arXiv subjects

Patrick Bruno

Publications and source records attributed to Patrick Bruno.

30 records · Page 2Linked to original sources

arXiv2002

Absence of spontaneous magnetic order at non-zero temperature in one- and two-dimensional Heisenberg and XY systems with long-range interactions

The Mermin-Wagner theorem is strengthened so as to rule out magnetic long-range order at T>0 in one- or two-dimensional Heisenberg and XY systems with long-range interactions decreasing as R^{-alpha} with a sufficiently large exponent alpha. For oscillatory interactions, ferromagnetic long-range order at T>0 is ruled out if alpha >= 1 (D=1) or alpha > 5/2 (D=2). For systems with monotonically decreasing interactions ferro- or antiferromagnetic long-range order at T>0 is ruled out if alpha >= 2D.

cond-mat.stat-mech↗

arXiv2001

Ab-initio calculations of exchange interactions, spin-wave stiffness constants, and Curie temperatures of Fe, Co, and Ni

We have calculated Heisenberg exchange parameters for bcc-Fe, fcc-Co, and fcc-Ni using the scalar-relativistic spin-polarized Green function technique within the tight-binding linear muffin-tin orbital method and by employing the magnetic force theorem to calculate total energy changes associated with a local rotation of magnetization directions. We have also determined spin-wave stiffness constants and found the dispersion curves for metals in question employing the Fourier transform of calculated Heisenberg exchange parameters. Curie temperatures were calculated both in the mean-field approximation and within the Green function random phase approximation. The latter results were found to be in a better agreement with available experimental data.

cond-mat.mes-hall↗

arXiv2000

Magnetic Scanning Tunneling Microscopy with a Two-Terminal Non-Magnetic Tip: Quantitative Results

We report numerical simulation result of a recently proposed \{P. Bruno, Phys. Rev. Lett {\bf 79}, 4593, (1997)\} approach to perform magnetic scanning tunneling microscopy with a two terminal non-magnetic tip. It is based upon the spin asymmetry effect of the tunneling current between a ferromagnetic surface and a two-terminal non-magnetic tip. The spin asymmetry effect is due to the spin-orbit scattering in the tip. The effect can be viewed as a Mott scattering of tunneling electrons within the tip. To obtain quantitative results we perform numerical simulation within the single band tight binding model, using recursive Green function method and Landauer-Büttiker formula for conductance. A new model has been developed to take into account the spin-orbit scattering off the impurities within the single-band tight-binding model. We show that the spin-asymmetry effect is most prominent when the device is in quasi-ballistic regime and the typical value of spin asymmetry is about 5%.

cond-mat.mes-hall↗

arXiv2000

Oscillatory Curie temperature of two-dimensional ferromagnets

The effective exchange interactions of magnetic overlayers Fe/Cu(001) and Co/Cu(001) covered by a Cu-cap layer of varying thickness were calculated in real space from first principles. The effective two-dimensional Heisenberg Hamiltonian was constructed and used to estimate magnon dispersion laws, spin-wave stiffness constants, and overlayer Curie temperatures within the mean-field and random-phase approximations. Overlayer Curie temperature oscillates as a function of the cap-layer thickness in a qualitative agreement with a recent experiment.

cond-mat.mes-hall↗

arXiv2000

Ab initio theory of the interlayer exchange coupling

Ab initio formulations of the interlayer exchange coupling (IEC) between two, in general non-collinearly aligned magnetic slabs embedded in a non-magnetic spacer are reviewed whereby both the spacer and the magnetic slabs as well as their interfaces may be either ideal or random. These formulations are based on the spin-polarized surface Green function technique within the tight-binding linear muffin-tin orbital method, the Lloyd formulation of the IEC, and the coherent potential approximation using the vertex-cancellation theorem. We also present an effective method for the study of the temperature dependence of the IEC. The periods, amplitudes, and phases are studied in terms of discrete Fourier transformations, the asymptotic behavior of the IEC is briefly discussed within the stationary-phase method. Numerical results illustrating the theory are presented.

