Magnetic Two-dimensional Materials Research Articles

Electrical detection of spin pumping in van der Waals ferromagnetic Cr2Ge2Te6 with low magnetic damping

The discovery of magnetic order in atomically-thin van der Waals materials has strengthened the alliance between spintronics and two-dimensional materials. An important use of magnetic two-dimensional materials in spintronic devices, which has not yet been demonstrated, would be for coherent spin injection via the spin-pumping effect. Here, we report spin pumping from Cr2Ge2Te6 into Pt or W and detection of the spin current by inverse spin Hall effect. The magnetization dynamics of the hybrid Cr2Ge2Te6/Pt system are measured, and a magnetic damping constant of ~ 4–10 × 10−4 is obtained for thick Cr2Ge2Te6 flakes, a record low for ferromagnetic van der Waals materials. Moreover, a high interface spin transmission efficiency (a spin mixing conductance of 2.4 × 1019/m2) is directly extracted, which is instrumental in delivering spin-related quantities such as spin angular momentum and spin-orbit torque across an interface of the van der Waals system. The low magnetic damping that promotes efficient spin current generation together with high interfacial spin transmission efficiency suggests promising applications for integrating Cr2Ge2Te6 into low-temperature two-dimensional spintronic devices as the source of coherent spin or magnon current.

Controllable synthesis of two-dimensional magnetic FeS nanoplates with high Curie temperature and unique magnetotransport properties

The discovery of magnetic two-dimensional materials has ignited intensive research interest for the potential development in innovative spintronic devices. Herein, ultrathin FeS nanoplates with various thicknesses are obtained through a chemical vapor deposition method and their magnetic properties are comprehensively studied. The obtained material displays trapezoidal shapes on SiO2/Si substrate and triangular shapes on large-area WSe2 nanosheets. A high degree of crystallinity and flat surface of FeS nanoplates are verified by Raman spectra, X-ray photoelectron spectroscopy analysis, X-ray diffraction patterns and annular dark-field aberration-corrected scanning transmission electron microscopy. Then magneto-transport studies illustrate that the resistance displays a bowtie shape after sweeping a full cycle of magnetic fields, which shows a decreasing trend with an increasing temperature but persists to 300 K. Thickness-dependent analysis demonstrates that the bowtie shape of resistance gradually reduces as the thickness of nanoplates increases. Angle-dependent measurements reveal that resistance shows apparent anisotropic properties. At last, vibrating sample magnetometer results show as-grown FeS nanoplates feature magnetism at room temperature. Our study demonstrates that FeS nanoplates grown by chemical vapor deposition methods provide an ideal system to research magnetic properties at the two-dimensional limit, providing new opportunities for the next-generation spintronic devices.

Spin transport property of Cr2C based nanoscale devices: A first principle study

Edge states and interfacial design are important influencing factors for spin transport property modulation of nanoscale devices. Electronic structure and spin charge transmission path of both armchair/zigzag Cr2C nanoribbon and single-porphyrin molecular junctions with Cr2C nanoribbon electrodes are studied based on spin polarized density functional theory and non-equilibrium Green's function method. The results show that both zigzag and armchair oriented Cr2C nanoribbons have intrinsic magnetic half-metallic behavior. Volt-ampere characteristics of zigzag Cr2C nanodevices are linear, and that of armchair Cr2C nanodevice shows obvious saturation effect and negative differential resistance effect (NDR). These two cases maintain both strong spin polarization current intensity and 100% spin filtering efficiency (SFE). In addition, the intercalation of porphyrin molecule significantly reduces electron accumulation near the Fermi level in case of armchair Cr2C nanodevice which retains 100% spin polarization behavior. This work expands a potential application of intrinsic magnetic two-dimensional materials in field of molecular spintronic devices.

Interlayer Coupling Induced Sharp Increase of the Curie Temperature in a Two-Dimensional MnSn Multilayer.

The MnSn monolayer synthesized recently is a novel two-dimensional ferromagnetic material with a hexagonal lattice, in which three Mn atoms come together to form a trimer, making it remarkably different from other magnetic two-dimensional materials. Most impressively, there occurs a sharp increase of the Curie temperature from 54 to 225 K when the number of layers increases from 1 to 3. However, no quantitative explanation has been reported in previous studies. Herein, by means of the first-principles calculation method and the Monte Carlo method, we demonstrate that strong interlayer ferromagnetic coupling plays an essential role in enhancing its critical temperature, which acts as a magnetic field to stabilize the ferromagnetism in MnSn multilayers. Our work not only explains the sharp increase of the Curie temperature of the MnSn film in experiments but also reveals that the interlayer coupling is a new routine to achieve high-temperature ferromagnetism in two-dimensional materials.

