Half-metallic State Research Articles

Engineering magnetic anisotropy and exchange couplings in double transition metal MXenes via surface defects

Two-dimensional (2D) materials have been experimentally proven to manifest almost all types of material properties observed in bulk materials. However, 2D magnetism was elusive until recently. In this work, we used an approach that synergistically uses density functional theory, and Monte Carlo methods to investigate the magnetic and electronic properties of magnetic double transition metal MXene alloys (Hf2MnC2O2 and Hf2VC2O2) by exploiting realistic surface terminations via creating surface defects including oxygen vacancies and H adatoms. We found that introducing surface oxygen vacancies or hydrogen adatoms is able to modify the electronic structures, magnetic anisotropies, and exchange couplings. Depending on the defect concentration, a ferromagnetic half-metallic state can be realized for both Hf2VC2O2 and Hf2MnC2O2. Bare Hf2VC2O2 exhibits easy-axis anisotropy, whereas bare Hf2MnC2O2 exhibits easy-plane anisotropy; however, defects can change the latter to easy-axis anisotropy, which is preferable for spintronics applications. The considered defects were found to modify the magnetic anisotropy by as much as 300%. Defects also produce an inhomogeneous pattern of exchange couplings, which can further enhance the Curie temperature. In particular, Hf2MnC2O2H0.22 was predicted to have a Curie temperature of about 171 K due to a combination of easy-axis anisotropy and a connected network of enhanced exchange couplings. Our calculations suggest a route toward engineering exchange couplings and magnetic anisotropy to improve magnetic properties.

Tunable magnetism in layered CoPS3 by pressure and carrier doping

Despite extensive research on recently discovered layered ferromagnetic (FM) materials, their further development is hampered by the limited number of candidate materials with desired properties. As a much bigger family, layered antiferromagnetic (AFM) materials represent excellent platforms to not only deepen our understanding of fundamental physics but also push forward high-performance spintronics applications. Here, by systematic first-principles calculations, we demonstrate pressure and carrier doping control of magnetic properties in layered AFM CoPS3, a representative of transition metal phosphorus trichalcogenides. In particular, pressure can drive isostructural Mott transition, in sharp contrast to other transition metal thiophosphates. Intriguingly, both pressure and carrier doping can realize the long-sought FM half-metallic states with 100% spin polarization percentage, which is good for improving the injection and detection efficiency of spin currents among others. Moreover, the Mott transition is accompanied by instantaneous spin-crossover (SCO) in CoPS3, and such cooperative SCO facilitates the implementation of fast-response reversible devices, such as data storage devices, optical displays and sensors. We further provide an in-depth analysis for the mechanisms of FM half-metallicity and SCO. Tunable magnetism in layered AFM materials opens vast opportunities for purposeful device design with various functionalities.

Three-dimensional Weyl hourglass networks in the nonsymmorphic half-metal Mg2VO4

Fermions with hourglass dispersions have been hotly studied in a nonmagnetic system currently. However, magnetic ones have been rarely discussed because of the scarcity of excellent candidate materials. Here, promoted by first-principles and symmetry analysis, we propose the presence of novel hourglass nodal networks in a nonsymmorphic half-metal ${\mathrm{Mg}}_{2}{\mathrm{VO}}_{4}$ that is constructed by three touching hourglass nodal lines at mutually intersecting planes in the extended Brillouin zone. Unlike previous hourglass semimetals in nonmagnetic materials, the material proposed here has a ferromagnetic half-metallic state; thus, the hourglass nodal networks are fully spin polarized. We also show that the hourglass nodal networks manifest spin-polarized drumhead surface states. We demonstrate the hourglass nodal networks are enforced by nonsymmorphic symmetry, and effective models are constructed. We further show that the novel topological phase of ${\mathrm{Mg}}_{2}{\mathrm{VO}}_{4}$ is robust against lattice strain and electron correlation effects. Our work provides a good platform to investigate novel hourglass fermions from nonsymmorphic symmetries in magnetic system.

