Fabrication and structural characterization of highly ordered sub-100-nm planar magnetic nanodot arrays over 1cm2 coverage area

Abstract

Porous alumina masks are fabricated by anodization of aluminum films grown on both semiconducting and insulating substrates. For these self-assembled alumina masks, pore diameters and periodicities within the ranges of 10-130 and 20-200 nm, respectively, can be controlled by varying anodization conditions. 20 nm periodicities correspond to pore densities in excess of 10 12 per square inch, close to the holy grail of media with 1 Tbit/ in. 2 density. With these alumina masks, ordered sub-100-nm planar ferromagnetic nanodot arrays covering over 1 cm 2 were fabricated by electron beam evaporation and subsequent mask lift-off. Moreover, exchange-biased bilayer nanodots were fabricated using argon-ion milling. The average dot diameter and periodicity are tuned between 25 and 130 nm and between 45 and 200 nm, respectively. Quantitative analyses of scanning electron microscopy ͑SEM͒ images of pore and dot arrays show a high degree of hexagonal ordering and narrow size distributions. The dot periodicity obtained from grazing incidence small angle neutron scattering on nanodot arrays covering ϳ2.5 cm 2 is in good agreement with SEM image characterization. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2356606͔ INTRODUCTIONNanostructured magnets have attracted great attention recently due to their unique magnetic properties which are completely different from those of continuous films and bulk materials. [1][2][3][4] In addition, nanoscale studies may shed light on the heavily investigated mechanism of exchange bias 5-9 existing in ferromagnet ͑FM͒-antiferromagnet ͑AF͒ bilayers. Fabrication of sub-100-nm FM single layer and FM/AF bilayer nanostructures offers a great opportunity for research on fundamental magnetism at the nanoscale.A variety of methods are used for nanofabrication, including electron beam and optical lithography. The low throughput and high cost of electron beam lithography lead to limited applicability for the fabrication of nanopatterns with large coverage area. Although optical lithography has high throughput, the smallest size of features that can be produced is limited by the long optical wavelength. Among many nanofabrication methods, 10-15 a promising technique is based on masks with sub-100-nm self-assembled pores. Such masks, for example, can be fabricated by the anodization of aluminum under appropriate experimental conditions. [16][17][18][19][20][21][22] Arrays of nanopores with tunable pore diameter covering more than 1 cm 2 demonstrate the potential application of porous alumina ͑AlO x ͒ masks for nanostructure fabrication. Previously, alumina membranes obtained from the anodization of thick ͑Ͼ100 m͒ aluminum foils were mostly used for the fabrication of nanowires. [23][24][25][26][27][28][29][30] These nanowires were grown by electrodeposition, which cannot be easily adapted for the fabrication of nanodots with controlled and uniform thickness. Moreover, this method requires transfer of the alumina membranes onto a substrate. Insufficient adhesion between the membrane and substrate,...

Temperature induced single domain–vortex state transition in sub-100nm Fe nanodots

“…20,21 The nanodots have been grown on silicon substrates with a diameter of 67± 13 nm and a thickness of 20 nm, similar to those reported earlier. 19 The Fe dots thus made are polycrystalline, capped with a Ag layer.…”

Deconvoluting reversal modes in exchange-biased nanodots

“…23 The nanodot size is 67±13 nm, and coexistence of SD and VS modes is expected from our prior studies on unbiased Fe dots. 24,25 The dot center-to-center spacing is roughly twice the dot diameter, as illustrated in Fig.…”

Development of vortex state in circular magnetic nanodots: Theory and experiment

“…19 Powder x-ray diffraction shows that the hexagonally ordered Fe dots are polycrystalline. By controlling the anodization process ͑i.e., electrochemical parameters͒ and using double anodization 20 dot diameters and periodicities in the range 25-150 nm with narrow distributions ͑10-15 standard deviation͒ are produced.…”

“…9,19,[25][26][27] Earlier calculations have shown that when the side-to-side distance in an array is larger than a single-dot diameter ͑i.e., the center-tocenter distance larger than twice the diameter͒ the dots interact weakly. 9 Since the dots are polycrystalline, we neglect the crystalline anisotropies.…”