Abstract
A new magnetic force microscopy (MFM) technique for measuring alternating magnetic field (ac magnetic field) was proposed by using frequency modulation (FM) phenomenon of tip oscillation. We detected a narrowband FM phenomenon in the tip oscillation of a high-coercivity MFM tip by applying an ac magnetic field to the tip by using a metal-in-gap (MIG) type ring head. In the experiment, the MFM tip was driven at a constant frequency fc near the resonant frequency of the cantilever by a piezoelectric element, and the ac magnetic field with a frequency fm up to 10 kHz was applied to the MFM tip. Two sideband spectra with a frequency of fc±fm were observed by applying ac magnetic field. The intensity of sideband spectra increased linearly with increasing applied current value of the MIG head. It was observed that FM occurred from the analysis of frequency spectra of the MFM signals. This FM phenomenon is caused by the force applied on the cantilever by the field gradient of the head. The FM phenomenon is understood by solving the differential equation for the harmonic oscillator of which the effective spring constant is changed periodically by ac magnetic field. The FM phenomenon can be used for imaging ac magnetic fields.
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