Spin Channel Research Articles

Photoinduced Spin Injection and Ferromagnetism in 2D Group III Monochalcogenides.

Strong light-matter interactions in low-dimensional materials offer an opportunity for flexible property-tuning by optical switching. Herein, we exploit photoexcitation for spin injection into semiconductors by rationally designing heterojunctions having distinct dynamic behavior for photocarriers in two spin channels. As a proof-of-concept, we trigger homogeneous magnetism in a group III monochalcogenide monolayer (MX with M = In, Ga; X = S, Se) by placing it on a ferromagnetic CrI3 substrate under light illumination. Our time-dependent ab initio nonadiabatic molecular dynamics simulations reveal fast electron-hole separation for the majority spin channel but rapid recombination for the minority spin channel at this heterostructure. The majority carriers cause hole doping and strong ferromagnetic ordering in the MX sheet, with magnetic moment tunable by the injected carriers' concentration. The interplay between photoexcited hole carriers, the Van Hove singularity of MX monolayers, and interfacial charge transfer provides essential physical insights for nondestructively manipulating charge and spin in two-dimensional semiconductors via light switching.

Novel bipolar magnetic semiconductor materials formed by adsorption of methyl or halomethyles on the graphene: A spin-polarized density functional theory study

Exploring magnetic materials with electrically controlled spin polarization and room temperature magnetic ordering is of particular importance for applications in spintronic devices. In this study, we present a detailed theoretical study on the structural, electronic, and magnetic features of pristine graphene on which methyl(CH3) or a halomethyl (CH2F, CH2Cl, CH2Br, or CH2I) radical has been adsorbed. The framework of our calculations is dispersion-corrected density functional theory with due account of the spin degree of freedom within the Quantum-ESPRESSO package. The investigation starts with a complete discussion of the geometrical characteristics, followed by a thorough study of the manner of charge transfer and redistribution, electronic band structures, and density of states. The calculated adsorption energies and distances demonstrate the adsorption process to be of the chemisorption type. The adsorption of each radical on graphene is accompanied by a net charge transfer, changing the work function of the resulting compounds and modulating the electronic and magnetization characteristics of pristine graphene. Considerations of the calculated spin-distinct electronic band structure and density of states support the latter conclusion. The presence of unequal band gaps in the up and down spin channels also corroborates the aforementioned compounds' bipolar magnetic semiconductor nature.

Pressure-induced collapse of ferromagnetism in nickel

Transition metals Fe, Co, and Ni are the canonical systems for studying the effect of external perturbations on ferromagnetism. Among these, Ni stands out as it undergoes no structural phase transition under pressure. Here we have investigated the long-debated issue of the pressure-induced magnetization drop in Ni from first principles. Our calculations confirm an abrupt quenching of magnetization at high pressures not associated with any structural phase transition. We find that the pressure substantially enhances the crystal-field splitting of Ni-$3d$ orbitals, driving the system toward a new metallic phase violating the Stoner Criterion for ferromagnetic (FM) ordering. Analyzing the charge populations in each spin channel, we show that the next-nearest-neighbor interactions play a crucial role in quenching FM ordering in Ni and materials alike.

Investigation of nodal line spin-gapless semiconductors using first-principles calculations

Nodal line spin-gapless semiconductors (NLSGSs) are a new type of topological spintronic material with possible high Curie temperature, 100% spin polarization, high carrier mobility, gapless nodal line states at the Fermi level in one spin channel, and spin-polarized drumhead-like surface states.

