First-principles prediction of two-dimensional MnOX (X = Cl, Br) monolayers: the half-metallic multiferroics with magnetoelastic coupling

Enhanced ferromagnetism, perpendicular magnetic anisotropy and high Curie temperature in the van der Waals semiconductor CrSeBr through strain and doping

Two-dimensional (2D) intrinsic van der Waals ferromagnetic semiconductor (FMS) crystals with strong perpendicular magnetic anisotropy and high Curie temperature (TC) are highly desirable and hold great promise for applications in ultrahigh-speed spintronic devices. Here, we systematically investigated the effects of a biaxial strain ranging between -8% and +8% and doping with different charge carrier concentrations (≤0.7 electrons/holes per unit cell) on the electronic structure, magnetic properties, and TC of monolayer CrSeBr by combining first-principles calculations and Monte Carlo (MC) simulations. Our results demonstrate that the pristine CrSeBr monolayer possesses an intrinsic FMS character with a band gap as large as 1.03 eV, an in-plane magnetic anisotropy of 0.131 meV per unit cell, and a TC as high as 164 K. At a biaxial strain of only 0.8% and a hole density of 5.31 × 1013 cm-2, the easy magnetization axis direction transitions from in-plane to out-of-plane. More interestingly, the magnetic anisotropy energy and TC of monolayer CrSeBr are further enhanced to 1.882 meV per unit cell and 279 K, respectively, under application of a tensile biaxial strain of 8%, and the monolayer retains its semiconducting properties throughout the entire range of investigated strains. It was also found that upon doping monolayer CrSeBr with holes with a concentration of 0.7 holes per unit cell, the perpendicular magnetic anisotropy and TC are increased to 0.756 meV per cell and 235 K, respectively, and the system tends to become metallic. These findings will help to advance the application of 2D intrinsic ferromagnetic materials in spintronic devices.

Hole-Doping-Induced Perpendicular Magnetic Anisotropy and High Curie Temperature in a CrSX (X = Cl, Br, I) Semiconductor Monolayer

A large perpendicular magnetic anisotropy and a high Curie temperature (TC) are crucial for the application of two-dimensional (2D) intrinsic ferromagnets to spintronic devices. Here, we investigated the electronic and magnetic properties of carrier-doped Van der Waals layered CrSX (X = Cl, Br, I) ferromagnets using first-principles calculations. It was found that hole doping can increase the magnitude of the magnetic anisotropy energy (MAE) and change the orientation of the easy magnetization axis at small doping amounts of 2.37 × 1013, 3.98 × 1012, and 3.33 × 1012/cm2 for CrSCl, CrSBr, and CrSI monolayers, respectively. The maximum values of the MAE reach 57, 133, and 1597 μeV/u.c. for the critical hole-doped CrSCl, CrSBr, and CrSI with spin orientation along the (001) direction, respectively. Furthermore, the Fermi energy level of lightly hole-doped CrSX (X = Cl, Br, I) moves into the spin-up valence band, leading to the CrSX (X = Cl, Br, I) magnetic semiconductor monolayer becoming first a half-metal and then a metal. In addition, the TC can also be increased up to 305, 317, and 345 K for CrSCl, CrSBr, and CrSI monolayers at doping amounts of 5.94 × 1014, 5.78 × 1014, and 5.55 × 1014/cm2, respectively. These properties suggest that the hole-doping process can render 2D CrSX (X = Cl, Br, I) monolayers remarkable materials for application to electrically controlled spintronic devices.

Two-dimensional ferroelastic and ferromagnetic NiOX (X = Cl and Br) with half-metallicity and a high Curie temperature

Two-dimensional (2D) multiferroic materials with distinctive properties, such as half-metallicity, high Curie temperature (TC), and magnetoelastic coupling, hold potential applications in novel nanoscale spintronic devices, but they are rare. Using density functional theory (DFT) calculations and evolutionary algorithms, we identify new types of 2D NiOX (X = F, Cl and Br) monolayers that are stable in energy, dynamics, thermodynamics, and mechanics. Among them, NiOF is an indirect-gap antiferromagnetic (AFM) semiconductor, while NiOCl and NiOBr are half-metallic materials with ferromagnetic (FM) ordering with a TC of 671 and 692 K and in-plane magnetic anisotropy energies (MAEs) of 541 and 609 μeV per Ni along the x-axis and y-axis, respectively. Notably, ferroelasticity is another important feature of NiOCl and NiOBr monolayers with energy barriers of 234.0 and 151.5 meV per atom, respectively. Moreover, the in-plane magnetic easy axis is strongly coupled to the lattice direction. The coexistence of high ferromagnetism, ferroelasticity, half-metallicity, and magnetoelastic coupling renders NiOCl and NiOBr monolayers great potential for future nanodevices.