Spin-orbit coupling effect on the electronic structure of Sr2FeHfO6 alloy for spintronics application

Strong spin–orbit coupling (SOC) in Sr2FeHfO6 (SFHO) alloy has been predicted by means of full-potential linear augmented plane wave method in order to study the effect on electronic structure. The tetragonal (I4/mmm) SFHO with SOC has an up-spin energy of ~1.2 eV and a total magnetic moment of ~2.03 μB mainly coming from the Fe (1.63 μB), and the SOC effect was prominent in SFHO due to the magnetic moment of 3d-Fe orbitals. The XAFS spectroscopy technique was used in order to analyze the electronic structure and optical properties of SFHO. The absorption spectra with Fe as a source atom shows a peak appearing around 7118 eV. SFHO with SOC falls within the half metallic state in the up-spin direction resulting significant increase of the total magnetic moment. This half-metallicity strong SOC in SFHO demonstrate much spin polarized electrons around the Fermi level, hence, this material is very potential in future memory devices and spintronic applications.

Tuning Magnetism of Metal Porphyrazine Molecules by a Ferroelectric In2Se3 Monolayer.

Electric field tuning of magnetism is highly desirable for nanoelectronics, but volatility in electron spin manipulation presents a major challenge that needs urgent resolution. Here, we show by first-principles calculations that magnetism of metal porphyrazine (MPz) molecules can be effectively tuned by switching ferroelectric polarization of an adjacent In2Se3 monolayer. The magnetic moments of TiPz and VPz (MnPz, FePz, and CoPz) decrease (increase) at one polarization but remain unchanged at reversed polarization. This intriguing phenomenon stems from distinct metal d-orbital occupation caused by electron transfer and energy-level shift associated with the polarization switch of the In2Se3 monolayer. Moreover, the ferroelectric switch also tunes the underlying electronic properties, producing a metallic, half-metallic, or semiconducting state depending on polarization. These findings of robust ferroelectric tuning of magnetism and related electronic properties in MPz-adsorbed In2Se3 hold great promise for innovative design and implementation in advanced magnetic memory storage, sensor, and spintronic devices.

Structural, electronic, magnetic and thermoelectric properties of Full-Heusler Fe2MnSi: Ab initio calculations

The linear wave augmented plane waves method at full potential using the functional density theory and applying the approximation methods “GGA-PBE, PBE-sol, mBJ and GGA + U” has been used to investigate the electronic, thermoelectric, magnetic and transport properties of Full-Heusler Fe2MnSi. The optimized and the most stable crystal structure at equilibrium is face-centered cubic with space group and lattice parameters a = 5.591 Å. The half metallic character is observed with an indirect band gap. To determine the stability of this material, the elastic parameters have been computed. Moreover, the Seebeck coefficients and the electrical conductivity have been obtained. It is found that the spin polarization (SP) reaches 100% for Fe2MnSi due to its half metallic state.

Impact of uniaxial strain on the electronic and magnetic properties of Sr2CrReO6

Among the double perovskite oxides, Sr2CrReO6 has been attractive a lot of attention due its half-metallic ferrimagnetic nature and the highest magnetic transition temperature of 635 K. Here, we systematically investigated the influence of uniaxial strain along [100]-direction on the electronic and magnetic properties of double perovskite oxide Sr2CrReO6 using first-principle calculations. Our results show that a critical tensile strain of +2% leads the system to the ferromagnetic metallic state from half-metallic ferrimagnetic with a spin-polarization of 64%. Because, tensile strain decreases the anti-ferromagnetic coupling between Cr and Re ions, which results in ferromagnetic ordering. The admixture of Cr/Re dx2−y2, dxz, and dyz orbitals are mainly responsible for metallicity. Along with this, the total magnetic moment gets enhanced and almost close to an integral value of 5 μB for higher tensile strains. On the other hand, compressive strains exhibit no substantial impact on the physical properties of Sr2CrReO6. Moreover, it is also found that spin-polarization optimized with a further increase of tensile strains. Close to half-metallic ferromagnetic state due to tensile strain, this material shows a wide range of scientific and technological applications in spintronics.

D0 Half-Metallic Ferromagnetism in GeNaZ (Z = Ca, Sr, and Ba) Ternary Half-Heusler Alloys: an Ab initio Investigation