DFT insights into half metallic ferromagnetism of new cubic PerovskiteVYO3 for spintronics application

The electronic structure, magnetic, optical, thermodynamic and thermoelectric properties of Vanadium-based perovskite VYO3 system are systematically investigated via first-principle computations. FP-LAPW method within PBE-GGA and LSDA, based on the density functional theory (DFT), executed in the WIEN2k-19.1 package is used for the computations. To improve the calculations of the electronic structure as well as magnetic characteristics, the Trans-Blaha modified Becke-Johnson (TB-mBJ) exchange potential is used, which can precisely calculate the energy gap of solids. The computed electronic structure reveals the half-metallic ferromagnetic nature for the investigated compound with total magnetic moment of 2.00008μB. The ferromagnetic phase of VYO3 is energetically more stable with negative cohesive energy, according to our findings. This agrees well with the available similar experimental as well as theoretical calculations. The estimated magnetic moments in VYO3 originate especially from the V-3d state electrons. A semiconductor gap (2.19 eV) has been obtained in minority spin channel while in majority spin channel, the metallic character is dominating, and therefore VYO3 exhibits half-metallic behaviour. The 2D electronic charge density designs in [110] plane indicate the existence of ionic bonding among (O2–-V3+- O2–) and (O2–-Y2+- O2–) bonds to construct all VYO3crystals. The thermodynamic quantities such as coefficient of thermal expansion, Grüneisen parameter, Debye temperature, specific heat capacity have also been analysed up to 1200 K and 80 GPa using Gibbs program. We have used TB-mBJ approximation to evaluate the optical properties by computing the complex dielectric function using with, other parameters are derived. Finally, we have also examined the transport properties i.e., Seebeck coefficient, electrical conductivity, power factor in the range 100–1200 K by semi-classical Boltzmann transport theory.

Engineering High-Spin State Cobalt Cations in Spinel Znco2o4 for Spin Channel Propagation and Electrocatalytic Activity Enhancement in Li-O2 Battery

Engineering High-Spin State Cobalt Cations in Spinel Znco2o4 for Spin Channel Propagation and Electrocatalytic Activity Enhancement in Li-O2 Battery

Electronic structure, magnetoresistance and spin filtering in graphene|2 monolayer-CrI33|graphene van der Waals magnetic tunnel junctions.

In the pursuit of designing van der Waals magnetic tunneling junctions (vdW-MTJs) with two-dimensional (2D) intrinsic magnets, as well as to quantitatively reveal the microscopic nature governing the vertical tunneling pathways beyond the phenomenological descriptions on CrI3-based vdW-MTJs, we investigate the structural configuration, electronic structure and spin-polarized quantum transport of graphene|2 monolayer(2ML)-CrI3|graphene heterostructure with Ag(111) layers as the electrode, using density functional theory (DFT) and its combination of non-equilibrium Green's function (DFT-NEGF) methods. The in-plane lattice of CrI3 layers is found to be stretched when placed on the graphene (Gr) layer, and the layer-stacking does not show any site selectivity. The charge transfer between CrI3 and Gr layers make the CrI3 layer lightly electron-doped, and the Gr layer hole-doped. Excitingly, the inter-layer hybridization between graphene and CrI3 layers render the CrI3 layer metallic in the majority spin channel, giving rise to an insulator-to-half-metal transition. Due to the metallic/insulator characteristics of the spin-majority/minority channel of the 2ML-CrI3 barrier in vdW-MTJs, Gr|2ML-CrI3|Gr heterostructures exhibit an almost perfect spin filtering effect (SFE) near the zero bias in parallel magnetization, a giant tunneling magnetoresistance (TMR) ratio up to 2 × 104%, and remarkable negative differential resistance (NDR). Our results not only give an explanation for the observed giant TMR in CrI3-based MTJs but also show the direct implications of 2D magnets in vdW-heterostructures.

