R. B. Muniz

A resistor network theory of the giant magnetoresistance in magnetic superlattices

An explanation of the giant magnetoresistance effect observed in some magnetic superlattices is given in terms of an equivalent network of resistors. The model leads to a simple analytic formula that relates the magnetoresistance to the spin-dependent electron mean-free paths and thickness of the magnetic and nonmagnetic layers in the superlattice. The formula is used to study the giant magnetoresistance of Co/Cu and Fe/Cr superlattices, and is also used to predict the most favorable values of parameters for a large magnetoresistance effect. >

Spin-orbit coupling and spin waves in ultrathin ferromagnets: The spin-wave Rashba effect

We present theoretical studies of the influence of spin-orbit coupling on the spin-wave excitations of the Fe monolayer and bilayer on the W110 surface. The Dzyaloshinskii-Moriya interaction is active in such films by virtue of the absence of reflection symmetry in the plane of the film. When the magnetization is in plane, this leads to a linear term in the spin-wave dispersion relation for propagation perpendicular to the magnetization, to produce a dispersion curve similar in nature to that found for electrons on semiconducting surfaces when the Rashba coupling is active. We also show spin-polarized electron-loss spectroscopy response functions that illustrate the role of spin-orbit coupling in such measurements. In addition to the modifications of the dispersion relations for spin waves, the presence of spin-orbit coupling in the W substrate leads to a substantial increase in the linewidth of the spin-wave modes. The formalism we have developed applies to a wide range of systems, and the particular system explored in the numerical calculations provides us with an illustration of phenomena which will be present in other ultrathin ferromagnet/substrate combinations.

Emergence of local magnetic moments in doped graphene-related materials

Motivated by recent studies reporting the formation of localized magnetic moments in doped graphene, we investigate the energetic cost for spin polarizing isolated impurities embedded in this material. When a well-known criterion for the formation of local magnetic moments in metals is applied to graphene we are able to predict the existence of magnetic moments in cases that are in clear contrast to previously reported density-functional theory (DFT) results. When generalized to periodically repeated impurities, a geometry so commonly used in most DFT calculations, this criterion shows that the energy balance involved in such calculations contains unavoidable contributions from the long-ranged pairwise magnetic interactions between all impurities. This proves the fundamental inadequacy of the DFT assumption of independent unit cells in the case of magnetically doped low-dimensional graphene-based materials. We show that this can be circumvented if more than one impurity per unit cell is considered, in which case the DFT results agree perfectly well with the criterion-based predictions for the onset of localized magnetic moments in graphene. Furthermore, the existence of such a criterion determining whether or not a magnetic moment is likely to arise within graphene will be instrumental for predicting the ideal materials for future carbon-based spintronic applications.

Dynamic RKKY interaction in graphene

The growing interest in carbon-based spintronics has stimulated a number of recent theoretical studies on the RKKY interaction in graphene, based on which the energetically favourable alignment between magnetic moments embedded in this material can be calculated. The general consensus is that the strength of the RKKY interaction in undoped graphene decays as 1/D 3 or faster, where D is the separation between magnetic moments. Such an unusually fast decay for a 2-dimensional system suggests that the RKKY interaction may be too short ranged to be experimentally observed in graphene. Here we show in a mathematically transparent form that a far more long ranged interaction arises when the magnetic moments are taken out of their equilibrium positions and set in motion. We not only show that this dynamic version of the RKKY interaction in graphene decays far more slowly but also propose how it can be observed with currently available experimental methods. PACS numbers:

Theory of local dynamical magnetic susceptibilities from the Korringa-Kohn-Rostoker Green function method

Within the framework of time-dependent density functional theory combined with the Korringa-Kohn-Rostoker Green function formalism, we present a real space methodology to investigate dynamical magnetic excitations from first-principles. We set forth a scheme which enables one to deduce the correct effective Coulomb potential needed to preserve the spin-invariance signature in the dynamical susceptibilities, i.e. the Goldstone mode. We use our approach to explore the spin dynamics of 3d adatoms and different dimers deposited on a Cu(001) with emphasis on their decay to particle-hole pairs.

Dynamical magnetic excitations of nanostructures from first principles.

Within time-dependent density functional theory, combined with the Korringa-Kohn-Rostoker Green functions, we devise a real space method to investigate spin dynamics. Our scheme enables one to deduce the Coulomb potential which assures a proper Goldstone mode is present. We illustrate with application to 3d adatoms and dimers on Cu(100).

Theory of oscillatory exchange in magnetic multilayers: effect of partial confinement and band mismatch

Abstract In an earlier work a theory of the oscillatory exchange coupling between two ferromagnetic layers across a nonmagnetic spacer was proposed. It relied on size-quantization of the energy of electrons of one spin confined in the spacer (complete confinement model). Detailed tight-binding calculations of the coupling between Fe layers across Cr(001) using five d orbitals and the complete confinement model are reported. The coupling exhibits oscillations with several different periods but the amplitude and phase do not agree with experiment. An exactly solvable hole gas model and new tight-binding results are presented which demonstrate that the oscillation period is unaffected but the amplitude and phase depend critically on the degree of confinement. In particular, the amplitude is reduced dramatically for a weak confinement appropriate to iron. The dependence of the coupling on the occupancy of the spacer layer d band is also investigated. It is shown that a mismatch between the ferromagnet and spacer layer d bands, which increases with increasing number of holes in the spacer, leads to a systematic variation of the coupling strength across the transition metal series in agreement with experiment.

Exponential behavior of the interlayer exchange coupling across nonmagnetic metallic superlattices

It is shown that the coupling between magnetic layers separated by non-magnetic metallic superlattices can decay exponentially as a function of the spacer thickness

Carbon nanotube: A low-loss spin-current waveguide

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Graphene-based spin-pumping transistor

, as opposed to the usual