Low-dimensional half-metallic materials: theoretical simulations and design

Exploring half-metallic ferromagnetism and thermoelectric properties of Tl2WX6 (X = Cl and Br) double perovskites

Half-metallic semiconductors typically exhibit 100% spin polarization at the Fermi level which makes them desired materials for spintronic applications. In this study, we reported a half-metallic ferromagnetic nature in vacancy-ordered double perovskites Tl2WX6 (X = Cl and Br). The magnetic, electronic, and thermoelectric properties of the material are studied by the use of density functional theory (DFT). For the calculations of exchange-correlation potential, PBE-sol is employed while more accurate electronic band structure and density of states (DOS) are calculated by the mBJ potential. Both materials exhibited structural stability in the cubic structure with Fm3̄m space-group. The mechanical stability is confirmed by their computed elastic constants while their thermodynamic stability is attested by negative formation energy. The spin-based volume optimization suggested the ferromagnetic nature of the materials which is further confirmed by the negative value of the exchange energy Δ x(pd). Moreover, computed magnetic moment value for Tl2WCl6 and Tl2WBr6 is 2 μB and the majority of this comes from W. The spin-polarized band structure and DOS confirmed that both materials are half-metallic and at the Fermi level they exhibit 100% spin polarization. Furthermore, in the spin-down state, materials behave as semiconductors with wide bandgaps. Lastly, the thermoelectric properties are evaluated by the BoltzTrap code. The thermoelectric parameters which include the Seebeck coefficient, electrical conductivity, thermal conductivity, power factor, and figure of merit (ZT) are investigated in the range of temperatures from 200 to 800 K. The half-metallic ferromagnetic and thermoelectric characteristics make these materials desired for spintronics and thermoelectric applications.

Recent progress in 2D bipolar magnetic semiconductors

Bipolar magnetic semiconductor (BMS) is a class of magnetic semiconductors, whose valence band maximum and conduction band minimum are fully spin-polarized with opposite spin directions. Due to the special energy band, half-metallicity can be easily obtained in BMS by gate voltage, and the spin polarization can be reversed between spin-up and down when the gate voltage switches from positive to negative. BMSs have great potential applications in spintronic devices, such as the field-effect spin valves, spin filters and spin transistors,etc. With the rapid progress of the two-dimensional (2D) magnetic materials, researchers have identified a series of potential intrinsic 2D BMS materials using high-throughput computational methods. Additionally, methods such as doping, application of external stress, introduction of external fields, stacking of interlayer antiferromagnetic semiconductors, and construction of Janus structures have endowed existing materials with BMS properties. This paper reviews the research progress of 2D BMS. These advancements provide crucial guidance for the design and synthesis of BMS materials and offer innovative pathways for the future development of spintronics.

Surface functionalization of graphene-like boron arsenide monolayer: a first-principles study

In this work, the effects of hydrogen (H) and oxygen (O) adsorption on the electronic and magnetic properties of graphene-like boron arsenide (BAs) monolayer are investigated using first-principles calculations. Pristine monolayer is a non-magnetic two-dimensional (2D) material, exhibiting direct gap semiconductor character with band gap of 0.75 (1.18) eV as calculated by generalized gradient approximation with Perdew-Burke-Ernzerhof (HSE06) functional. Four high-symmetry adsorption sites are considered, including on-top of B atom (TB), on-top of As atom (TAs), on-top of hollow site (TH), and on-top of bridge site (Tbridge). Using the criterion of adsorption energy, it is found thatTBandTbridgesites are favorable adsorption sites for H and O adatom, respectively. The analysis of electronic interactions indicate the charge transfer from host BAs monolayer to both adatoms. H adsorption conducts to the emergence of magnetic semiconductor nature in BAs monolayer with a total magnetic moment of 1.00 μB. Herein, the magnetism is originated mainly from H adatom and its neighbor As atoms. In contrast, the non-magnetic nature of BAs monolayer is preserved upon absorbing O atoms. In this case, the energy gap exhibits a slight reduction of 4%. Further, the effects of adatom coverage are also analyzed. The presented results suggest an effective modification of ground state electronic properties, as well as induction of new feature-rich properties to make new multifunctional 2D materials from non-magnetic BAs monolayer.

