A. Pérez-Junquera

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.

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.

Crossed ratchet effects on magnetic domain walls: geometry and transverse field effects

Domain wall propagation across a 2D array of asymmetric holes is strongly dependent on the domain wall configuration: i.e. on whether the wall is flat or kinked. This results in interesting crossed ratchet and asymmetric accommodation effects that have been studied as a function of geometry and transverse field. Micromagnetic simulations have shown that the observation of crossed ratchet effects is easier for arrow than for triangular holes due to a larger field range in which kink propagation is the preferred mode for domain wall motion. Also, it has been found that dc transverse fields can produce a significant enhancement of the easy axis asymmetric accommodation and, also, that ac transverse fields can be rectified by the crossed ratchet potential.

Study of domain wall propagation in nanostructured CoPt multilayers by using antisymmetric magnetoresistance

Domain wall propagation has been studied in perpendicular anisotropy CoPt multilayers patterned by e-beam lithography into 5 μm wide wires. Positive and negative peaks appear in time resolved magnetoresistance curves, associated to the different directions of domain wall propagation along the wires. The field dependence of domain wall velocity is well described by a creep model of a 1D wall in the presence of weak disorder with critical exponent μ=1/4.