Minority Spin Band Gap Research Articles

Sb surface terminated MnSb devices in the niccolite phase

The magneto-electronic properties of ferromagnetic MnSb grown by molecular beam epitaxy can be dominated by the presence of a surface state in the minority spin bandgap when the surface is Sb-terminated. The material resistivity is 120 µΩ.cm at 295 K, and although this is determined by the majority spin population, the anisotropic magnetoresistance, dependent on minority spins, is ∼0.24% for the Sb-terminated devices with Mn-terminated devices showing ∼0.02%. At 295 K, the extraordinary Hall constant is 0.5 Ω/T for the Sb-terminated surface and 1.5 Ω/T for the Mn-terminated surface with the extraordinary Hall constant and anisotropic magnetoresistance behaving with an anomalous temperature dependence between 295 and 1.5 K. The dominant MnSb structural phase on the GaAs (001) orientation is naturally doped p-type with a carrier density ∼1 × 1022 cm−3 determined by the normal Hall effect after the extraordinary Hall effect has saturated at higher fields than ∼2 T. Spintronic device possibilities are discussed, particularly the spin-light emitting diode and magnetic nano-structures. A natural p-type doping in MnSb limits the devices to dominant hole carrier effects although there is compatibility with both III–V and Si–Ge materials for hybrid device possibilities.

The spin-state transition in ACo2O4 spinels (A = Be, Mg, Ca, Cd, Zn)

The magnetic and electronic properties of the Co-based spinel oxides ACo2O4 (A = Be, Mg, Ca, Zn, Cd) were studied within GGA + U approach. It was found that the Co3+ ion is in a low-spin state due to the effect of the crystal field of octahedral symmetry. It is shown that Co3+ ion undergoes a spin-state transition into the high-spin state under the critical pressure of P = −10 GPa – −20 GPa. This pressure-induced spin-state transition is caused by the redistribution of electrons between the t2g- and eg-orbitals arising with increasing interatomic distances. The role of interatomic distances between Co3+ ion and its ligands is discussed. Thin-film form also favors the appearance of a high-spin state of Co3+ ion. At the same critical pressure, there is a sharp increase in the majority spin bandgap and a sharp decrease in the minority spin bandgap. These findings allow manipulating the spin state of Co3+ ions and bandgap width through the pressure or strain arising in thin films.

Electronic, Structural, Mechanical, and Thermodynamic Properties of CoYSb (Y = Cr, Mo, W) Half-Heusler Compounds as Potential Spintronic Materials

We used density functional theory (DFT) calculations to investigate the structural, electronic, magnetic, mechanical, and thermodynamic properties of CoYSb (Y = Cr, Mo and W) compounds. These are XYZ type half-Heusler alloys, which also exist in the face centred cubic MgAgAs-type structure and conform to F4¯3m space group. We computed these properties in three different atomic arrangements known as Type-I, Type-II, and Type-III phases. In all these phases, the alloys were found to be in the ferromagnetic state. Furthermore, the calculated electronic band structure and the total electronic density of states indicated a metallic behavior in CoWSb, nearly half-metallic behavior in CoMoSb, and half-metallic behavior in CoCrSb, with a minority-spin band gap of 0.81 eV. Furthermore, the calculated mechanical properties predicted an anisotropic behavior of these alloys in their stable phase. Finally, due to its high Debye temperature value, CoCrSb shows stronger covalent bonding than CoMoSb and CoWSb, respectively.

Half-metallicity in Fe2MnSi and Mn2FeSi heusler compounds: A comparative ab initio study

First-principles calculations of the structural, electronic, and magnetic properties of full-Heusler compounds Fe2MnSi and Mn2FeSi in regular L21 and inverse XA structures have been performed using density functional theory (DFT) within generalized gradient approximation (GGA) and SCAN functionals. All compounds indicate half-metallic properties with the minority spin bandgap. The causes for the appearance of half-metallic bandgap and the difference in the electronic and magnetic properties of Heusler compounds were studied and analyzed in terms of the local environment. It is shown, that the half-metallic bandgap determines by the behavior of the t2g-electrons of A- and C-sites atoms. The behavior of the compounds under pressure was also considered. The high sensitivity of the magnetic moments on atoms A, C and the bandgap to pressure is discussed. The transition from regular to inverse structure is predicted at the negative pressure.

