EFFECTS OF BAR LENGTH ON SWITCHING FIELD OF NANOSCALE NICKEL AND COBALT BARS FABRICATED USING LITHOGRAPHY

The switching behavior of isolated nanoscale nickel and cobalt bars, w hich were fabricated using electron-beam lithography, was studied as a function of bar length. The bars have a 35 nm thickness, a 100 nm wid th, and a length varying from 200 nm to 5 mu m. Magnetic force microsc opy showed that except for the Ni bars with a length equal to or less than 250 nm, all other as-fabricated bars were single domain. Unlike t he bar width dependence, the switching field of the single-domain bars was found to first increase with the bar length, then decrease after reaching a peak. The peak switching field and the corresponding bar le ngth are 640 Oe and 1 mu m for Ni and 1250 Oe and 2 mu m for Co, respe ctively. The nonmonotonic length dependence suggests that the magnetiz ation switching may be quasicoherent in the short bars and incoherent in the long bars, and that the exchange coupling is much stronger in C o bars than in Ni bars. Furthermore, the switching field of 1-mu m-lon g Co bars was found to increase monotonically as the bar width decreas es, reaching 3000 Oe at a 30 nm width. (C) 1996 American Institute of Physics.