Superparamagnetic transitions in ultrathin film NiFe nanolines

We found that the coercivity of electron-beam patterned nanoscale NiFe and Co lines (width ranging from 55 nm to 1 μm) has a distinct inverse-width dependence when the film thickness is greater than 3.5 nm. This inverse-width dependence is consistent with a nucleation picture in which the magnetization reversal is controlled by a small nucleus, which spreads across the width of the nanoline but is independent of the length. However, the inverse-width dependence breaks down for the narrowest NiFe lines at room temperature in films below 3.5 nm in thickness. In this thickness regime the coercivity drops rapidly as a function of decreasing width as the controlling nucleus size approaches the superparamagnetic limit. The Arrhenius–Neel equation for this equivalent volume very effectively models the data. As the temperature is decreased, the coercivity of the narrowest lines rapidly increases and the inverse-width trend is recovered.