Julian M. Gonzalez

Magnetization configurations and reversal of magnetic nanotubes with opposite chiralities of the end domains

For thick soft magnetic nanotubes with an anisotropy axis directed along the nanotube length the equilibrium energy ground states present magnetization configurations with opposite rotating senses in two tube ends (B-state), referring as antiparallel chiralities of the end vortex domains. For nanotubes with outer radius R of 50 nm, 100 nm and 150 nm, and length L = (2.5–20)R the B-state remanent magnetization and the reversal field dependence on tube thickness and anisotropy strength are studied by using both two-dimensional simulation and analytic methods. The equilibrium states, the hysteresis loops and the switching field values calculated numerically are presented as the functions of tube size and material parameters. For the short nanotubes the domain walls patterns, such as transverse walls and vortex walls, nucleating in the tube center, as well as the hysteresis loops of the nanotubes with transverse walls are presented. The numerical results are interpreted by a simple analytical model in which t...

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.

Short range order of FeCo and FeNi amorphous alloys from stress and/or field induced magnetic anisotropy☆

Abstract Stress and/or field induced magnetic anisotropies by current annealing of (Fe 1− x Co x ) 75 Si 15 B 10 (Fe-rich, x ⩽ 0.5) amorphous alloy ribbons have been studied. The results of magnetic anisotropy induced by magnetic field and tensile stress applied simultaneously to amorphous ribbons show that this anisotropy becomes reinforced and depends on both composition and annealing parameters. This enhancement makes the anisotropy induced by stress + field to be different from the addition to the anisotropies separately induced by either stress or field under the same annealing conditions. Moreover this enhancement seems to be much lower than that obtained in (Fe 1− x Ni x ) 80 B 20 (Fe-rich). The compositional dependence of index n between the induced magnetic anisotropic K ind ( T ann ) and the saturation magnetization M s ( T ann ) has been also performed. The experimental values obtained of this index n differ from that predicted by the theory of directional ordering or atomic pairs of the magnetic anisotropy. This difference may be attributed to the contribution of the FeNi or FeCo tetrahedral holes of Bernal.

Spin torque and critical currents for magnetic vortex nano-oscillator in nanopillars

We calculated the main dynamic parameters of the spin polarized current induced magnetic vortex oscillations in nanopillars, such as the range of current density, where a vortex steady oscillations exist, the oscillation frequency and orbit radius. We accounted for both the non-linear vortex frequency and non-linear vortex damping. To describe the vortex excitations by the spin polarized current we used a generalized Thiele approach to motion of the vortex core as a collective coordinate. All the calculation results are represented via the free layer sizes, saturation magnetization, Gilbert damping and the degree of the spin polarization of the fixed layer. Predictions of the developed model can be checked experimentally.