Quasi-static switching of a nanomagnet can become increasingly difficult and power consuming if its lateral size is reduced and switching by Oersted fields is considered. To overcome this "burning" problem different schemes are currently explored, e.g., thermally assisted switching. Recently it has been found that irradiation of microwaves can also reduce the quasi-static switching field. The microscopic details of this non-linear process are not yet fully understood. Our recent investigations on both microscopic Permalloy rings and arrays of permalloy nanowires show that excitation of specific spin-wave excitations leads to efficient switching. The resonant microwave absorption process will allow to address nanomagnets separately which exhibit a different magnetic configuration and thereby different spin wave eigenfrequencies. However, we have observed that efficient microwave assisted switching is not provided by all spin-wave excitations, but only selected ones. A predictive theory still needs to be developed.

Microwave assisted switching of rings from the vortex (blue color) to the onion (red) state. The white phase boundary line indicates the critical microwave field hrf for switching. It is found to depend on the frequency of the microwave source. Resonant behavior is found around 2.7 GHz and 5.5 GHz for the specific ring parameters and applied magnetic field H. At these frequencies only a small microwave field is needed to initiate the switching. The total field for switching is reduced if compared to the quasi-static value. The dataset summarizes more than 8000 irradiation experiments. Rings had an outer diameter of 2 µm and a width of 500 nm.