Imaging the switching behavior of superparamagnetic nanoislands by spin-polarized scanning tunneling microscopy

In the past, spin-polarized scanning tunneling microscopy (SP-STM) was mainly applied to static domain configurations that do not vary in time. Here, we show that SP-STM may also be used to image the thermal switching behavior of superparamagnetic nanoislands. Special experimental care has to be taken in order to allow the unambiguous interpretation of the obtained data. Most important, the imaging of superparamagnetic particles requires the use of antiferromagnetic probe tips as the stray field of ferromagnetic tips may modify the sample's intrinsic switching behavior. Our results show that Fe monolayer islands on Mo(110) switch thermally when their area is smaller than 40 nm2. Dipolar coupling between adjacent islands is observed at small inter-particle distance. A pronounced shape dependence is found that confirms existing but yet unverified analytical predictions. The first experiments performed on Fe double-layer islands on W(001) also show thermal switching events, but no clear-cut size dependence is found.