Tetragonal and uniaxial strains in pristine and doped half-Heusler AuMnSn alloy

Using the first-principles density functional theory calculations, we examined the effect of tetragonal strain (t-strain) and uniaxial strain (u-strain) on the electronic and magnetic properties of pristine and Sb-doped half-Heusler AuMnSn alloy. Our result shows that ferromagnetic half-Heusler AuMnSn (AMS) exhibits half metallic ferromagnetism (HMF) only from −8% to −10% u-strain. On the other hand, the Sb-doped half-Heusler AuMnSn (AMSS) exhibits HMF from −2% to 4% t-strain and from −4% to 2% u-strain while the width of the minority spin band gap remains unaltered. We also observed changes in the magnetic moments as the applied strain changes from compressive to tensile strain. Our result suggests the possibility of epitaxial growth of thin-film of half-metallic AMSS on a semiconductor substrate, even with a small lattice mismatch.

Half-metallic surfaces in thin-film Ti2MnAl0.5Sn0.5

Materials exhibiting a high degree of spin polarization in electron transport are in demand for applications in spintronics—an emerging technology utilizing a spin degree of freedom in electronic devices. Room-temperature half-metals are considered ideal candidates, as they behave as an insulator for one spin channel and as a conductor for the other spin channel. In addition, for nano-size devices, one has to take into account possible modification of electronic structure in thin-film geometry, due to the potential presence of surface/interface states. It has been shown that typically these states have a detrimental impact on half-metallicity, i.e. their presence results in reduced spin-polarization. Here, we employ density functional calculations to explore an inverse Heusler compound, Ti2MnAl0.5Sn0.5, which exhibits half-metallic electronic structure in bulk geometry. In particular, this material behaves as a regular metal for majority-spin, and as a semiconductor for minority-spin states. We show that in thin-film geometry, the type of termination surface has a decisive effect on half-metallicity of this material. In particular, we analyze six possible termination configurations, and show that for four of them, energy states emerge in the minority-spin band gap, significantly reducing the spin polarization of Ti2MnAl0.5Sn0.5. At the same time, our calculations indicate that two termination surfaces preserve half-metallic properties of this material. This result is somewhat unexpected, as most of the available literature reports reduction of the spin-polarization due to the presence of surface states. Thus, our results show that a judicious choice of the termination surface may be a crucial factor in nano-device applications, where highly spin-polarized current is needed.

Effects of pressure and strain on spin polarization of IrMnSb

A high degree of spin polarization in electron transport is one of the most sought-after properties of a material which can be used in spintronics—an emerging technology utilizing a spin degree of freedom in electronic devices. An ideal candidate to exhibit highly spin-polarized current would be a room temperature half-metal, a material which behaves as an insulator for one spin channel and as a conductor for the other spin channel. In this paper, we explore a semi-Heusler compound, IrMnSb, which has been reported to exhibit pressure induced half-metallic transition. We confirm that the bulk IrMnSb is a spin-polarized metal, with dominant contribution to electronic states at the Fermi energy from majority-spin electrons. Application of a uniform pressure shifts the Fermi level into the minority-spin energy gap, thus demonstrating pressure induced half-metallic transition. This behavior is explained by the reduction of the exchange splitting of the spin bands consistent with the Stoner model for itinerant magnetism. We find that the half-metallic transition is suppressed when instead of uniform pressure the bulk IrMnSb is exposed to biaxial strain. This suppression of half-metallicity is driven by the epitaxial strain induced tetragonal distortion, which lifts the degeneracy of the Mn 3d t2g and eg orbitals and reduces the minority-spin band gap under compressive strain, thus preventing half-metallic transition. Our calculations also indicate that in thin film geometry, surface states emerge in the minority-spin band gap, which has detrimental for practical applications impact on the spin polarization of IrMnSb.

Half-metallic Co-based quaternary Heusler alloys for spintronics: Defect- and pressure-induced transitions and properties

