Detecting Ferromagnetic Resonance of Single Nanostructures with the Microresonator

Nowadays the magnetic characterization of nanosized objects is of major interest, as they are used in the field of nanosized magnetic recording [1] and spintronic devices. With conventional resonance methods, e.g. in an X-band cavity, the minimum number of spins, which can be detected, is about 1012 for permalloy [2]. Therefore, only large arrays of small nanoobjects can be detected. Here, the specific characteristics of small nanoobjects disappear in the averaged signal [3]. Hence, for the analysis of single nanoobjects a much higher sensitivity is required.
We developed a microresonator to measure the ferromagnetic resonance (FMR) of single nanosized objects [4], [5]. We optimized the device for different sample sizes. We show that it is possible to measure FMR on a single Cobalt nanodot with a diameter of 139 nm and 25 nm thickness. Taking the signal-to-noise ratio into account, we extrapolate the detection limit to 105 spins. Not only the uniform excitation mode, but also signals of standing spinwaves like e.g. edgemodes can be observed. Their state can be visualized and interpreted with micromagnetic simulations (OOMMF) [6]. We show results of a variety of nanosized samples and their interpretation.

REFERENCES
[1]C. T. Rettner, et. al., IEEE Trans. Magn., 38, 4 (2002).
[2] Poole, Electron Spin Resonance, New York: Wiley, (1983).
[3] J. M. Shaw, et.al., J. Appl. Phys. 108, 093922 (2010).
[4] A. Banholzer, et al., Nanotechnology 22, 295713 (2011).
[5] R. Narkowicz et al., Rev. Sci. Instrum. 79, 084702 (2008).
[6] http://math.nist.gov/oommf/