Regulating the electronic properties of the WGe2N4 monolayer by adsorption of 4d transition metal atoms towards spintronic devices

We study the regulation of the electronic and spin transport properties of the WGe2N4 monolayer by adsorbing 4d transition metal atoms (Y-Cd) using density functional theory combined with non-equilibrium Green's function. It is found that the adsorption of transition metal atoms (except Pd, Ag and Cd atoms) can introduce a magnetic moment into the WGe2N4 monolayer. Among the transition metal atoms, the adsorption of Nb and Rh atoms transforms WGe2N4 from a semiconductor to a half-metal and a highly spin-polarized semiconductor, respectively. The half-metallic Nb-adsorbed WGe2N4 system is selected to investigate the spin transport properties, and a high magnetoresistance ratio of 107% is achieved. In both parallel and antiparallel magnetization configurations, the spin filtering efficiency reaches close to 100% in the whole bias range, and the antiparallel magnetization configuration exhibits a dual spin filtering effect with a rectification ratio of up to 104. Our study predicts that the adsorption of 4d transition metal heteroatoms is an effective method to regulate the electronic and magnetic properties of WGe2N4 towards high-performance spintronic devices.

Intrinsic Ferromagnetism in 2D Fe2H with a High Curie Temperature

The rational design of ferromagnetic materials is crucial for the development of spintronic devices. Using first-principles structural search calculations, we have identified 73 two-dimensional transition metal hydrides. Some of them show interesting magnetic properties, even when combined with the characteristics of the electrides. In particular, the P3̅m1 Fe₂H monolayer is stabilized in a 1T-MoS₂-type structure with a local magnetic moment of 3 μ_B per Fe atom, whose robust ferromagnetism is attributed to the exchange interaction between neighboring Fe atoms within and between sublayers, leading to a remarkably high Curie temperature of 340 K. On the other hand, it has a large magnetic anisotropic energy and spin-polarization ratio. Interestingly, the above room-temperature ferromagnetism of the Fe₂H monolayer is well preserved within a biaxial strain of 5%. The structure and electron property of surface-functionalized Fe₂H are also explored. All of these interesting properties make the Fe₂H monolayer an attractive candidate for spintronic nanodevices.

High Curie Temperature and Intrinsic Ferromagnetic Half-Metallicity in Mn2X3 (X = S, Se, Te) Nanosheets

Two-dimensional (2D) intrinsic half-metallic materials with room-temperature ferromagnetism, sizable magnetic anisotropy energy (MAE), and wide half-metallic gap are excellent candidates for pure spin generation, injection, and transport in nanospintronic applications. However, until now, such 2D half metallicity has been rarely observed in experiment. In this work, by using first-principles calculations, we design a series of such materials, namely, Mn₂X₃ (X = S, Se, Te) nanosheets, which could be obtained by controlling the thickness of synthesized α-MnX(111) nanofilm to a quintuple X-Mn-X-Mn-X layer. All these nanosheets are dynamically and thermally stable. Electronic and magnetic studies reveal they are intrinsic half metals with high Curie temperatures between 718 and 820 K, sizable MAEs with -1.843 meV/Mn for Mn₂Te₃ nanosheet, and wide half-metallic gaps from 1.55 to 1.94 eV. Above all, the outstanding features of Mn₂X₃ nanosheets make them promising in fabricating nanospintronic devices working at room temperature.

Thickness Dependent Magnetic Transition in Few Layer 1T Phase CrTe2

Room temperature two-dimensional (2D) ferromagnetism is highly desired in practical spintronics applications. Recently, 1T phase CrTe₂ (1T-CrTe₂) nanosheets with 5 and thicker layers have been successfully synthesized, which all exhibit the properties of ferromagnetic (FM) metals with Curie temperatures around 305 K. However, whether the ferromagnetism therein can be maintained when continuously reducing the nanosheet's thickness to monolayer limit remains unknown. Here, through first-principles calculations, we explore the evolution of magnetic properties of 1 to 6 layer CrTe₂ nanosheets and several interesting points are found: First, unexpectedly, monolayer CrTe₂ prefers a zigzag antiferromagnetic (AFM) state with its energy much lower than that of FM state. Second, in 2 to 4 layer CrTe₂, both the intralayer and interlayer magnetic coupling are AFM. Last, when the number of layers is equal to or greater than 5, the intralayer and interlayer magnetic coupling become FM. Such highly thickness dependent magnetism provides a new perspective to control the magnetic properties of 2D materials.

