Spin-flip Gap Research Articles

Effect of uniaxial strain on the electronic and magnetic properties of N2SrRb: a DFT study

ABSTRACT Using the first-principles density functional theory calculations, we examined the effect of uniaxial strain on the electronic and magnetic properties of full-Heusler alloy N2SrRb. Our results show that N2SrRb is a ferromagnetic full-Heusler alloy, and it exhibits half metallic properties with 100% spin polarisation when no strain is applied. The half-metallic and magnetic properties of N2SrRb were retained under the ranges of tensile and compressive strain applied. The spin flip gap was found to increase as the tensile strain increases, while it decreases as the compressive strain increases. The mechanical properties show that the full-Heusler alloy is mechanically stable and having a ductile characteristics

Where is the Quantum Spin Nematic?

We provide strong evidence of the spin-nematic state in a paradigmatic ferro-antiferromagnetic J_{1}-J_{2} model using analytical and density-matrix renormalization group methods. In zero field, the attraction of spin-flip pairs leads to a first-order transition and no nematic state, while pair repulsion at larger J_{2} stabilizes the nematic phase in a narrow region near the pair-condensation field. A devil's staircase of multipair condensates is conjectured for weak pair attraction. A suppression of the spin-flip gap by many-body effects leads to an order-of-magnitude contraction of the nematic phase compared to naïve expectations. The proposed phase diagram should be broadly valid.

Half-metallic, magneto-optic, and thermoelectric properties of CoRuVZ (Z=Al, Ga)

Based on the density functional theory, structural, phonon stability, half-metallic, optical, magneto-optical, and thermoelectric properties of CoRuVAl and CoRuVGa compounds have been calculated. In the ferromagnetic phase, these compounds have equilibrium volumes. Elastic coefficients and their related parameters indicate the elastic stability of these compounds. Moreover, the B/G ratio indicates their ductile behavior. Phonon scattering in the first Brillouin zone in both compounds indicates dynamic stability. The electrical properties of these compounds with GGA+U+mBJ approximation demonstrate half-metallic behavior with a spin-flip gap of 1.5 eV and 1.2 eV for CoRuVAl and CoRuVGa, respectively. Kerr and Epsilon angles of these compounds have significant peaks in the visible region, representing large polarization leading to proper magneto-optical behavior in the visible region. Thermoelectric parameters such as the figure of merit and Seebeck coefficients indicate that these compounds are suitable for thermoelectric applications at room temperature.

Electronic, magnetic, and optical properties of bulk and (1 1 1)-surfaces of CoMnZnSi quaternary Heusler alloy

The structural stability, electronic structure, and magnetic and optical properties of CoMnZnSi quaternary Heusler alloy (QHA) in the bulk and (111)-slab forms have been investigated by performing density functional theory (DFT) calculations. Formation and cohesive energies, and elastic constants confirm that the bulk CoMnZnSi is chemically and mechanically stable at an equilibrium lattice parameter of 5.81 Å. The calculated elastic constants also indicate that this QHA has ductile and anisotropic features. We investigate and discuss the bonding behavior from charge density distribution and density of states. These calculations in the bulk phase show a perfect half-metallic behavior with an integer value of magnetic moment (4 µB) and a large spin-flip gap of 0.39 eV. On the other hand, the Co (111) and Si (111)-slabs exhibit semi-metallic nature at the Fermi level. These results indicate that CoMnZnSi qualifies for spintronic applications. In the bulk and Co (111), Zn (111), and Si (111)-surfaces, the magnetic moment of Co and Mn atoms are ferromagnetically aligned, while the ferrimagnetic alignment of Co with Mn has been found in Mn (111)-surface. We also investigate the optimal conductivity, dielectric functions, reflectivity, absorption coefficient, refractive index, and loss function to understand the underlying optical properties of this alloy.

Preserving the half-metallicity at the full Heusler alloy Sc2CrSi surfaces as well as Sc2CrSi/HgTe (1 1 1) interface

Using the first principle study within the density functional theory, the electronic and magnetic properties of the bulk on (111), (110) and (001) surfaces of the full-Heusler alloy and the Sc2CrSi/HgTe (111) interfaces were verified. In this article, our calculations showed that the compound exhibits half-metallic ferromagnetism with an energy gap of 0.544 eV and a spin-flip gap of 0.1 eV in the majority spin channel at an equilibrium lattice constant of 6.39 Å. The total magnetic moment of the studied compound was calculated according to the Slater rule to be 2 μB. The results confirm that the half-metallicity of bulk is completely conserved at the Sc(2)Si-terminated (001) surface, Sc(1)-, and Si-terminated (111) surfaces, but destroyed at Cr- and Sc(2)-terminated (111) surface, the Sc(1)Cr- terminated (001) surface, and Sc(1)Sc(2)CrSi-terminated (110) Surface. From the above-mentioned characteristics of Sc2CrSi exhibit that this alloy is an adequate promising candidate for spin electronics implementations. Moreover, during the calculation of the interfacial adhesion energies, it is found it that Sc(1)-Te and Si-Te configurations are more stable among the others. Regrettably, interfacial configurations show that the half-metallicity of bulk Sc2CrSi is destroyed for all four possible configurations.