Heusler compounds offer potential as spintronic devices due to their spin-polarization and half-metallicity properties, where electron spin-majority (minority) manifold exhibits states (band gap) at the electronic chemical potential, yielding full spin-polarization in a single manifold. Yet, Heuslers often exhibit intrinsic disorder that degrades its half-metallicity and spin-polarization. Using density-functional theory, we analyze the electronic and magnetic properties of equiatomic Heusler ($L$2$_{1}$) CoMnCrAl and CoFeCrGe alloys for effects of hydrostatic pressure and intrinsic disorder (thermal antisites, binary swaps, and vacancies). Under pressure, CoMnCrAl undergoes a metallic transition, while half-metallicity in CoFeCrGe is retained for a limited range. Antisite disorder between Co-Al pairs in CoMnCrAl and Co-Ge pairs in CoFeCrGe is energetically the most favored, and retain half-metallic character in Co-excess samples. However, Co-deficient samples undergo a transition from half-metallic to metallic, with a discontinuity in the saturation magnetization. For binary swaps, configurations that compete with the ground state are identified and show no loss of half-metallicity; however, the minority-spin bandgap and magnetic moments vary depending on the atoms swapped. For single binary swaps, there is a significant energy cost in CoMnCrAl but with no loss of half metallicity. Although a few configurations in CoFeCrGe energetically compete with the ground statei, however the minority-spin bandgap and magnetic moments vary depending on the atoms swapped. These informations should help in controlling these potential spintronic materials.

Half-metallicity and magnetism of Ti2[formula omitted] Cox[formula omitted] Siy inverse Heusler alloys

Half-metallicity and magnetism of Ti2Ni1−x CoxAl1−y Siy, which are obtained by Co/Si substitutions for Ni/Al of inverse Heusler alloy Ti2NiAl, are investigated by first-principle calculations based on density functional theory (DFT). The optimized lattice constants of the doped systems all conform to the Vegard law as the increase of the impurity concentration, and the magnetic moments obey the Slater-Pauling rule when the half-metallicity is retained. The defect formation energies of the codoped systems are lower than those of the monodoped systems due to the charge compensation effects, thus the Co+Si codoping is more favorable in energy than the Co/Si monodoping. Furthermore, for the Co and Si monodoped systems, the Co monodoping retains the minority-spin bandgap unchanged although the Fermi level moves towards high energy region, and the Si monodoping leads to the minority-spin bandgap narrowing and even the loss of half-metallicity at the high concentration, while for the Co+Si codoped systems, the majority of the codoped compounds obviously show more stable half-metallicity and the minority-spin gap get widened. In particular, the minority-spin band gap of the codoped compounds Ti2Ni0.5Co0.5Al0.5Si0.5, Ti2Ni0.25Co0.75Al0.5Si0.5, and Ti2NiCo Al0.25Si0.75 are widened distinctly and their Fermi level are adjusted to the middle of the minority-spin gap, indicating that they possess robust half-metallicity and thus they are promising candidates for spintronics applications.

Highly spin-polarized Dirac fermions at the graphene/Co interface

The interface of graphene with ferromagnets is highly relevant for spintronics, because graphene on Co(0001) shows a largely intact Dirac cone and strong hybridization with Co $3d$ states breaking the sublattice symmetry that had been considered mutually exclusive. Here we show by spin- and angle-resolved photoemission that the Dirac cone and Dirac point are also highly spin polarized $(\ensuremath{\sim}\ensuremath{-}25%)$, which reinforces the puzzling issue of a strong graphene-substrate interaction. The problem is solved by our ab initio calculations which show that (i) the upper and lower halves of the Dirac cone belong to different sublattices and (ii) one half is spin polarized by spin-dependent hybridization because it is situated at the edge of a minority-spin band gap of the Co substrate.

Probing the electronic states of high-TMR off-stoichiometric Co2MnSi thin films by hard x-ray photoelectron spectroscopy

The tunnel magnetoresistance ratio (TMR) of fully epitaxial magnetic tunnel junctions with an off-stoichiometric Co${}_{2}$MnSi Heusler alloy has been shown to exhibit a systematic dependence on Mn content, reaching 1135% at 4.2 K for Co${}_{2}$Mn${}_{1.29}$Si. In this paper, we explain the behavior of the observed TMR ratio using ab initio calculations and hard x-ray photoelectron spectroscopy (HAXPES). For the Mn-deficient samples, we show that the the drop of the TMR is caused by Co antisite atoms, which impose extra states into the minority-spin band gap. On the other hand, Mn-excess composition shows nearly half-metallic behavior. This result can be intuitively understood since both Co${}_{2}$MnSi and Mn${}_{2}$CoSi are theoretically predicted to be half-metallic ferromagnets.

