Minority Spin Carriers Research Articles

Magneto-electronic and optoelectronic attributes of half-Heusler VXPt (X = Br, Se) alloys: A first-principles study

High-spin polarized magnetic materials might be useful in spintronics applications. The spin-polarized magnetic, optical, electronic and structural attributes of half-Heusler VXPt (X = Br, Se) alloys were theoretically explored and calculated using the WIEN2k code. The ground states of the structures are optimized, and the bulk moduli and lattice constants are computed. The values obtained from the formation energies show their stability. For VBrPt, an energy gap is present in major and minority spin carriers, while for VSePt, major spin carriers exhibit an energy gap and metallic character is seen in minority spin carriers. The band structures are further illustrated deeply through the investigation of density of states. The electron density graphs depict the ionic bonding in both alloys. The magnetic moments are additionally assessed, and the recorded values support the magnetic characteristics of VBrPt and VSePt. The assessment of how the studied alloys react to light is determined by computing various parameters. The imaginary component and absorption coefficient anticipate substantial light absorption within the visible region. The calculated outcomes unveiled the potential of the examined alloys for applications in spintronics.

Spin Seebeck effect and thermal properties of zigzag graphene nanoribbons with edge magnetism

Zigzag graphene nanoribbons (GNRs), demonstrating edge magnetism, are fascinating materials for electronic and spintronic applications. In this paper, we investigate with explicit consideration of electron-phonon scattering the Seebeck effect and thermal conductivity at various carrier densities in zigzag GNRs, which undergo antiferromagnetic, ferromagnetic, and paramagnetic transitions. Seebeck coefficients are spin dependent in ferromagnetic zigzag GNRs and can have opposite signs for the majority and the minority spin carriers, which enables the separation of spin carriers spatially under a thermal gradient. The electronic thermal conductivity is small compared to the lattice thermal conductivity, except for GNRs in the ferromagnetic state. A 100% increase in thermal conductivity is expected at antiferromagnetic to ferromagnetic transition. These results demonstrate that zigzag GNRs are also materials of high interest for spin caloritronics.

Spin Seebeck effect in the 2D ferromagnetic CrPbTe3

Spin Seebeck effect has been mostly explored in bulk magnetic materials, but the efficiency is not large enough for device application. Recently, we have found that the 2D ferromagnetic CrPbTe3 layer has rather a high Curie temperature of 110 K. Thus, we have investigated the spin dependent Seebeck effect using the Boltzmann transport approach. We have found the largest carrier mobility and relaxation time in the minority spin electron carrier system due to the spin dependent effective mass and deformation potential constant. The onset energy of the electrical conductivity shows spin and carrier type dependency. Thus, both effective spin and effective charge Seebeck coefficients are originated from the majority spin carrier in the hole doped system whereas the minority spin carrier controls the effective spin and effective charge Seebeck coefficient in the electron doped system. We have obtained the maximum spin Seebeck coefficient of 1320 μV/K at 50 K. This value is suppressed at 100 K, but we still find a large spin Seebeck coefficient of 715 μV/K.

Shift of the collective mode of polarized graphene on a substrate using spin-sensitive response theory

The growing precision of optical and scattering experiments necessitates a better understanding of the influence of damping onto the collective mode of sheet electrons. As spin-polarized systems are of particular interest for spintronic applications, we here report spin-sensitive linear response functions of graphene, which give access to chargeand spin-density related excitations. We further calculate the reflectivity of graphene on an SiO2 surface, a setup used in s-wave scanning near-field microscopy. Increasing the partial spin-polarization of the graphene charge carriers leads to a significant broadening and shift of the plasmon mode, due to single-particle interband transitions of the minority spin carriers. We also predict an antiresonance in the longitudinal magnetic response function, similar to that of semiconductor heterostructures.

Hot Electrons and Hot Spins at Metal–Organic Interfaces

AbstractA model is developed to describe the electron transport properties of hot electron devices based on organic semiconductors. For the first time, the simulations cover all the different processes the carriers experience in the device, which allows disentangling various effects on the transport characteristics. The model is compared to experimental measurements and excellent agreement is found. In addition, the model includes the electron spin and is thus able to describe a hot spin transistor. In this device, a spatial variation of the spin diffusion length is found, which scales inversely proportional to the variation of the electron density. The spin current can be increased by increasing the hot electron energy and by decreasing the image charge barrier without changing the spin diffusion length. Unprecedented insight into the effect of interfacial disorder at the metal–organic interface on charge and spin transport is provided. Finally, conditions are established, where majority and minority spin carriers propagate in opposite directions, increasing the spin current relative to the charge current and the occurrence of pure spin currents is analyzed.

