  Focused-electron-beam-induced deposition is a promising technique for
patterning nanomagnets for spin qubit control in a single step. We fabricate
cobalt nanomagnets in such a process, obtaining cobalt contents and saturation
magnetizations comparable to or higher than those typically obtained using
electron-beam lithography. We characterize the nanomagnets using transmission
electron microscopy and image their stray magnetic field using scanning NV
magnetometry, finding good agreement with micromagnetic simulations. The
magnetometry reveals the presence of magnetic domains and halo side-deposits,
which are common for this fabrication technique. Finally, we estimate dephasing
times for electron spin qubits in the presence of disordered stray fields due
to these side-deposits.