cond-mat.mes-hall↗

arXiv1999

Theory of interlayer exchange interactions in magnetic multilayers

This paper presents a review of the phenomenon of interlayer exchange coupling in magnetic multilayers. The emphasis is put on a pedagogical presentation of the mechanism of the phenomenon, which has been successfully explained in terms of a spin-dependent quantum confinement effect. The theoretical predictions are discussed in connection with corresponding experimental investigations.

cond-mat.mes-hall↗

arXiv1999

Geometrically constrained magnetic wall

The structure and properties of a geometrically constrained magnetic wall in a constriction separating two wider regions are investigated theoretically. They are shown to differconsiderably from those of an unconstrained wall, so that the geometrically constrained magnetic wall truly constitutes a new kind of magnetic wall, besides the well known Bloch and Neel walls. In particular, the width of a constrained wall cann become very small if the characteristic length of the constriction is small, as is actually the case in an atomic point contact. This provides a simple, natural explanation for the large magnetoresistance observed in ferromagnetic atomic point contacts.

cond-mat.mes-hall↗

arXiv1999

Interlayer exchange coupling: Preasymptotic corrections

In the asymptotic limit, the interlayer exchange coupling decays as $D^{-2}$, where $D$ is the spacer thickness. A systematic procedure for calculating the preasymptotic corrections, i.e., the terms of order $D^{-n}$ with $n \ge 3$, is presented. The temperature dependence of the preasymptotic corrections is calculated. The results are used to discuss the preasymptotic corrections for the Co/Cu/Co(001) system.

cond-mat.mes-hall↗

arXiv1999

Influence of disorder on the perpendicular magnetoresistance of magnetic multilayers

The effect of disorder on the perpendicular magnetoresistance of magnetic multilayers is investigated theoretically. Various kinds of disorder are considered: (i) interface substitutional disorder and (ii) bulk disorder in the various layers and in the leads. The calculations are based upon the non-equilibrium Green's function formalism, together with the recursion method for calculating the real-space Green's function.

cond-mat.mes-hall↗

arXiv1999

Theory of interlayer exchange coupling

This paper contains the notes of lectures on the theory of interlayer exchange coupling presented at the 30-th Ferienschule of the Institut fuer Festkoerperforschung, Forschungszentrum Juelich, March 1999.

cond-mat.mes-hall↗

arXiv1998

The combined effect of temperature and disorder on interlayer exchange coupling in magnetic multilayers

We study the combined effect of temperature and disorder in the spacer on the interlayer exchange coupling. The temperature dependence is treated on ab initio level. We employ the spin-polarized surface Green function technique within the tight-binding linear muffin-tin orbital method and the Lloyd formulation of the IEC. The integrals involving the Fermi-Dirac distribution are calculated using an efficient method based on representation of integrands by a sum of complex exponentials. Application is made to Co/Cu_{100-x}M_x/Co(001) trilayers (M=Zn, Au, and Ni) with varying thicknesses of the spacer.

cond-mat.mes-hall↗

arXiv1998

Range dependence of interlayer exchange coupling

We have considered the effects of non magnetic impurities and interface roughness on the interlayer coupling between magnetic layers in metallic multilayers. The two types of defects alter the interlayer coupling in quite different ways. Elastic electron scattering by impurities in the non magnetic spacer layers between magnetic layers produces an exponential decay of the coupling with a characteristic decay length that is considerably longer than the ``global'' transport mean free path for the spacer layer with its surrounding interfaces. Interfacial roughness leads to an attenuation of the coupling that is related to the width of the roughness in relation to the Fermi wavelength; roughness does not alter the range dependence of the coupling. For certain types of electrical transport, e.g., for current perpendicular to the plane of the layers, the scattering from interface roughness and impurities in the spacer layers contribute on an equal footing to the exponential decay of the electron propagators, i.e., global mean free path. We show that interface roughness and impurities in the spacer layer affect the interlayer coupling differently.

cond-mat.mes-hall↗