Plasmons in spin-polarized graphene: A way to measure spin polarization

We study the collective charge excitations (plasmons) in spin polarized graphene, and derive explicit expressions for their dispersion in the undamped regime. From this, we are able to calculate the critical wave vector beyond which the plasmon enters the electron-hole continuum, its quality factor decreasing sharply. We find that the value of the critical wave vector is strongly spin polarization-dependent, in a way that has no analog in ordinary two-dimensional electron gases. The origin of this effect is in the coupling between the plasmon and the inter-band electron-hole pairs of the minority spin carriers. We show that the effect is robust with respect to the inclusion of disorder and we suggest that it can be exploited to experimentally determine the spin polarization of graphene.

K-edge x-ray dichroism investigation of Fe1−Co Si: Experimental evidence for spin polarization crossover

Both Fe and Co K-edge x-ray magnetic circular dichroism (XMCD) have been employed as element-specific probes of the magnetic moments in the composition series of the disordered ferromagnet Fe1−xCoxSi (for x=0.2, 0.3, 0.4, 0.5). A definitive single peaked XMCD profile occurs for all compositions at both Fe and Co K-edges. The Fe 4p orbital moment, deduced from the integral of the XMCD signal, has a steep dependence on x at low doping levels and evolves to a different (weaker) dependence at x≥0.3, similar to the behavior of the magnetization in the Co composition range studied here. It is systematically higher, by at least a factor of two, than the corresponding Co orbital moment for most of the composition series. Fine structure beyond the K-edge absorption (limited range EXAFS) suggests that the local order (atomic environment) is very similar across the series, from the perspective of both the Fe and Co absorbing atom. The variation in the XMCD integral across the Co composition range has two regimes, that which occurs below x=0.3 and then evolves to different behavior at higher doping levels. This is more conspicuously present in the Fe contribution. This is rationalized as the evolution from a half-metallic ferromagnet at low Co doping to that of a strong ferromagnet at x>0.3 and as such, spin polarization crossover occurs. The Fermi level is tuned from the majority spin band for x<0.3 where a strongly polarized majority spin electron gas prevails, to a regime where minority spin carriers dominate at higher doping. The evolution of the Fe-derived spin polarized (3d) bands, indirectly probed here via the 4p states, is the primary determinant of the doping dependence of the magnetism in this alloy series.

Fluorinated boron nitride nanotube quantum dots: a spin filter.

Spin filtering requires a selective transmission of spin-polarized carriers. A perfect spin filter allows all majority (or minority) spin carriers to pass through a channel while blocking the minority (or majority) carriers. The quest for a novel low-dimensional metal-free magnetic material that would exhibit magnetism at a higher temperature with an excellent spin filtering property has been intensively pursued. Herein, using a first-principles approach, we demonstrate that the fluorinated boron nitride nanotube (F-BNNT) quantum dot, which is ferromagnetic in nature, can be used as a perfect spin filter with efficiency as high as 99.8%. Our calculation shows that the ferromagnetic spin ordering in F-BNNT is stable at a higher temperature. Comparison of the conductance value of the F-BNNT quantum dot with that of the pristine BNNT quantum dot reveals a significantly higher conductance in F-BNNT, which is in very good agreement with the experimental report (Tang, C., et al. J. Am. Chem. Soc. 2005, 127, 6552).

First-principles investigations of Mn doped zinc-blende ZnO based magnetic semiconductors: Materials for spintronic applications

ZnO based magnetic semiconductors are intensively investigated because of showing strong potential as base materials for spintronic devices. In this study, full potential linearized augmented plane-wave plus local orbitals FP-L(APW+lo) scheme of computation is used to explore the structural, electronic and magnetic properties of Manganese-doped ZnO based magnetic semiconductors in zinc-blende (ZB) phase. For comprehensive understanding of Mn-doping effect on ZnO, several compositions of Mn:ZnO for 12.5%, 25%, 37.5%, 50%, 62.5%, 75% and 87.5% of Mn concentration are investigated. Our obtained results show a successful induction of the magnetic moment (MM) by Mn-doping into the ZnO matrix, without any geometrical deformation. However a marginal increase in the value of lattice constants is found up to 25% concentration of Mn for this system and for above compositions, it tends to decrease revealing the formation of secondary phases. It is also found that Mn:ZnO system favors ferromagnetic coupling for 12.5% and 25% of Mn contents that has been switched to anti-ferromagnetic coupling for higher Mn contents. The spin polarized electronic structure of Mn:ZnO system was calculated within the generalized gradient approximation (GGA). In addition, the Hubbard parameter was also employed to improve the electronic band structure calculations. The calculations performed at GGA+U level related to electronic band structures show zero energy gap for majority spin carriers, whereas a considerable energy gap is noted for minority spin carriers. This distinguished response of Mn:ZnO system to majority and minority spin carriers, in terms of resistivity and conductivity, highlights its importance for diverse applications as based material in spintronic devices like spin dependent transports, currents and other spin based electronic applications.

