2D Magnetic Materials Research Articles

Two-dimensional Janus transition-metal dichalcogenides with intrinsic ferromagnetism and half-metallicity

Searching experimental feasible two-dimensional (2D) ferromagnetic crystals with room-temperature magnetic order and high spin-polarization is one key for the development of next-generation spintronic devices. Inspired by the recent experimental achievement for the synthesis of Janus MoSSe monolayer with out-of-plane symmetry, here, by ab initio calculation, we demonstrate this feasibility with the discovery of intrinsic 2D ferromagnetic Janus TMDs monolayer (TMXX′, TM = V, Cr and Mn; X, X′ = S, Se and Te, X ≠ X′) with large spin-polarization and high Curie temperature. Particularly, the MnSSe monolayer show half-metal with 100% spin polarization and wide half-metallic gap. A Curie temperature as high as 400 K has been found for VSSe materials. The formation energy calculation for Janus TMDs can be comparable with the TMDs monolayer. The dynamical stability for these 2D crystals also have been confirmed by phonon spectrum calculation. Our study presents a new class of 2D magnetic materials for future spintronics and valleytronics.

Epitaxially grown monolayer VSe2: an air-stable magnetic two-dimensional material with low work function at edges

Recent experimental breakthroughs open up new opportunities for magnetism in few-atomic-layer two-dimensional (2D) materials, which makes fabrication of new magnetic 2D materials a fascinating issue. Here, we report the growth of monolayer VSe2 by molecular beam epitaxy (MBE) method. Electronic properties measurements by scanning tunneling spectroscopy (STS) method revealed that the as-grown monolayer VSe2 has magnetic characteristic peaks in its electronic density of states and a lower work-function at its edges. Moreover, air exposure experiments show air-stability of the monolayer VSe2. This high-quality monolayer VSe2, a very air-inert 2D material with magnetism and low edge work function, is promising for applications in developing next-generation low power-consumption, high efficiency spintronic devices and new electrocatalysts.

Annealing effect on the ferromagnetism of MoS2 nanoparticles

Novel properties of two-dimensional (2D) materials have drawn extensive attention in past decade. Magnetic property is one of the research focuses since it can add extra spin freedom for the manipulation of 2D material properties. The magnetism in 2D materials can be originated from defects (i.e. vacancies, zigzag defects). In this work, MoS2 nano particles were annealed at various temperatures (from 600 till 1000 °C) in an argon environment to explore the post-annealing effect on the magnetic properties. The results indicate that annealing may induce tunable ferromagnetism associated with the annealing temperature. The ferromagnetic signal is enhanced with the increase in annealing temperature which may attribute to the defects pertaining to the sulfur vacancies and edge terminated structure induced by annealing.

Effects of hydrostatic pressure on spin-lattice coupling in two-dimensional ferromagnetic Cr2Ge2Te6

Spin-lattice coupling plays an important role in both formation and understanding of the magnetism in two-dimensional magnetic semiconductors (2DMS). In this paper, the steady pressure effects on the lattice structure, Raman resonances, and magnetization of a 2DMS Cr2Ge2Te6 have been studied by both experiments and first principles calculations. It is found that the bond length of Cr-Cr decreases, the angle of Cr-Te-Cr diverges from 90°, and the Raman modes Eg3 and Ag1 show an increase with the application of external pressure. Consequently, the magnetic phase transition temperature TC decreases from 66.6 K to 60.6 K (∼9%) as the pressure increases from 0 to 1 GPa. These pressure effects not only confirm the existence of strong spin-lattice coupling but also reveal the detailed information about the lattice deformation effect on the magnetic properties in such 2DMS, which would be a benefit for the further understanding and manipulation of the magnetism in 2D materials.

Hole-Doping Induced Ferromagnetism in Monolayer SnO: A First-Principles Study

Two dimensional (2D) materials are currently attracting considerable interest due to their promising applications in future nanoelectronic devices. Magnetic 2D materials are also gaining attention, for their possible use in novel spintronic devices [1]. Recently, hole-doping in monolayer SnO has been theoretically predicted to induce a paramagnetic to ferromagnetic phase transition in this 2D material, for a typical hole density of about 5x1013/cm2 [2]. Magnetism in monolayer SnO arises from an exchange splitting of electronic states at the top of the valence band, where the density of states exhibits a sharp van Hove singularity, resulting in a so-called Stoner (magnetic) instability [2]. This behavior is typical of materials with “Mexican-hat” energy band edges (as predicted for the valence band edge of monolayer SnO), and has also been predicted in e.g. monolayer GaSe [3]. In this work, the possibility to induce hole doping in monolayer SnO by intrinsic and extrinsic defects is investigated, using first-principles simulations, based on density functional theory. It is found that Sn vacancies generate spin-polarized gap states near the valence band edge of monolayer SnO, and that these point defects are behaving like acceptors. A typical density of 5x1013/cm2 of Sn vacancies induces a paramagnetic to ferromagnetic phase transition in the material, with a magnetic moment density of 1 Bohr magneton/hole. Substitutional doping of Sn atoms by e.g. In or Zn atoms is also predicted to result in hole-doping of monolayer SnO, and also leads to a ferromagnetic order in the 2D material. The possibility to induce a ferromagnetic phase transition in monolayer SnO by electrostatic doping will also be discussed. [1] W. Han, APL Mater. 4, 032401 (2016). [2] L. Seixas et al., Phys. Rev. Lett. 116, 206803 (2016). [3] T. Cao et al., Phys. Rev. Lett. 114, 236602 (2015).

