  We combine top-down and bottom-up nanolithography to optimize the coupling of
small molecular spin ensembles to $1.4$ GHz on-chip superconducting resonators.
Nanoscopic constrictions, fabricated with a focused ion beam at the central
transmission line, locally concentrate the microwave magnetic field. Drops of
free-radical molecules have been deposited from solution onto the circuits. For
the smallest ones, the molecules were delivered at the relevant circuit areas
by means of an atomic force microscope. The number of spins $N_{\rm eff}$
effectively coupled to each device was accurately determined combining Scanning
Electron and Atomic Force Microscopies. The collective spin-photon coupling
constant has been determined for samples with $N_{\rm eff}$ ranging between $2
\times 10^{6}$ and $10^{12}$ spins, and for temperatures down to $44$ mK. The
results show the well-known collective enhancement of the coupling proportional
to the square root of $N_{\rm eff}$. The average coupling of individual spins
is enhanced by more than four orders of magnitude (from $4$ mHz up to above
$180$ Hz) when the transmission line width is reduced from $400$ microns down
to $42$ nm, and reaches maximum values near $1$ kHz for molecules located on
the smallest nanoconstrictions. This result opens promising avenues for the
realization of magnetic spectroscopy experiments at the nanoscale and for the
development of hybrid quantum computation architectures based on molecular spin
qubits.
