16 research outputs found

    Interlayer Exchange Coupling Mediated by Valence Band Electrons

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    The interlayer exchange coupling mediated by valence band electrons in all-semiconductor IV-VI magnetic/nonmagnetic superlattices is studied theoretically. A 3D tight-binding model, accounting for the band and magnetic structure of the constituent superlattice components is used to calculate the spin-dependent part of the total electronic energy. The antiferromagnetic coupling between ferromagnetic layers in EuS/PbS superlattices is obtained, in agreement with the experimental evidences. The results obtained for the coupling between antiferromagnetic layers in EuTe/PbTe superlattices are also presented.Comment: 8 pages, 6 figures, to be submitted to Phys.Rev.

    Electronic structure of Li2Pd3B and Li2Pt3B

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    Li2Pd3B is known to be superconducting, while the isotypical Li2Pt3B compound is not. Electronic structures of Li2Pd3B and Li2Pt3B have been calculated in order to obtain an insight into this surprising difference, through an analysis of the differences in the band structures. The electronic structures of these systems were obtained using the Full Potential Linear Augmented Plane Wave plus local orbitals (FP-LAPW+lo) method and it was found that four bands cross the Fermi level (EF). Out of these four bands, only two bands contribute significantly to the density of states at the EF. One of these bands is a hole band and the other an electron band. Thus at least a two-band model is required for studying the electronic properties of the Pd and Pt compounds. These two bands are rather narrow and hence the coulombic correlations effects can be significant.Comment: 18 pages, 8 figures, submitted to Physica

    Resonant tunnel magnetoresistance in double-barrier magnetic tunnel junctions: A non-equilibrium Green's function study

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    The non-equilibrium Green's function technique is used to study the transport characteristics of double-barrier magnetic tunnel junctions. The exchange coupling strength of the electrodes is found to be crucial in deciding the magnetoresistance characteristics of these devices. At sufficiently large values of the magnetic coupling strength, the device is found to exhibit resonant tunnel magnetoresistance and its magnitude is found to be large. The existence of pure spin currents in these devices when there is antiferromagnetic coupling between the end electrodes is found to be the primary cause of resonant tunnel magnetoresistance. The influence of the band occupation of the electrodes and the many-body interaction present in the electrode regions on the spin current and magnetoresistance are also studied

    Middle-layer ferromagnetism-induced transition of the tunnel magneto-resistance in double-barrier magnetic tunnel junctions: A non-equilibrium Green's function study

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    Using the non-equilibrium Green's function modeling of the transport characteristics of tunnel devices, we have found the middle-layer ferromagnetism-induced transition of the tunnel magneto-resistance in double-barrier magnetic tunnel junctions. It is observed from our study that even a weak ferromagnetism of the middle metallic layers is capable of promoting resonant tunnel magneto-resistance in these devices and the strength of the ferromagnetism is found to have strong influence on the bias dependence of the resonant tunnel magneto-resistance. The spin-up and spin-down currents flow in opposite directions for certain band occupancies and at certain bias voltage ranges when there is antiferromagnetic coupling between the electrodes of the tunnel junction. Resonant tunnel magneto-resistance occurs when the net current (sum of spin-up and spin-down currents) becomes very small at situations mentioned above. We have further studied the influence of band occupation of the electrode layers and the many-body interactions present in the electrode region on the spin current and magneto-resistance of these devices

    Evolution of the Kondo insulating gap in Fe<SUB>1-x</SUB>Ru<SUB>x</SUB>Si

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    Electrical resistivity measurements are reported in Fe1-xRuxSi (x=0.0 to 0.30) as a function of temperature (4.2 K to 300 K) and as a function of temperature and pressure (0-5 GPa) for x=0.04 and 0.1. The temperature-dependent resistivity data have been analyzed based on a Model Density of States (DOS) taking into account both electron correlations and disorder. Using this model DOS and temperature-dependent mobility, the conductivity as a function of temperature is calculated and fitted to the experimental data for several external pressures and Ru concentrations. The parameters of the fit as a function of x, show extremal values at the Ru composition x=0.06. An effective band gap (Δeff) obtained from these parameters is seen to decrease with Ru substitution up to x=0.06 beyond which it increases again. Band-structure calculations performed for several stoichiometric compositions of x in Fe1-xRuxSi, indicate a broadening of the bands and an overall increase in the gap from 0.13 eV for x=0.0 to 0.3 eV for x=1.0. The existence of localized states within the gap leads to the description of the transport behavior at low temperature (5-25 K) in terms of variable range hopping (VRH) mechanism. The VRH parameter T0 decreases by more than three orders of magnitude for x up to 0.06, beyond which it increases again. It is surmised that with Ru substitution, (FeRu)Si goes over from a Kondo insulator to a conventional band semiconductor via an intermediate metallic phase. The results of temperature-dependent resistivity under high pressure in the samples with compositions x=0.04 and 0.1 are also reported and analyzed based on the model