Magnetic antidots

 

In recent years there have been considerable efforts to achieve miniaturization of magnetic structures and devices. Most of these efforts have focused on increasing the storage density by reducing the size of the bits. Several techniques are currently being used to prepare ordered magnetic nanostructures including various lithography techniques. Another alternative approach to achieve nanopatterned media is the use of anodized Al templates. In this case, a two-step anodization process has been successfully used following the method introduced by Masuda and Fukuda. The first anodization process generates a porous alumina layer with a disordered surface and an ordered array of hollows at the alumina–aluminium interface. By removing the alumina layer and performing a second anodization process in the remaining nanostructured Al sample, an ordered alumina membrane with parallel aligned self-assembled nanopores is obtained.

 

Various routes have been proposed to replicate this ordering of the template where the final replicated nanostructures consist of highly ordered glassy nanohole or nanochannel arrays. Furthermore, the replica/antireplica process is currently used to fabricate arrays of nanowires or antidot structures. In particular, metal nanohole arrays based on the replication of an anodic alumina membrane have been recently reported using a polymeric material in the intermediate process.

 

Magnetic antidots are currently a topic intensively investigated because they are promising candidates for a new generation of electronic devices. Furthermore, the magnetic antidots can play an important role in magnonics and can be excellent candidates for ultra-high density magnetic recording media, due to the absence of the superparamagnetic effect in these systems. The presence of holes in a thin film produces a specific dipolar field, which controls the nucleation and propagation of domain walls. Thus, by varying the size of the holes in the antidots, one could control which types of domain walls can be propagated on antidots. Not only that, it also has been shown that varying the size of the holes in an array of antidots also affects the magnetoresistance, permeability, and in general, all their magnetic properties. A clear example of this is the increase in coercivity observed in arrays of antidots compared to a continuous film. This is due to a change in the way the system reverses its magnetization as a function of the size of the holes. Besides, by varying the holes dimensions, symmetry of the lattice and external magnetic field it is possible to tune the frequencies of ferromagnetic resonance modes in antidot arrays. Finally, another system that is of interest is the arrays of nanostructured antidots obtained by self-assembling polystyrene nanospheres.

 

In this topic, we have investigated the magnetic properties of permalloy [1,2] and cobalt [3] magnetic antidot arrays with different hole sizes. Importantly, these articles considered the synthesis of antidots, morphological and magnetic characterization, and theoretical study by micromagnetic simulations. Indeed, in order to investigate the influence of the natural disorder of the net of holes, and the imperfections of the circularity of each hole, we have utilized three-dimensional modeling using the SEM image as a bitmap. The process consists of transforming a SEM image in a black and white image, which can be read for the OOMMF package.

 

[1] J. L. Palma, C. Gallardo, L. Spinu, J. M. Vargas, L. S. Dorneles, J. C. Denardin, J. Escrig, Magnetic properties of Fe20Ni80 antidots: Pore size and array disorder, J. Magn. Magn. Mater. 344, 8-13 (2013).

[2] R. L. Rodríguez-Suárez, J. L. Palma, E. O. Burgos, S. Michea, J. Escrig, J. C. Denardin, C. Aliaga, Ferromagnetic resonance investigation in permalloy magnetic antidot arrays on alumina nanoporous membranes, J. Magn. Magn. Mater. 350, 88-93 (2014).

[3] S. Michea, J. L. Palma, R. Lavín, J. Briones, J. Escrig, J. C. Denardin, R. L. Rodríguez, Tailoring the magnetic properties of cobalt antidot arrays by varying the pore size and degree of disorder, J. Phys. D: Appl. Phys. 47, 335001 (2014).