Two-dimensional Magnetic Materials Research Articles

Chemical Vapor Deposition of Two-Dimensional Magnetite Nanosheets and Raman Study of Heat-Induced Oxidation Reaction

Two-dimensional magnetic materials exhibiting antiferromagnetic properties at room temperature offer significant potential for developing next generation spintronic nanodevices. This work presents controlled synthesis of two-dimensional magnetite (Fe3O4) nanosheets utilizing lost cost and convenient chemical vapor deposition (CVD) technique, Raman spectroscopy study of laser-heating and annealing induced oxidation reaction. X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy, and Raman spectroscopy characterizations demonstrated that Fe3O4 nanosheets down to about 10 nm with good crystalline quality, surface uniformity, and minimal defects can be obtained by optimizing the growth parameters in our CVD synthesis. Both laser-heating and annealing studies of Fe3O4 nanosheets indicated that ferrimagnetic Fe3O4 nanosheets gradually change to antiferromagnetic α-Fe2O3 nanosheets at about 400 °C. Our study showed that Raman spectroscopy provides a convenient and powerful method for estimating heat-induced temperature and investigating oxidation reaction.

Modulating the electronic and optical properties of CrI3/In2Se3 van der Waals heterostructures by external fields

The van der Waals heterostructures (vdWH) composed of two-dimensional (2D) magnetic materials and non-magnetic atomic layers provide promising candidates for innovative device design. Here, we construct the CrI3/α-In2Se3 vdWH and regulate its band structure, magnetism and optical absorption through external fields. The intrinsic vdWH possesses type-Ⅱ band alignment with 0.38 eV band gap. As a result of proximity effects and superexchange interaction, magnetism is induced in the α-In2Se3 layer. The Curie temperature of the vdWH with FM coupling has increased to 99.3 K. Interestingly, with the increasing of the applied electric field, the band alignment exhibits a transition from type-Ⅱ to type-Ⅰ and then back to type-Ⅱ. When the electric field is −4.31 V/nm, the vdWH convert from semiconductor to half-metal. Moreover, the applying biaxial and vertical strain can influence the energy level of hybridization, and transferred charges. For example, the increased magnetic moment of the CrI3 monolayer increased by four times when the distance gap is decreased by 0.5 Å. These properties can offer novel strategies for engineering band structure and regulating magnetic properties, thereby enabling researchers to design tunable multiferroic vdWH devices.

Strain-induced high-Chern-number spin-unlocked edge states in monolayer MnAsO3 with intrinsic quantum anomalous Hall effect

Two-dimensional intrinsic magnetic topological materials that can realize device miniaturization have attracted significant attention recently based on their chiral dissipationless edge states. However, since the experimental observation of quantum anomalous Hall effect (QAHE) is still limited by low temperature, high operating temperature and large nontrivial gap are urgently needed. Here, monolayer MnAsO3 is predicted to be a room-temperature intrinsic magnetic topological material with high Chern number C = 3 based on first-principles calculations, which offers the possibility of achieving high-speed and low-energy-consumption electron transport in the future. Furthermore, the large and experimental feasible nontrivial gap up to 79.09 meV is obtained under compressive strain modulation. Moreover, the high-Chern-number topological phase transition and strain-induced spin-unlocked edge states are observed, indicating the possibility of tuning the electron transport of QAHE. All these findings suggest that monolayer MnAsO3 is a suitable and promising material for fabricating low-energy-consumption spintronics devices.

Orbital-Ordering Driven Simultaneous Tunability of Magnetism and Electric Polarization in Strained Monolayer VCl3

Two-dimensional (2D) van der Waals magnetic materials have promising and versatile electronic and magnetic properties in the 2D limit, indicating a considerable potential to advance spintronic applications. Theoretical predictions thus far have not ascertained whether monolayer VCl3 is a ferromagnetic (FM) or anti-FM monolayer; this also remains to be experimentally verified. We theoretically investigate the influence of potential factors, including C 3 symmetry breaking, orbital ordering, epitaxial strain, and charge doping, on the magnetic ground state. Utilizing first-principles calculations, we predict a collinear type-III FM ground state in monolayer VCl3 with a broken C 3 symmetry, wherein only the former two of three t 2g orbitals (a 1g, eg2π and eg1π ) are occupied. The atomic layer thickness and bond angles of monolayer VCl3 undergo abrupt changes driven by an orbital ordering switch, resulting in concomitant structural and magnetic phase transitions. Introducing doping to the underlying Cl atoms of monolayer VCl3 without C 3 symmetry simultaneously induces in- and out-of-plane polarizations. This can achieve a multiferroic phase transition if combined with the discovered adjustments of magnetic ground state and polarization magnitude under strain. The establishment of an orbital-ordering driven regulatory mechanism can facilitate deeper exploration and comprehension of magnetic properties of strongly correlated systems in monolayer VCl3.

