  We present a study by ferromagnetic resonance at microwave Q band of two
sheets of cobalt nanoparticles obtained by annealing SiO2 layers implanted with
cobalt ions. This ex- perimental study is performed as a function of the
applied magnetic field orientation, tempera- ture, and dose of implanted cobalt
ions. We demonstrate that each of those magnetic sheet of cobalt nanoparticles
can be well modelled by a nearly two dimensional ferromagnetic sheet hav- ing a
reduced effective saturation magnetization, compared to a regular thin film of
cobalt. The nanoparticles are found superparamagnetic above around 210 K and
ferromagnetic below this blocking temperature. Magnetostatic calculations show
that a strong magnetic field gradient of around 0.1 G/nm could be produced by a
ferromagnetic nanostripe patterned in such magnetic sheet of cobalt
nanoparticles. Such a strong magnetic field gradient combined with electron
para- magnetic resonance may be relevant for implementing an intermediate scale
quantum computer based on arrays of coupled electron spins, as previously
reported (Eur. Phys. J. B (2014) 87, 183). However, this new approach only
works if no additional spin decoherence is introduced by the spin waves
exitations of the ferromagnetic nanostructure. We thus suggest theoretically
some possible magnetic anisotropy engineering of cobalt nanoparticles that
could allow to suppress the spin qubit decoherence induced by the unwanted
collective excitation of their spins.