Computational study of phase stability, electronic, magnetic and thermoelectric properties of a new quaternary Heusler alloy CrCoIrGa

Exploring new materials with interesting physical properties plays a vital role in the development of modern technologies. Herein, using density functional theory based first-principles calculations, we present a study on the structural, electronic, magnetic and thermoelectric properties of a new quaternary Heusler alloy CrCoIrGa. The structure stability determined in terms of cohesive energy, formation energy and phonons dispersion curves confirms that alloy is stable and can be considered in experiments. The structural optimization in non-magnetic, ferromagnetic and antiferromagnetic state proves that alloy is ferromagnetic in the ground state with optimized lattice constant of 5.93 Å. The electronic band structure and density of states show that alloy is nearly half-metal (NHF) with 96% spin-polarization at the Fermi level. The effect of pressure on the electronic and magnetic properties investigated in the range 0–25 GPa, reveals that complete half-metallic character at the Fermi level can be obtained for pressure ≥10 GPa with a half-metallic gap of about 1 eV in the minority spin channel and a magnetic moment (Mt) of 3.0 μB/f.u., in agreement with Slater-Pauling rule (Mt = Zt-24). In addition, the electronic structures determined using DFT+U approach exhibit nearly half-metallic property such that amount of spin-polarization increases with increasing Hubbard parameter U. Finally, thermoelectric properties are examined by computing Seebeck coefficient (S), electrical conductivity (σ/τ), electronic thermal conductivity (κ) and power factor (PF) up to 800 K temperature. The positive value of S conveys that heat transport is carried by positive charge carriers. The room temperature values of S, σ/τ, κe/τ and PF are found to be about 5.9 μV/K, 3.06 × 1020 Ω−1m−1s−1, 2.27 × 1015 WK−1m−1s−1 and 1 WK−2m−1s−1, respectively. Collectively, these findings suggest that CrCoIrGa could play a useful role for the development of spintronics and energy production applications.

Generalized dynamical mean-field theory of two-sublattice systems with long-range interactions and its application to study charge and spin correlations in graphene

We investigate magnetic and charge correlations in graphene by using the formulation of extended dynamical mean-field theory (E-DMFT) for two-sublattice systems. First, we map the average non-local interaction onto the effective static interaction between different sublattices, which is treated together with the local interaction within an effective "two-orbital" local model. The remaining part of the non-local interaction is considered by introducing an effective retarded interaction within the E-DMFT approach. The non-local susceptibilities in charge and spin channel are further evaluated in the ladder approximation. We verify the applicability of the proposed method to describe the effect of uniformly screened long-range Coulomb potential $\propto 1/r$, as well as screened realistic long-range electron interaction [T. O. Wehling et al., Phys. Rev. Lett. 106, 236805 (2011)] in graphene. We show that the developed approach describes a competition of semimetal, spin density wave (SDW), and charge-density-wave (CDW) correlations. The obtained phase diagram is in a good agreement with recent results of functional renormalization group (fRG) for finite large graphene nanoflakes and scaling analysis of quantum Monte Carlo data on finite clusters. Similarly to the previously obtained results within the fRG approach, the realistic screening of Coulomb interaction by $\sigma$ bands causes moderate (strong) enhancement of critical long-range interaction strength, needed for the SDW (CDW) instability, compared to the results for the uniformly screened Coulomb potential.

Mechanical Stability, Electronic And Magnetic Properties Of Half- Heusler FeCrAs Alloy For Spintronics Application

The search for functional materials in spintronic devices has become a major component of material research in recent times. The structural, elastic, mechanical, electronic and magnetic properties of half-Heusler FeCrAs alloy (HHFCA) have been examined adopting spin-polarized density functional theory calculations. Our result shows that the hexagonal structure is the high pressure phase of the FeCrAs alloy while the half-Heusler structure is the more stable phase at ambient pressure. Also, the HHFCA is mechanically stable and exhibits half-metallic ferromagnetism besides an indirect band gap in the minority spin channel. The total magnetic moment in one formula unit of the alloy is 1.00 μB, in agreement with the Slater-Pauling rule and the bulk of the magnetic moment contributed by the Cr atoms. Furthermore, high Curie temperature of ~ 1000 K has been obtained for the HHFCA which suggests that it is a promising material for spintronic applications.

