J. d'Albuquerque e Castro

Fundamental oscillation periods of the interlayer exchange coupling beyond the RKKY approximation

A general method for obtaining the oscillation periods of the interlayer exchange coupling is presented. It is shown that it is possible for the coupling to oscillate with additional periods beyond the ones predicted by the RKKY theory. The relation between the oscillation periods and the spacer Fermi surface is clarified, showing that non-RKKY periods do not bear a direct correspondence to the Fermi surface. The interesting case of an FCC(110) structure is investigated, unmistakably proving the existence and relevance of non-RKKY oscillations. The general conditions for the occurrence of non-RKKY oscillations are also presented.

Magnetism of nanosized metallic Co-clusters

The magnetization of a small ferromagnetic particle as a function of the temperature is investigated. The possibility of occurrence of vortex-like excitations within the cluster, giving rise to a non-saturated magnetization even at low temperatures, is pointed out.

Hole concentration in a diluted ferromagnetic semiconductor

We consider a mean-field approach to the hole-mediated ferromagnetism in III-V Mn-based semiconductor compounds in order to discuss the dependence of the hole density on that of Mn sites in Ga1-xMnxAs. The hole concentration, p, as a function of the fraction of Mn sites, x, is parametrized in terms of the product m*Jpd2 (where m* is the hole effective mass and Jpd is the Kondo-like hole/local-moment coupling), and the critical temperature Tc. By using experimental data for these quantities, we have established the dependence of the hole concentration on x, which can be associated with the occurrence of a re-entrant metal-insulator transition taking place in the hole gas. We also calculated the dependence of the Mn magnetization on x, for different temperatures (T), and found that as T increases, the width of the composition-dependent magnetization decreases dramatically, and that the magnetization maxima also decrease in magnitude, indicating the need for quality control of the Mn doping level in diluted magnetic semiconductor devices.

Effects of impurities on the exchange coupling in magnetic metallic multilayers

In this work the variation of the exchange coupling between magnetic layers across a non-magnetic spacer layer, due to impurities of the magnetic material in the spacer, is calculated. The impurities are treated in the dilute limit and beyond it. It is shown that the change per impurity is comparable to the unperturbed coupling per atom, and that it oscillates as a function of spacer layer thickness with the same period as that coupling, though with a different phase. The most favourable magnetic moment orientation of the impurity is found for some spacer layer thicknesses and impurity positions. It is also shown that the short-period oscillations of the coupling may be much more affected by the presence of impurities than the long-period ones.

Complex magnetic reversal modes in low-symmetry nanoparticles

A detailed numerical analysis of the magnetization reversal processes in T-shaped nanoparticles has been carried out. Attention has been focused on the influence of the symmetry of the particle on the formation, propagation, and interaction of internal magnetic structures such as domain walls, vortices, and antivortices. Results show that the lower the degree of symmetry of the particle, the more complex the reversal process is. Thus, symmetry represents an additional ingredient to control the magnetic properties of ferromagnetic nanoparticles.

Simulational study of annealing effects in multilayers

Abstract The influence of annealing on the interface quality of multilayers presenting “back-diffusion” effects is simulated using an Ising lattice gas model. We show that interface roughness has a minimum as a function of the annealing temperature ( T a ). The probability of breaking the layers is obtained as a function of T a for different layer thicknesses. We have also calculated the average number of not alike (first and second) neighbors of an atom, which can be accessed by nuclear magnetic resonance experiments.

Magnetoresistance in granular metallic systems

The magnetic and transport properties of ferromagnetic clusters embedded in a metallic host are studied. A simple model is presented to explain the observed behaviour of the magnetoresistance in such systems. The role played by both the dipolar interaction between the clusters and the ratio of the electronic mean free path in the non-magnetic material to the average inter-cluster distance is clarified. Finally, a suggestion is made of systems in which the best magnetoresistance ratios are expected to be found.

Non-RKKY oscillations of exchange coupling in magnetic multilayers

Abstract It is shown how the exchange coupling between two ferromagnetic planes embedded in an infinite non-magnetic metal, regarded as a function of the distance between the planes, may contain important components which oscillate with periods not predicted by RKKY theory. The interesting case of fcc(110) structure with a Cu-like Fermi surface is discussed in detail.

Exchange stiffness constant of two-dimensional Ni and Fe

Abstract The exchange stiffness constant D 2d of unsupported (100) nickel monolayer is calculated in the tight-binding and random phase approximations. The tight-binding parameters are determined from a fit to ground-state spin density functional band structure calculations. The relative stability of the bulk and monolayer systems is discussed by comparing the results for D 2d and bulk D 3d . Preliminary results for (100) iron monolayer are also reported.

Enhanced magnetoresistance and spin-filter effects in magnetic heterostructures

We show that ballistic transport through a pair of superlatticed magnetic junctions connected by a low-density-of-carriers conducting medium can provide an efficient spin filter and cause an enhanced magnetoresistive response.