Minority Spin Band Research Articles

Tuning optical and ferromagnetic properties of thin GdN films by nitrogen-vacancy centers

AlN/GdN/AlN double heterostructures were grown on c-sapphire substrates using a reactive rf sputtering method under high vacuum conditions. The optical absorption spectrum of the GdN shows a clear fundamental band edge of GdN around 800 nm; this transition is attributed to the minority spin band energy of GdN at the X point. Nitrogen vacancy centers cause a blue-shift of the optical band edge of GdN, which could be ascribed to both the band filling, and the electron-hole interactions resulting from the free carriers generated by nitrogen vacancies. Temperature-dependent magnetization measurements demonstrate a clear change in the magnetization values of GdN with respect to the N2 partial pressure. Nitrogen vacancy centers in the thin GdN film raise the Curie temperature from 31 K to 39 K, which has been accurately measured by the Arrott plots.

Scanning tunneling microscopy investigation of electronic properties of cobalt nanoislands supported by Cu–9at.%Al(111)

Growth and surface electronic states of Co nanoislands supported by Cu–9at.%Al(111) are investigated by low temperature scanning tunneling microscopy. Deposition of about 0.25 monolayer of Co atoms causes the formation of flat Co nanoislands with thicknesses ranging from monolayer to triple layer. Scanning tunneling spectroscopy measurements reveal that a Tamm-type surface state exists on the Co islands and its energy varies with the thicknesses and stacking manners. In addition, density functional theory calculations conclude that the surface states of the mono- and bilayer nanoislands mainly originate from the hybridization between Co d bands and sp bands of the substrate and the Co d3z2−r2 minority-spin bands, respectively.

New Heusler Compounds Cr2YSb (Y=Fe, Co and Ni): Magnetic and Electronic Properties

The electronic and magnetic properties of the Cr2YSb (Y=Co, Fe, and Ni) Heusler alloys with both CuHg2Ti-type and AlCu2Mn-type structures have been investigated using first-principles calculations based on density functional theory (DFT). Two nearly half-metallic ferrimagnets (HMFs), Cr2CoSb, and Cr2FeSb in CuHg2Ti-type structure, are predicted. The energy gap lies in the minority spin band for both alloys. The calculated total spin magnetic moments are 2μ B and 1μ B per unit cell for Cr2CoSb and Cr2FeSb alloys, respectively, which are in good agreement with the Slater–Pauling relation. For these alloys, the magnetic moments of Y and Cr(B) are antiparallel to that of Cr(A) and all of these moments increase with increasing lattice constant. It was also found that the half-metallic properties of Cr2CoSb and Cr2FeSb are unaffected to the lattice distortion and the half-metallicity can be obtained within the wide range of 5.52–6.07 A and 5.96–6.16 A for Cr2CoSb and Cr2FeSb alloys, respectively.

Ferromagnetic properties of GdN thin films studied by temperature dependent circular polarized spectroscopy

Magneto-optical properties of GdN thin films have been investigated by employing temperature dependent optical circular dichroism spectroscopy and angular dependent resonance field measurement. Spin-splitting in the band structure of GdN thin films has been evaluated by optical circular dichroism absorbance spectra, and the manifested spin-splitting energy ascribed to the difference between majority and minority spin band states in GdN. The plot of left circular polarization and right circular polarization bandgap reflects the half-hysteresis loop (positive side) trend, which is evidenced by magnetization measurements. The angular dependent resonance field measurements showed strong magnetic anisotropy along in-plane of GdN, which is attributed to the disturbance in the spin alignment in GdN. We demonstrate that the ferromagnetic properties depend on the film thickness. These results provide a pathway to control the spin ordering using circularly polarized light and the magnetic anisotropy.

Microstructure, magnetic and electronic transport properties of polycrystalline γ′-Fe4N films

The single-phase γ′-Fe4N films were fabricated using reactive sputtering. The x-ray diffraction peaks from γ′-Fe4N(111), (200) and (311) indicate that the films are γ′-Fe4N. The grain size increases with the increase of film thickness (t), and the grains grow with a columnar structure. All of the films are soft ferromagnetic at room temperature. The saturation magnetization decreases with the increasing temperature, and satisfies the modified Bloch's spin wave theory. The electrical transport properties show a metallic conductance mechanism, and the room-temperature resistivity decreases with the increasing t, revealing that the electron scattering increases with the decrease of t. The magnetoresistance (MR) evolves from positive to negative with the increase of temperature, and the transition temperature decreases with the increase of t. The positive MR at 5K increases with the increasing t. The complex MR should be dominated by Lorentz force effect, the suppression of the electron scattering, and the shift of minority and majority spin bands under a magnetic field.

