G. O. G. Rebouças

Nucleation of vortex pairs in exchange biased nanoelements

We report a theoretical investigation of interface effects in the magnetic order of interface biased iron and Permalloy™ elliptical nano-elements. Contrary to intuition, there is a partial pinning of the interface layer, favoring double vortex states along the hysteresis loop. Interface biasing affects the relative chirality and the distance of the vortices. Unbiased nanoelements may nucleate vortex pairs with the same chirality separated by an antivortex. For interface biased nanoelements the vortex pair forms with opposite chirality separated by a magnetic domain.

Controlling the core-to-core distance of vortex pairs in exchange-biased iron elliptical nanoelements

We report a theoretical study of vortex pairs in exchange-biased elliptical iron nanoelements. We show that the remanent state may be tailored to fit vortex pairs with opposite chiralities separated by a diamond-like domain. Flat nanoelements with lateral dimensions ranging from 115 nm × 425 nm to 195 nm × 425 nm have the core-to-core distance tunable by the interface field strength.

Depinning field of a periodic domain wall array in vicinal nanowires

We report a theoretical investigation of the magnetic states and depinning field of a periodic array of head-to-head domain walls of flat Fe rectangular nanowires, exchange coupled with a vicinal two-sublattice uniaxial antiferromagnetic substrate. We show that for strong interface exchange energy, domain walls are pinned at interface steps perpendicular to the antiferromagnetic easy axis, separating terraces with opposite interface exchange field. The array sequence, which alternates head-to-head and tail-to-tail domain walls, may form a structure with alternate chirality or with the same chirality. The domain wall dipolar field affects the chirality sequence, which is tunable by the geometrical constraints and the strength of the interface exchange field. The depinning field of 10 nm thick, 1 μm long wires, with widths of 100 and 200 nm, is of the order of the interface field strength, and the depinning process involves domain wall motion and the transversal displacement of a periodic array of vortices.

Thermal hysteresis of interface biased dipolar coupled nanoelements

We report a theoretical investigation of thermal hysteresis of a pair of interface biased elliptical iron nanoelements, separated by an ultrathin layer of nonmagnetic material. The thermal hysteresis originates in the strong dipolar interaction, and is tunable by the nature of the low temperature state and the eccentricity of the nanoelements. The width of the thermal hysteresis varies from 500 K to 100 K for lateral dimensions of 125 nm × 65 nm and 145 nm × 65 nm.

Dipolar effects on the magnetic phases of superparamagnetic clusters

We report a theoretical study of the impact of dipolar interactions on the room temperature magnetic phases of superparamagnetic nanoparticles confined in spherical and ellipsoidal clusters. We consider Fe3O4 nanoparticles with size ranging from 9 nm to 12 nm, arranged with uniform density in hundred nanometer-sized clusters. We show that one may have a large enhancement of the initial susceptibility for ellipsoidal clusters of high eccentricity, as required for most biomedical applications. Spherical clusters display a reduction of the initial susceptibility, due to the early nucleation of new magnetic phases. In densely packed systems, the dipolar interaction may lead to thermal stabilization of the individual nanoparticle moments, while keeping the cluster superparamagnetic, with a vanishingly small magnetic moment in the absence of an external field. The theoretical model is used to discuss recent findings on quasi-one-dimensional arrays of superparamagnetic Fe and Co nanoparticles, and on spherical clusters of superparamagnetic Fe3O4 nanoparticles.We report a theoretical study of the impact of dipolar interactions on the room temperature magnetic phases of superparamagnetic nanoparticles confined in spherical and ellipsoidal clusters. We consider Fe3O4 nanoparticles with size ranging from 9 nm to 12 nm, arranged with uniform density in hundred nanometer-sized clusters. We show that one may have a large enhancement of the initial susceptibility for ellipsoidal clusters of high eccentricity, as required for most biomedical applications. Spherical clusters display a reduction of the initial susceptibility, due to the early nucleation of new magnetic phases. In densely packed systems, the dipolar interaction may lead to thermal stabilization of the individual nanoparticle moments, while keeping the cluster superparamagnetic, with a vanishingly small magnetic moment in the absence of an external field. The theoretical model is used to discuss recent findings on quasi-one-dimensional arrays of superparamagnetic Fe and Co nanoparticles, and on spherical c...

Excitations of interface pinned domain walls in constrained geometries

We report a theoretical investigation of the equilibrium pattern and the spectra of head-to-head and Neel domain walls of flat Fe and Py stripes, exchange coupled with a vicinal antiferromagnetic substrate. We show that the domain wall excitation spectrum is tunable by the strength of the interface field. Furthermore, strong interface coupling favors localized wall excitations.

Confinement of magnetic vortex and domain walls in dipolar coupled concentric nanocylinders

Ferromagnetic nanostructures have recently attracted extensive research interest, for fundamental studies of magnetism in confined geometries, and a variety of emerging applications, such as spin logic devices, magnetic sensors and magnetic nano-oscillators. Vortices and domain walls are issues of current interest in these systems . Key applications benefit from the possibility of tailoring the vortex core and the domain wall magnetic patterns. We report a theoretical study of the magnetic phases of core-shell nanocylinders, consisting of a thin Fe (Py) cylindrical core, surrounded by a few nanometers thick coaxial Py (Fe) cylindrical shell, separated by a thin nonmagnetic cylindrical layer . The Fe uniaxial anisotropy is along the X-axis direction in the core-shell surface.

Tailoring the vortex core in confined magnetic nanostructures

We present a study of vortex formation in interface biased nano-sized disk and square nanoelements of Fe and PyTM. For small lateral dimensions, the circular nanoelements have smaller vortex core diameter than square nanoelements with equal top surface area. For surface area ranging from 1900 nm2 to 6700  nm2, the vortex core diameter of 30 nm thick Fe (PyTM) nanoelements varies from 32 nm (36 nm) to 36 nm (48 nm). Interface effects are stronger for PyTM nanoelements.

Domain wall pinning at F/AF interface defects

In this article, we discuss the nature of the domain wall nucleated in a single step defect in a F/AF bilayer. We have found that for ultra-thin F films the surface pattern is a replica of the interface pattern.