Morphology-Dependent Room-Temperature Ferromagnetism in Undoped ZnO Nanostructures

Bandgap lowering in mixed alloys of Cs3Bi2-xSbxBr9 perovskite powders

Lead-free metal halide perovskites have received widespread attention due to their composition of minimal hazardous components, excellent air stability, and long carrier lifetimes. However, the majority of the lead-free metal halide perovskites, such as Cs3Bi2Br9, have wide bandgaps, which limits their photoelectric in solar cells and optoelectronic devices. To address this issue, attempts have been made to adjust the bandgap through material alloying. Based on a solution approach, a pure phase of Cs3Bi2-xSbxBr9 crystals has been synthesized, with the alloying parameter x changing over the full range of composition. It is found that the mixed alloy has a smaller bandgap than pure Bi-based and Sb-based perovskites, with the smallest bandgap of 2.22 eV near x = 1, and there is a phenomenon of bandgap bowing throughout the alloying process. The electronic structure of Cs3Bi2-xSbxBr9 has been investigated using DFT calculations and the bandgap bowing of Cs3Bi2-xSbxBr9 is deduced to be related to the type-II band alignment between the Cs3Bi2Br9 and Cs3Sb2Br9. Owing to the mismatch of s and p orbital energies of Bi and Sb, the mixed alloy has a smaller bandgap. Our work demonstrated a method for achieving bandgap reduction and explained the phenomenon of bandgap bowing by pairing materials into type-II band alignment, which may also be found in other lead-free metal perovskites.

Progress and Prospects in Metallic FexGeTe2 (3 ≤ x ≤ 7) Ferromagnets

Thermal fluctuations in two-dimensional (2D) isotropy systems at non-zero finite temperatures can destroy the long-range (LR) magnetic order due to the mechanisms addressed in the Mermin-Wanger theory. However, the magnetic anisotropy related to spin-orbit coupling (SOC) may stabilize magnetic order in 2D systems. Very recently, 2D FexGeTe2 (3 ≤ x ≤ 7) with a high Curie temperature (TC) has not only undergone significant developments in terms of synthetic methods and the control of ferromagnetism (FM), but is also being actively explored for applications in various devices. In this review, we introduce six experimental methods, ten ferromagnetic modulation strategies, and four spintronic devices for 2D FexGeTe2 materials. In summary, we outline the challenges and potential research directions in this field.

Strain Engineering of Intrinsic Ferromagnetism in 2D van der Waals Materials

Since the discovery of the low-temperature, long-range ferromagnetic order in monolayers Cr2Ge2Te6 and CrI3, many efforts have been made to achieve a room temperature (RT) ferromagnet. The outstanding deformation ability of two-dimensional (2D) materials provides an exciting way to mediate their intrinsic ferromagnetism (FM) with strain engineering. Here, we summarize the recent progress of strain engineering of intrinsic FM in 2D van der Waals materials. First, we introduce how to explain the strain-mediated intrinsic FM on Cr-based and Fe-based 2D van der Waals materials through ab initio Density functional theory (DFT), and how to calculate magnetic anisotropy energy (MAE) and Curie temperature (TC) from the interlayer exchange coupling J. Subsequently, we focus on numerous attempts to apply strain to 2D materials in experiments, including wrinkle-induced strain, flexible substrate bending or stretching, lattice mismatch, electrostatic force and field-cooling. Last, we emphasize that this field is still in early stages, and there are many challenges that need to be overcome. More importantly, strengthening the guideline of strain-mediated FM in 2D van der Waals materials will promote the development of spintronics and straintronics.

The Progress on Magnetic Material Thin Films Prepared Using Polymer-Assisted Deposition

Polymer-assisted deposition (PAD) has been widely used in the preparation of high-quality oxides and sulfides for basic research and applications. Specifically, diverse PAD-prepared magnetic material thin films such as ZnO, Ga₂O₃, SrRuO₃, LaCoO₃, LaMnO₃, Y₃Fe₅O₁₂, MoS₂, MoSe₂, and ReS₂ thin films have been grown, in which thickness-dependent, strain-modulated, doping-mediated, and/or morphology-dependent room-temperature ferromagnetism (RTFM) have been explored. Inspired by the discovery of intrinsic low-temperature FM in two-dimensional (2D) systems prepared using mechanical exfoliation, the search for more convenient methods to prepare 2D ferromagnetic materials with high-temperature FM has seen explosive growth, but with little success. Fortunately, the very recent synthesis of 2D NiO by PAD has shed light on this challenge. Based on these abovementioned developments, the difficulties of PAD when preparing a-few-nanometer single-crystalline materials and the opportunities in PAD for novel materials such as chiral magnetic soliton material Cr_1/3NbS₂ are discussed.

Recent Progress in Research on Ferromagnetic Rhenium Disulfide

Since long-range magnetic ordering was observed in pristine Cr₂Ge₂Te₆ and monolayer CrCl₃, two-dimensional (2D) magnetic materials have gradually become an emerging field of interest. However, it is challenging to induce and modulate magnetism in non-magnetic (NM) materials such as rhenium disulfide (ReS₂). Theoretical research shows that defects, doping, strain, particular phase, and domain engineering may facilitate the creation of magnetic ordering in the ReS₂ system. These predictions have, to a large extent, stimulated experimental efforts in the field. Herein, we summarize the recent progress on ferromagnetism (FM) in ReS₂. We compare the proposed methods to introduce and modulate magnetism in ReS₂, some of which have made great experimental breakthroughs. Experimentally, only a few ReS₂ materials exhibit room-temperature long-range ferromagnetic order. In addition, the superexchange interaction may cause weak ferromagnetic coupling between neighboring trimers. We also present a few potential research directions for the future, and we finally conclude that a deep and thorough understanding of the origin of FM with and without strain is very important for the development of basic research and practical applications.

Strain-Modulated Magnetism in MoS2

Since the experiments found that two-dimensional (2D) materials such as single-layer MoS2 can withstand up to 20% strain, strain-modulated magnetism has gradually become an emerging research field. However, applying strain alone is difficult to modulate the magnetism of single-layer pristine MoS2, but applying strain combined with other tuning techniques such as introducing defects makes it easier to produce and alter the magnetism in MoS2. Here, we summarize the recent progress of strain-dependent magnetism in MoS2. First, we review the progress in theoretical study. Then, we compare the experimental methods of applying strain and their effects on magnetism. Specifically, we emphasize the roles played by web buckles, which induce biaxial tensile strain conveniently. Despite some progress, the study of strain-dependent MoS2 magnetism is still in its infancy, and a few potential directions for future research are discussed at the end. Overall, a broad and in-depth understanding of strain-tunable magnetism is very necessary, which will further drive the development of spintronics, straintronics, and flexible electronics.