The thermal switching behavior of individual in-plane magnetized Fe/W(110) nanoislands is investigated by a combined study of variable-temperature spin-polarized scanning tunneling microscopy and Monte Carlo simulations. Even for islands consisting of less than 100 atoms the magnetization reversal takes place via nucleation and propagation. The Arrhenius prefactor is found to strongly depend on the individual island size and shape, and based on the experimental results a simple model is developed to describe the magnetization reversal in terms of metastable states. Complementary Monte Carlo simulations confirm the model and provide new insight into the microscopic processes involved in magnetization reversal of smallest nanomagnets.

Switching the magnetization of a magnetic bit by injection of a spin-polarized current offers the possibility for the development of innovative high-density data storage technologies. We show how individual superparamagnetic iron nanoislands with typical sizes of 100 atoms can be addressed and locally switched using a magnetic scanning probe tip, thus demonstrating current-induced magnetization reversal across a vacuum barrier combined with the ultimate resolution of spin-polarized scanning tunneling microscopy. Our technique allows us to separate and quantify three fundamental contributions involved in magnetization switching (i.e., current-induced spin torque, heating the island by the tunneling current, and Oersted field effects), thereby providing an improved understanding of the switching mechanism.

A full magnetic characterization of bulk Cr tips has been achieved using spin-polarized scanning tunneling microscopy at low temperature. A detailed bias-dependent study of the spatial magnetic sensitivity on the system of 1.5 monolayers of Fe/W(110) reveals that all magnetic directions in space are sensed over a wide bias range, thereby indicating a canted magnetization direction being a typical feature of bulk Cr tips. Consequently, using Cr as tip material allows any standard scanning tunneling microscope setup to be extended by the spin-polarized mode.

The influence of a high spin-polarized tunnel current onto the switching behavior of a superparamagnetic nanoisland on a nonmagnetic substrate is investigated by means of spin-polarized scanning tunneling microscopy. A detailed lifetime analysis allows for a quantification of the effective temperature rise of the nanoisland and the modification of the activation energy barrier for magnetization reversal, thereby using the nanoisland as a local thermometer and spin-transfer torque analyzer. Both the Joule heating and spin-transfer torque are found to scale linearly with the tunnel current. The results are compared to experiments performed on lithographically fabricated magneto-tunnel junctions, revealing a very high spin-transfer torque switching efficiency in our experiments.

Current-induced magnetization switching of thermally quasistable magnetic nanoislands is demonstrated using a spin-polarized scanning tunneling microscope. The magnetization of an individual Fe nanoisland consisting of about 40 atoms on a W(110) surface is reversibly switched between two quasistable states by the application of spin-polarized tunnel current pulses without an applied magnetic field. The pulse length is shown to be crucial for a high switching efficiency. Sweeping the tunnel current from the nanoampere to the microampere regime allows for the determination of critical switching currents.