Robust Intrinsic Ferromagnetism and Half Semiconductivity in Stable Two-Dimensional Single-Layer Chromium Trihalides (1507.07275v2)

Abstract: Two-dimensional (2D) intrinsic ferromagnetic (FM) semiconductors are crucial to develop low-dimensional spintronic devices. Using density functional theory, we show that single-layer chromium trihalides (SLCTs) (CrX$_3$,X=F, Cl, Br and I) constitute a series of stable 2D intrinsic FM semiconductors. A free-standing SLCT can be easily exfoliated from the bulk crystal, due to a low cleavage energy and a high in-plane stiffness. Electronic structure calculations using the HSE06 functional indicate that both bulk and single-layer CrX$_3$ are half semiconductors with indirect gaps and their valence bands and conduction bands are fully spin-polarized in the same spin direction. The energy gaps and absorption edges of CrBr$_3$ and CrI$_3$ are found to be in the visible frequency range, which implies possible opt-electronic applications. Furthermore, SLCTs are found to possess a large magnetic moment of 3$\mu_B$ per formula unit and a sizable magnetic anisotropy energy. The magnetic exchange constants of SLCTs are then extracted using the Heisenberg spin Hamiltonian and the microscopic origins of the various exchange interactions are analyzed. A competition between a near 90$^\circ$ FM superexchange and a direct antiferromagnetic (AFM) exchange results in a FM nearest-neighbour exchange interaction. The next and third nearest-neighbour exchange interactions are found to be FM and AFM respectively and this can be understood by the angle-dependent extended Cr-X-X-Cr superexchange interaction. Moreover, the Curie temperatures of SLCTs are also predicted using Monte Carlo simulations and the values can further increase by applying a biaxial tensile strain. The unique combination of robust intrinsic ferromagnetism, half semiconductivity and large magnetic anisotropy energies renders the SLCTs as promising candidates for next-generation semiconductor spintronic applications.

• The paper demonstrates that single-layer chromium trihalides exhibit robust intrinsic ferromagnetism and half semiconductivity through advanced DFT calculations.
• It confirms material stability via phonon dispersion and ab initio molecular dynamics while revealing tunable magnetic properties under biaxial strain.
• Monte Carlo simulations predict enhanced Curie temperatures, underscoring the potential of these 2D materials for next-generation spintronic and optoelectronic devices.

Overview of Robust Intrinsic Ferromagnetism and Half Semiconductivity in Stable Two-Dimensional Single-Layer Chromium Trihalides

This paper presents a comprehensive paper on single-layer chromium trihalides (CrX$_3$, X = F, Cl, Br, I), demonstrating that these materials are stable two-dimensional (2D) intrinsic ferromagnetic semiconductors. The research is grounded in density functional theory (DFT) calculations, focusing on the potential applications of these materials in 2D spintronics, a cutting-edge area in materials science and condensed matter physics.

Structure and Stability

The paper highlights that single-layer chromium trihalides can be efficiently exfoliated from their bulk form due to their relatively low cleavage energy and high in-plane stiffness. The stability of these materials is confirmed through phonon dispersion calculations and ab initio molecular dynamics simulations, indicating that they can withstand thermal agitation at room temperature without degradation. These findings underscore the robustness of the 2D crystals, making them feasible for practical applications.

Electronic and Optical Properties

The electronic structure analysis, conducted using the HSE06 functional, reveals that both bulk and single-layer forms of CrX$_3$ are half semiconductors with indirect band gaps. The spin-polarized band structure shows that the valence bands and conduction bands are fully polarized in the same spin direction, which is critical for spintronic devices. Notably, CrBr$_3$ and CrI$_3$ exhibit band gaps and absorption edges within the visible spectrum, indicating potential for optoelectronic applications.

Magnetic Properties

A distinctive feature of single-layer CrX$_3$ is its robust intrinsic ferromagnetism. The magnetic moment is calculated to be 3$\mu_B$ per formula unit. The paper explores the magnetic exchange interactions using the Heisenberg spin Hamiltonian framework, revealing that the nearest-neighbor exchange interaction is predominantly ferromagnetic due to a competitive balance between direct antiferromagnetic and indirect (superexchange) ferromagnetic interactions. The Curie temperatures, predicted via Monte Carlo simulations, are shown to rise with applied biaxial tensile strain, suggesting tunable magnetic properties.

Implications and Future Directions

The unique combination of ferromagnetism, half semiconductivity, and tunability under strain makes single-layer CrX$_3$ promising candidates for next-generation spintronic and magneto-optic devices. The paper suggests that future research could explore the effect of external stimuli (such as electric fields and chemical doping) on the electronic and magnetic properties of these materials, potentially leading to novel device functionalities.

In conclusion, the findings presented establish single-layer chromium trihalides as a significant addition to the growing inventory of 2D materials, with practical implications for the development of spintronic technologies. The paper's detailed theoretical insights lay a strong foundation for experimentalists to explore these materials further, moving towards their integration into advanced technological applications.