x=0.25$).

Figures (16)

• Figure 1: (a) The atomic arrangement in Cr$_3$Te$_4$. The hexagonal/pseudo-hexagonal Cr$_\text{h}$ layers (in magenta) are common to the Cr$_{1+x}$Te$_2$ system, while the occupied interstitial Cr$_\text{i}$ sites (in blue) vary among different phases. (b) Approximate phase diagram for Cr$_{1+x}$Te$_2$. Phase boundaries are estimated from the data in Table II of Ipser et al.ipse83. Letters denote symmetry: T:trigonal, M:monoclinic, and H:hexagonal. The circles and stars indicate the FM ($T_\text{C}$) and AFM ($T_\text{N}$) ordering temperatures, respectively, as listed in Table \ref{['tab']}. The red square denotes $T_\text{C}$, which was measured by magnetization liu19.
• Figure 2: (a-e) Rocking curves at 350 K for a selection of structural Bragg peaks illustrating the varying separation of peaks A and B. Legends are common across all the panels. The fit (red line) to rocking curves consists of three peaks: A (seagreen), B (orange), and C (blue), simulated by Gaussian lineshapes of fixed width, and a global linear background (black). The vertical dashed lines show the position of A and B peaks, and their difference is highlighted in the panels by A$-$B. (f) A schematic highlighting the orientation of phase B with respect to phase A in the ($H$ 0 $L$) reciprocal space. Their $c^*$ axes are co-aligned and represented by $c^*_\text{AB}$. The $a^*$ axis associated with the B peaks is rotated by 180$^\circ$ around the $c^*$ axis relative to the $a^*$ axis of the A peaks, such that $a^*_\text{A}$ and $- a^*_\text{B}$ are aligned, and their angular difference is equal to $\beta_{\rm A}+\beta_{\rm B}-180^\circ$.
• Figure 3: Rocking curves at 4 K (circles) and 350 K (squares) for (0 0 $-2$), (2 0 0), and (2 0 $-2$). Letters A, B, and C highlight the locations of peaks from the respective phases.
• Figure 4: Mesh scans for (a) (2 0 $-2$) and (b) (1.5 0 1.5) based on the indexing of phase A. (c) Scan along ${\bf Q}$ = (1.5 0 $L$)$_{\rm A}$; green arrow points to the absence of a peak for phase A at (1.5 0 $-1.5$).
• Figure 5: Comparison of the the structural refinement (red line) from the 350 K NPD data (black line) (the weighted-profile factor $R_\text{wp} = 5.05$) for the $\delta = -0.10$ sample. Lower panel shows the difference between calculated intensities and the data.