First-principles approach based on density functional theory was applied to investigate the structural, elastic, phonon dynamics, thermodynamics, electronic, and magnetic properties of GeNaZ (where Z = Ca, Sr and Ba) ternary Heusler alloys. The Perdew-Burke-Ernzerhof generalized gradient approximation (GGA) and the modified Becke-Johnson (mBJ-GGA) were used for the exchange-correlation energy and potential. The GeNaZ ternary Heusler alloys were found to be half-metallic and stable in the ferromagnetic α phase. The alloys were found to satisfy the criteria for dynamic and elastic stabilities. Thus, they are dynamically and elastically stable. The calculated spin-polarized electronic band structure and density of states using GGA (mBJ-GGA) reveal that the minority spin channel has metallic character and the majority spin channel having an electronic band gap of 0.26 (0.59), 0.39 (0.66), and 0.08 (0.33) eV for the GeNaCa, GeNaSr, and GeNaBa ternary Heusler alloys, respectively. The considered alloys all result in a stable ferromagnetic half-metallic ground state structure with a magnetic moment of 1 μB/cell. The density of states and the spin-charge density of these alloys show that the magnetization originates primarily from the 4p states of the Ge atoms. The GeNaZ ternary Heusler alloys do not have any magnetic elements, and their induced magnetism makes them very promising materials for spintronic devices.

Electronic Structure and Magnetic Anisotropy of Single-Layer Rare-Earth Oxybromide.

The discovery of intrinsic magnetism in two-dimensional (2D) limit has triggered increasing investigations in layered magnetic materials. However, most of the available candidates involves 3d transition metals, while the layered rare-earth magnetic materials are largely unexplored at present. Here, we proposed a series of 2D rare-earth magnetic semiconductors REOBr (RE = Tb, Dy, Ho, Er and Tm) with large magnetic moments and magnetic anisotropy energies using the PBE + U method. Our calculations indicate a half-metallic meta-stable state and a low-energy semi-conducting ground state in these 4f single-layers, which can be characterized by the location of the two-fold degenerate x(x2 – 3y2) orbital. The dynamical stability of single-layer REOBr is further confirmed using phonon dispersions. The predicted energy gaps ranging from 2.47 to 4.26 eV decrease with the atomic number of the rare-earth element. Meanwhile, very large spin moments and orbital moments up to 6.018 and 2.872 μB are found, which seem to be insensitive to the magnetic state. Furthermore, the magnetic anisotropy energies are evaluated and understood by a fourth-order non-uniaxial anisotropy mode. Diverse anisotropy energy landscapes including easy cone, easy plane, and easy axis are found, and an extremely high magnetic anisotropy energy of about 8 meV per RE atom is found in the single-layer DyOBr. Our investigations provide a unique insight into layered rare-earth magnetic materials and suggest the single-layer REOBr as competing candidates for low-dimensional data storage applications.

Topological superconductivity in carbon nanotubes with a small magnetic flux

We show that a one-dimensional topological superconductor can be realized in carbon nanotubes, using a relatively small magnetic field. Our analysis relies on the intrinsic curvature-enhanced spin-orbit coupling of the nanotubes, as well as on the orbital effect of a magnetic flux threaded through the nanotube. Tuning experimental parameters, we show that a half-metallic state may be induced in the nanotube. Coupling the system to an Ising superconductor, with an appreciable spin-triplet component, can then drive the nanotube into a topological superconducting phase. The proposed scheme is investigated by means of real-space tight-binding simulations, accompanied by an effective continuum low-energy theory, which allows us to gain some insight on the roles of different terms in the Hamiltonian. We calculate the topological phase diagram and ascertain the existence of localized Majorana zero modes near the edges. Moreover, we find that in the absence of a magnetic field, a regime exists where sufficiently strong interactions drive the system into a time-reversal-invariant topological superconducting phase.

Orbital ordering and magnetism in layered Perovskite Ruthenate Sr2RuO4

Local density approximation plus on-site Coulomb interaction U electronic structure calculations reveal that layered perovskite oxide Sr2RuO4 exhibits the ferromagnetic (FM) half-metallic ground state, which is nearly degenerate with the antiferromagnetic (AFM) phase with a slightly higher total energy. The nearly degenerate FM/AFM total energies provide a reasonable explanation for the experimentally observed spin-fluctuation. In addition, a dumbbell-shape 4d − t2g recombined dxz − dyz orbital ordering on the Ru sublattice is obtained owing to the on-site Coulomb interaction U associated with the elongated RuO6 octahedron local structure. The discovered orbital ordering is robust against the spin-orbit interaction as well as the surface terminations. Our findings unravel the on-site Coulomb correlation as the driving force of the Ru-4d orbital ordering as well as the inherent magnetic degeneracy.