Effects of spin-wave excitations in half-metallic materials

Finite-temperature magnetic properties of half-metallic materials are investigated on the basis of the first-principles Korringa-Kohn-Rostoker Green's function method. Influences of spin-wave excitations on the electronic structure are examined in the framework of the coherent potential approximation. The calculations show that the half-metallicity is easily destroyed due to an appearence of low-energy electron excited states in the minority-spin band gap at finite temperature. On the other hand, calculated dc conductivity shows that the 100$%$ spin-polarized conduction occurring in half-metallic materials hardly deteriorates even in such a temperature.

Electronic structures of Heusler alloy Co2FeAl1−x Si x surface

We have calculated the electronic structures of Co2FeAl1−xSix(101) surface using first-principles method based on the density functional theory. Because of the surface effect, the minority spin band gap at the Fermi level disappears at the surface of bulk Co2FeAl1−xSix. However, beneath the surface, the minority spin gap opens at the Fermi level, which indicates that the electronic structures of Co2FeAl1−xSix(101) become close to that of bulk phase. Accordingly, the Co2FeAl1−xSix(101) surface is a composite tri-layer structure that corresponds to the weakening of half-metallic property in Co2FeAl1−xSix films. Even though, the spin polarization of Co2FeAl1−xSix(101) surface is still larger than that of Co2FeAl or Co2FeSi materials, making Co2FeAl1−xSix a promising spintronics material.

Magnetic Properties of Boron-Doped Co$_{2}$FeSi: A First-Principles Study

A new half-metallic Heusler alloy candidate boron-doped Co2FeSi is predicted by first-principles calculations based on density functional theory. It is found that the lattice parameter of Co2FeSi1-xBx decreases linearly with the increasing x. The magnetic moment shows a similar trend and is close to that of Co2FeAl at the extreme component x = 1. The electronic structure studies indicate that with the increasing x, the Fermi level shifts from the bottom of conduction band to the top of valence band in the minority spin states. When B atoms substitute 50% of Si atoms, the Fermi level locates in the middle of the minority spin band gap, which suggests the structure to be an ideal half-metallic compound processing 100% spin polarization. According to the calculations, B-doped Co2FeSi is a promising candidate for spintronic materials.

Graphene Spintronics: The Role of Ferromagnetic Electrodes

We report a first principles study of spin transport under finite bias through a graphene-ferromagnet (FM) interface, where FM = Co(111), Ni(111). The use of Co and Ni electrodes achieves spin efficiencies reaching 80% and 60%, respectively. This large spin filtering results from the materials specific interaction between graphene and the FM which destroys the linear dispersion relation of the graphene bands and leads to an opening of spin-dependent energy gaps of ≈0.4-0.5 eV at the K points. The minority spin band gap resides higher in energy than the majority spin band gap located near E(F), a feature that results in large minority spin dominated currents.

A better ferrimagnetic half-metal LuCu 3Mn 4O 12: Predicted from first-principles investigation

Electronic structure calculations based on density functional theory (DFT) within the generalized gradient approximation (GGA) and GGA+ U for manganite cuprate compound LuCu 3Mn 4O 12 have been performed, using the full-potential linearized augmented plane wave method. The calculated results indicate that LuCu 3Mn 4O 12 is ferrimagnetic and half-metallic in both GGA and GGA+ U calculations. The minority-spin band gap is 0.7 eV within GGA, which is larger than that of LaCu 3Mn 4O 12 (0.3 eV), indicating its better half-metallicity. Further, the minority-spin gap enlarges from 0.7 to 2.8 eV with U taken into account, and simultaneously the Fermi level being shifted to the middle of the gap, making the half-metallic energy gap to be 1.21 eV. These results demonstrate that electronic correlation effect enhances the stability of half-metallic property. These facts make this system interesting candidates for applications in spintronic devices.

Surface effects on the magnetic properties of Co 2FeAl(0 0 1): An ab initio study

Electronic structures of the Co 2FeAl(0 0 1) surface are studied theoretically via first-principles calculations based on density functional theory. It is found that the minority spin band gap at the Fermi level in bulk Co 2FeAl disappears at the surface due to space localization of the states. However, beneath the surface, the density of states of individual atoms shows a trend of minority spin gap opening at the Fermi level, which indicates that the electronic structures become close to that of bulk Co 2FeAl. The termination of FeAl is more favorable for spin polarization of Co 2FeAl films than that of Co. Accordingly, we present a composite tri-layer model to illustrate the fading of the half-metallic property in Co 2FeAl films against the ideal character in bulk materials.