Orbital Design of Two-Dimensional Transition-Metal Peroxide Kagome Crystals with Anionogenic Dirac Half-Metallicity

Assembling p orbital ferromagnetic half-metallicity and a topological element, such as a Dirac point at the Fermi level, in a single nanomaterial is of particular interest for long-distance, high-speed, and spin-coherent transportation in nanoscale spintronic devices. On the basis of the tight-binding model, we present an orbital design of a two-dimensional (2D) anionogenic Dirac half-metal (ADHM) by patterning cations with empty d orbitals and anions with partially filled p-type orbitals into a kagome lattice. Our first-principles calculations show that 2D transition-metal peroxides h-TM₂(O₂)₃ (TMO₃, TM = Ti, Zr, Hf), containing group IVB transition-metal cations [TM]⁴⁺ bridged with dioxygen anions [O₂]^8/3- in a kagome structure, are stable ADHMs with a Curie temperature over 103 K. The 2/3 filled π* orbitals of dioxygen anions are ferromagnetically coupled, leading to p orbital ferromagnetism and a half-metallic Dirac point right at the Fermi level with a Fermi velocity reaching 2.84 × 10⁵ m/s. We proposed that 2D h-TM₂(O₂)₃ crystals may be extracted from ABO₃ bulk materials containing 2D TMO₃ layers.

First-principles investigation of a new 2D magnetic crystal: Ferromagnetic ordering and intrinsic half-metallicity

The development of two-dimensional (2D) magnetic materials with half-metallic characteristics is of great interest because of their promising applications in spintronic devices with high circuit integration density and low energy consumption. Here, by using density functional theory calculations, ab initio molecular dynamics, and Monte Carlo simulation, we study the stability, electronic structure, and magnetic properties of a OsI₃ monolayer, of which crystalline bulk is predicted to be a van der Waals layered ferromagnetic (FM) semiconductor. Our results reveal that the OsI₃ monolayer can be easily exfoliated from the bulk phase with small cleavage energy and is energetically and thermodynamically stable at room temperature. Intrinsic half-metallicity with a wide bandgap and FM ordering with an estimated T_C = 35 K are found for the OsI₃ monolayer. Specifically, the FM ordering can be maintained under external biaxial strain from -2% to 5%. The in-plane magnetocrystalline anisotropy energy of the 2D OsI₃ monolayer reaches up to 3.89 meV/OsI₃, which is an order larger than that of most magnetic 2D materials such as the representative monolayer CrI₃. The excellent magnetic features of the OsI₃ monolayer therefore render it a promising 2D candidate for spintronic applications.

Strain robust spin gapless semiconductors/half-metals in transition metal embedded MoSe2monolayer

The realization of spin gapless semiconductor (SGS) and half-metal (HM) behavior in two-dimensional (2D) transition metal (TM) dichalcogenides is highly desirable for their applications in spintronic devices. Here, using density functional theory calculations, we demonstrate that Fe, Co, Ni substitutional impurities can not only induce magnetism in MoSe2monolayer, but also convert the semiconducting MoSe2to SGS/HM system. We also study the effects of mechanical strain on the electronic and magnetic properties of the doped monolayer. We show that for all TM impurities we considered, the system exhibits the robust SGS/HM behavior regardless of biaxial strain values. Moreover, it is found that the magnetic properties of TM-MoSe2can effectively be tuned under biaxial strain by controlling the spin polarization of the 3dorbitals of Fe, Co, Ni atoms. Our findings offer a new route to designing the SGS/HM properties and modulating magnetic characteristics of the TM-MoSe2system and may also facilitate the implementation of SGS/HM behavior and realization of spintronic devices based on other 2D materials.

R3c-type LnNiO3 (Ln = La, Ce, Nd, Pm, Gd, Tb, Dy, Ho, Er, Lu) half-metals with multiple Dirac cones: a potential class of advanced spintronic materials

In the past three years, Dirac half-metals (DHMs) have attracted considerable attention and become a high-profile topic in spintronics becuase of their excellent physical properties such as 100% spin polarization and massless Dirac fermions. Two-dimensional DHMs proposed recently have not yet been experimentally synthesized and thus remain theoretical. As a result, their characteristics cannot be experimentally confirmed. In addition, many theoretically predicted Dirac materials have only a single cone, resulting in a nonlinear electromagnetic response with insufficient intensity and inadequate transport carrier efficiency near the Fermi level. Therefore, after several attempts, we have focused on a novel class of DHMs with multiple Dirac crossings to address the above limitations. In particular, we direct our attention to three-dimensional bulk materials. In this study, the discovery via first principles of an experimentally synthesized DHM LaNiO3 with many Dirac cones and complete spin polarization near the Fermi level is reported. It is also shown that the crystal structures of these materials are strongly correlated with their physical properties. The results indicate that many rhombohedral materials with the general formula LnNiO3 (Ln = La, Ce, Nd, Pm, Gd, Tb, Dy, Ho, Er, Lu) in the space group R 3 c are potential DHMs with multiple Dirac cones.