Spin scattering in manganese doped nickel

Spin scattering characteristics of nickel doped with manganese are compared to nickel doped with iron and chromium in nickel. Conclusions are made based on the spin characteristics of the nickel samples doped with manganese with varying percentages in a host nickel crystal. Manganese doping increases the magnetic exchange splitting of nickel, similar to doping with iron and opposite to doping with chromium. On the other hand, manganese doping behaves different from iron with respect the ratio of the majority to minority spin carriers and the ratio of their lifetimes. While iron doping affects both ratios strongly, manganese and chromium doping do not.

Spin Coulomb Drag in a One-Dimensional Spin-Polarized Conductor

The spin Coulomb drag is a distinctive feature of spin-polarized transport. The current of majority spins can induce a current of minority spin carriers via the transconductivity. The friction is caused by the Coulomb interaction between up- and down-spin. This interaction reduces the current but does not change the spin-polarization. We calculate the conductivities and the transconductivity for a spin-polarized interacting 1-D electron gas for a lattice without inversion symmetry in the presence of nonmagnetic impurities. Due to the Luttinger liquid properties, the temperature dependence of the transport correlation functions follows power laws of with nonuniversal exponents.

Is the nature of itinerant ferromagnetism playing a role in the competition between spinpolarization and singlet pair correlations?

We consider the competition between spin singlet pairing and itinerant ferromagnetismwhose magnetization is yielded by a relative shift of the bands with opposite spinpolarization or by asymmetric spin-dependent bandwidths. Within the framework of theexact solution of an extended version of the reduced BCS model, the structure of thecoexisting state is shown to have general features that are not related to the character ofthe ferromagnetism. The role of different types of ferromagnet is then investigated for theproximity effect in a system made of a bilayer junction with a spin singlet superconductorinterfaced with a ferromagnet in the clean limit. We show that the qualitative behaviour ofthe proximity effect does not depend on the nature of the ferromagnetism. Differencesemerge at the borderline with the half-metallic regime. For the spin-dependentbandwidth type of ferromagnetism the pairing amplitude exhibits an oscillatingbehaviour until the density of the minority spin carrier becomes almost zero. Thecrossover from an oscillating to an exponentially damped profile occurs awayfrom the half-metallic limit when a spin exchange type ferromagnet is considered.

Bilayer junction with chiral p-wave superconductor and itinerant ferromagnet: Role of distinct mechanisms for the generation of spin imbalance

We have investigated the behavior of a bilayer junction made of a p-wave chiral superconductor and an itinerant ferromagnet, under the assumption of high barrier transparency and in the clean limit. For the ferromagnet two distinct mechanisms for the generation of spin imbalance have been taken into account: a Stoner-like one originated by the presence of a constant exchange field, and a mass asymmetry one generated by a narrowing of the bandwidth of the minority spin carriers with respect to the majority ones. By solving the Bogoliubov-de Gennes equations, we show that the two mechanisms for ferromagnetism lead to different features as concerns the formation at the interface of dominant and sub-dominant superconducting components as well as their propagation in the ferromagnetic side.

Scattering by atomic spins and magnetoresistance in dilute magnetic semiconductors

We studied electrical transport in magnetic semiconductors, which is determined by scattering of free carriers off localized magnetic moments. We calculated the scattering time and the mobility of the majority- and minority-spin carriers with both the effects of thermal spin fluctuations and of spatial disorder of magnetic atoms taken into account. We discuss the role of the above effects on magnetoresistance of nondegenerate semiconductors where magnetic impurities are electrically active or neutral. The application of the external magnetic field suppresses the thermodynamic spin fluctuations thus promoting negative magnetoresistance. Simultaneously, scattering off the built-in spatial fluctuations of the atomic spin concentrations may increase with the magnetic field. The latter effect is due to the growth of the magnitude of random local Zeeman splittings with the magnetic field. It promotes positive magnetoresistance. The enhancement of spin-dependent scattering by the external magnetic field is especially large in the materials with electrically active magnetic impurities.

