In magnetic resonance force microscopy (MRFM) a small magnetic particle attached to the end of a cantilever is utilized to localize the magnetic excitation of the sample under investigation. The sample is excited using microwave radiation which is either amplitude or frequency modulated at the cantilever resonance frequency. The field gradient of the tip generates a force on the cantilever, which, due to the high Q factor of the cantilevers typically used in MRFM-experiments, leads to a measurable oscillation amplitude of the cantilever.
An experimental setup for MRFM-experiments at liquid Helium temperatures is shown in the figure on the right. The magnetic cantilever is positioned using a combination of a three axis inertial piezo drive (AttoCube ANPxyz50) and a four quadrant piezo tube for scanning. This setup enables both the detection of electron spin resonance (ESR) and ferromagnetic resonance (FMR). FMR spectra obtained with an MRFM setup are somewhat more complicated than regular FMR spectra, as they contain not only information about the average properties of the sample, but also local information on the dynamic magnetic properties of the part of the sample right underneath the magnetic tip. A spectrum of a sample consisting of permalloy discs (1.5mum diameter, 1.8 mum apart and 50nm thick) is shown in the following figure.

The two different contributions that constitute the spectrum, i.e. from dots far away and directly underneath the magnetic tip, can easily be separated by measuring the distance dependence of the spectra. As the dots far away from the tip are subject to a very small additional field from the magnetic tip this part of the spectrum can be used to measure the average dynamic properties of the sample just like in conventional FMR experiments. Another way of confirming that the minima in the spectrum above are indeed caused by the ferromagnetic resonance of the dot right underneath the tip, is to laterally scan the sample surface for a fixed external magnetic field in this range and measure the cantilever amplitude as a function of position. This way one obtains a map of the force acting on the cantilever at different lateral positions, as shown in the next figures.