Hard ferromagnetism in van der Waals Fe3GaTe2 nanoflake down to monolayer

Two-dimensional (2D) magnetic materials are of not only fundamental scientific interest but also promising candidates for numerous applications. However, so far only a few intrinsic magnets with long-ranged order down to the 2D limit have been experimentally established. Here, we report that the intrinsic 2D ferromagnetism can be realized in van der Waals (vdW) Fe3GaTe2 nanoflake down to monolayer. By measuring the Hall resistance and magnetoresistance, we demonstrate that the Fe3GaTe2 monolayer exhibits 2D hard ferromagnetism with record-high Cure temperature (Tc) of 240 K for the monolayer of known intrinsic ferromagnets. Both of square-shaped hysteresis loops with near-vertical jump in anomalous Hall effect (AHE) and the negative magnetoresistance (NMR) behavior with an applied out-of-plane magnetic field reveal robust perpendicular magnetic anisotropy (PMA) in Fe3GaTe2 nanoflakes down to the monolayer limit. Furthermore, we find the intrinsic mechanism that stems from the Berry curvature of electronic bands dominates AHE of nanoflakes in the low temperature range. Our results not only provide an excellent candidate material for next-generation spintronic applications, but also open up a platform for exploring physical mechanisms in 2D ferromagnetism.

Ultra-thin ferromagnets with large magnetic anisotropy by assembling MnCl3 superatoms on SbAs monolayer

Two-dimensional (2D) magnetic materials with large magnetic anisotropy are crucial for developing thermally stable, miniaturized, and low-power spintronic devices. However, naturally existing 2D materials with intrinsic anisotropic magnetization are scarce. Employing a bottom-up primitive assembly approach and first-principles calculations, we find that the adsorption of MnCl3 superatoms not only induces magnetism in non-magnetic monolayer SbAs but also hardens it (large out-of-plane magnetic anisotropy). The MnCl3/SbAs system becomes a ferromagnetic half-metal with high Curie temperature. Interestingly, the delocalized p orbitals of Sb/As not only mediate the long-range magnetic exchange but also contribute to the significant magnetic anisotropic energy. The magnetic exchange and anisotropy of the system can be efficiently modulated by changing the assembly mode of MnCl3 on the SbAs surface. This study presents an alternative concept and method for the development of 2D ferromagnetic materials with uniaxial magnetic anisotropy, which holds promise for spintronic device applications.

Magnon–phonon coupling modulation via dimensional reduction in thin antiferromagnet MnPSe3 nanoribbons

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.

Magnetic phase tuning in patronite-based VS4 nanowires

Magnetic semiconductors have the potential to push the boundaries of emerging technologies like spintronics. They could change the way we process and store data by leveraging not just the electron charges, but also their intrinsic spin. Materials with low dimensionality, specifically one- or two-dimensional materials, are key to this advancement. Within this category, one-dimensional semiconductors with either antiferromagnetic or ferromagnetic properties are particularly interesting for their electronic and magnetic properties. However, creating and maintaining stable ferromagnetic ordering in these semiconductors is not straightforward. There are significant challenges, such as issues due to the disorder of dopants in dilute magnetic semiconductors, or stability problems in two-dimensional magnetic materials like chromium triiodide (CrI3). In this work, we show a potential solution in VS4 nanowires, a one-dimensional material found in patronite minerals. These nanowires can have four magnetic orderings: two antiferromagnetic (AFM1 and AFM2), one ferrimagnetic (FI), and one ferromagnetic (FM) ordering, which we can access using different strains. We observe two magnetic phase transitions with strains in the plastic (non-linear) regime. The nanowire transitions from AFM1 to AFM2 with ɛz=6.8 %, and from AFM2 to FM with ɛz=11.8 %. This ability to tune the magnetic phases in a semiconductor material opens up a vast range of opportunities for their integration into spintronic devices. We believe that the range of magnetic phases in a one-dimensional semiconductor, like VS4 nanowires, could lead to the enhanced control of both charge and spin in nanotechnology devices.

