A combined first principles study of the structural, magnetic, and phonon properties of monolayer CrI3

Abstract

The first magnetic 2D material discovered, monolayer (ML) CrI3, is particularly fascinating due to its ground state ferromagnetism. However, because ML materials are difficult to probe experimentally, much remains unresolved about ML CrI3’s structural, electronic, and magnetic properties. Here, we leverage Density Functional Theory (DFT) and high-accuracy Diffusion Monte Carlo (DMC) simulations to predict lattice parameters, magnetic moments, and spin–phonon and spin–lattice coupling of ML CrI3. We exploit a recently developed surrogate Hessian DMC line search technique to determine CrI3’s ML geometry with DMC accuracy, yielding lattice parameters in good agreement with recently published STM measurements—an accomplishment given the ∼10% variability in previous DFT-derived estimates depending upon the functional. Strikingly, we find that previous DFT predictions of ML CrI3’s magnetic spin moments are correct on average across a unit cell but miss critical local spatial fluctuations in the spin density revealed by more accurate DMC. DMC predicts that magnetic moments in ML CrI3 are 3.62 μB per chromium and −0.145 μB per iodine, both larger than previous DFT predictions. The large disparate moments together with the large spin–orbit coupling of CrI3’s I-p orbital suggest a ligand superexchange-dominated magnetic anisotropy in ML CrI3, corroborating recent observations of magnons in its 2D limit. We also find that ML CrI3 exhibits a substantial spin–phonon coupling of ∼3.32 cm−1. Our work, thus, establishes many of ML CrI3’s key properties, while also continuing to demonstrate the pivotal role that DMC can assume in the study of magnetic and other 2D materials.

Density Functional Theory Study of Controllable Optical Absorptions and Magneto-Optical Properties of Magnetic CrI3 Nanoribbons: Implications for Compact 2D Magnetic Devices

“…All DFT results in Table show approximately the same magnetic moment of 0.21–0.26 μ B of the edge I atoms (atom indices 10–13), while LSDA-U, PBE, SCAN, and r 2 SCAN give similar values (∼3.0–3.2 μ B ) for Cr atoms (see Figure e for atom indexes), and both mTASK and TASK give values (∼3.5–3.6 μ B ) close to those of PBE-U (∼3.3 μ B ). The mTASK value is very close to the DMC (diffusion Monte Carlo) value (3.62 μ B ) for Cr in monolayer CrI 3 . All DFT results here give the moments on Cr and edge I atoms pointing to the positive y direction and perpendicular to the ribbon plane, while the relatively small moments on other nonedge I atoms point to the negative y direction, indicating that those I atoms have a weak antiferromagnetic coupling with Cr and edge I atoms).…”

Intrinsic Ferromagnetism of Two-Dimensional (2D) MnO2 Revisited: A Many-Body Quantum Monte Carlo and DFT+U Study

“…As previously mentioned, we used the geometry obtained with SCAN (FM and U = 0) for the subsequent DMC calculations because the calculated lattice constant (for the 2 × 2 × 1 supercell) of 5.696 Å is identical with the experimental value of 5.69(1) Å. DMC has the zero-variance property which means that as the trial wave function approaches the exact ground state (i.e., the exact nodal surface), the statistical fluctuations in the energy reduce to zero . There have been instances where various sophisticated, oftentimes expensive methods have been used to optimize the nodal surface of the trial wave function. − However, similar to other DMC studies of correlated magnetic materials, ,,,, we used a PBE+ U approach where the Hubbard U value was used as a variational parameter to optimize the nodal surface by using DMC (for the FM and AFM states of 2D MnO 2 ). The fact that we can determine the optimal U parameter variationally by using DMC makes our DMC results more reliable than DFT+ U , where in DFT+ U the U parameter is usually arbitrarily chosen or fitted to experimental data.…”

“…Diffusion Monte Carlo (DMC) is a correlated electronic structure method that has had demonstrated success for the electronic and magnetic properties of a variety of 2D and bulk systems. − This method has a weaker dependence on the starting Hubbard parameter and density functional and can successfully achieve results with an accuracy beyond the mean-field approximation . For example, DMC has successfully been used to calculate the spin superexchange in the correlated cuprate Ca 2 CuO 3 , has been used to successfully predict the magnetic structure in FeSe when DFT methods disagreed, has been applied to bulk polymorphs of MnO 2 to achieve band gap and lattice constant values in excellent agreement with experiment, and has been applied to study the excitation energies of Mn 4+ doped phosphors (both Mn-based studies used the same RRKJ pseudopotentials we used in our work , ).…”

Mn2NT2 (T = O, F) Nanosheets for Electrically and Thermally Driven Magnetic Tunnel Junctions

“…Design strategies for ferromagnetic materials open up new directions for building unusual electronic states and designing superconducting devices with better performance . Several 2D magnets such as Fe 3 GeTe 2 , CrCl 3 , and CrBr 3 have been made experimentally, and the resulting devices have excellent properties including controllable magnetism, enormous magnetoresistance, and energy-saving switching. − Nevertheless, low Curie temperatures or slow spin polarization rates limit the application of these 2D magnetic systems. Therefore, 2D intrinsic ferromagnetic (FM) materials combining substantial spin polarization and high Curie temperature are particularly important and interesting. − …”