Ultralong lifetime for fully photogenerated spin-polarized current in two-dimensional ferromagnetic/nonmagnetic semiconductor heterostructures

Photogenerated fully spin-polarized current has attracted considerable attention in spintronics owing to its high efficiency for information processing but low energy consumption. However, the short spin carrier lifetime of the primary materials significantly impedes the practical application of spin-polarized current and presents a major challenge. Here, on the basis of density functional theory and nonadiabatic molecular dynamics simulations, we design two prototypes of two-dimensional (2D) ferromagnetic/nonmagnetic semiconductor heterostructures ($\mathrm{Cr}{\mathrm{I}}_{3}/\mathrm{Mo}{\mathrm{Se}}_{2}$ and $\mathrm{Cr}{\mathrm{Br}}_{3}/\mathrm{Mo}{\mathrm{S}}_{2}$) with demonstrated ultralong photogenerated spin carrier lifetime. It is shown that the photogenerated spin holes can be effectively injected from the ferromagnetic layer to the nonmagnetic layer, and fully spin-polarized electrons will be confined in the ferromagnetic layer due to their type-II spin-polarized channel and large spin-flip gap. More surprisingly, the photogenerated spin carrier lifetime of these heterostructures is up to the time scale of nanoseconds. Such an ultralong spin carrier lifetime is mainly originated from small nonadiabatic coupling induced by weak spin electron-phonon interaction and slow nuclear velocity. Our results provide insights into the design of 2D semiconductor spintronic devices with an ultralong fully spin-polarized current lifetime.

New p-type sp-based half-Heusler compounds LiBaX(X = Si, Ge) for spintronics and thermoelectricity via ab-initio calculations

We have investigated the structural, electronic, magnetic, elastic and thermoelectric properties of the sp-based half-Heusler LiBaX (X = Si and Ge) alloys using the full-potential linearized augmented plane wave method of density functional theory. Two approaches are employed to approximate the correlation-exchange potential, namely the generalized-gradient approximation and the modified Becke–Johnson. Both compounds free of transition metals are ferromagnetic, half metallic with a large spin-flip gaps which are appropriate for Spintronic applications. Their half-metallicity is preserved under large lattice constant and pressure change which make them promising candidates for spintronic devices. The transport properties reveal high figure of merit values making these compounds great thermoelectric candidates. Moreover, the studied half-Heuslers are mechanically stable and both of them exhibit negative formation energy and cohesion which indicate the possibility of synthesizing and stabilizing these cheap and available materials. Up to date, no experimental or theoretical studies have been carried out regarding LiBaSi, while for LiBaGe, thermoelectric properties have not been investigated yet.

A study half-metallic surfaces of the full-Heusler Sc2CrGe compound and the interface of Sc2CrGe/InSb (111)

Our study showed that full-Heusler Sc2CrGe displays half metallic ferromagnetism with an energy gap of 0.57 eV and a spin-flip gap of 0.12 eV in the majority spin channel at an optimization equilibrium lattice constant of 6.5 Å. In this study, we investigate extensively the electronic and magnetic features of the (111), (001), and (110) surfaces and the interface with the semiconductor InSb (111) via applying the first-principles computations of the density functional theory. The atomic density of states confirms that the half-metallicity discovered in the bulk Sc2CrGe is kept at the Sc(2)Ge-terminated (001), Sc(1)-, and Ge-terminated (111) surfaces, but it is lost at the Sc(2)- and Cr-terminated (111), Sc(1)Cr- terminated (001), and Sc2CrGe (110) surfaces. For the Sc2CrGe/InSb (111) interface, the bulk half-metallicity is destroyed at the Sc(1)-In, Sc(1)-Sb, and Ge-In configurations while the Ge-Sb configuration exhibits a full spin polarization. Moreover, the computed interfacial adhesion energy shows that the Sc(1)-Sb shape is more stable than the others. The calculated magnetic moments of interface atoms (Sc and Ge) decrease compared to the corresponding bulk values.

