Room temperature ferromagnetism and antiferromagnetism in two-dimensional iron arsenides

Ultrahigh Néel Temperature Antiferromagnetism and Ultrafast Laser-Controlled Demagnetization in a Dirac Nodal Line MoB3 Monolayer

Two-dimensional (2D) antiferromagnetic (AFM) materials boasting a high Néel temperature (TN), high carrier mobility, and fast spin response under an external field are in great demand for efficient spintronics. Herein, we theoretically present the MoB3 monolayer as an ideal 2D platform for AFM spintronics. The AFM MoB3 monolayer features a symmetry-protected, 4-fold degenerate Dirac nodal line (DNL) at the Fermi level. It demonstrates a high magnetic anisotropy energy of 865 μeV/Mo and an ultrahigh TN of 1050 K, one of the highest recorded for 2D AFMs. Importantly, we reveal the ultrafast demagnetization of AFM MoB3 under laser irradiation, which induces a rapid transition from a DNL semimetallic state to a metallic state on the time scale of hundreds of femtoseconds. This work presents an effective method for designing advanced spintronics using 2D high-temperature DNL semimetals and opens up a new idea for ultrafast modulation of magnetization in topological semimetals.

Iron-arsenide monolayers as an anode material for lithium-ion batteries: a first-principles study

This theoretical investigation delves into the structural, electronic, and electrochemical properties of two hexagonal iron-arsenide monolayers, 1T-FeAs and 1H-FeAs, focusing on their potential as anode materials for lithium-ion batteries. Previous studies have highlighted the ferromagnetic nature of 1T-FeAs at room temperature. Our calculations reveal that both phases exhibit metallic behaviour with spin-polarized electronic band structures. Electrochemical studies show that the 1T-FeAs monolayer has better ionic conductivity for Li ions than the 1H-FeAs phase, attributed to a lower activation barrier of 0.38 eV. This characteristic suggests a faster charge/discharge rate. Both FeAs phases exhibit comparable theoretical capacities (374 mA h g-1), outperforming commercial graphite anodes. The average open-circuit voltage for maximum Li atom adsorption is 0.61 V for 1H-FeAs and 0.44 V for 1T-FeAs. The volume expansion over the maximum adsorption of Li atoms on both phases is also remarkably less than the commercially used anode material such as graphite. Furthermore, the adsorption of Li atoms onto 1H-FeAs induces a remarkable transition from ferromagnetism to anti-ferromagnetism, with minimal impact on the electronic band structure. In contrast, the original state of 1T-FeAs remains unaffected by Li adsorption. To summarize, both 1T-FeAs and 1H-FeAs monolayers have potential as promising anode materials for lithium-ion batteries, offering valuable insights into their electrochemical performance and phase transition behaviour upon Li adsorption.

Two-dimensional Janus SVAN2 (A = Si, Ge) monolayers with intrinsic semiconductor character and room temperature ferromagnetism: tunable electronic properties via strain and an electric field

In the context of developing next-generation information technology, two-dimensional materials with inherent ferromagnetism, a Curie temperature above room temperature, and significant magnetic anisotropy hold great promise. In this work, we employed first-principles calculations to investigate a novel two-dimensional Janus structure, namely SVAN2 (A = Si, Ge). Our findings reveal that these structures are not only dynamically and thermally stable, but also exhibit semiconductor properties alongside their ferromagnetic states. The Janus SVSiN2 monolayer exhibits an in-plane easy axis, while the SVGeN2 monolayer shows an out-of-plane easy axis, both characterized by a significant magnetic anisotropy energy (129 and 172 μeV, respectively). Notably, through Monte Carlo simulation, we found that the Curie temperature of the SVSiN2 monolayer is 330 K, which is higher than room temperature. Finally, by applying biaxial strain and an external electric field, we successfully regulated the electronic properties of the SVAN2 (A = Si, Ge) monolayers, enabling a transition from semiconductor to half-metallic behavior. These remarkable electronic and magnetic properties make the Janus SVAN2 (A = Si, Ge) monolayers promising candidate materials for spin electron applications.

