Large magneto-optical effects and magnetic anisotropy energy in two-dimensional Cr$_2$Ge$_2$Te$_6$

TL;DR

This study uses ab initio calculations to reveal large magneto-optical effects and significant magnetic anisotropy energies in atomically thin Cr$_2$Ge$_2$Te$_6$ films, highlighting their potential for spintronic applications.

Contribution

It provides the first detailed analysis of magneto-optical effects and magnetic anisotropy energies in 2D Cr$_2$Ge$_2$Te$_6$ films, including monolayer, bilayer, trilayer, and bulk, using density functional theory.

Findings

Large MAEs (~0.1 meV/Cr) in 2D Cr$_2$Ge$_2$Te$_6$

Significant Kerr rotations up to 1.0 degree in visible range

Faraday rotations up to 120 degrees per micrometer

Abstract

By performing systematic $ab$ $initio$ density functional calculations, here we study two relativity-induced properties of atomically thin ferromagnetic (FM) Cr$_2$Ge$_2$Te$_6$ films [monolayer (ML), bilayer (BL) and trilayer (TL) as well as bulk], namely, magnetic anisotropy energy (MAE) and magneto-optical (MO) effects. Competing contributions of both magneto-crystalline anisotropy energy (C-MAE) and magnetic dipolar anisotropy energy (D-MAE) to the MAE, are computed. Calculated MAEs of these materials are large, being in the order of $\sim$0.1 meV/Cr. Interestingly, we find that the out-of-plane magnetic anisotropy is preferred in all the systems except the ML where an in-plane magnetization is favored because here the D-MAE is larger than the C-MAE. Crucially, this explains why long-range FM order was observed in all the few-layer Cr$_2$Ge$_2$Te$_6$ except the ML because the out-of-plane magnetic anisotropy would open a spin-wave gap and thus suppress magnetic fluctuations so that long-range FM order could be stabilized at finite temperature. In the visible frequency range, large Kerr rotations up to $\sim$1.0 deg in these materials are predicted and they are comparable to that observed in famous MO materials such as PtMnSb and Y$_3$Fe$_5$O$_{12}$. Moreover, they are $\sim$100 times larger than that of 3$d$ transition metal MLs deposited on Au surfaces. Faraday rotation angles in these 2D materials are also large, being up to $\sim$120 deg/$\mu$m, and are thus comparable to the best-known MO semiconductor Bi$_3$Fe$_5$O$_{12}$. These findings thus suggest that with large MAE and MO effects, atomically thin Cr$_2$Ge$_2$Te$_6$ films would have potential applications in novel magnetic, MO and spintronic nanodevices.