Spin reorientations in structurally metastable, disordered, and hexagonal Cr7Te8

Abstract

Vapor deposited two-dimensional Cr$_{7}$Te$_{8}$ displays unusual temperature dependent Hall effect properties, including a room temperature anomalous Hall effect, sign reversals of the Hall resistivity on cooling, and a peak in the Hall resistivity at low temperatures. The two dimensional Cr$_{7}$Te$_{8}$ heterostructures that form the basis of these measurements are hexagonal in structure. We study the magnetic and structural properties of bulk Cr$_{7}$Te$_{8}$ synthesized by quenching from 1000 $^{\circ}$C with the goal of relating the magnetic, structural, and electronic properties. This quenched phase is metastable, hexagonal, and displays different magnetic properties from the slow-cooled and more thermodynamically stable monoclinic phase. High-resolution x-ray diffraction of the quenched hexagonal phase finds a first-order transition to a lower symmetry monoclinic phase on \textit{heating} above $\sim$ 550 K. Magnetic susceptibility measurements of the quenched hexagonal phase reveal ferromagnetic ordering above room temperature, along with the two distinct transitions at $\sim$ 220~K and $\sim$ 70~K. Through neutron diffraction studies, we find the $\sim$ 220 K anomaly is a spin reorientation transition of the ferromagnetically aligned magnetic moments and the $\sim70$ K feature represents a transition from a high temperature ferromagnet to a low temperature antiferromagnet. We suggest that these magnetic transitions are related to changes in the unit cell dimensions and are connected to the temperature dependent Hall resisitivity studied in two-dimensional heterostructures. This implies a link between structural, magnetic, and electronic properties in the ``pseudo" two-dimensional chromium tellurides.

Figures (6)

• Figure 1: Crystal Structure of Cr$_{7}$Te$_{8}$ from the $c$-axis and crystal structure of Cr$_{7}$Te$_{8}$ with $(a)$ displaying the slow-cooled ordered monoclinic phase. The hexagonal Ni-As type structure with disordered chromium vacancies from quenching is illustrated in $(b)$. Results of the Rietveld refinement of Cr$_{7}$Te$_{8}$ are listed Tables 1 and 2. The corresponding temperature dependent magnetic susceptibility for the ordered (slow-cooled) monoclinic phase $(c)$ and disordered (quenched) phase $(d)$ are plotted.
• Figure 2: Structure analysis using the Rietveld refinements of powder sample of Cr$_{7}$Te$_{8}$ at two different temperatures using monochromatic Cu K-$\alpha$ radiation. $(a)$, $(c)$ demonstrate the phase change from hexagonal to mixture of monoclinic and hexagonal as a function of temperature and $(b)$ shows the peak splitting evident at higher angles supporting the phase change from hexagonal to mixed phase. The left panel $(e-f)$ shows the change in the peak as the phase changes from monoclinic to hexagonal as a function of temperature. The coexistence of both phases at high temperatures is indicative of a first order transition from hexagonal to monoclinic on heating.
• Figure 3: The temperature dependence of lattice constants, the volume and the Cr-Cr bond distance fitted against the hexagonal space group. A marked contraction is observed in the c lattice and an expansion is measured in the a lattice up to 360 K.
• Figure 4: Combined neutron diffraction patterns of data taken at 5 K, 150 K, 298 K, and 400 K. A new anti-ferromagnetic contribution is observed in the 150 K diffraction data and a peak attributed to a k vector of $(\frac{1}{2}, \frac{1}{2}, 0)$ in the 5 K data.
• Figure 5: Neutron diffraction patterns at 5 K (a), 150 K (b), and 298 K (c) for a powder sample of Cr$_{7}$Te$_{8}$.The experimental data are drawn as grey dots. Rietveld refinement calculations are shown as Dark blue, light blue, and salmon for 5 K, 150 K, and 298 K respectively.