Using nanofabrication processes, we have synthesized a mesoscopic assembly of interacting nanomagnets, with statistical properties quantitatively described by an Ising-like spin Hamiltonian. This artificial realization of a spin model has been imaged in real space using a synchrotron-based magnetic imaging technique. The magnetic con ! gurations we obtain after demagnetizing the spin assembly fit well with predictions from Monte Carlo simulations. In particular, we observe the emergence of a phase where spins fluctuate while classical magnetic charges, associated with these spins, crystallize. This phase was predicted theoretically, but never observed experimentally. Our results open the way to the investigation of a wide variety of classical spin models within the concept of the “lab on a chip”. (Full text)