Theoretical Prediction of Half-Metallic Properties of CrHfCoZ (Z = Ga, In, Tl, Si) Alloys

Using first-principles calculations, we investigate systematically the stability, electronic and magnetic characteristics for the CrHfCoZ (Z = Ga, In, Tl, Si) alloys. Stability is estimated in views of thermodynamics, mechanical and dynamics points. Electronic calculations show that CrHfCoZ alloys have half-metallic behavior, and their integral magnetic moments obey the Slater-Pauling rule. Further, we discuss the formation of spin-flip gap along with integer magnetic moment according to the density of states. In addition, we plot the Fermi surface to illustrate the electronic behavior around the Fermi level of CrHfCoZ alloys. Finally, the changes of conduction band maximum and valence band minimum dependent on the lattice constants are also discussed. Our research may provide a potential applications in spintronic devices.

New equiatomic quaternary Heusler compounds without transition metals KCaBX (X = S and Se): Robust half-metallicity and optical properties

It is known that high spin-polarization and magnetism can be found even in materials with neither transition metals nor rare earths. In this paper, we report results of the structural design, electronic structure, magnetic and optical properties of new equiatomic quaternary Heusler (EQH) KCaBX (X = S and Se) compounds. Electron exchangecorrelation interactions are described by the Wu-Cohen (WC) functional and Tran-Blaha modified Becke-Johnson exchange (mBJ) potential. Ferromagnetic ordering is stable for the cubic structure of space group F43 m in which the K, Ca, B and X atoms are located at 4c, 4d, 4a and 4b Wyckoff positions, respectively. Quaternaries at hand exhibit a perfect spin-polarization around the Fermi level, which is a result of the half-metallicity with metallic spin-up channel and semiconductor spin-dn channel. The ferromagnetic half-metallic and spin-flip band gaps are 2.648(2.470) and 0.673(0.526), respectively, for KCaBS(KCaBSe). Both studied compounds have a total magnetic moment of 2.000 μB. Additionally, the strain effect on the electronic and magnetic properties is also examined. Finally, the optical properties of the KCaBX alloys are investigated for energies up to 25 eV. Optical spectra show the metallic behavior at extremely low energies and semiconductor nature at higher energies. Interestingly, KCaBS and KCaBSe exhibit prospective absorption properties with a quite large absorption coefficient in the ultraviolet regime.

Mn2CoX (X = P and As) full-Heusler compounds for spintronic applications: Half-metallicity and elastic properties

Structural stability, half-metallicity, magnetism and elastic properties of the Mn2CoP and Mn2CoAs full-Heusler compounds are investigated using the density functional theory (DFT). Materials crystallize in the inverse Heusler structure with confirmed thermodynamic, structural and dynamical stabilities. Band structures show the half-metallicity of materials, with metallic spin-up channel and semiconductor spin-dn channel. Obtained ferromagnetic and spin-flip gaps are 0.965(1.174) and 0.311(0.113) eV, respectively, when X atom is P(As). A total magnetic moment of 4 (μB) is obtained, being produced mainly by the Mn(3d)-Co(3d) interactions. Strain effect on the electronic and magnetic properties is also examined. Results suggest the promising spintronic applicability of materials studied here. Elastic properties calculations indicate that these compounds are mechanically stable and elastically ductile, where the former is more resistant than the later.

Thermoelectric and half-metallic behavior of the Co2TaAl: a DFT study

The electronic, thermoelectric, optical and thermodynamic stability of the full-Heusler compound Co2TaAl is investigated under hydrostatic pressure in the range of (25.4 to − 26.3 GPa) within the framework of the density functional theory and the semi-classical Boltzmann theory using the generalized gradient approximation. The Co2TaAl compound in the minority spin is an n-type semiconductor with an indirect band gap of about 0.78 eV along Γ–X direction and a spin-flip gap of 0.6 eV. Applying stress led to decreasing the band gap turning the material to a p-type semiconductor; however, it remained n-type but with zero spin-flip gap under strain. The dimensionless Merit coefficient for the spin dn channel of Co2TaAl at equilibrium pressure obtained a large value of 0.98 at temperatures below 500 K. The optical properties of this compound, including refraction and reflection coefficients, absorption spectra, and real and imaginary parts of the dielectric function are studied in detail.

