  A recent advance in improving the spatial resolution of magnetic force
microscopy (MFM) uses as sensor tips carbon nanotubes grown at the apex of
conventional silicon cantilever pyramids and coated with a thin ferromagnetic
layer. Magnetic images of high density vertically recorded media using these
tips exhibit a doubling of the spatial frequency under some conditions. Here we
demonstrate that this spatial frequency doubling is due to the switching of the
moment direction of the nanotube tip. This results in a signal which is
proportional to the absolute value of the signal normally observed in MFM. Our
modeling indicates that a significant fraction of the tip volume is involved in
the observed switching, and that it should be possible to image very high bit
densities with nanotube magnetic force sensors.