Possible origin of ferromagnetism in pristine magnesium oxide film

The electronic structure and magnetism of pristine magnesium oxide (MgO) (001) surface have been studied using first principle calculations based on density functional theory (DFT). Our results reveal that isolated magnesium (Mg) vacancy (VMg) can produce a magnetic moment of about 1.99 μB. The magnetic moment mainly comes from p-orbitals of oxygen atoms adjacent to VMg. The surface with one VMg will show half-metallic properties. Studies of magnetic coupling show that the surface with two VMg's will be in a spin singlet state (S = 0) at the distance of 2.98 or 8.42 Å between two VMg's, while the two VMg's are coupled ferromagnetically at the distance of 4.21, 5.96, or 6.66 Å. As the distance between two VMg's increases, the system will change from semiconductor state to half-metallic state, and finally to metal state. These findings are significant for d0 ferromagnetism in MgO film and useful for spintronic application.

Spin-valley polarized quantum anomalous Hall effect and a valley-controlled half-metal in bilayer graphene

We investigate topological phases of bilayer graphene subject to antiferromagnetic exchange field, interlayer bias, and irradiated by light. We discover that at finite bias and light intensity the system transitions into a previously unknown spin-valley polarized quantum anomalous Hall (SVP-QAH) insulator state, for which the subsystem of one spin is a valley Hall topological insulator (TI) and that of the other spin is a QAH insulator. We assess the TI phases occurring in the system by analytically calculating the spin-valley dependent Chern number, and characterize them by considering edge states in a nanoribbon. We demonstrate that the SVP-QAH edge states lead to a unique spin rectification effect in a domain wall. Along the phase boundary, we observe a bulk half-metal state with Berry's phase of 2\pi.

Functionalisation of hexagonal boron phosphide (h-BP) monolayer via atomic adsorption

ABSTRACTIn this study, we investigate the adsorption properties of Fe, Co, Ni, Cu, Zn, In, Tl, Ar atoms on hexagonal boron phosphide monolayer (h-BP) using density functional theory within both GGA and LDA functionals. Bare h-BP is a direct gap semiconductor with planar structure. The adsorption of the atoms on h-BP exhibits a large variety of electronic properties like semiconducting, metallic, and half-metallic states. Fe- and Ni-adsorbed h-BPs show semiconducting character with decreased band gaps. Ni atom is strongly adsorbed on the surface giving largest adsorption energy observed in this work. Fe-adsorbed system is a semiconducting ferromagnet with 1.95 magnetic moment. Co adsorption results in a half-metallic behaviour with 1.00 net magnetic moment and a perfect spin polarisation at Fermi level. Cu, In, and Tl adsorbed h-BP systems show metallic character. The results obtained show that h-BP surface can be functionalised via adsorption of related single atoms and can be suitable for various applications in optoelectronics and spintronics.

Effect of hybridization on half metallic properties of some diluted chalcopyrites for spintronic applications

Based on the ab initio electronic structure calculations performed by Korring–Kohn–Rostoker method combined with the coherent potential approximation, the effect of transition metals impurities on the electronic states and magnetic properties of ZnXAs2 (X = Sn, Ge) chalcopyrites has been investigated. Electronic density of states, Curie temperature TC, ferromagnetic (FM), antiferromagnetic (AF) and spinglass-like (DLM) energies and their variation as well as crystal field splitting energy ΔCF and exchange interactions Δe and are all discussed in detail. The analysis of the densities of states of Zn(Sn, TM)As2 and Zn(Ge, TM)As2 shows that these compounds exhibit half-metallic character and high-spin ferromagnetic state. The importance of different exchange mechanisms like double exchange, ferromagnetic and antiferromagnetic super-exchange in Zn(Sn, TM)As2 and Zn(Ge, TM)As2 chalcopyrites is also discussed. The result as derived in this report shows that and doped ZnSnAs2 and ZnGeAs2 chalcopyrites exhibit a stable ferromagnetic half metallic state, whereas doping manifests antiferromagnetic and spinglass-like states. These results make Zn(Sn, TM)As2 and Zn(Ge, TM)As2 chalcopyrites useful for spintronic applications.

Electronic and magnetic properties of van der Waals ferromagnetic semiconductor VI3

Magnetic van der Waals materials with intrinsic magnetic properties provide the ideal platform for exploring magnetism in the low-dimensional limit. In this work, we investigate the electronic and magnetic properties of the ${\mathrm{VI}}_{3}$ material, a new member of the ferromagnetic van der Waals materials. First-principles results confirm the Mott-insulator state as its electronic ground state. The half-metallic state as reported in the literature is a metastable state, with a total energy 0.2--0.3 eV per ${\mathrm{VI}}_{3}$ higher than the Mott-insulator state. Magnetocrystalline anisotropy calculations confirm an out-of-plane magnetic easy axis of the ${\mathrm{VI}}_{3}$ monolayer. We predict the interlayer exchange coupling of the ${\mathrm{VI}}_{3}$ bilayer to be determined by the interlayer stacking order, ferromagnetic at the most stable and antiferromagnetic at the metastable stacking orders, respectively, reminiscent of the ${\mathrm{CrI}}_{3}$ material.

