Spin wave free spectrum and magnetic field gradient of nanopatterned planes of ferromagnetic cobalt nanoparticles: key properties for magnetic resonance based quantum computing

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 experimental study is performed as a function of the applied magnetic field orientation, temperature, 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 having 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 paramagnetic resonance may be relevant for implementing an intermediate scale quantum computer based on arrays of coupled electron spins, as previously reported [J. Tribollet, Eur. Phys. J. B 87, 183 (2014)]. 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 electron spin qubit decoherence induced by the collective magnetic excitation of those nanoparticles.