Chemically Engineering Magnetic Anisotropy of 2D Metalloporphyrin.

Continuous miniaturization of magnetic units in spintronics devices inspires efforts to search for novel 2D magnetic materials with giant magnetic anisotropy energy (MAE). Through systematic first‐principles calculations, large MAE of 24 meV in W or Re embedded 2D polyporphyrin frameworks is found. Interestingly, the MAE can be enhanced up to 60 meV, through replacing the hydrogen atoms on the edges of the Re based 2D polyporphyrin framework by hydroxyl and amino radicals. Analysis of the electronic structures reveals that the enhancement of MAE is mainly attributed to charge redistributions and energy shifts of Re 5d orbitals induced by the functional radicals. The findings pave a new and feasible way for tailoring the magnetic properties of magnetic organic materials to fulfill the criteria for applications in spintronics devices at high temperature.

Tuning magnetic properties in quasi-two-dimensional ferromagnetic Fe3−yGe1−xAsxTe2 (0 ⩽ x ⩽ 0.85)

We report that Fe3GeTe2 can form a wide solid solution by substitution of As for Ge, providing an opportunity to tune the magnetic and electronic properties in this 2D material. The crystal structure, physical properties and electronic structure of iron-deficient solid solution Fe3−yGe1−xAsxTe2 (0 ⩽ x ⩽ 0.85) are studied. We found that the Curie temperature can substantially change from 177 K to 33 K and resistivity decreases by about 30% with the arsenic doping x from 0 to 0.85. First principles calculations demonstrate that the elongation of Fe(1)–Fe(1) dumb-bells along c axis is essentially responsible for decreasing the integrated spin density of states below Fermi level and weakening spin polarization, resulting in a decrease of Curie temperature. Our study reveals the magnetism manipulation can be realized via modification of bondlengths in 2D magnetic materials.

Large edge magnetism in oxidized few-layer black phosphorus nanomeshes

The formation and control of a room-temperature magnetic order in two-dimensional (2D) materials is a challenging quest for the advent of innovative magnetic- and spintronic-based technologies. To date, edge magnetism in 2D materials has been experimentally observed in hydrogen (H)-terminated graphene nanoribbons (GNRs) and graphene nanomeshes (GNMs), but the measured magnetization remains far too small to allow envisioning practical applications. Herein, we report experimental evidences of large room-temperature edge ferromagnetism (FM) obtained from oxygen (O)-terminated zigzag pore edges of few-layer black phosphorus (P) nanomeshes (BPNMs). The magnetization values per unit area are ~100 times larger than those reported for H-terminated GNMs, while the magnetism is absent for H-terminated BPNMs. The magnetization measurements and the first-principles simulations suggest that the origin of such a magnetic order could stem from ferromagnetic spin coupling between edge P with O atoms, resulting in a strong spin localization at the edge valence band, and from uniform oxidation of full pore edges over a large area and interlayer spin interaction. Our findings pave the way for realizing high-efficiency 2D flexible magnetic and spintronic devices without the use of rare magnetic elements.

Tunable magneto-optical effects in hole-doped group-IIIA metal-monochalcogenide monolayers

