Magnetic and Magnetoresistive Properties of Thin Films Patterned by Self-Assembling Polystyrene Nanospheres
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Self-assembling of polystyrene nanospheres (PNs) is a powerful technique for preparing large area (several mm2) nanostructured thin films. Compared to conventional lithographic techniques (e.g. EBL), which have more resolution and are more versatile, but are limited to very small surface areas, self-assembling PN allows preparation of large nanostructured samples. This technique limits the shaping to only circular dot and antidot geometries which can be obtained in the hexagonal close-packed configuration.
In this paper, the self-assembling PN preparation technique will be thoroughly discussed, and the magnetic and magnetoresistive properties of dot and antidot arrays of Ni80Fe20, Ni, Co and Fe-based amorphous alloys will be investigated. All dot and antidot arrays have been obtained from monolayers of PN on Si substrates. The initial diameter of the spheres is 500 nm, and is reduced to 250–400 nm by reactive ion etching. The typical thickness of the magnetic material is approximately 10–30 nm for antidot samples, and in the 30–80 nm range for dot samples.
Both dot and antidot systems have been studied by means of scanning electron microscopy, atomic force and magnetic force microscopy, and alternating gradient field magnetometry, to record hysteresis loops. On antidot samples, magnetoresistance measurements have been carried out in the 5–300 K temperature range. The expected anisotropic magnetoresistance effect is observed, superimposed to a giant magnetoresistance effect on some systems. Co antidots also display an exchange bias effect below approximately 150 K, as evidenced by both magnetoresistance and low temperature hysteresis loop measurements. On dot samples, a typical vortex structure is observed, which depends on the material thickness and mean dot size. Preliminary magnetoresistance measurements on dot samples are also presented.
KeywordsElectron Beam Lithography Magnetic Force Microscopy Domain Configuration Magnetic Force Microscopy Image Magnetoresistance Measurement
This work has been partially performed at NanoFacility Piemonte, INRIM, a laboratory supported by Compagnia di San Paolo.
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