Room-temperature ferromagnetism of chromium-doped molybdenum disulfide synthesized via chemical vapor deposition

Two-dimensional (2D) magnetic materials have triggered tremendous interest in recent years due to their remarkable potential applications in magnetic storage and spintronics devices. Heat dissipation is of great significance in stability and durability of increasingly integrated magnetic devices. However, little investigation of thermal transport has been carried out in 2D magnetic materials and a comprehensive understanding of the underlying mechanism is still lacking. We experimentally demonstrate the thermal conductivity measurement of MnPSe3 nanoribbons and find a nonmonotonic thickness dependence, which is attributed to the phonon confinement effect in thin nanoribbons. The peaks of measured thermal conductivity are found to be modified with increasing thickness due to the magnon–phonon coupling. We propose that the magnon–phonon scattering rate increases with increasing thickness and causes a huge suppression in thermal conductivity. This study will deepen the understanding of the thermal properties of 2D magnetic materials and will benefit thermal management in designing magnetic devices.

It is currently technologically important to predict new two-dimensional (2D) ferromagnetic materials for next-generation information storage media. However, discovered 2D ferromagnetic materials are still rare. Here, we explored the fact that 2D transition metal borides are potential room-temperature 2D ferromagnetic materials. By performing first-principles calculations, we found that the CrB monolayer is a ferromagnetic (FM) metal, while the FeB monolayer is a typically antiferromagnetic (AFM) semiconductor. Interestingly, both CrB and FeB monolayers are FM metals with a moderate magnetic anisotropy energy by saturating with functional groups. Monte Carlo simulations show that the Curie temperature (Tc) of the CrB monolayer is about 520 K, which is further increased to 580 K and 570 K through -F and -OH chemical modification, while Tc is about 250 K, 275 K and 300 K for the FeBF, FeBO and FeBOH monolayer, respectively. Thus, the 2D transition metal borides have great potential applications in information storage devices.

Two-dimensional (2D) magnetic materials with magnetic anisotropy can form magnetic order at finite temperature and monolayer limit. Their macroscopic magnetism is closely related to the number of layers and stacking forms, and their magnetic exchange coupling can be regulated by a variety of external fields. These novel properties endow 2D magnetic materials with rich physical connotation and potential application value, thus having attracted extensive attention. In this paper, the recent advances in the experiments and theoretical calculations of 2D magnets are reviewed. Firstly, the common magnetic exchange mechanisms in several 2D magnetic materials are introduced. Then, the geometric and electronic structures of some 2D magnets and their magnetic coupling mechanisms are introduced in detail according to their components. Furthermore, we discuss how to regulate the electronic structure and magnetism of 2D magnets by external (field modulation and interfacial effect) and internal (stacking and defect) methods. Then we discuss the potential applications of these materials in spintronics devices and magnetic storage. Finally, the encountered difficulties and challenges of 2D magnetic materials and the possible research directions in the future are summarized and prospected.

Recent innovations in 2D magnetic materials and their potential applications in the modern era

A review on two-dimensional (2D) magnetic materials and their potential applications in spintronics and spin-caloritronic

Two-dimensional (2D) ferromagnetic (FM) materials have great potential for applications in next-generation spintronic devices. Since most 2D FM materials come from van der Waals crystals, stabilizing them on a certain substrate without killing the ferromagnetism is still a challenge. Through systematic first-principles calculations, we proposed a new family of 2D FM materials which combines TaX (X = S, Se or Te) monolayer and Al2O3(0001) substrate. The TaX monolayers provide magnetic states and the Al2O3(0001) substrate stabilizes the former. Interestingly, the Al2O3(0001) substrate leads to a metal-to-insulator transition in the TaX monolayers and induces a band gap up to 303 meV. Our study paves the way to explore promising 2D FM materials for practical applications in spintronics devices.

Two-dimensional (2D) ferromagnetic materials (FMs) are potentially the material foundation for future spintronics devices. However, at present, the Curie temperature (TC) of most 2D FM is relatively low and cannot meet the need for practical applications. Nowadays, CrTe2 thin films grown by molecular beam epitaxy (MBE) are reported to be room-temperature ferromagnetic only on graphene substrate instead of 3D substrates. In this work, we report high-quality Bi-doped CrTe2 (BixCr1−xTe2) thin films grown on conventional substrates of GaAs(111)B by MBE. Magnetotransport measurements reveal strong ferromagnetism of all the films, with out-of-plane magnetic anisotropy. More importantly, as more Bi atoms are doped into the film, the Curie temperature increases and reaches 305 K at x = 0.1. This improvement is a step forward for its application in spintronics and other fields.

