The correlation between the microstructure and the magnetization switching field of individual Co/Pt multilayer nanodots with Pt or Ta/Pt underlayers has been investigated by transmission electron microscopy and magnetic force microscopy. The insertion of a 2-nm-thick Ta underlayer promoted the Co/Pt  crystal axis orientation and improved the mean switching field and the switching field distribution of nanodot arrays with a diameter of 70nm from 1.4 to 6.9 kOe and from 22 to 8%, respectively. Electron microscopy of individual dots has revealed that all the nanodots without the Ta layer contain (200)-oriented grains, which possibly work as nucleation sites owing to a small perpendicular magnetic anisotropy. However, no structural difference is found among the dots having the same underlayers, suggesting the existence of other factors affecting the switching field distribution.
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