Strain induced electronic and magnetic properties of 2D magnet CrI3: a DFT approach

In the post-graphene era, out of several monolayer 2D materials, Chromium triiodide ([Formula: see text]) has triggered an exotic platform for studying the intrinsic ferromagnetism and large anisotropy at the nanoscale regime. Apart from that, its strong spin-orbit coupling of I also plays a key role in tailoring the electronic properties. In this work, the composition of compressive and tensile strain (uniaxial as well as biaxial) upto 12% have been applied to study the variation of the electronic and magnetic properties of [Formula: see text] employing density functional theory in (LDA+U) exchange correlation scheme. The stability limits of the structures under the influence of strains have been carried out via the deformation potential (DP) and stress-strain relation. For compressive strains in specific directions, the down-spin band gap is seen to be decreasing steadily. The magnetic moment computed from the density of states (DOS) is enhanced significantly under the influence of compressive strain. However, it has been observed that after the application of strain in some specific crystal directions, the magnetic moment of monolayer [Formula: see text] remains almost unchanged. Thus, with the help of strain, the tunning band gap along with underlying characteristic ferromagnetism of this material can unfold a new avenue for potential usage in spintronic devices.

Toward Room-Temperature Magnetic Semiconductors in Two-Dimensional Ferrimagnetic Organometallic Lattices

Obtaining room-temperature magnetically ordered two-dimensional (2D) semiconductors is urgently needed for high-speed nanospintronic devices but remains a big challenge. Here, we propose a potential route to solve this issue by constructing ferrimagnetic semiconductors in 2D metal organic frameworks, taking advantage of the high Curie temperature of ferrimagnetic semiconductors and easy tunability of metal organic frameworks. The proposal is confirmed by first-principles design of 2D metal organic frameworks with conjugated electron acceptors diketopyrrolopyrrole (DPP) as organic linkers and transition metal Cr (V) as nodes. The robust ferrimagnetic ordering comes from the strong direct exchange interaction between d-electron magnetic moments on transition metals and charge transfer-induced p-electron magnetic moments on DPPs, which can be modulated facilely by reducing the d-p orbital interaction distance via moderate compressive strain or increasing the d-p orbital charge transfer through introducing electron-withdrawing groups into the DPP moiety.

Transition-Metal Dihydride Monolayers: A New Family of Two-Dimensional Ferromagnetic Materials with Intrinsic Room-Temperature Half-Metallicity

Two-dimensional (2D) ferromagnetic materials with intrinsic half-metallicity are highly desirable for nanoscale spintronic applications. Here, we predict a new and stable family of 2D transition-metal dihydride (MH₂; M = Sc, Ti, V, Cr, Fe, Co, Ni) monolayers with novel properties. Our density functional theory computation shows that CoH₂ and ScH₂ monolayers are ferromagnetic metals, while the others are antiferromagnetic semiconductors. In particular, the CoH₂ monolayer is a perfect half-metal with a wide spin gap of 3.48 eV. The ScH₂ monolayer can also possess half-metallicity through hole doping. Most importantly, our Monte Carlo simulations show that the CoH₂ monolayer possesses an above-room-temperature Curie point (339 K), while that of the ScH₂ monolayer can also reach 160 K. A synthetic approach is proposed to realize CoH₂ and ScH₂ monolayers in the laboratory. Notably, their half-metallicity can be well maintained on substrates. The new family of MH₂ monolayers are promising functional materials for spintronic applications due to their novel magnetic properties.

Half-metallicity in a honeycomb–kagome-lattice Mg3C2 monolayer with carrier doping

A transition from an anti-ferromagnetic semiconductor to a ferromagnetic half metal can be induced by carrier doping in the honeycomb–kagome-lattice Mg₃C₂ monolayer.