Two-dimensional (2D) materials with large linear magnetoresistance (LMR) are very interesting owing to their potential application in magnetic storage or sensor devices. Here, we report the synthesis of 2D MoO2 nanoplates grown by a chemical vapor deposition (CVD) method and observe large LMR and nonlinear Hall behavior in MoO2 nanoplates. As-obtained MoO2 nanoplates exhibit rhombic shapes and high crystallinity. Electrical studies indicate that MoO2 nanoplates feature a metallic nature with an excellent conductivity of up to 3.7 × 107 S m-1 at 2.5 K. MoO2 nanoplates display a large LMR of up to 455% at 3 K and -9 T. A thickness-dependent LMR analysis suggests that LMR values increase upon increasing the thickness of nanoplates. Besides, nonlinearity has been found in the magnetic-field-dependent Hall resistance, which decreases with increasing temperatures. Our studies highlight that MoO2 nanoplates are promising materials for fundamental studies and potential applications in magnetic storage devices.

Intrinsic two-dimensional (2D) magnetic materials with room-temperature ferromagnetism and air stability are highly desirable for spintronic applications. However, the experimental observations of such 2D or ultrathin ferromagnetic materials are rarely reported owing to the scarcity of these materials in nature and for the intricacy in their synthesis. Here, we report a successful controllable growth of ultrathin γ-Fe2O3 nanoflakes with a variety of morphologies tunable by the growth temperature alone using a facile chemical vapor deposition method and demonstrate that all ultrathin nanoflakes still show intrinsic room-temperature ferromagnetism and a semiconducting nature. The γ-Fe2O3 nanoflakes epitaxially grown on α-Al2O3 substrates take a triangular shape at low temperature and develop gradually in lateral size, forming eventually a large-scale γ-Fe2O3 thin film as the growth time increases due to a thermodynamic control process. The morphology of the nanoflakes could be tuned from triangular to stellated, petaloid, and dendritic crystalloids in sequence with the rise of precursor temperature, revealing a growth process from thermodynamically to kinetically dominated control. Moreover, the petaloid and dendritic nanoflakes exhibit enhanced coercivity compared with the triangular and stellated nanoflakes, and all the nanoflakes with diverse shapes possess differing electrical conductivity. The findings of such ultrathin, air-stable, and room-temperature ferromagnetic γ-Fe2O3 nanoflakes with tunable shape and multifunctionality may offer guidance in synthesizing other non-layered magnetic materials for next-generation electronic and spintronic devices.

In this project, two-dimensional (2D) materials are classified into the categories 2D magnetic materials and 2D non-magnetic materials. The 2D magnetic materials are further classified into two categories, namely 2D ferromagnetic materials and 2D anti-ferromagnetic materials. It is important to identify 2D ferromagnetic materials because they are used in various important applications such as data storage mechanisms and spintronic devices. Raw data from the 2D material databases such as 2DMatpedia was used in the project. Classical machine learning involving algorithms such as decision trees and support vector machines were used to classify the data obtained from the above databases into the respective categories anti-ferromagnetic, ferromagnetic and non-magnetic materials. Techniques such as deep learning and convolutional neural networks were also employed to classify the data. The best results for classification of the two-dimensional data into magnetic and non-magnetic materials were obtained for the classical machine learning algorithm, gradient boost algorithm with an accuracy of 93.37%. The most important features of the materials for determining the magnetic properties were band gap, number of magnetic atoms, maximum ionic character, presence of chromium and presence of manganese in that order. The magnetic materials classified were further classified into ferromagnetic and anti-ferromagnetic materials. The best accuracy for this process was obtained to be 94.44% using the classical machine learning algorithm, random forest classifier. The average ionic character, maximum average ionic character, yang-omega, and band-center were the key features for determining whether a material is ferromagnetic or anti-ferromagnetic. A web interface was deployed for the model and can be found here: https://twodferromagnetism-model.herokuapp.com. The code used for obtaining the results can be found here: https://github.com/RiyaBOT/ME4101A-ShethRiyaNimish.