Mott transition and superexchange mechanism in magnetically doped XSi2N4 caused by large 3d orbital onsite Coulomb interaction

Magnetic two-dimensional materials have great potential in the preparation of nanoelectronic and spintronic devices. Recently, a new two-dimensional material with no corresponding bulk-structure, ${\mathrm{MoSi}}_{2}{\mathrm{N}}_{4}$, has been prepared, attracting public attention to the ${XA}_{2}{Z}_{4}$ series of materials. Naturally, doping with $3d$ magnetic element will result in various magnetic orderings. In our work, we replaced the Mo element with different transition-metal elements whose property varies from magnetic to nonmagnetic and screened out a variety of two-dimensional exchange scenarios. Then, we provided an analysis that shows the existence of a huge on-site Coulomb interaction at $3d$ orbitals which is strongly related to the Mott transition and therefore causes the superexchange interaction.

Achieving High-Temperature Ferromagnetism by Means of Magnetic Ion Dimerization in the Graphene-like Mn2N6C6 Monolayer

Magnetic two-dimensional materials have potential application in next-generation electronic devices and have stimulated extensive interest in condensed matter physics and material fields. However, how to realize high-temperature ferromagnetism in two-dimensional materials remains a great challenge in physics. Herein, we propose an effective approach that the dimerization of magnetic ions in two-dimensional materials can enhance the exchange coupling and stabilize the ferromagnetism. Manganese carbonitride Mn$_2$N$_6$C$_6$ with a planar monolayer structure is taken as an example to clarify the method, in which two Mn atoms are gathered together to form a ferromagnetic dimer of Mn atoms and further these dimers are coupled together to form the overall ferromagnetism of the two-dimensional material. In Mn$_2$N$_6$C$_6$ monolayer, the near-room-temperature ferromagnetism with the Curie temperature of 272.3 K is determined by solving Heisenberg model using Monte Carlo simulations method.

Iron-rich talc as air-stable platform for magnetic two-dimensional materials

Intrinsically magnetic layered materials – especially monolayers – suffer from the lack of ambient stability and mostly exhibit magnetic ordering only at cryogenic temperatures. These restrains impose a great challenge for the integration of two-dimensional magnetic materials into future technologies. We propose to overcome this by exploiting phyllosilicates, such as iron-rich talc. Via combined magnetic force microscopy in applied external magnetic fields, superconducting quantum interference device magnetometry, first-principle calculations, and structural analysis, we demonstrate that incorporated iron ions in talc are in a very robust high spin state, resulting in a weak ferromagnetic behavior at room temperature. Iron-rich talc can be thinned down to a monolayer, remaining fully stable under ambient conditions, and retaining magnetic properties even in monolayers. Finally, we propose iron-rich end members of the phyllosilicates as very promising platforms for air-stable magnetic monolayers.

Synthesis, engineering, and theory of 2D van der Waals magnets

The recent discovery of magnetism in monolayers of two-dimensional van der Waals materials has opened new venues in materials science and condensed matter physics. Until recently, two-dimensional magnetism remained elusive: Spontaneous magnetic order is a routine instance in three-dimensional materials but it is not a priori guaranteed in the two-dimensional world. Since the 2016 discovery of antiferromagnetism in monolayer FePS3 by two groups and the subsequent demonstration of ferromagnetic order in monolayer CrI3 and bilayer Cr2Ge2Te6, the field changed dramatically. Within several years of scientific discoveries focused on 2D magnets, novel opportunities have opened up in the field of spintronics, namely spin pumping devices, spin transfer torque, and tunneling. In this review, we describe the state of the art of the nascent field of magnetic two-dimensional materials focusing on synthesis, engineering, and theory aspects. We also discuss challenges and some of the many different promising directions for future work, highlighting unique applications that may extend even to other realms, including sensing and data storage.

Data-driven studies of magnetic two-dimensional materials

We use a data-driven approach to study the magnetic and thermodynamic properties of van der Waals (vdW) layered materials. We investigate monolayers of the form hbox {A}_2hbox {B}_2hbox {X}_6, based on the known material hbox {Cr}_2hbox {Ge}_2hbox {Te}_6, using density functional theory (DFT) calculations and machine learning methods to determine their magnetic properties, such as magnetic order and magnetic moment. We also examine formation energies and use them as a proxy for chemical stability. We show that machine learning tools, combined with DFT calculations, can provide a computationally efficient means to predict properties of such two-dimensional (2D) magnetic materials. Our data analytics approach provides insights into the microscopic origins of magnetic ordering in these systems. For instance, we find that the X site strongly affects the magnetic coupling between neighboring A sites, which drives the magnetic ordering. Our approach opens new ways for rapid discovery of chemically stable vdW materials that exhibit magnetic behavior.