Because of unusual properties and fascinating prospects for next-generation device applications, two-dimensional (2D) materials have attracted enormous attention since graphene was discovered in 2004. Among the 2D materials beyond graphene, group-IIIA metal-monochalcogenide (MX) monolayers (MLs), are receiving increasing interests because their excellent applications on electronics and optoelectronics. Recently, ferromagnetism and half-metallicity have been predicted in hole-doped GaS and GaSe MLs, which promise exciting potentials for semiconductor spintronics. Detection and measurement of spontaneous magnetization in these 2D materials will be essential for their spintronic applications. The magneto-optical (MO) effects not only are a powerful probe of magnetism in 2D materials but also have valuable applications in high-density data-storage technology. Furthermore, anomalous Hall effect is not only an ideal transport probe of itinerant magnetism but also of considerable current interest because of its topological nature. Here we perform a systematic first-principles density functional study on the MO Kerr and Faraday effects as well as such important magnetic and transport properties as magneto-crystalline anisotropy energy (MAE) and anomalous Hall conductivity (AHC) of all hole-doped MX (M = Ga, In; X = S, Se, Te) MLs. In this paper, we report the following important findings: (a) gate-tunable MO effects in MX MLs in a broad range of hole concentration; (b) large Kerr and Faraday rotation angles with Kerr angles comparable to well-known MO 3d-transition-metal multilayers and Faraday angles being among the largest ones reported; (c) tunable MAE and large AHC, making MX MLs suitable for magnetic memory devices current-driven via spin-transfer torque and also promising materials for magnetic field nanosensors with high sensitivity. Superior MO characteristics, together with the other interesting properties, would make MX MLs an excellent family of 2D materials for semiconductor MO and spintronic nanodevices.

Metallic and Magnetic 2D Materials Containing Planar Tetracoordinated C and N

The top monolayers of surface carbides and nitrides of Co and Ni are predicted to yield new stable 2D materials upon exfoliation. These 2D phases are p4g clock reconstructed, and contain planar tetracoordinated C or N. The stability of these flat carbides and nitrides is high, and ab initio molecular dynamics at a simulation temperature of 1800 K suggest that the materials are thermally stable at elevated temperatures. The materials owe their stability to local triple aromaticity (π, σ-radial, and σ-peripheral) associated with binding of the main group element to the metal. All predicted 2D phases are conductors, and the two alloys of Co are also ferromagnetic, a property especially rare among 2D materials. The preparation of 2D carbides and nitrides is envisioned to be done through surface deposition and peeling, possibly on a metal with a larger lattice constant for reduced affinity.

Raman spectroscopy of atomically thin two-dimensional magnetic iron phosphorus trisulfide (FePS3) crystals

Metal phosphorous trichalcogenide is an important group of layered two-dimensional (2D) materials with potentially diverse applications in low-dimensional magnetic and spintronic devices. Herein we present a comprehensive investigation on the lattice dynamics and spin–phonon interactions of mechanically exfoliated atomically thin 2D magnetic material—iron phosphorus trisulfide (FePS3) by Raman spectroscopy and first principle calculations. Layer-number and temperature dependent Raman spectroscopy suggests a magnetic persistence in FePS3 even down to monolayer regime through the spin–phonon coupling, while the Néel temperature decreases from 117 K in bulk to 104 K in monolayer sample. Our studies advocate the intriguing magnetic properties in 2D crystals and suggest that FePS3 is a promising candidate material for future magnetic applications.

Metallic and Magnetic 2D Materials Containing Planar Tetracoordinated C and N

Metallic and Magnetic 2D Materials Containing Planar Tetracoordinated C and N

Two-dimensional magnetic boron

We predict a two-dimensional (2D) antiferromagnetic (AFM) boron (designated as M-boron) by using ab initio evolutionary methodology. M-boron is entirely composed of B20 clusters in a hexagonal arrangement. Most strikingly, the highest valence band of M-boron is isolated, strongly localized, and quite flat, which induces spin polarization on each cap of the B20 cluster. This flat band originates from the unpaired electrons of the capping atoms, and is responsible for magnetism. M-boron is thermodynamically metastable and is the first cluster-based 2D magnetic material in the elemental boron system.

Tuning magnetism of monolayer GaN by vacancy and nonmagnetic chemical doping

In view of important role of inducing and manipulating the magnetism in 2D materials for the development of low-dimensional spintronic devices, the magnetism of GaN monolayer with Ga vacancy and nonmagnetic chemical doping are investigated using first-principles calculations. It is found that pure GaN monolayer has graphene-like structure and is nonmagnetic. While, a neutral Ga vacancy can induce 3μB intrinsic magnetic moment, localized mainly on the neighboring N atoms. Interestingly, after one Mg or Si atom doping in g-GaN with Ga vacancy, the magnetic moment can be modified to 4μB or 2μB respectively due to the change in hole number. Meantime, Mg-doped g-GaN with Ga vacancy shows half-metal character. With the increasing of doping concentrations, the magnetic moment can be further tuned. The results are interesting from a theoretical point of view and may open opportunities for these 2D GaN based materials in magnetic devices.

Atomically Thin Transition-Metal Dinitrides: High-Temperature Ferromagnetism and Half-Metallicity.

High-temperature ferromagnetic two-dimensional (2D) materials with flat surfaces have been a long-sought goal due to their potential in spintronics applications. Through comprehensive first-principles calculations, we show that the recently synthesized MoN2 monolayer is such a material; it is ferromagnetic with a Curie temperature of nearly 420 K, which is higher than that of any flat 2D magnetic materials studied to date. This novel property, made possible by the electron-deficient nitrogen ions, render transition-metal dinitrides monolayers with unique electronic properties which can be switched from the ferromagnetic metals in MoN2, ZrN2, and TcN2 to half-metallic ones in YN2. Transition-metal dinitrides monolayers may, therefore, serve as good candidates for spintronics devices.

