  We review how a magnetic resonance force microscope (MRFM) can be applied to
perform ferromagnetic resonance (FMR) spectroscopy of \emph{individual}
sub-micron size samples. We restrict our attention to a thorough study of the
spin-wave eigen-modes excited in permalloy (Py) disks patterned out of the same
43.3 nm thin film. The disks have a diameter of either 1.0 or $0.5 \mu$m and
are quasi-saturated by a perpendicularly applied magnetic field. It is shown
that \emph{quantitative} spectroscopic information can be extracted from the
MRFM measurements. In particular, the data are extensively compared with
complementary approximate models of the dynamical susceptibility: i) a 2D
analytical model, which assumes an homogeneous magnetization dynamics along the
thickness and ii) a full 3D micromagnetic simulation, which assumes an
homogeneous magnetization dynamics below a characteristic length scale $c$ and
which approximates the cylindrical sample volume by a discretized
representation with regular cubic mesh of lateral size $c=3.9$ nm. In our
analysis, the distortions due to a breaking of the axial symmetry are taken
into account, both models incorporating the possibility of a small misalignment
between the applied field and the normal of the disks.