Symmetry and magnetic direction dependent spin/valley splitting and anomalous Hall conductivity of antiferromagnetic monolayer MnPTe3

Due to the time reversal symmetry breaking or spin-valley coupling, there are intrinsic valley splittings in two-dimensional magnetic materials, which are closely related to the symmetry and magnetic order. Based on MnPTe3 monolayers with different symmetries, we present an understanding of direction dependent spin/valley splitting and anomalous Hall conductivity from the perspective of orbital interactions. Unlike T-MnPTe3 with central inversion symmetry, in-plane magnetization leads to further non-degeneracy of valley electrons of H–MnPTe3, and the splitted valley states are spin polarized, which is perpendicular to the in-plane magnetization direction. The model and first-principle calculations consistently indicate that the splitting of the valley states is almost proportional to sinθxz, which originates from the difference in spin-up and spin-down interactions introduced by the breaking of spatial inversion symmetry. Regardless of spin degeneracy, non-zero anomalous Hall conductivity occurs with out-of-plane magnetization, and this direction dependent anomalous Hall effect is caused by mirror symmetry breaking from the neighboring Te atoms and spin-orbit coupling. Magnetization direction is an important means of regulating valley/spin splittings and anomalous Hall effect, which has certain significance for realizing spin or valley polarized Hall effect and the research of spintronic or valleytronic devices.

Large tunneling magnetoresistance and its high bias stability in Weyl half-semimetal based lateral magnetic tunnel junctions

Magnetic tunnel junctions (MTJs) based on novel states of two-dimensional (2D) magnetic materials will significantly improve the value of the tunneling magnetoresistance (TMR) ratio. However, most 2D magnetic materials exhibit low critical temperatures, limiting their functionality to lower temperatures rather than room temperature. Moreover, most MTJs experience the decay of TMR ratio at large bias voltages within a low spin injection efficiency (SIE). Here, we construct a series of MTJs with Weyl half-semimetal (WHSM, e.g. MnSiS3, MnSiSe3, and MnGeSe3 monolayers) as the electrodes and investigate the spin-dependent transport properties in these kind of lateral heterojunctions by employing density functional theory combined with non-equilibrium Green’s function method. We find that an ultrahigh TMR (∼109%) can be obtained firmly at a small bias voltage and maintains a high SIE even at a large bias voltage, and MnSiSe3 monolayer is predicted to exhibit a high critical temperature. Additionally, we reveal that the same structure allows for the generation of fully spin-polarized photocurrent, irrespective of the polarization angle. These findings underscore the potential of WHSMs as candidate materials for high-performance spintronic devices.

Turning Nonmagnetic Two-Dimensional Molybdenum Disulfides into Room-Temperature Ferromagnets by the Synergistic Effect of Lattice Stretching and Charge Injection.

Exploring two-dimensional (2D) room-temperature magnetic materials in the field of 2D spintronics remains a formidable challenge. The vast array of nonmagnetic 2D materials provides abundant resources for exploration, but the strategy to convert them into intrinsic room-temperature magnets remains elusive. To address this challenge, we present a general strategy based on surface halogenation for the transition from nonmagnetism to intrinsic room-temperature ferromagnetism in 2D MoS2 based on first-principles calculations. The derived 2D halogenated MoS2 are half-semimetals with a high Curie temperature (TC) of 430-589 K and excellent stability. In-depth mechanistic studies revealed that this marvelous nonmagnetism-to-ferromagnetism transition originates from the modulation of the splitting as well as the occupation of the Mo d orbitals by the synergy of lattice stretching and charge injection induced by the surface halogenation. This work establishes a promising route for exploring 2D room-temperature magnetic materials from the abundant pool of 2D nonmagnetic counterparts.