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.

First-principles study on the (111) surface of half-metallic rocksalt VPo

In this work, we have probed the structural, electronic, and magnetic properties of the (111) surface of rocksalt (RS) VPo. The first-principles full-potential linearized augmented plane-wave method is used. Bulk VPo is found to be a half-metallic ferromagnet at equilibrium lattice constant (0.59nm) with a total spin magnetic moment of 3.0μB per formula unit. At the same equilibrium lattice constant, the V-terminated (111) surface preserves the half-metallic characteristics of the bulk VPo, however, the surface states destroy the gap in the minority spin band of the Po-terminated (111) surface. We also discuss the surface stability.

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.

Origin of the Z−28 rule in Mn2Cu-based Heusler alloys: A comparing study

The origin of the Slater–Pauling curve Mt=Zt−28 (Here Mt is the total spin moment and Zt is the number of valence electrons) in half-metallic Heusler alloys Mn2CuZ (Z=Ge and Sb) has been studied in detail. In Mn2CuZ the half-metallic gap has a similar origin like half-Heusler alloys. The Cu atom acts as an electron “donator” in Mn2CuZ, which contributes five d-electrons to the minority spin band of Mn2CuZ. So there are 14 valence electrons in the minority band of Mn2CuZ below the Fermi level. This is the origin of the S–P curve Mt=Zt–28. Finally, it is found that, by partial doping of Cu to the vacant site of half-metallic half-Heusler alloys, the magnetic moments of these can be tuned without destroying the half-metallicity. This can be a possible way to design new half-metals.

Doppler broadening of annihilation radiation measurements on3dand4fferromagnets using polarized positrons

We measured the Doppler broadening of annihilation radiation (DBAR) spectra of $3d$ (Fe, Co, and Ni) and $4f$ (Gd, Tb, and Dy) ferromagnets under a magnetic field by using spin-polarized positrons from a ${}^{68}$Ge-${}^{68}$Ga source. The results showed that the DBAR spectra of these metals have notably different magnetic-field dependences. The differences among Fe, Co, and Ni reflect that the upper minority spin bands of Fe and Co are nearly empty while those of Ni are still mostly occupied. For the rare-earth metals instead of the inner $4f$ electrons, $5d$ electrons that mediate the exchange interaction of the $4f$ electrons are primarily responsible for the magnetic-field effects on the DBAR spectra. Furthermore, the magnetic-field effects on the DBAR spectra of Gd, Tb, and Dy vanished above the Curie temperatures of the magnetic-phase transition for these metals.

Ab initio study of the electronic and magnetic properties of Sr2−xLaxFe1+y/2Mo1−y/2O6 double perovskites

Using the first-principles full potential linearized augmented plane wave method, the electronic structure of Sr2−xLaxFe1+y/2Mo1−y/2O6 (SLFMO) double-perovskite systems is investigated for x = 1/2 and 1 and for y =+ 1 and − 1. Substituting Sr atoms by La atoms allows one to tune the electrons added into the minority spin band and enhances the half-metal feature—according to the rigid band shift model—even if the magnetization decreases with increasing La concentration. By taking into account the chemical disorder on the Fe and Mo sites, resulting from the introduction of La as shown experimentally, it is shown (i) that a supplemental magnetization reduction occurs because the magnetic moment on the Fe antisite is opposite to the one on the Fe regular sites and (ii) that the half-metal feature is preserved in SLFMO for x = 1/2 and 1 contrary to the case for SFMO. Finally, because surface or interface Fe deficiency should have a more limited impact on the spin polarization than for SFMO, SLFMO/SrTiO3 multilayers are investigated in order to confirm this prediction by determining the spin polarization at the interface, which is found to remain high.

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.