Half-metallic properties of Zr2CrAl ferrimagnetic full-Heusler compound, investigated in tetragonal, orthorhombic and rhombohedral crystal structures

The research efforts focused on the implementation of Heusler materials into spintronic heterostructures triggered the investigations on the half-metallic and magnetic properties on Zr2CrAl ferrimagnetic compound. The theoretical investigations based on Density Functional Theory were performed on the tetragonal, orthorhombic and rhombohedral distortions of the typical inverse-Heusler cubic crystalline structure, previously reported. The compound exhibits half-metallic characteristics for all deformations considered. The slight variations in volume of the unit cell do not destroy the high spin polarization state. The band gap from minority spin channel slightly decreases in orthorhombic and rhombohedral deformations. The total magnetic moment calculated per formula unit is integer (equal to 1μB/f.u.) and follows the Slater-Pauling rule for all cases investigated. The antiferromagnetic coupling between the Zr and Cr atoms was preserved. The functionality of ferrimagnetic full-Heusler alloys to be used as overlayers in the production of epitaxially deposited heterostructures on substrates with crystalline structures other than cubic was proven.

Effect of pressure on dynamical stability, electronic and thermoelectric properties of equiatomic quaternary Heusler alloy LaCoTiGe

The equiatomic quaternary Heusler alloys have tremendous applications in many optoelectronic, thermoelectric, and spintronic devices. The proper functioning of these alloys requires the study of lattice dynamics at different values of pressure. Therefore, the lattice dynamical stability of Lanthanum-based LaCoTiGe quaternary Heusler alloy in the face-centered cubic phase has been explored under the effect of pressure by employing density functional perturbation theory followed by density functional theory. The lattice dynamical stability of the face-centered cubic phase of LaCoTiGe is computed with a uniform increase in pressure equal to 2.0 GPa. The pressure-dependent modes are observed in the F-43m phase at the ‘Г’ point. The calculated electronic band structure by implementing density functional theory shows the half-metallic character with a band-gap of the order 0.43 eV for stable LaCoTiGe in the minority spin channel. To evaluate the thermoelectric properties of LaCoTiGe at 2.0 GPa the Boltzmann approximation and constant time relaxation are incorporated as implemented in the BoltzTraP code. The half-metallic behavior of LaCoTiGe results in the high value of the Seebeck coefficient with its maximum value of −50 μV/K at 800K. At relatively higher values of temperature i.e., of the order of 800K, the calculated electronic thermoelectric figure of merit ZT for this material is 0.12, which is much higher than that of other reported similar quaternary Heusler alloys. Based on the results obtained, the Lanthanum-based quaternary Heusler alloy LaCoTiGe could be a suitable candidate for spintronic devices to operate under harsh environmental conditions.

Half Metallic Ferromagnetism and Transport Properties of Zinc Chalcogenides ZnX2Se4 (X = Ti, V, Cr) for Spintronic Applications.

In ferromagnetic semiconductors, the coupling of magnetic ordering with semiconductor character accelerates the quantum computing. The structural stability, Curie temperature (Tc), spin polarization, half magnetic ferromagnetism and transport properties of ZnX2Se4 (X = Ti, V, Cr) chalcogenides for spintronic and thermoelectric applications are studied here by density functional theory (DFT). The highest value of Tc is perceived for ZnCr2Se4. The band structures in both spin channels confirmed half metallic ferromagnetic behavior, which is approved by integer magnetic moments (2, 3, 4) μB of Ti, V and Cr based spinels. The HM behavior is further measured by computing crystal field energy crystal, exchange energies x(d), x (pd) and exchange constants (No and No). The thermoelectric properties are addressed in terms of electrical conductivity, thermal conductivity, Seebeck coefficient and power factor in within a temperature range 0–400 K. The positive Seebeck coefficient shows p-type character and the PF is highest for ZnTi2Se4 (1.2 × 1011 W/mK2) among studied compounds.

