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, which were fabricated using electron-beam lithography, was studied as a function of bar length. The bars have a 35 nm thickness, a 100 nm width, and a length varying from 200 nm to 5 μm. Magnetic force microscopy 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 the 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 length and 640 Oe and 1 μm for Ni and 1250 Oe and 2 μm for Co, respectively. The nonmonotonic length dependence suggests that the magnetization switching may be quasicoherent in the short bars and incoherent in the long bars, and that the exchange coupling is much stronger in Co bars than in Ni bars. Furthermore, the switching field of 1-μm-long Co bars was found to increase monotonically as the width decreases, reaching 3000 Oe at a 30 nm width.