Synthesis of magnetic two-dimensional materials by chemical vapor deposition

The development of magnetic two-dimensional (2D) materials in its infancy has generated an enormous amount of attention as it offers an ideal platform for the exploration of magnetic properties down to the 2D limit, paving the way for spintronic devices. Due to the nonnegligible advantages including time efficiency and simplified process, the facile bottom-up chemical vapor deposition (CVD) is regarded as a robust method to fabricate ultrathin magnetic nanosheets. Recently, some ultrathin magnets possessing fascinating properties have been successfully synthesized via CVD. Here, the recent researches toward magnetic 2D materials grown by CVD are systematically summarized with special emphasis on the fabrication methods. Then, heteroatoms doping and phase transition induced in CVD growth to bring or tune the magnetic properties in 2D materials are discussed. Characterizations and applications of these magnetic materials are also discussed and reviewed. Finally, some perspectives in need of urgent attention regarding the development of CVD-grown magnetic 2D materials are proposed.

First-principles study of magnetism in some novel MXene materials.

Magnetic two-dimensional materials have gained considerable attention in recent years due to their special topologies and promising applications in electronic and spintronic devices. As a new family of two-dimensional materials, MXene materials may have unusual magnetic properties. In this work, the structural stabilities and electronic properties of 1H and 1T type pristine M2C (M = Sc, Ti, Fe, Co, Ni, Cu, Zn) MXenes with different magnetic configurations were calculated and compared. The critical temperatures of the magnetic MXenes were evaluated through Monte Carlo simulations using the spin–exchange coupling parameters. The results suggest that the ground-state 1T-Ti2C and 1T-Fe2C, 1H-Co2C MXenes are antiferromagnetic or ferromagnetic materials with high Néel or Curie temperatures. Different from the other pristine M2C MXenes with metallic properties, indirect band gaps were found for the 1T-Ti2C and 1T-Ni2C MXenes, which may be useful for their application in information storage or sensors. The findings are expected to promote the development of novel devices based on MXenes and their magnetic properties.

Two-dimensional Dy doped MoS2 ferromagnetic sheets

Magnetic two-dimensional materials have attracted immense consideration as spintronic devices. However, design and synthesis of the magnetic two-dimensional MoS2 crystals with more controllable electronic structure and better conductivity are still great challenges. Here, we make a breakthrough to synthesize the MoS2:Dy sheets with robust adjustable room-temperature ferromagnetic properties by a gas-liquid chemical deposition method. The as-synthesized samples are characterized by XRD, Raman, TEM, HRTEM and XPS. Furthermore, the magnetic properties of the samples are also investigated by VSM and Squid. The maximum observed saturation magnetization is 0.023 emu/g. The origin of ferromagnetism in the samples is investigated by using the first-principles calculations based on the density functional theory. The ferromagnetism is mainly related to the exchange interactions among the S 3p, Mo 4d and Dy 5d orbits. The results indicate that the transformation of the localized charges can effectively engineer and manipulate the magnetic properties of 2D materials, making MoS2 a potential candidate for spintronics and electronic applications and providing a new perspective on other 2D layered materials in spintronics application.

A two-dimensional Fe-doped SnS2 magnetic semiconductor

Magnetic two-dimensional materials have attracted considerable attention for their significant potential application in spintronics. In this study, we present a high-quality Fe-doped SnS2 monolayer exfoliated using a micromechanical cleavage method. Fe atoms were doped at the Sn atom sites, and the Fe contents are ∼2.1%, 1.5%, and 1.1%. The field-effect transistors based on the Fe0.021Sn0.979S2 monolayer show n-type behavior and exhibit high optoelectronic performance. Magnetic measurements show that pure SnS2 is diamagnetic, whereas Fe0.021Sn0.979S2 exhibits ferromagnetic behavior with a perpendicular anisotropy at 2 K and a Curie temperature of ~31 K. Density functional theory calculations show that long-range ferromagnetic ordering in the Fe-doped SnS2 monolayer is energetically stable, and the estimated Curie temperature agrees well with the results of our experiment. The results suggest that Fe-doped SnS2 has significant potential in future nanoelectronic, magnetic, and optoelectronic applications.