Half-metallic properties in Co2XSn (X = Ti, V and Cr) full-Heusler compound

First-principles calculations based on density functional theory (DFT) confirm the half-metallic ferromagnetism in both [Formula: see text] and [Formula: see text], and the nearly half-metallic ferromagnetism in [Formula: see text] Heusler alloys with the [Formula: see text]-type structure [Formula: see text]. The electronic band structures and density of states (DOS) calculations of the [Formula: see text] and [Formula: see text] compounds show that the spin-up electrons are metallic, whereas the spin-down bands are semiconducting with a gap of 0.47 eV and 0.53 eV, respectively, with 0.21 eV and 0.36 eV as a spin-flip gap, respectively. The [Formula: see text] and [Formula: see text] Heusler were half-metal compounds with magnetic moment of [Formula: see text] and [Formula: see text] at the equilibrium lattice constants [Formula: see text] Å and [Formula: see text] Å, respectively, which agrees with the Slater–Pauling rule, and have 100% polarization for a wide range of lattice parameters. The [Formula: see text] is a nearly half-metal (NHF) compound with magnetic moment of [Formula: see text] and 92.9% polarization at the equilibrium lattice constants [Formula: see text] Å and acquire half-metal behavior under the pressure 16.70 GPa.

Magnetism and half-metallicity in bulk and (100), (111)-surfaces of Co2ScSb full Heusler alloy for spintronic applications

The half-metallic nature with 100% spin-polarization at the surface needs to be conserved to implement the Heusler alloys in spintronic devices. Therefore, we studied the electronic and magnetic properties in bulk and (100), (111)-surfaces of the Co2ScSb Heusler alloy. We performed ab-initio calculations utilizing the GGA and GGA + U approximations. The Co2ScSb Heusler alloy is found to be stable in the ferromagnetic state with an optimized lattice parameter of 6.19 Å. From elastic constant calculations, the alloy considered is mechanically stable with ductile character. The real phonon frequency in whole Brillouin zone confirms the dynamical stability of Co2ScSb alloy. It was found that the half-metallic nature preserved by both approximations and GGA + U gives a larger energy band gap of 0.65 eV and spin-flip gap of 0.10 eV for bulk Co2ScSb alloy. From the investigated layer-density of states, we observed the half-metallicity of 0.13 and 0.06 eV with 100% spin-polarization in ScSb (100) and Sc (111)-surfaces respectively. However, the half-metallic nature is destroyed in CoCo (100), Co (111) and Sb (111)-surfaces because of the surface states presented at the Fermi level. We found that the atomic moments of the S9 layer for five terminations are precisely close to the corresponding bulk values. This work proves that the GGA + U approximation is an appropriate technique to find the half-metallic character in thin-film Heusler alloys.

Intriguing p-orbital magnetic semiconductors and carrier induced half-metallicity in surface oxygen-functionalized two-dimensional [formula omitted] ([formula omitted]) crystals

Two-dimensional (2D) ferromagnetic semiconductors with robust magnetism are urgently desired for nanoscale spintronics applications. However, it remains a challenge to realize them experimentally. In this work, we proposed intriguing 2D p-orbital ferromagnetic semiconductors X2NO2 (X=Ca,Sr) monolayer with 3μB magnet per unit under O surface termination using first-principles calculations. The Ca2NO2 monolayer is bipolar magnetic material (BMS) with spin-flip gap 0.24 eV, and the Sr2NO2 is half-semiconductor (HSC) with spin-flip gap 0.31 eV, which is large enough to prevent the spin-flip transition. The Curie temperature can reach to 206 and 239 K, respectively, due to the superexchange interaction between N- ions. The values are much higher than the boiling point of liquid nitrogen (77 K) and comparable to that of the reported ScCl monolayer. Moreover, the half metals are obtained via carrier doping for both the Ca2NO2 and Sr2NO2 monolayers. In addition, the half-metallic completely spin-polarized direction of BMS Ca2NO2 monolayer can be controlled by carrier doping type. Furthermore, the magnetism of X2NO2 monolayer is derived from p orbital, without the involvement of conventional transition metals or rare earth atoms. This is advantageous for high-speed and long-distance spin-polarization transport. These results suggest that the X2NO2 monolayer can develop spin field effect transistor for information processing and storage, and open opportunities for designing new ferromagnetic semiconductors.

Investigation of half-metallic and magnetic phase transition in Co2TiZ (Z = Al, Ga, In) Heusler alloys

ABSTRACTThe half-metallic and ferromagnetic properties of Co2TiZ (Z = Al, Ga, In) Heusler alloys are investigated. The structural stability is analyzed among the two possible structures, namely, L21 (Cu2MnAl phase) and XA (Hg2CuTi phase) structures. It is found that these alloys are stable in L21 (Cu2MnAl phase) structure. The electronic structure of Co2TiZ reveals that these alloys exhibit half-metallic ferromagnetism with small spin-flip gap at the minority spin state. As the pressure is increased, ferromagnetic to non-magnetic phase transition is observed at the pressures of 380.9, 363.0 and 317.8 GPa for Co2TiAl, Co2TiGa and Co2TiIn, respectively. Half-metallic to metallic phase transition occurs in Co2TiAl, Co2TiGa and Co2TiIn at the pressures of 76.5 GPa, 73.1 GPa and 63.9 GPa respectively.

