  In this work, we present a detailed investigation of the magnetic properties
of cobalt nanospheres grown on cantilever tips by Focused Electron Beam Induced
Deposition (FEBID). The cantilevers are extremely soft and the cobalt
nanospheres are optimized for Magnetic Resonance Force Microscopy (MRFM)
experiments, which implies that the cobalt nanospheres must be as small as
possible while bearing high saturation magnetization. It is found that the
cobalt content and the corresponding saturation magnetization of the
nanospheres decrease for nanosphere diameters below 300 nm. Electron holography
measurements show the formation of a magnetic vortex state in remanence, which
nicely agrees with magnetic hysteresis loops performed by local magnetometry
showing negligible remanent magnetization. As investigated by local
magnetometry, optimal behavior for high-resolution MRFM has been found for
cobalt nanospheres with diameter of $\approx$ 200 nm, which present atomic
cobalt content of $\approx$ 83 at% and saturation magnetization of 106 A/m,
around 70% of the bulk value. These results represent the first comprehensive
investigation of the magnetic properties of cobalt nanospheres grown by FEBID
for application in MRFM.