Two-dimensional superhard silicon nitrides with widely tunable bandgap, high carrier mobility and hole-doping-induced robust magnetism

The search for new forms of the traditional bulk materials to enrich their interactions and properties is an attractive subject in two-dimensional (2D) materials. In this work, novel tetra-hexa-mixed coordinated 2D silicon nitrides (Si3N4) and their analogues are systematically investigated via density functional theory. The results show the global minimum 2D structure, Si3N4 (T-aa), is a highly chemically and thermally stable superhard semiconductor with a wide indirect bandgap (about 6.0 eV), which is widely adjustable under both biaxial strain and vertical electric field. It also possesses anisotropic high carrier mobility, up to 5490 cm2 V-1 s-1 at room temperature. Besides, its nitride analogues of group IVA (Si, Ge, Sn, and Pb) exhibit diverse electronic structures with regular bandgap distribution. Remarkably, some nitride analogues display linearly increasing robust magnetism with hole doping. The theoretical Curie temperatures of Si3N4 and Sn3N4 with hole doping (1h+ per unit cell) are 298 and 180 K, respectively. The Si3N4 (T-aa) and its analogues have a variety of excellent properties to be potentially applied in various fields, e.g., semiconductor electronics, spintronics, high-temperature structural materials, and superhard materials.

Two-Dimensional Cr2X3S3 (X = Br, I) Janus Semiconductor with Intrinsic Room-Temperature Magnetism

The exploration of two-dimensional (2D) semiconductors with intrinsic room-temperature magnetism for use in nanoscale spintronic devices is of particular interest. Recently, the ferromagnetic CrX₃ monolayer (X = Br, I) has received growing attention, but low critical temperature hinders its practical applications in spintronics. Here, using first-principles calculations, we report 2D Cr₂X₃S₃ (X = Br, I) Janus semiconductors with room-temperature magnetism by replacing one layer of halogon atoms with sulfur atoms in CrX₃ monolayer. Our results demonstrate that Cr₂Br₃S₃ and Cr₂I₃S₃ Janus crystals are ferrimagnetic semiconductors, that maintain their magnetic order, with a direct bandgap of 1.19 and 0.61 eV and high critical temperature of 387 and 447 K, respectively. The residual unpaired p electrons on the S anions lead to a strong direct-exchange interaction between the Cr and S atoms. Moreover, their room-temperature magnetism is robust under biaxial strain, while the bandgap can be remarkably modulated with strain. The novel magnetic properties in 2D Cr₂X₃S₃ Janus magnetic semiconductors give them promising applications in spintronics.

Charge transfer and strain tuned antiferromagnetism in the two-dimensional CrCl3/[Mo2C(-O)]2 heterojunction

Magnetic ordering in two-dimensional materials with atomic level thickness has been one of the most important issues in condensed matter physics and material science. Most previous studies have focused on the two-dimensional ferromagnetic systems, while the antiferromagnetic systems have been much less touched. Here, by using first-principles calculations and Monte Carlo simulation, a two-dimensional antiferromagnetic heterojunction: CrCl3/[Mo2C(-O)]2, is predicted, by tuning the electronic distribution. The ferromagnetic coupling between the Cr-Cr atoms in the CrCl3/(Mo2C)2 heterostructure is enhanced by the transferred electrons from Mo2C, which will occupy the t2g orbits of Cr. With the O adsorbed on the Mo2C, the Cr-Cl bond length increases and the superexchange interaction is decreased. The magnetic ground state changes to antiferromagnetism. More interestingly, under a moderate compressive biaxial strain, its Néel temperature of CrCl3/(Mo2C-O)2 can be significantly increased for the enhanced direct exchange of Cr-Cr atom with a value of 146 K. The heterojunction is useful for two-dimensional spintronic logic, ultrafast magnetodynamic devices and information storage for new generation computer devices.