Spin amplification by controlled symmetry breaking for spin-based logic

Spin amplification is one of the most critical challenges for spintronics and spin-based logic in order to achieve spintronic circuits with fan-out. We propose a new concept for spin amplification that will allow a small spin current in a non-magnetic spin channel to control the magnetization of an attached ferromagnet. The key step is to bring the ferromagnet into an unstable symmetric state (USS), so that a small spin transfer torque from a small spin current can provide a magnetic bias to control the spontaneous symmetry breaking and select the final magnetization direction of the ferromagnet. Two proposed methods for achieving the USS configuration are voltage-controlled Curie temperature (VC-TC) and voltage-controlled magnetic anisotropy (VC-MA). We believe the development of new 2D magnetic materials with greater tunability of VC-TC and VC-MA will be needed for practical applications. A successful realization of spin amplification by controlled symmetry breaking will be important for the implementation of existing spin-logic proposals (e.g. ‘all spin logic’) and could inspire alternative ideas for spintronic circuits and devices.

Room-Temperature Ordered Spin Structures in Cluster-Assembled Single V@Si12 Sheets

Since most of the existing pristine two-dimensional (2D) materials are either intrinsically nonmagnetic or magnetic with small magnetic moment per unit cell and weak strength of magnetic coupling, introducing transition metal atoms in various nanosheets has been widely used for achieving a desired 2D magnetic material. However, the problem of surface clustering for the doped transition metal atoms is still challenging. Here we demonstrate via first-principles calculations that the recently experimentally characterized endohedral silicon cage V@Si12 clusters can construct two types of single cluster sheets exhibiting hexagonal porous or honeycomb-like framework with regularly and separately distributed V atoms. For the ground state of these two sheets, the preferred magnetic coupling is found to be ferromagnetic due to a free-electron-mediated mechanism. By using external strain, the magnetic moments and strength of magnetic coupling for these two sheets can be deliberately tuned, which would be propitious to their advanced applications. More attractively, our first-principles molecular dynamics simulations show that both the structure and strength of ferromagnetic coupling for the pristine porous sheet are stable enough to survive at room temperature. The insights obtained in this work highlight a new avenue to achieve 2D silicon-based spintronics nanomaterials.

Novel Magnetic Monolayers of Transition Metal Silicide

Two-dimensional (2D) magnetic materials are important for the potential applications in spintronics and magnetic storage. Twenty transition metal silicide (TMSi2) monolayers have been studied by first-principles calculations. Seven TMSi2 monolayers, i.e., TiSi2, VSi2, CrSi2, MnSi2, FeSi2,NbSi2, and MoSi2, are found to be magnetic. Among them, the formation energies of ferromagnetic TiSi2 and antiferromagnetic FeSi2 monolayers are lower than those of silicene. Meanwhile, a large magnetic moment of 2.512 μB per Mn atom is obtained in monolayer MnSi2 sheet, which has the same formation energy of silicene. Therefore, these three novel magnetic monolayers hold promise in future 2D magnetic devices.

Electronic and magnetic properties of an AlN monolayer doped with first-row elements: a first-principles study

Two-dimensional (2D) magnetic materials are the focus of one of the most active areas of nano-materials research.

Enhanced Stability of Skyrmions in Two-Dimensional Chiral Magnets with Rashba Spin-Orbit Coupling

Recent developments have led to an explosion of activity on skyrmions in three-dimensional (3D) chiral magnets. Experiments have directly probed these topological spin textures, revealed their nontrivial properties, and led to suggestions for novel applications. However, in 3D the skyrmion crystal phase is observed only in a narrow region of the temperature-field phase diagram. We show here, using a general analysis based on symmetry, that skyrmions are much more readily stabilized in two-dimensional (2D) systems with Rashba spin-orbit coupling. This enhanced stability arises from the competition between field and easy-plane magnetic anisotropy and results in a nontrivial structure in the topological charge density in the core of the skyrmions. We further show that, in a variety of microscopic models for magnetic exchange, the required easy-plane anisotropy naturally arises from the same spin-orbit coupling that is responsible for the chiral Dzyaloshinskii-Moriya interactions. Our results are of particular interest for 2D materials like thin films, surfaces, and oxide interfaces, where broken surface-inversion symmetry and Rashba spin-orbit coupling naturally lead to chiral exchange and easy-plane compass anisotropy. Our theory gives a clear direction for experimental studies of 2D magnetic materials to stabilize skyrmions over a large range of magnetic fields down to T=0.