Disentangling electronic, lattice, and spin dynamics in the chiral helimagnet Cr1/3NbS2

We investigate the static and ultrafast magneto-optical response of the hexagonal chiral helimagnet $Cr_{1/3}NbS_{2}$ above and below the helimagnetic ordering temperature. The presence of a magnetic easy plane contained within the crystallographic ab-plane is confirmed, while degenerate optical pump-probe experiments reveal significant differences in the dynamic between the parent, $NbS_{2}$, and Cr-intercalated compounds. Time resolved magneto-optical Kerr effect measurements show a two-step demagnetization process, where an initial, sub-ps relaxation and subsequent buildup ($\tau > 50$ ps) in the demagnetization dynamic scale similarly with increasing pump fluence. Despite theoretical evidence for partial gapping of the minority spin channel, suggestive of possible half metallicity in $Cr_{1/3}NbS_{2}$, such a long demagnetization dynamic likely results from spin lattice-relaxation as opposed to minority state blocking. However, comparison of the two-step demagnetization process in $Cr_{1/3}NbS_{2}$ with other 3d intercalated transition metal dichalcogenides reveals a behavior that is unexpected from conventional spin-lattice relaxation, and may be attributed to the complicated interaction of local moments with itinerant electrons in this material system.

Effective modulation of spin accumulation using a ferromagnetic/nonmagnetic bilayer spin channel

A lateral spin valve consisting of highly spin-polarized CoFeAl electrodes with a CoFeAl/Cu bilayer spin channel has been developed. Despite a large spin absorption into the CoFeAl capping channel layer, an efficient spin injection and detection using the CoFeAl electrodes enable us to observe a clear spin valve signal. We demonstrate that the nonlocal spin accumulation signal is significantly modulated depending on the relative angle of the magnetizations between the spin injector and absorber. The observed modulation phenomena is explained by the longitudinal and transverse spin absorption effects into the CoFeAl channel layer with the spin resistance model.

Investigating the magneto‐electronic, structural, mechanical, and thermodynamic properties of filled skutterudite NdRu4Sb12 and EuRu4Sb12: A first‐principles perspective

AbstractThe computational approaches are valuable in predicting the ground state properties of crystalline material. In this study, using the density functional theory, different physical properties of filled skutterudite NdRu4Sb12 and EuRu4Sb12 have been examined. The structural optimization signifies that both the materials are most stable in the ferromagnetic phase and have a lattice constant of 9.27 and 9.39 Å, respectively, which are comparable with already reported results. The spin‐polarized band structure reveals that these materials are metallic for both types of spin electrons. The low density of states around the Fermi level of the spin minority channel corresponding to the significant density of states at the Fermi level of the majority spin channel signifies the presence of high spin‐polarization. The Nd and Eu‐based materials possess a magnetic moment of 3.58 and 6.78 , respectively. The non‐negative elastic constants satisfying the Born stability criteria corroborate the mechanical stability of the materials. Furthermore, we have computed thermodynamic properties like Specific heat, Debye temperature, so on and studied their temperature and pressure dependence.

Electrically switchable van der Waals magnon valves

Van der Waals magnets have emerged as a fertile ground for the exploration of highly tunable spin physics and spin-related technology. Two-dimensional (2D) magnons in van der Waals magnets are collective excitation of spins under strong confinement. Although considerable progress has been made in understanding 2D magnons, a crucial magnon device called the van der Waals magnon valve, in which the magnon signal can be completely and repeatedly turned on and off electrically, has yet to be realized. Here we demonstrate such magnon valves based on van der Waals antiferromagnetic insulator MnPS3. By applying DC electric current through the gate electrode, we show that the second harmonic thermal magnon (SHM) signal can be tuned from positive to negative. The guaranteed zero crossing during this tuning demonstrates a complete blocking of SHM transmission, arising from the nonlinear gate dependence of the non-equilibrium magnon density in the 2D spin channel. Using the switchable magnon valves we demonstrate a magnon-based inverter. These results illustrate the potential of van der Waals anti-ferromagnets for studying highly tunable spin-wave physics and for application in magnon-base circuitry in future information technology.

