J. d’Albuquerque e Castro

Stability of magnetic configurations in nanorings

The relative stability of the vortex, onion, and ferromagnetic phases in nanorings is examined as a function of the ring geometry. Total energy calculations are carried out analytically, based on simple models for each configuration. Results are summarized by phase diagrams, which might be used as a guide to the production of rings with specific magnetic properties.

Magnetic properties of layered nanorings

The magnetic structure of nanorings consisting of alternate layers of magnetic and nonmagnetic materials is investigated as a function of their geometry. Phase diagrams giving the relative stability of characteristic internal magnetic configurations of the rings are obtained. Attention is focused on the condition for occurrence of the vortex configurations, in which case the layered structure might be used to produce magnetoresistive random access memories.

Magnetic behavior of nanoparticles in patterned thin films

The magnetic behavior of truncated conical nanoparticles in patterned thin films is investigated as a function of their size and shape. Using a scaling technique, phase diagrams giving the relative stability of characteristic internal magnetic structures of the particles are obtained. The role of the uniaxial anisotropy in determining the magnetic properties of such systems is discussed, and a simple method for establishing its strength is proposed.

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

Enhanced magnetoresistance effect in layered systems

N

Magnetic susceptibility of exchange-disordered antiferromagnetic finite chains

, as opposed to the usual

Model for spin coupling disorder effects on the susceptibility of antiferromagnetic nanochains

N^{-2}

Static and dynamical properties of elliptical quantum corrals

decay. This effect is due to the lack of constructive contributions to the coupling from extended states across the spacer. The exponential behavior is obtained by properly choosing the distinct metals and the superlattice unit cell composition.

Multi-stability in low-symmetry magnetic nanoparticles

Magnetoresistance ratios several orders of magnitude higher than those of conventional multilayers may be obtained with much smaller saturation fields in magnetic layers separated by a periodically modulated structure. Conditions for the occurrence of such effect, as well as the possible use of these systems as spin-filter devices and magnetic logical gates, are discussed.

Modeling anisotropic magnetoresistance in layered antiferromagnets

The low-temperature behavior of the static magnetic susceptibility �(T) of exchange-disordered antiferromagnetic spin chains is investigated. It is shown that for a relatively small and even number of spins in the chain, two exchange distributions which are expected to occur in nanochains of P donors in silicon lead to qualitatively distinct behaviors of the low-temperature susceptibility. As a consequence, magnetic measurements might be useful to characterize whether a given sample meets the requirements compatible with Kane’s original proposal for the exchange gates in a silicon-based quantum computer hardware. We also explore the dependence of �(T) on the number of spins in the chain as it increases towards the thermodynamic limit, where any degree or distribution of disorder leads to the same low-temperature scaling behavior. We identify a crossover regime where the two distributions of disorder may not be clearly differentiated, but the characteristic scaling of the thermodynamic limit has not yet been reached.