Magnetic properties engineering of nanopatterned cobalt antidot arrays

Abstract

We report on the study of arrays of 60 nm wide cobalt antidots, nanopatterned using focused ion beam milling. Square and hexagonal symmetry arrays have been studied, with varying antidot densities and lattice constant from 150 up to 300 nm. We find a strong increase of the arrays' magnetic coercivity with respect to the unpatterned film, which is monotonic as the antidot density increases. Additionally, there is a strong influence of the array symmetry to the in-plane magnetic anisotropy: square arrays exhibit fourfold symmetry and hexagonal arrays exhibit sixfold symmetry. The above findings are corroborated by magnetic imaging and micromagnetic modeling, which show the magnetic structure of the arrays to depend strongly on the array morphology.

Nanopatterned hard/soft bilayer magnetic antidot arrays with long-range periodicity

“…In the high DVD cases (lattice constant 240 nm), square symmetry MAAs show the highest coercivity along the direction of the second antidot neighbors, that is along 45 o ; in contrast, for hexagonal symmetry MAAs, the highest anisotropy is obtained along 0 o , the direction of the first antidot neighbors. For square symmetry MAAs these results are in agreement with our previous study of FIB nanopatterned MAAs of pure Co (17). However, the opposite behavior is observed for hexagonal symmetry MAAs; we attribute this to the subtle coercivity differences introduced by the Py layers.…”

“…Moreover, the two studies were performed on MAA samples deposited using different sputtering systems and most importantly, different substrates; as a result, the properties of the films are not necessarily identical. In the low DVD cases (lattice constant 360 nm), no particular systematic dependence of coercivity to anisotropy is observed; this is also in agreement with our previous work on a single Co film (17): as the DVD increases, the array symmetry becomes more important. Furthermore, it should be noted that bottom-up fabricated MAAs onto porous anodic alumina substrates -deposited simultaneously to the ones of this study -do not show any preferred in-plane orientation, as a result of the short-range order of the MAAs in the bottom-up approach (13).…”

“…between 0º and 90º for square arrays and between 0º and 60º for hexagonal ones. The results, summarized in figure 5, are in agreement with previous studies showing a monotonic dependence of coercivity on the DVD of the MAAs (17), whereas coercivity values of more than 700 Oe in Co MAAs having 0.58 DVD and more than 500 Oe in Py MAAs having 0.69 DVD, are reported in the literature (21). Furthermore, it is worth noting that the coercivity values obtained in this study are much higher than those obtained for MAAs fabricated under the same conditions (identical equipment and sputtering parameters) but onto porous anodic alumina, which were reported in our previous publication (13).…”

“…In the case of hexagonal arrays, the first neighbor directions for square arrays are 0º, 60º and 120º, whilst the second neighbor directions are 30º, 90º and 150º. Disk-shaped MAAs are patterned to avoid any macroscopic shape anisotropy (17); their diameter is 15 μm and they are surrounded by a 0.5 μm-wide circular trench to magnetically decouple the MAAs from the rest of the continuous multilayer. Non-patterned regions confined by a trench were also fabricated for the sake of comparison.…”

“…Non-patterned regions confined by a trench were also fabricated for the sake of comparison. Antidots can be considered as defects acting as domain-wall (DW) pinning centers during magnetization reversal (17), thus, the defect volume density (DVD) of the MAAs in correlation to the hole diameter is of great importance to their study. Taking into account the MAA geometry (17) and considering an antidot diameter of 40 nm (as measured in figure 3 at half depth), the DVD is given by the formula for the square…”