Atomic Spin Scattering and Giant Magnetoresistance in Magnetic Semiconductors

We studied electrical transport in dilute magnetic semiconductors, which is determined by scattering free carriers by localized magnetic moments. While calculating the scattering time and the mobility of the majority and minority-spin carriers, we took into account both the effects of thermal spin fluctuations and of the spatial disorder of magnetic atoms. These effects are responsible for the magnetic-field dependence of electrical resistivity. Namely, the application of the external magnetic field suppresses the thermodynamic spin fluctuations, thus promoting negative magnetoresistance (MR). Simultaneously, scattering by built-in spatial fluctuations of the atomic spins increases with the magnetic field. The latter effect is due to the growth of the magnitude of random local Zeeman splittings with the magnetic field. It promotes positive MR. We discuss the role of the above effects on MR of semiconductors where magnetic impurities are isoelectronic

Mobility of Charge Carriers in Dilute Magnetic Semiconductors

AbstractWe studied electrical transport in dilute magnetic semiconductors, which is determined by scattering of free carriers by localized magnetic moments. In our calculations of the scattering time and the mobility of the majority and minority-spin carriers we took into account both the effects of thermal spin fluctuations and of built-in spatial disorder of the magnetic atoms. These effects are responsible for the magnetic-field dependence of the mobility of the charge carriers. The application of the external magnetic field suppresses the thermodynamic spin fluctuations thus increasing the mobility. Simultaneously, depending on the type of the carriers and on parameters of the impurity potential, scattering by built-in spatial fluctuations of the atomic spins increases or decreases with the magnetic field. The latter effect is due to the change in the magnitude of the random local Zeeman splitting with the magnetic field. We discuss the role of the above effects on mobility and magnetoresistance of semiconductors where magnetic impurities are electrically active or neutral.

Crossover of magnetoresistance from negative to positive in the heterojunction composed of La0.82Ca0.18MnO3 and 0.5wt% Nb-doped SrTiO3

We report the magnetotransport properties of the heterojunction composed of La0.82Ca0.18MnO3 (LCMO) and 0.5wt% Nb-doped SrTiO3 (SNTO). At temperature below 120K, the heterojunction only exhibits a negative magnetoresistance (MR) independent of bias current. At temperatures above 120K, with increasing bias current a crossover of MR from negative to positive is observed. This intriguing crossover of MR can be ascribed to the dominant spin character changing from the majority spin carriers to the minority spin carriers at the LCMO-SNTO interface due to their occupation in the complicated bands of La0.82Ca0.18MnO3.

Spin-Filter Effect in Magnetite Nanowire

Spin-dependent electron transport in individual magnetite (Fe3O4) nanowires contacted with normal metallic electrodes was investigated. Such a configured device demonstrated a spin-filter effect, that is, only the minority spin carriers can transport through the magnetite nanowire due to its negative spin polarization. An anomalous positive magnetoresistance approximately 7.5% is observed at room temperature. Moreover, the magnetoresistance can be controlled via bias voltage.

Doping profile vs spin-resolved subband structure and spontaneous magnetization of selectively Mn-doped DMS narrow quantum wells

The interrelation between magnetic properties and doping profile of selectively Mn-doped (p-AlGaAs)/(GaAs, Mn)/(p-AlGaAs) quantum wells (QWs) is studied by analyzing the self-consistent spin-resolved subband structures of the system. The effective potentials felt by the majority and minority spin carriers in the QW depend markedly on the doping profiles of the Mn ions and impurities in the barriers. Various doping profiles of the magnetic ions are considered to vary the overlap of the carriers and magnetic ions in the QW, and the resulting spin-resolved carrier distribution, subband structure, and ferromagnetic tendency of the system are examined. Our result shows that (1) Curie temperature depends sensitively on the width and location of Mn doping inside the QW for a given 2D density of Mn ions and (2) modulation doping of acceptors in the barriers does not necessarily enhance the T c of a given Mn-doped narrow QW structure.

Observation of two distinct magnetic states in [formula omitted] via the dHvA effect under pressure

We have found an anomaly in magnetic field dependence of the de Haas-van Alphen (dHvA) frequency of ZrZn 2 . Pressure dependence of this anomaly suggests an existence of two distinct ferromagnetic states and then a “nested” phase diagram similar to that of UGe 2 . The Stoner exchange parameter and the exchange splitting energy are 400 and 66 meV, respectively, which are derived from comparing the field dependence of the dHvA frequency difference with the magnetization. The two parameters do not change in both the low and slightly higher average moment states, suggesting that the phase transition between these states is ascribed to the change in the difference of majority and minority spin carrier densities.