Electric control of optically-induced magnetization dynamics in a van der Waals ferromagnetic semiconductor

Electric control of magnetization dynamics in two-dimensional (2D) magnetic materials is an essential step for the development of novel spintronic nanodevices. Electrostatic gating has been shown to greatly affect the static magnetic properties of some van der Waals magnets, but the control over their magnetization dynamics is still largely unexplored. Here we show that the optically-induced magnetization dynamics in the van der Waals ferromagnet Cr2Ge2Te6 can be effectively controlled by electrostatic gates, with a one order of magnitude change in the precession amplitude and over 10% change in the internal effective field. In contrast to the purely thermally-induced mechanisms previously reported for 2D magnets, we find that coherent opto-magnetic phenomena play a major role in the excitation of magnetization dynamics in Cr2Ge2Te6. Our work sets the first steps towards electric control over the magnetization dynamics in 2D ferromagnetic semiconductors, demonstrating their potential for applications in ultrafast opto-magnonic devices.

Nonvolatile Multistate Manipulation of Topological Magnetism in Monolayer CrI3 through Quadruple-Well Ferroelectric Materials.

Nonvolatile multistate manipulation of two-dimensional (2D) magnetic materials holds promise for low dissipation, highly integrated, and versatile spintronic devices. Here, utilizing density functional theory calculations and Monte Carlo simulations, we report the realization of nonvolatile and multistate control of topological magnetism in monolayer CrI3 by constructing multiferroic heterojunctions with quadruple-well ferroelectric (FE) materials. The Pt2Sn2Te6/CrI3 heterojunction exhibits multiple magnetic phases upon modulating FE polarization states of FE layers and interlayer sliding. These magnetic phases include Bloch-type skyrmions and ferromagnetism, as well as a newly discovered topological magnetic structure. We reveal that the Dzyaloshinskii-Moriya interaction (DMI) induced by interfacial coupling plays a crucial role in magnetic skyrmion manipulation, which aligns with the Fert-Levy mechanism. Moreover, a regular magnetic skyrmion lattice survives when removing a magnetic field, demonstrating its robustness. The work sheds light on an effective approach to nonvolatile and multistate control of 2D magnetic materials.

Quantum-state engineering in cavity magnomechanics formed by two-dimensional magnetic materials

Cavity magnomechanics has become an ideal platform to explore macroscopic quantum effects. Bringing together magnons, phonons, and photons in a system, it opens many opportunities for quantum technologies. It was conventionally realized by an yttrium iron garnet, which exhibits a parametric magnon–phonon coupling mˆ†mˆ(bˆ†+bˆ) , with mˆ and bˆ being the magnon and phonon modes. Inspired by the recent realization of two-dimensional (2D) magnets, we propose a cavity magnomechanical system using a 2D magnetic material with both optical and magnetic drivings. It features the coexisting photon–phonon radiation-pressure coupling and quadratic magnon–phonon coupling mˆ†mˆ(bˆ†+bˆ)2 induced by the magnetostrictive interaction. A stable squeezing of the phonon and bi- and tri-partite entanglements among the three modes are generated in the regimes with a suppressed phonon number. Compared with previous schemes, ours does not require any extra nonlinear interaction and reservoir engineering and is robust against the thermal fluctuation. Enriching the realization of cavity magnomechanics, our system exhibits its superiority in quantum-state engineering due to the versatile interactions enabled by its 2D feature.

First-principles study of bilayer hexagonal structure CrN2 nanosheets: A ferromagnetic semiconductor with high Curie temperature and tunable electronic properties

Two-dimensional magnetic materials have been increasingly studied and discussed in the field of spintronics due to their unique electronic properties, high spin polarizability, and a variety of magnetic properties. In this paper, we report a new two-dimensional bilayer hexagonal monolayer material bilayer hexagonal structure (BHS)-CrN2 by first-principles calculations. The BHS-CrN2 nanosheet is an intrinsic ferromagnetic semiconductor material, and the Curie temperature obtained by Monte Carlo simulation is 343 K. The absence of a significant imaginary frequency in the phonon spectrum indicates the dynamic stability of BHS-CrN2. After ab initio molecular dynamics simulation, the supercell of BHS-CrN2 remains a complete structure, indicating its thermal stability. The calculated elastic moduli satisfy the Born–Huang criterion, indicating that the BHS-CrN2 system has good mechanical stability. Interestingly, the compressive strain and O atom doping can transform the electronic structure of BHS-CrN2 from a semiconductor to a half-metal, and the Curie temperature of BHS-CrN2 can be further increased to 1059 K when a 5% tensile strain is applied. Furthermore, the BHS-CrN2 in the ferromagnetic state shows a significant in-plane magnetic anisotropy energy of 0.01 meV per Cr, and the CrP2 and CrAs2 show a large out-of-plane magnetic anisotropy energy of 0.207 and 0.988 meV per Cr, respectively. The results show that the intrinsic ferromagnetic semiconductor BHS-CrN2 has good stability, high Curie temperature, and tunable magnetic properties, which is a promising material for room-temperature spintronic devices.