Instability of the Parent Phase in Nearly Ordered Fe<sub>3</sub>Pt Invar Alloys

The martensitic transformation in a nearly ordered Fe3Pt is weak first order, and the martensite phase shows a large magnetic field-induced strain of several percent. We have investigated the origin of this transformation from its electronic structure. A characteristic feature in the electronic structure is the existence of a relatively high peak in the density of states of the minority spin band just below the Fermi energy. This peak splits into two peaks by tetragonal distortion, and one of them shifts to lower energy by the distortion, suggesting that the band Jahn-Teller effect is the main cause for the transformation.

Electronic structure and magnetism in full-Heusler compound Mn 2ZnGe

The first-principle calculations within density functional theory are used to investigate the electronic structure and magnetism of the Mn 2ZnGe Heusler alloy with CuHg 2Ti-type structure. The half-metallic ferrimagnets (HMFs) in Mn 2ZnGe are predicted. The energy gap lies in the minority-spin band for the Mn 2ZnGe alloy. The calculated total spin magnetic moment is − 2 μ B per unit cell for Mn 2ZnGe alloy, the magnetic moments of Zn and Mn(B) are antiparallel to that of Mn(A), and we also found that the half-metallic properties of Mn 2ZnGe are insensitive to the dependence of lattice within the wide range of 5.69 and 5.80 Å where exhibiting perfect 100% spin polarization at the Fermi energy.

First-principles study of magnetism and electronic structureof Sb-containing half-Heusler alloys

Using the first-principles density functional theory, we calculate the crystal structures, magnetisms and electronic structures of Sb-containing half-Heusler alloys XYSb(X=Ni, Pd, Pt; Y=Mn, Cr). The calculation results show that alloy NiMnSb is half-metal and the others are metals at equilibrium lattice constant. The contribution of the spin magnetic moment of Y element to the total moment is largest for all alloys. It is found that the Fermi level of the minority spin band shifts closer to the bottom of spin-down conduction band with atomic number of X element reducing. The Fermi level moves up due to the compressive strain, away from p bands of Sb atom. Under the compresive stress, PtMnSb, PdMnSb and NiCrSb can induce metal half-metal transitions.

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.

Theory of tunneling magnetoresistance of Fe/GaAs/Fe(001) junctions

We investigate theoretically spin-dependent transport through an epitaxial Fe/GaAs/Fe(001) tunnel junction with and without spin-orbit interaction. Calculations neglecting spin-orbit interaction and the effect of $d$ orbitals on the GaAs band structure predict that the tunneling magnetoresistance (TMR) should increase with increasing thickness of GaAs barrier and approach values close to the perfect spin-valve limit. However, when $d$ orbitals and, in particular, spin-orbit interaction is included the TMR ratio saturates rapidly with GaAs thickness to a rather modest value of about 30% when the Fermi level ${E}_{F}$ lies in the middle of the GaAs gap. This unexpectedly small value cannot be explained by spin-orbit interaction alone. It is shown that the underlying reason for this is the presence of a resonance in the minority-spin band structure of the Fe/GaAs/Fe trilayer lying close to the center of the gap. Investigation of the dependence of the TMR on the height of the GaAs barrier (position of the Fermi energy ${E}_{F}$ in the gap) shows that the TMR of a perfect junction is strongly enhanced when ${E}_{F}$ lies at the resonance in the minority-spin channel. However, we show that any small asymmetry of the junction removes the TMR peak and reduces the TMR to small values, of the order of 50%, for a rather large interval of values of ${E}_{F}$ in the vicinity of the middle of the GaAs gap. We thus conclude that the spin-orbit coupling leads to saturation of the TMR with GaAs thickness, but the saturation value is determined by the presence of the resonance.

Ferromagnetic Systems with Hopping Interaction

We analyze the influence of hopping interaction on magnetic ordering. Scattering scheme of the Hubbard III approximation with included inter-site kinetic electron–electron correlation is used. The hopping interaction and inter-site correlation lead to two spin dependent effects: the band width correction and the band-shift correction. The band-shift correction factor causes an exchange splitting between the spin-up and spin-down spectrum, and its role is similar to the exchange interaction in the classic Stoner model. The spin dependent band width correction enhanced strongly by the inter-site kinetic correlation lowers the kinetic energy of electrons by decreasing the majority spin band width for some electron occupations with respect to the minority spin band width. The results show that in the case of the symmetrical density of states there is only ferromagnetic enhancement. For the strongly asymmetrical density of states there is a ferromagnetic transition.

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.