Theoretical study of electronic, magnetic and structural properties of Mo and W based group V (N, P, As, Sb and Bi) compounds

Magnetic and electronic structure calculations were carried out for WC-, MnP-, NaCl- and zinc blende (ZB)-type Mo and W based group V compounds, TMX (TM = Mo and W; X = N, P, As, Sb and Bi), using the tight-binding linear muffin-tin orbital (TB-LMTO) method. For these compounds in these four structures, the total energy has been calculated as a function of volume in spin-polarization and non-spin-polarization calculations. From the total energy calculations, it is observed that these compounds favour the structures for which there is octahedral co-ordination for the anion rather than tetrahedral co-ordination. For the large volume expansion, these compounds prefer tetrahedral co-ordination compared to octahedral co-ordination. Moreover, these compounds in the ZB-type structure are found to exhibit half-metallic property with a magnetic moment of 3.00 μ B per formula unit under expansion of volume. The equilibrium lattice constant, bulk modulus, heat of formation, magnetic moment, spin-flip-gap and minority spin bandgap are calculated and compared with available results. The band structure and density of states are presented.

Evidence of local moment formation in Co-based Heusler alloys

In this work we use a combination of x-ray magnetic circular and linear dichroism (XMCD and XMLD) techniques to examine the formation of local moments in Heusler alloys of the composition Co{sub 2}MnX (where X=Si or Al). The existence of local moments in a half-metallic system is reliant upon the band gap in the minority-spin states. By utilizing the element-specific nature of x-ray techniques we are able to explore the origin of the minority-spin band gap in the partial density of states (PDOS), via the degree of localization of moments on Co and Mn atoms. We observe a crucial difference in the localization of the Co moment when comparing Co{sub 2}MnSi (CMS) and Co{sub 2}MnAl (CMA) films that is consistent with the predicted larger minority-spin gap in the Co PDOS for CMS. These results provide important evidence for the dominant role of the Co minority-spin states in realizing half-metallic ferromagnetism (HMF) in these Heusler alloys. They also demonstrate a direct method for measuring the degree of interfacial HMF in the raw materials without the need for fabricating spin-transport devices.

Mn2CoZ(Z=Al,Ga,In,Si,Ge,Sn,Sb)compounds: Structural, electronic, and magnetic properties

We study the electronic structures and magnetic properties of ${\mathrm{Mn}}_{2}\mathrm{Co}Z$ $(Z=\mathrm{Al},\mathrm{Ga},\mathrm{In},\mathrm{Si},\mathrm{Ge},\mathrm{Sn},\mathrm{Sb})$ compounds with ${\mathrm{Hg}}_{2}\mathrm{Cu}\mathrm{Ti}$-type structure using first-principles full-potential linearized-augmented plane-wave calculations. It is found that the compounds with $Z=\mathrm{Al}$, Si, Ge, Sn, and Sb are half-metallic ferrimagnet. Experimentally, we successfully synthesized the ${\mathrm{Mn}}_{2}\mathrm{Co}Z$ $(Z=\mathrm{Al},\mathrm{Ga},\mathrm{In},\mathrm{Ge},\mathrm{Sn},\mathrm{Sb})$ compounds. Using the x-ray diffraction method and Rietveld refinement, we confirm that these compounds form ${\mathrm{Hg}}_{2}\mathrm{Cu}\mathrm{Ti}$-type structure instead of the conventional $L{2}_{1}$ structure. Based on the analysis on the electronic structures, we find that there are two mechanisms to induce the minority-spin band gap near the Fermi level, but only the $d\text{\ensuremath{-}}d$ band gap determines the final width of the band gap. The magnetic interaction is quite complex in these alloys. It is the hybridization between the $\mathrm{Mn}(C)$ and Co atom that dominates the magnitude of magnetic moment of the Co atom and the sign of the $\mathrm{Mn}(B)\text{\ensuremath{-}}\mathrm{Co}$ exchange interaction. The ${\mathrm{Mn}}_{2}\mathrm{Co}Z$ alloys follow the Slater-Pauling rule ${M}_{H}={N}_{V}\ensuremath{-}24$ with varying $Z$ atom. It was further elucidated that the molecular magnetic moment ${M}_{H}$ increases with increasing valence concentration only by decreasing the antiparallel magnetic moment of $\mathrm{Mn}(C)$, while the magnetic moments of $\mathrm{Mn}(B)$ and Co are unaffected.