Electronic and Magnetic Properties of Fe2−xCoxTiSi (x = 0, 0.5, 1, 1.5, 2) Heusler Alloys

The first principles calculations are carried out to understand the half metallic and ferromagnetic properties of Fe2−xCoxTiSi (x = 0, 0.5, 1, 1.5, 2) full Heusler alloys. The stable structure is predicted as L21 structure (Cu2MnAl phase). The lattice constant value of Fe2−xCoxTiSi Heusler compounds increases as the value of x increases. These alloys exhibit half metallic ferromagnetism (except Fe2TiSi) with small spin flip gap at the minority spin state. At high pressure, semiconductor to metallic phase transition is observed in Fe2TiSi while half metallic to metallic phase transition is observed in Fe1.5Co0.5TiSi, Fe1.0 Co1.0TiSi, Fe0.5Co1.5TiSi and Co2TiSi. The spin polarization of these compounds (except Fe2TiSi) is 100%. Hence, these compounds are suitable for spin injection devices in spintronics.

Experimental realization and magnetotransport properties of half-metallic Fe2Si

Thin films of the high Curie temperature intermetallic ferromagnet β-Fe2Si were synthesized via molecular beam epitaxy. Investigation using X-ray diffraction and atomic force microscopy shows a hexagonal crystal structure and a smooth topography. Theoretically, Fe2Si has been predicted to exhibit uniquely desirable magnetotransport properties. We report on these properties experimentally, including the ordinary Hall coefficient R0 and anomalous Hall resistivity ρxyAH. The compound is found to be a soft Heisenberg ferromagnet with temperature dependent magnetization based on the thermal excitation of spin waves. We present a detailed look into the contributions to its longitudinal resistivity, which due to the presence of a spin-flip gap ΔkB, indicating a half-metallic band structure. The correct scaling relations between these components of the resistivity tensor (ρxx and ρxy) are also discussed.

Investigation of half-metallicity and magnetism of bulk and (111)-surfaces of Fe2MnP full Heusler alloy

The structural, electronic, magnetic, elastic and phonon properties of bulk and electronic and magnetic properties of (111)-surfaces of AlCu2Mn-type Fe2MnP Heusler alloy have been investigated by using density functional theory based on the generalized gradient approximation scheme. The cohesive energy confirmed the stability of the alloy considered at an equilibrium lattice constant of 5.5628 Å. We observed a half-metallic nature in the bulk alloy with energy band gaps of 0.34 eV and spin-flip gaps of 0.09 eV with 100% spin-polarization in the minority spin electrons. The computed magnetic moments for the Fe2MnP alloy are found to be 4 μB/f.u. The investigated elastic constants, phonon dispersion and density phonon states showed that the Fe2MnP alloy is mechanically and dynamically stable. We also found that the alloy is ductile from the Poisson's and B/G ratios. The half-metallic ferromagnetic nature is preserved in the P (111)-terminated slab among all terminations with 100% spin-polarization which is a crucial property of spin devices, since it can be optimize their performance. The spin-polarization clearly decreases by nearly 75% for Fe (111) and Mn (111)-terminated slabs due to surface states located below the Fermi level. Moreover, the computed atomic magnetic moments of (111)-slabs of the Fe2MnP Heusler alloy are extremely close to the atomic bulk values.

Electronic and magnetic properties of Fe2−xCoxTiGe (x = 0, 0.5, 1, 1.5, 2) full Heusler alloys

Half metallic and magnetic properties of Fe2TiGe and Co2TiGe full Heusler alloys are studied. The structural stability between the two possible structures, namely, L21 (Cu2MnAl phase) and XA (Hg2CuTi phase) structures is analyzed. It is found that L21 phase is the stable phase for these alloys. The density of states predicted that Fe2TiGe is a semiconductor and Co2TiGe is half-metallic. When cobalt is substituted for Fe, it is observed that Fe1.5Co0.5TiGe, Fe1.0Co1.0TiGe, Fe0.5Co1.5TiGe alloys exhibit half metallic ferromagnetism with small spin flip gap at the minority spin state. This is due to the presence of d7 electronic configuration within Co2+ in Fe2+ ions site. Half metallic to metallic phase transition is observed at high pressure. Our work paves the way for designing novel Heusler materials at normal and high pressure. Furthermore, it is observed that Co- doping is an effective strategy to improvise the ferromagnetic performance of Fe2TiGe full Heusler compounds.