Generation of terahertz transients from Co2Fe0.4Mn0.6Si -Heusler-alloy/normal-metal nanobilayers excited by femtosecond optical pulses

We generated pulses of electromagnetic radiation in the terahertz (THz) frequency range by optical excitation of ${\mathrm{Co}}_{2}{\mathrm{Fe}}_{0.4}{\mathrm{Mn}}_{0.6}\mathrm{Si}$ (CFMS)/normal-metal (NM) bilayer structures. The CFMS is a Heusler alloy showing a band gap in one spin channel and is therefore a half metal. We compared the THz emission efficiency in a systematic manner for four different CFMS/NM bilayers, where NM was either Pt, Ta, Cr, or Al. Our measurements show that the THz intensity is highest for a Pt capping. We also demonstrate the tunability of the THz amplitude by varying the magnetic field for all four bilayers. We attribute the THz generation to the inverse spin Hall effect. In order to investigate the role of the interface in THz generation, we measured the spin mixing conductance for each CFMS/NM bilayer using a ferromagnetic resonance method. We found that the spin-orbit coupling cannot completely describe the THz generation in the bilayers and that the spin transmission efficiency of the CFMS/NM interface and the spin diffusion length, as well as the oxidation of the NM layer, play crucial roles in the THz emission process.

Bipolar magnetic semiconducting behavior in VNbRuAl

We report the theoretical prediction of a class of spintronic materials, namely, bipolar magnetic semiconductors (BMSs), also supported by our experimental data. BMSs possess a unique band structure with unequal band gaps for spin-up and spin-down channels and thus are useful for tunable spin-transport-based applications such as spin filters. The valence band and conduction band in BMSs approach the Fermi level through opposite spin channels and hence facilitate reversible spin polarization that is controllable via applied gate voltage. We report the quaternary Heusler alloy VNbRuAl to exactly possess the band structure of a BMS. A rigorous normal x-ray diffraction (XRD) fitting along with synchrotron XRD data confirms that this alloy crystallizes in the LiMgPdSn structure with partial $\mathrm{B}2$ disorder. Transport measurements show a two-channel semiconducting behavior and a quasilinear dependence of negative magnetoresistance, indicating the possible semiconducting nature. The thermoelectric power data not only confirm the semiconducting nature but also give a strong indication of the BMS nature. Interestingly, VNbRuAl also appears to show features of a fully compensated ferrimagnetic (FCF) behavior with vanishing magnetization and significantly high ordering temperature ($>900 \mathrm{K}$). Theoretical simulations of the special quasirandom structure predict partial B2 disorder to be mainly responsible for the coexistent BMS and FCF-like behavior. This study opens up the possibility of finding another class of materials for antiferromagnetic spintronics, with great significance for both fundamental and applied fronts.

High-order one-dimensional (1D) fermion in ferromagnetic RbFeF3

Nodal line semimetals/metals (NLSMs), hosting band-crossings in one dimension, are expected to exhibit exotic physical phenomena, like drumhead surface states. The conventional NLSMs host a linear energy dispersion around it. Recently, it has been predicted that NLs hosting a high-order energy dispersion may also be stabilized in nonmagnetic systems. However, it still remains challenges to find their counterpart in magnetic systems. Here, based on first-principles calculation and theoretical analysis, we predict for the first-time that RbFeF3, a ferromagnetic material, has a high-order NL without spin-orbital coupling (SOC). We give the symmetry conditions which stabilizes the quadratic nodal lines (QNLs), thus an effective Hamiltonian is constructed to demonstrate its existence. Different from the nonmagnetic nodal lines, they come from the same single spin channel, achieving a 100% spin polarization. Interestingly, it exhibits a “torus” drumhead surface states in the whole Brillouin zone, which is quite different from that of linear NL. Under symmetry breaking, the QNL could be reduced into a pair of linear type-II NLs by tuning the magnetization direction. Our work provides a ideal magnetic material with high-order NL which has not been observed before.