M. Vélez

Asymmetric grazing incidence small angle x-ray scattering and anisotropic domain wall motion in obliquely grown nanocrystalline Co films.

Strong asymmetries have been observed in grazing incidence small angle x-ray scattering (GISAXS) in situ patterns obtained from 30 nm-thick nanocrystalline Co films prepared by oblique sputtering (15°-75° off-sample normal). These asymmetries have been qualitatively simulated by a simple model consisting of an ensemble of 8 nm-wide inclined Co nanocolumns. It is found that narrow inclined features appear in the diffuse background resembling those characteristic of faceted systems, which can be used to obtain straightforward non-destructive estimations of buried nanocolumnar grains inclination, even for oblique angles below 45°, when the stronger and broader asymmetric features of the pattern are not yet fully formed. Furthermore, using magneto-optical microscopy, a marked change in the magnetic domains nucleation and growth process has been observed in the sample prepared at 75°, with the stronger GISAXS asymmetries. Easy axis magnetization reversal starts by a random and homogeneous nucleation of small (∼μm) elongated domains aligned with the nanocolumns long axis and proceeds through the preferred propagation of head-to-head domain walls (DWs) along the applied field direction. This peculiar magnetic behavior indicates that the strongly anisotropic nanostructuring created by the oblique growth process is equivalent, from a magnetic point of view, to an array of self-assembled buried nanowires. These results show how GISAXS and magneto-optical microscopy can be combined as a powerful tool for correlating the morphology and magnetism of thin nanostructured systems.

Controlled nucleation of topological defects in the stripe domain patterns of lateral multilayers with perpendicular magnetic anisotropy

Work was supported by Spanish MICINN under Grant No. FIS2008-06249. R.M. and N.S. acknowledge support from UPV/EHU UFI11/23 and Basque Country Government Grant No. Etorek SE11-304. A.H.-R. acknowledges support from FCT of Portugal grant (Grant No. SFRH/BPD/90471/2012).

Controlled nucleation of topological defects in the stripe domain patterns of Lateral multilayers with Perpendicular Magnetic Anisotropy: competition between magnetostatic, exchange and misfit interactions

Work was supported by Spanish MICINN under Grant No. FIS2008-06249. R.M. and N.S. acknowledge support from UPV/EHU UFI11/23 and Basque Country Government Grant No. Etorek SE11-304. A.H.-R. acknowledges support from FCT of Portugal grant (Grant No. SFRH/BPD/90471/2012).

MFM observations of domain wall creep and pinning effects in amorphous CoxSi1- xfilms with diluted arrays of antidots

Magnetic force microscopy (MFM) has been used to analyse the behaviour of domain walls in uniaxial amorphous CoxSi1−x films patterned with diluted arrays of antidots by electron beam lithography. The walls are found to be pinned by the antidot array when the antidot density is high enough along the easy axis. The expansion of reversed nuclei under the influence of the tip stray field has been observed in several consecutive MFM images of the same area, showing how the competition between line tension effects and pinning by the patterned holes governs the creep motion of the 180° walls across the array of antidots.

Néel wall pinning on amorphous CoxSi1−x and CoyZr1−y films with arrays of antidots in the diluted regime

The magnetization reversal process has been studied in amorphous magnetic films patterned with ordered arrays of antidots in the diluted limit (i.e., with small enough antidot density so that the original film anisotropy is maintained and the flux closure structures around each dot are independent from each other). The role of the material parameters in the final behavior has been analyzed comparing the results on films made of two different Co-based amorphous alloys, CoxSi1−x and CoyZr1−y, that present a similar intrinsic uniaxial anisotropy but have a different saturation magnetization. The patterned holes are found to act as weak pinning centers for the motion of the Neel walls involved in the reversal process which results in an enhancement of coercivity in certain angular ranges. However, they are only effective in the material with the lower saturation magnetization (CoxSi1−x) which can be related with the different relative sizes of the patterned holes and the Neel walls in each case.

Deterministic propagation of vortex-antivortex pairs in magnetic trilayers

Thin perpendicular magnetic anisotropy films between two soft ferromagnetic layers have the nuclei for magnetization inversion at the bifurcations of their characteristic stripe domain pattern. The inverted nuclei induce vortex-antivortex pairs in the soft magnetic layers that exhibit a correlated motion extending several μm along the magnetic stripes during magnetization reversal. The sense of motion is completely determined by the topology of the magnetic bifurcations causing vortex-antivortex pairs to propagate in opposite senses depending on their polarities. This is a robust effect that might have practical applications. These findings are based on X-ray microscopy and micromagnetic calculations.

Imprinted labyrinths and percolation in Nd-Co/Nb magnetic/superconducting hybrids

Magnetization reversal processes have been studied in hybrid magnetic/superconducting Nd-Co/Nb bilayers by the comparison of out-of-plane magnetic hysteresis loops and superconducting phase diagrams as a function of magnetic layer thickness and of disorder in the magnetic layer induced by a nanostructured copolymer template. A good correlation is found between the regimes corresponding to percolation effects in the superconductor and to the transition from extended to confined superconductivity with the characteristic fields for reverse domain nucleation and fast domain expansion in the magnetic layer, indicating that superconductivity nucleates on the disordered network imprinted on the superconducting layer by the labyrinth domain structure of the magnetic layer. As disorder increases in the magnetic layer, percolation effects disappear from the superconducting transitions in agreement with a more homogeneous magnetization reversal process.

FLUX-FLOW RESISTIVITY AND VORTEX VISCOSITY OF HIGH-TC FILMS NEAR TC

The flux flow regime of high-T

Influence of boundary geometry in domain wall propagation in magnetic films with asymmetric holes: Micromagnetic calculations

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Antiferromagnetic coupling in amorphousCoxSi1−x∕Simultilayers

samples of different normal state resistivities is studied in the temperature range where the sign of the Hall effect is reversed. The scaling of the vortex viscosity with normal state resistivity is consistent with the Bardeen-Stephen theory. Estimates of the influence of possible mechanisms suggested for the sign reversal of the Hall effect are also given.