Spin reorientations in structurally metastable, disordered, and hexagonal Cr7Te8

TL;DR

This study investigates the magnetic, structural, and electronic properties of metastable hexagonal Cr7Te8, revealing spin reorientations and phase transitions that influence its unusual Hall effect behavior, with implications for 2D chromium tellurides.

Contribution

It provides new insights into the magnetic phase transitions and structural changes in metastable Cr7Te8, linking these to its electronic transport properties.

Findings

Metastable hexagonal Cr7Te8 exhibits ferromagnetism above room temperature.

A first-order transition from hexagonal to monoclinic structure occurs above 550 K.

Spin reorientation and magnetic transitions occur at 220 K and 70 K, affecting Hall effect properties.

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.