Two-dimensional magnetic materials for spintronic applications

Two-dimensional magnetic materials for spintronic applications

Synthesis Methods and Property Control of Two-Dimensional Magnetic Materials

Two-dimensional (2D) magnetic materials have been demonstrated to have excellent chemical, optical, electrical, and magnetic properties, particularly in the development of multifunctional electronic and spin electronic devices, showcasing tremendous potential. Therefore, corresponding synthesis techniques for 2D magnetic materials that offer high quality, high yield, low cost, time-saving, and simplicity are highly desired. This review provides a comprehensive overview of recent research advances in preparation of magnetic 2D materials, with a particular focus on the preparation methods employed. Moreover, the characteristics and applications of these magnetic materials are also discussed. Finally, the challenges and prospects of synthesis methods for magnetic 2D materials are briefly addressed. This review serves as a guiding reference for the controlled synthesis of 2D magnetic materials.

Optical transparency in 2D ferromagnetic WSe2/1T-VSe2/WSe2 multilayer with strain induced large anomalous Nernst conductivity

Transparent two-dimensional (2D) magnetic materials may bring intriguing features and are indispensable for transparent electronics. However, it is rare to find both optical transparency and room-temperature ferromagnetism simultaneously in a single 2D material. Herein, we explore the possibility of both these features in 2D WSe2/1T-VSe2 (1ML)/WSe2 and WSe2/1T-VSe2 (2ML)/WSe2 heterostructures by taking one monolayer (1ML) and two monolayers (2ML) of 1T-VSe2 using first-principles calculations. Further, we investigate anomalous Hall conductivity (AHC) and anomalous Nernst conductivity (ANC) using a maximally localized Wannier function. The WSe2/1T-VSe2 (1ML)/WSe2 and WSe2/1T-VSe2 (2ML)/WSe2 systems show Curie temperatures of 328 and 405 K. Under biaxial compressive strain, the magnetic anisotropy of both systems is switched from in-plane to out-of-plane. We find a large AHC of 1.51 e2/h and 3.10 e2/h in the electron-doped region for strained WSe2/1T-VSe2 (1ML)/WSe2 and WSe2/1T-VSe2 (2ML)/WSe2 systems. Furthermore, we obtain a giant ANC of 3.94 AK−1 m−1 in a hole-doped strained WSe2/1T-VSe2 (2ML)/WSe2 system at 100 K. Both WSe2/1T-VSe2 (1ML)/WSe2 and WSe2/1T-VSe2 (2ML)/WSe2 are optically transparent in the visible ranges with large refractive indices of 3.2–3.4. Our results may suggest that the WSe2/1T-VSe2/WSe2 structure possesses multifunctional physical properties and these features can be utilized for spintronics and optoelectronics device applications such as magnetic sensors, memory devices, and transparent magneto-optic devices at room temperature.

Exploration of the Two-Dimensional Ising Magnetic Materials in the Triangular Prismatic Crystal Field

Exploration of the Two-Dimensional Ising Magnetic Materials in the Triangular Prismatic Crystal Field

Spintronic devices based on topological and two-dimensional materials

Novel quantum materials such as topological materials, two-dimensional materials, create new opportunities for the spintronic devices. These materials can improve the charge-spin conversion efficiency, provide high-quality interface, and enhance the energy efficiency for spintronic devices. In addition, they have rich interactions and coupling effects, which provides a perfect platform for finding new physics and novel methods to control the spintronic properties. Many inspiring results have been reported regarding the research on topological materials and two-dimensional materials, especially the layered topological and two-dimensional magnetic materials, and their heterostructures. This paper reviews the recent achievements of these novel quantum materials on spintronic applications. Firstly the breakthroughs that topological materials have been made in spin-orbit torque devices is introduced, then two-dimensional magnetic materials and their performances in spintronic devices are presented, finally the research progress of topological materials/two-dimensional magnetic materials heterostructures is discussed. This review can help to get a comprehensive understanding of the development of these novel quantum materials in the field of spintronics and inspire new ideas of research on these novel materials.