Electronic structure and stability of two-dimensional bimetallic ferromagnetic semiconductor CrMoI<sub>6</sub>

Abstract

Two-dimensional magnetic semiconductors have received extensive attention due to their combination of magnetism, semi-conductivity and special two-dimensional structures, which also provide a new idea and platform for developing the nanometer spintronic and optoelectronic devices and also for conducting the related basic theoretical research. However, in addition to the common problems of two-dimensional magnetic semiconductor materials, such as volume manufacturing and environmental stability, the two-dimensional magnetic semiconductor materials have the unique difficulty, i.e. low Curie temperature, which makes it difficult to maintain ferromagnetic coupling at higher temperature. For example, the Curie temperature of the existing CrI<sub>3</sub> monolayer is lower than 45 K, while that of the Cr<sub>2</sub>Ge<sub>2</sub>Te<sub>6</sub> double-layer is only 20 K, which is far lower than the room temperature. Therefore, how to improve the Curie temperature of two-dimensional magnetic semiconductor materials through various approaches is one of the important issues that need to be resolved in this field of research. Based on the first-principles calculations, the exchange energies of a series of two-dimensional bimetallic iodides CrTMI<sub>6</sub> (TM denotes transition metal elements in fourth and fifth rows) constructed from the lattice of CrI<sub>3</sub> monolayer are preliminarily calculated and screened. Structures are fully relaxed until the force and the energy are converged to 0.01 eV/Å and 10<sup>–6</sup> eV, respectively, and the ferromagnetic CrMoI<sub>6</sub> monolayer is selected. Further calculations show that the band structure of the CrMoI<sub>6</sub> monolayer exhibits ideal semiconductor characteristics with a band gap of about 1.7 eV. At the same time, theoretical calculations with considering the spin-orbit coupling show that the CrMoI<sub>6</sub> monolayer has a considerable magnetic anisotropy (741.3 μeV/TM), and its easy axis is perpendicular to the two-dimensional plane. Monte Carlo simulation based on the Heisenberg model predicts that the Curie temperature of CrMoI<sub>6</sub> monolayer reaches 92 K, which is about twice that of the CrI<sub>3</sub> monolayer. The molecular dynamics and phonon spectrum calculations also prove that it has both thermal and kinetic stability. In addition, under the condition of applying compressive and tensile strain, its ferromagnetic coupling shows strong stability. This kind of magnetic transition metal halide which can be synthesized by alloying will further expand the family of two-dimensional magnetic materials and their applications in the field of spintronic devices.