Magnetization dynamics of a single Fe-filled carbon nanotube detected by ferromagnetic resonance

Designing future spintronic devices or entities requires the knowledge of their static and dynamic properties. However, the precise magnetic characterization of a single nanostructure like a nanowire or dot, e.g. by ferromagnetic resonance, is still an experimental challenge.
Broadband ferromagnetic resonance (FMR) or cavity-based FMR usually lack the necessary sensitivity to measure single micron-sized or smaller structures. Measurements of arrays of such elements often might be no alternative due to inhomogeneity either.
Here we show that microresonator FMR [1] can overcome these limitations. By decreasing the resonator size the filling factor is increased, therefore boosting the FMR sensitivity by several orders of magnitude.
This allows for measuring even a single Fe-filled carbon nanotube (Fe-CNT) [2]. The Fe-filling inside the CNT has a diameter of 42 nm with an initial length of several micrometers (see Fig. 1). In order to understand the origin of the various resonance peaks, a focused ion beam was used to shorten little-by-little the length of the Fe-CNT after each FMR measurement.
Furthermore, angle-dependent FMR measurements were performed to extract the anisotropy contributions (like shape and magnetocrystalline anisotropy). In addition, the measured narrow linewidth suggests that the Fe-filling is a well ordered crystal, as confirmed by transmission electron microscopy. Therefore, besides the magnetic characterization of a single nanostructure, the microresonator FMR can also provide ‘indirectly’ information about the crystalline structure.