Juan José del Val

α and β relaxation processes in internally plasticized poly (vinylchloride)

Abstract The α and β relaxation processes of poly (vinyl chloride) (PVC) have been studied by means of dielectric relaxation in samples of PVC internally plasticized by the substitution of chlorine atoms either by hydrogen ones or by the benzenethiolate group in order to investigate the properties of this polymer which has a relaxation function with one of the largest widths. In the α relaxation process the width of the relaxation function is decreased. The β relaxation temperature of the process is decreased. These facts, as well as X-ray diffraction results, suggest that the intermolecular interactions of PVC, due to the chlorine atoms, are diminished by the substitution process, making the molecular motions of PVC easier.

Limits for the vortex state spin torque oscillator in magnetic nanopillars: Micromagnetic simulations for a thin free layer

We report micromagnetic simulations of magnetization dynamics of a vortex state in the free layer of a circular nanopillar excited by the spin transfer torque effect of a perpendicular to the layer (dot) plane spin-polarized electrical current. The magnetization of the reference layer (polarizer) is assumed to be fixed. A new regime of the dynamic magnetization response to the current is reported: vortex expelling from the dot, subsequent in-plane magnetization oscillations in single domain state, and the vortex return with an opposite core polarization. We analyze conditions (limits of the vortex state as a nano-oscillator) to achieve steady magnetization oscillations corresponding to a gyrotropic motion of the vortex core in terms of the current intensity. These conditions are formulated via the critical currents and vary greatly with the magnetic damping parameter and the cell size used for micromagnetic simulations. The existing experiments on the current induced magnetization dynamics in nanopillars and nanocontacts are discussed.

Preparation and Characterization of Fe-Pt and Fe-Pt-(B, Si) Microwires

We have prepared Fe-Pt and Fe-Pt-(B, Si) microwires using the Taylor-Ulitovsky technique and studied their magnetic properties. Magnetic properties considerably depend on the metallic core composition. Microwires present either amorphous or a mixture of amorphous and nanocrystalline phases with a presence of face-centered cubic FePt3 (A1) disordered phase in the as-prepared state. We obtained microwires with coercivities from 4 to 140 Oe, which can be explained considering different metallic nucleus microstructures.