  We demonstrate the coupling of rare-earth ions locally implanted in a
substrate (Gd$^{3+}$ in Al$_{2}$O$_{3}$) to a superconducting NbN
lumped-element micro-resonator. The hybrid device is fabricated by a controlled
ion implantation of rare-earth ions in well-defined micron-sized areas, aligned
to lithographically defined micro-resonators. The technique does not degrade
the internal quality factor of the resonators which remain above $10^{5}$.
Using microwave absorption spectroscopy we observe electron-spin resonances in
good agreement with numerical modelling and extract corresponding coupling
rates of the order of $1$ MHz and spin linewidths of $50 - 65$ MHz.
