María Vélez

Crossed-Ratchet Effects for Magnetic Domain Wall Motion

We study both experimentally and theoretically the driven motion of domain walls in extended amorphous magnetic films patterned with a periodic array of asymmetric holes. We find two crossed-ratchet effects of opposite sign that change the preferred sense for domain wall propagation, depending on whether a flat or a kinked wall is moving. By solving numerically a simple phi(4) model we show that the essential physical ingredients for this effect are quite generic and could be realized in other experimental systems involving elastic interfaces moving in multidimensional ratchet potentials.

Topological defects and misfit strain in magnetic stripe domains of lateral multilayers with perpendicular magnetic anisotropy

Stripe domains are studied in perpendicular magnetic anisotropy films nanostructured with a periodic thickness modulation that induces the lateral modulation of both stripe periods and in-plane magnetization. The resulting system is the 2D equivalent of a strained superlattice with properties controlled by interfacial misfit strain within the magnetic stripe structure and shape anisotropy. This allows us to observe, experimentally for the first time, the continuous structural transformation of a grain boundary in this 2D magnetic crystal in the whole angular range. The magnetization reversal process can be tailored through the effect of misfit strain due to the coupling between disclinations in the magnetic stripe pattern and domain walls in the in-plane magnetization configuration.

Three-dimensional spin structure in exchange-biased antiferromagnetic/ferromagnetic thin films

A coexistence of lateral and in-depth domain walls in antiferromagnet/ferromagnet (AF/FM) thin films exhibiting double hysteresis loops (DHLs) is demonstrated. Comparison of single and DHLs together with local and global measurements confirms the formation of two oppositely oriented domains in the AF that imprint a lateral domain structure into the FM layer. Most significantly, the magnetization reversal mechanism within each opposite domain takes place by incoherent rotation of spring-like domain walls extending through the Ni thickness. Therefore, complex three-dimensional domain walls are created perpendicular and parallel to the AF/FM interface in exchange biased systems.

Capabilities and limitations of LA-ICP-MS for depth resolved analysis of CdTe photovoltaic devices

The analytical potential of ArF* excimer Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) is investigated for fast qualitative depth profile analysis of multi-layer CdTe photovoltaic (PV) devices. Critical parameters (e.g. laser fluence and laser repetition rate) are evaluated and optimized to reduce the aerosol mixing from consecutive laser shots and to achieve a low penetration rate. As a result, a high depth resolution (10 s of nm) is demonstrated through the analyses of superficial and embedded coatings. For instance, a layer with a thickness of 100 nm at a depth of 3 μm is successfully measured. Moreover, qualitative profiles of major and minor elements obtained by LA-ICP-MS are validated using reference techniques such as Secondary Ion Mass Spectrometry (SIMS) or Glow Discharge Time of Flight Mass Spectrometry (GD-TOFMS). Additionally, the shape and morphology of the laser-induced craters, after different numbers of laser shots, are investigated using mechanical profilometry and Atomic Force Microscopy (AFM), respectively.

Double percolation effects and fractal behavior in magnetic/superconducting hybrids

Perpendicular magnetic anisotropy ferromagnetic/superconducting (FM/SC) bilayers with a labyrinth domain structure are used to study nucleation of superconductivity on a fractal network, tunable through magnetic history. As clusters of reversed domains appear in the FM layer, the SC film shows a percolative behavior that depends on two independent processes: the arrangement of initial reversed domains and the fractal geometry of expanding clusters. For a full labyrinth structure, the behavior of the upper critical field is typical of confined superconductivity on a fractal network.

Fabrication and magnetic properties of nanostructured amorphous Nd?Co films with lateral modulation of magnetic stripe period

Amorphous Nd?Co films with perpendicular magnetic anisotropy have been nanostructured in a lateral magnetic multilayer geometry in order to analyse and modify in a controlled way the configuration of its characteristic stripe domains as well as their rotation processes. Magnetic force microscopy measurements reveal that, actually, the artificial thickness modulation results in size effects on the magnetic stripes, so that the stripe domains configuration can be tuned with the lateral multilayer periodicity, and, due to the consequent reduction of the rotatable magnetic anisotropy, it is possible to modify the stripe rotation processes for in-plane magnetization reversal.

Coercive and anisotropy fields in patterned amorphous FeSi submicrometric structures

Amorphous FexSi1−x films have been prepared on Si substrates in order to fabricate submicrometric magnetic structures with soft magnetic behavior. The magnetic properties compositional dependence of the unpatterned samples has been analyzed to select the Fe content (x=0.7) with the lowest coercive and anisotropy fields values. Arrays of Fe0.7Si0.3 lines have been fabricated by electron beam lithography combined with a liftoff technique, with typical dimensions of 200 nm linewidth and 1 μm line spacing. These arrays present coercive fields parallel to the line direction as small as 9 Oe.

Coercive fields of amorphous Co–Si films with diluted arrays of antidots

Diluted arrays of antidots have been patterned by electron beam lithography and an etching process on amorphous Co–Si films of well defined uniaxial anisotropy. The analysis of the angular dependence of the hysteresis loops shows that the antidot arrays present a similar uniaxial anisotropy to the unpatterned film, and that the main effect of patterning for this small antidot density appears as an enhancement in the coercivity. The observed easy axis coercive fields are consistent with the estimates for domain wall pinning by a non-magnetic inclusion surrounded by a closure domain structure. However, the angular dependence of the coercivity presents an anomalous behaviour that points to the existence of an anisotropic domain wall pinning mechanism of the antidot arrays.

Interplay between collective pinning and artificial defects on domain wall propagation in Co/Pt multilayers

The interplay between collective pinning on intrinsic structural defects and artificial pinning at a patterned hole is studied in magnetic multilayers with perpendicular anisotropy. The pinning strength of a patterned hole is measured through its efficiency to stop domain wall (DW) propagation into a consecutive unpatterned nanowire section (using antisymmetric magnetoresistance to detect the direction of DW propagation) whereas collective pinning is characterized by the field dependence of DW velocity. Close to room temperature, collective pinning becomes weaker than artificial pinning so that pinning at the hole compensates nucleation-pad geometry, blocking DW propagation across the nanowire.

Perpendicular magnetic anisotropy in Nd-Co alloy films nanostructured by di-block copolymer templates

Nd-Co amorphous magnetic films with perpendicular magnetic anisotropy have been grown on nanostructured templates prepared with self-organized di-block poly(styrene)-block-poly(4-vinylpyridine) copolymer layers with a periodic structure of 60 nm spaced pores. These templates modify both the magnetic film topography and mechanical strain on a local scale. The effect of these structural changes is particularly noticeable in the low thickness range of the magnetic films where the transition from in-plane to out-of plane magnetization takes place. The Nd-Co films grown on the copolymer template present lower perpendicular magnetic anisotropy and, also, stronger stripe domain pinning effects in comparison with reference films grown on flat Si substrates.