Tuning electronic structure and magnetic properties of Mn- and Fe-doped arsenene with biaxial strain

Effectively controlling and modulating the electronic structure and magnetism is critical for the application of spintronics devices which based on two dimensional (2D) materials. Herein, by using density functional theory (DFT), we investigate synergistic effect of Mn (or Fe) doping and biaxial strain on electronic structure and magnetism of arsenene system. Unstrained Mn-doped arsenene exhibits a half-metallic state, but unstrained Fe-doped structure is narrow and direct band-gap semiconductor. The emerging spin splitting is caused by orbital hybridization of 4p -3d. Therefore, by the Mn and Fe doing, the systems produce a net magnetic moment of 4.00 and 1.00 , respectively. Particularly, the room temperature ferromagnetism (RTFM) can be achieved in two doping structures. Under the condition of coexistence of impurity atom and biaxial strain, for the Mn doping, the systems are still half-metal materials and magnetism remain stable. While the Fe-doped system translates into a magnetic metal state due to the closure of band gap under the tensile strain of 4%. And the magnetic moments have a sudden transition, increase to 3.92, 5.00 and 5.00 under strains of 4%, 6% and 8%, respectively. In addition, the electronic structure and magnetism of Fe doping can also be affected by doping concentration. This work broadens the way about application of arsenene-based spintronic devices.

Electrical, magnetic and galvanomagnetic properties of Mn-based Heusler alloys

Half-metallic ferromagnets and spin gapless semiconductors are promising materials for spintronic devices since a high degree of the spin polarization of charge carriers can be realized in such materials. Spin gapless semiconductors make it possible to combine the properties of half-metallic ferromagnets with semiconductor characteristics and to perform fine tuning of the energy gap value. The Mn2MeAl (Me = Ti, V, Cr, Mn, Fe, Co, Ni) Heusler alloys can possess such features. We studied the electrical, magnetic and galvanomagnetic properties of the Mn2MeAl (Me = Ti, V, Cr, Mn, Fe, Co, Ni) Heusler alloys from 4.2 K to 900 K and in magnetic fields up to 100 kOe. The features in the electronic and magnetic properties of Mn2MeAl Heusler alloys were observed, which can be a manifestation of the electronic energy spectrum peculiarities with occurrence of the half-metallic ferromagnet and/or spin gapless semiconductor states.

Exotic States Emerged By Spin-Orbit Coupling, Lattice Modulation and Magnetic Field in Lieb Nano-ribbons

The Lieb nano-ribons with the spin-orbit coupling, the lattice modulation and the magnetic field are exactly studied. They are constructed from the Lieb lattice with two open boundaries in a direction. The interplay between the spin-orbit coupling, the lattice modulation and the magnetic field emerges various exotic ground states. With certain conditions of the spin-orbit coupling, the lattice modulation, the magnetic field and filling the ground state becomes half metallic or half topological. In the half metallic ground state, one spin component is metallic, while the other spin component is insulating. In the half topological ground state, one spin component is topological, while the other spin component is topological trivial. The model exhibits very rich phase diagram.

Site-Preference, Electronic, Magnetic, and Half-Metal Properties of Full-Heusler Sc2VGe and a Discussion on the Uniform Strain and Tetragonal Deformation Effects

A hypothetical full-Heusler alloy, Sc2VGe, was analyzed, and the comparison between the XA and L21 structures of this alloy was studied based on first-principles calculations. We found that the L21-type structure was more stable than the XA one. Further, the electronic structures of both types of structure were also investigated based on the calculated band structures. Results show that the physical nature of L21-type Sc2VGe is metallic; however, XA-type Sc2VGe is a half-metal (HM) with 100% spin polarization. When XA-type Sc2VGe is at its equilibrium lattice parameter, its total magnetic moment is 3 μ B , and its total magnetism is mainly attributed to the V atom. The effects of uniform strain and tetragonal lattice distortion on the electronic structures and half-metallic states of XA-type Sc2VGe were also studied. All the aforementioned results indicate that XA-type Sc2VGe would be an ideal candidate for spintronics studies, such as spin generation and injection.