In spintronics, there has been increasing interest in two-dimensional (2D) magnetic materials. The well-defined layered crystalline structure, interface conditions, and van der Waals stacking of these materials offer advantages for the development of high-performance spintronic devices. Spin-orbit torque (SOT) devices and the tunneling magnetoresistance (TMR) effect based on these materials have emerged as prominent research areas. SOT devices utilizing 2D magnetic materials can efficiently achieve SOT-driven magnetization switching by modulating the interaction between spin and orbital degrees of freedom. Notably, crystal structure symmetry breaking in 2D magnetic heterojunctions leads to field-free perpendicular magnetization switching and an extremely low SOT-driven magnetization switching current density of down to 106 A/cm2. This review provides a comprehensive overview of the construction, measurement, and mechanisms of 2D SOT heterojunctions. The TMR effect observed in 2D materials also exhibits significant potential for various applications. Specifically, the spin-filter effect in layered A-type antiferromagnets has led to giant TMR ratios approaching 19,000%. Here, we review the physical mechanisms underlying the TMR effect, along with the design of high-performance devices such as magnetic tunnel junctions (MTJ) and spin valves. This review summarizes different structural types of 2D heterojunctions and key factors that enhance TMR values. These advanced devices show promising prospects in fields such as magnetic storage. We highlight significant advancements in the integration of 2D materials in SOT, MTJ, and spin valve devices, which offer advantages such as high-density storage capability, low-power computing, and fast data transmission rates for Magnetic Random Access Memory and logic integrated circuits. These advancements are expected to revolutionize future developments in information technology.

Some recent experiments reported real two-dimensional (2D) ferromagnetic materials (FM) at nonzero Curie temperature. Given the vast material phase-space of 2D van der Waals heterostructures, it will be very exciting to search for 2D FMs with room or higher Curie temperatures. Here, the authors perform a systematic search by screening the nearly 200,000 material entries in the Inorganic Crystal Structure Database (ICSD). The search not only rediscovered the recent experimentally found 2D FMs, but also identified several new 2D FM candidates including one having above-room Curie temperature.

Recent theoretical and experimental discoveries of two-dimensional (2D) ferromagnetic (FM) materials have sparked intense interest for their potential applications in spintronics. 2D FM materials with high spin polarization are extremely desirable for future low-dimensional spintronics. Half-metallicity plays a key role in the development of such devices. Here, we reported a new 2D nanomagnet VClI2 using the first-principles based density functional theory calculations. VClI2 shows an exciting half-metallic character with a wide half-metallic gap of 0.4 eV. The ground state favors ferromagnetic coupling with a Curie temperature Tc of 21 K. The half-metallicity with a FM ground state is further achieved by the application of an external strain and by the combined effects of the strain and the electric field. A phase transition from a half-metallic → semiconductor → metal was further observed under different stimuli with an antiferromagnetic ground state. At Ez=7.5 V/nm and in the presence of η=5% strain, the calculated Tc is estimated at 35 K, which shows a 67% increment than the Tc observed in the unstrained condition. The fascinating and unique properties suggest that VClI2 is a promising two-dimensional ferromagnetic half-metal, which can be useful for applications in future memory devices to enrich the 2D magnetic materials library.

Ferromagnetic semiconductors (FMS) enable simultaneous control of both charge and spin transport of charge carriers, which have emerged as a class of highly desirable but rare material for applications in spin field-effect transistors and quantum computing. Organic-inorganic hybrid perovskite with high compositional adjustability and structural versatility can offer unique benefits in the design of FMS but has not been fully explored. Here we demonstrate a series of molecular FMSs based on two-dimensional organic-inorganic hybrid perovskite structure, namely (2ampy)CuCl4 , (3ampy)CuCl4 and (4ampy)CuCl4 , which exhibits high saturation magnetization, dramatic temperature-dependent conductivity change and tunable ferromagnetic resonance. Magnetic measurements revealed high saturation magnetization up to 18.56 emu/g for (4ampy)CuCl4 , which is one of the highest value among reported hybrid FMSs to date. Conductivity studies of the three FMSs demonstrate that the smaller adjacent octahedron distance in the two-dimensional layer results in higher conductivity. Systematic ferromagnetic resonance investigation shows that the gyromagnetic ratio and Landau factor values are strongly dependent on the types of organic cations used. This work demonstrates that two-dimensional hybrid perovskite materials can simultaneously possess both tunable long-range ferromagnetic ordering and semiconductivity, providing a straightforward strategy for designing and synthesizing high-performance intrinsic FMSs. This article is protected by copyright. All rights reserved.

Nonlinear optics, a critical branch of modern optics, presents unique potential in the study of two-dimensional (2D) magnetic materials. These materials, characterized by their ultra-thin geometry, long-range magnetic order, and diverse electronic properties, serve as an exceptional platform for exploring nonlinear optical effects. Under strong light fields, 2D magnetic materials exhibit significant nonlinear optical responses, enabling advancements in novel optoelectronic devices. This paper outlines the principles of nonlinear optics and the magnetic structures of 2D materials, reviews recent progress in nonlinear optical studies, including magnetic structure detection and nonlinear optical imaging, and highlights their role in probing magnetic properties by combining second harmonic generation (SHG) and multispectral integration. Finally, we discuss the prospects and challenges for applying nonlinear optics to 2D magnetic materials, emphasizing their potential in next-generation photonic and spintronic devices.